145
Biol Res 27: 1 4 5 - 1 5 7 ( 1 9 9 4 )
Phrenic nerve activity during artificial
ventilation at different body temperatures and its
relationships with carotid chemosensory activity
R I T U R R I A G A , C L A R R A I N and P Z A P A T A *
L a b o r a t o r y of N e u r o b i o l o g y , Catholic University of Chile, S a n t i a g o , Chile
While the chemoreceptor
discharges of carotid bodies in vitro are highly
dependent
on temperature,
these chemoreceptors
in situ contribute
only moderately
to the
ventilatory adjustment to changing body temperature
(T ), probably because of the
concomitant
and reverse changes in natural chemoreceptor
stimuli in
closed-loop
preparations.
Accordingly,
we studied the frequency
of carotid
chemosensory
discharge
(f ) and the phrenic
integrated
electroneurogram
(IENG .) in
pentobarbitone
anesthetized
cats, paralyzed
with alcuronium
and
artificially
ventilated, at three steady-state
levels ofT (35.5, 37.5 and 40.2° C), modifying
the
frequency
and volume of the ventilator to maintain P C0
within normal
range.
While fx increases along with T when P C0
is allowed tofluctuate freely, its mean
basal value was not consistently different at the three T 's studied under
controlled
conditions.
The amplitude of IENG.
was reduced and the frequency
of phrenic
inspiratory cycles was increased as T was raised from 35.5 to 37.5°C and then to
40.2° C. Brief J 00% 0 inhalations and iv injections of dopamine produced
minimal
depressions
of IENG
amplitude in hypothermia,
but pronounced
although
similar
depressions in normothermia
and hyperthermia.
Iv injections ofNaCN
augmentedf
and IENG
in dose related manner, and the relationships
between both
variables
showed larger changes in IENG
at the hypothermic
and normothermic
conditions
when expressed in absolute terms, but not when expressed in relative
terms.
Thus, the chemosensory
input is not consistently modified by thermal levels
under
controlled
ventilatory
conditions,
but the chemosensory
drive of the
ventilatory
output is less pronounced
in hypothermia.
The chemosensory
input is
similarly
affected by varying degrees of cytotoxic hypoxia at different T 's, but the
ventilatory
output is less vigorously increased in hyperthermia,
pointing to a decreased
reflex
gain in that
condition.
b
x
b
EI
b
Er
2
2
b
b
2
ph
x
ph
ph
b
K e y w o r d s : body temperature,
hyperthermic
hyperventilation,
carotid body, chemosensory
activity,
integrated electroneurogram,
phrenic
INTRODUCTION
T h e c h e m o s e n s o r y nerve discharge record­
ed from the carotid b o d y supervised in vitro
is highly d e p e n d e n t o n the t e m p e r a t u r e of the
m e d i u m w h e n the rest of the variables are
kept constant (Gallego et al, 1979; A l c a y a g a
et al, 1993). Similarly, transient increases
in the t e m p e r a t u r e of the blood circulating
*
chemoreceptors,
activity.
through o n e carotid p r o d u c e fast increments
in the frequency of c h e m o s e n s o r y discharges
(f ) recorded from the carotid (sinus) nerve
of cats ( M c Q u e e n and E y z a g u i r r e , 1 9 7 4 ) .
O t h e r w i s e , i n c r e a s i n g t h e t e m p e r a t u r e of
blood perfusing the vascularly isolated carotid
b o d i e s of d o g s l e a d s t o t r a n s i e n t r e f l e x
increases in breathing (Bernthal and W e e k s ,
1939).
x
Correspondence to: Dr P Zapata, Laboratory of Neurobiology, Catholic University of Chile, PO Box 114-D, Santiago 1,
Chile. Fax: (56-2)222-5515.
146
Biol Res 27: 145-157 (1994)
In spite of the a b o v e , recent observations
s h o w a significant b u t m o d e r a t e contribution
of the peripheral c h e m o r e c e p t o r s to the ven­
tilatory a d j u s t m e n t p r o d u c e d b y s u s t a i n e d
h y p e r t h e r m i a (Fadic et al, 1991). Furthermore,
only slight increases in f w e r e observed w h e n
raising b o d y t e m p e r a t u r e in t h e spontaneously
breathing cat (Loyola et al, 1991). Since in the
w h o l e a n i m a l , in steady-state conditions, the
v e n t i l a t o r y r e g u l a t i o n is m a i n t a i n e d b y
h o m e o s t a t i c closed l o o p s , the resultant hyper­
t h e r m i c hyperventilation m a y reduce the levels
of the c h e m i c a l stimuli acting u p o n the carotid
and aortic b o d i e s , and partially cancels the
direct h y p e r t h e r m i c effect o n c h e m o r e c e p t o r
activity (Eyzaguirre and Zapata, 1984).
Alternatively, it is possible that the hyper­
thermia could m o d u l a t e the central gain of the
chemosensory input o n the inspiratory neurons.
In fact, h y p e r t h e r m i a still induces a strong
t a c h y p n e a after bilateral carotid n e u r o t o m y
(Fadic etal, 1991).
x
T h e e x p e r i m e n t s reported here w e r e in­
tended to a n s w e r the apparent discrepancy in
the a b o v e results. W e studied the contribution
of the peripheral chemoreceptors to the efferent
p h r e n i c nerve activity in the cat anesthetized
with s o d i u m p e n t o b a r b i t o n e , paralyzed and
artificially ventilated. In this preparation, the
ventilatory regulation d e p e n d s o n the p u m p
i m p o s e d ventilation, w h i c h allows the levels
of the respiratory g a s e s to be maintained or
modified, independently of the p h r e n i c neural
output. F u r t h e r m o r e , sodium pentobarbitone
modifies the h o m e o t h e r m i c regulation, m a k ­
ing the b o d y c o r e t e m p e r a t u r e ( T ) m o r e de­
pendent on environmental temperature changes
(Refinetti and Carlisle, 1989).
b
O b s e r v a t i o n s o n the afferent input from
arterial c h e m o r e c e p t o r s were restricted to the
c h e m o s e n s o r y afferences from the carotid
bodies. T h i s is based on the fact that the carotid
bodies play a predominant role in the excitation
of ventilatory reflexes induced by cytotoxic
h y p o x i c stimulation (Serani and Zapata, 1981)
and in the c h e m o s e n s o r y drive of ventilation
u n d e r resting conditions (Eugenin et al, 1989).
It m u s t b e n o t e d that the s e c t i o n of o n e
glossopharyngeal nerve, required for recording
from the cut carotid nerve, does not modify
significantly the basal ventilatory variables in
p e n t o b a r b i t o n e anesthetized cats ( E u g e n i n et
al, 1990).
METHODS
E x p e r i m e n t s w e r e performed o n 12 adult m a l e
cats, w e i g h i n g 2.96 ± 0.16 k g ( m e a n ± S E M ) ,
and anesthetized w i t h s o d i u m p e n t o b a r b i t o n e
4 0 m g / k g , ip. Additional d o s e s (12 m g iv)
w e r e given w h e n n e c e s s a r y to m a i n t a i n a light
level of surgical anesthesia (stage III, p l a n e 2)
ascertained b y the a b s e n c e of w i t h d r a w a l re­
flexes to strong p r e s s u r e o n the p a w s , with
persistence of patellar reflexes.
T h e cats w e r e placed in supine position.
The body core temperature (T ) was monitor­
ed c o n t i n u o u s l y t h r o u g h a t h e r m i s t o r p r o b e
introduced 5 0 m m into t h e rectum and con­
nected to a tele-thermometer. T h e initial rectal
temperature was 37.4 ± 0.2°C. T h e T w a s then
m a i n t a i n e d s u c c e s s i v e l y at s t e a d y - s t a t e
values of 3 5 . 5 , 37.5 and 4 0 . 2 + 0.2°C, with a
regulated heating p a d , placed u n d e r the cat,
and controlled a u t o m a t i c a l l y b y a t h e r m o regulator driven from the p r o b e . T o attain the
h y p e r t h e r m i c condition, additional h e a t was
provided by an o v e r h e a d l a m p (20 c m a b o v e
the a b d o m i n a l wall) controlled b y a d i m m e r .
Care w a s t a k e n not to e x c e e d 5 0 ° C of surface
t e m p e r a t u r e at the e x p o s e d sites of the skin.
T h e e x p e r i m e n t s w e r e p e r f o r m e d u n d e r the
following prevailing c o n d i t i o n s : r o o m t e m ­
perature 2 8 . 3 ± 1.4° C, relative h u m i d i t y 62.5
+ 1.5 %, a t m o s p h e r i c pressure 7 4 0 . 3 ± 0.2
Torr.
b
b
T h e trachea w a s cannulated per os with a
flexible tube (3.8 m m I D , 4.4 m m O D ) . A fine
(PE-10) t u b i n g w a s introduced d e e p into the
tracheal cannula for c o n t i n u o u s s a m p l i n g of
air and recording of the tracheal C 0 pressure
( P C 0 ) t h r o u g h an infrared g a s analyzer.
A l v e o l a r v e n t i l a t i o n w a s e s t i m a t e d from
breath-by-breath end-tidal C 0
pressure
( P C 0 ) , calculated after subtracting the H 0
pressure in moist tracheal air (47 T o r r ) from
the barometric pressure.
2
T
2
2
E T
2
2
T h e right s a p h e n o u s vein was cannulated
for administration of d r u g s . S y s t e m i c arterial
pressure was recorded from the right femoral
artery t h r o u g h a cannula ( P E - 1 0 0 ) filled with
h e p a r i n 5 0 I U / m l in s a l i n e s o l u t i o n , and
connected to a pressure transducer. H e a r t rate
was counted electronically using a t a c h o g r a p h
triggered by the arterial p r e s s u r e p u l s e .
T h e cats were paralyzed by iv administration
of a l c u r o n i u m c h l o r i d e ( 1 0 0 - 2 0 0 u g / k g
147
Biol Res 27: 1 4 5 - 1 5 7 ( 1 9 9 4 )
initially, and subsequently 5 0 - 1 0 0 |ig/kg every
3 0 m i n ) a n d artificially v e n t i l a t e d w i t h a
positive-pressure ventilator. The tracheal
c a n n u l a w a s c o n n e c t e d to a Y - s h a p e d t u b e .
O n e a r m w a s c o n n e c t e d to the v e n t i l a t o r
t h r o u g h a h e a t e d Fleisch p n e u m o t a c h o g r a p h
( n u m b e r 00) and to a P T - 5 volumetric pressure
t r a n s d u c e r for r e a d i n g inspiratory flow. Tidal
volume ( V ) was obtained by electronic
integration of the flow signal. T h e other end
of the Y - t u b e w a s c o n n e c t e d t o a p i e c e of
flexible t u b i n g e n d i n g at the b o t t o m of a bottle
c o n t a i n i n g w a t e r (5-8 cm height) to allow
expiration and e s c a p e of the extra inspiratory
air. A t the b e g i n n i n g of the artificial ven­
tilation, t h e inspiratory v o l u m e w a s fixed to
obtain a P C 0 v a l u e similar to the p r e v i o u s
one, r e c o r d e d d u r i n g s p o n t a n e o u s breathing.
T h e expiratory side of the system w a s blocked
for 1 or 2 c y c l e s from t i m e to t i m e (5 m i n ) to
p r o d u c e a u g m e n t e d inspirations (simulated
g a s p s ) , intended to m a i n t a i n an a d e q u a t e al­
v e o l a r surfactant. Since h y p e r t h e r m i a raised
P C 0 , the frequency of the ventilator w a s
a u g m e n t e d to k e e p the P C 0 l o w .
T h e p h y s i o l o g i c a l variables w e r e display­
ed o n a p o l y g r a p h a n d a m u l t i p l e b e a m
oscilloscope. R a w signals w e r e stored on a
digital video system (through an analog-digital
converter) for latter analyses.
T
E T
E T
2
2
E T
Recordings
of afferent
2
and efferent
fibers
T h e right carotid and p h r e n i c nerves w e r e
e x p o s e d t h r o u g h a lateral longitudinal incision
in the n e c k . T h e b r a n c h of the p h r e n i c n e r v e
arising from the C 5 root w a s dissected free
of the e p i n e u r i u m , cut caudally, placed o n a
pair of b i p o l a r Pt-Ir electrodes and covered
w i t h w a r m mineral oil. T h e efferent neural
d i s c h a r g e s w e r e preamplified, amplified, full
w a v e rectified ( R E N G ) and integrated
( I E N G ) . T h e efferent signals w e r e also fed
to an amplitude w i n d o w discriminator and the
i n s t a n t a n e o u s frequency ( f . ) of the standard­
ized pulses w a s obtained electronically.
In five e x p e r i m e n t s , the carotid (sinus)
nerve w a s dissected and cut, and the ipsilatcral
g a n g l i o g l o m e r a l nerves were also cut.
Barodenervation was performed through
section and crush of n e r v e filaments b e t w e e n
the e m e r g e n c e of the internal carotid artery
and the carotid b o d y (Zapata et al, 1969). After
p h
p h
excision of the e p i n e u r i u m , the n e r v e w a s
placed o n a pair of Pt-Ir e l e c t r o d e s and cover­
ed with w a r m m i n e r a l oil. T h e signals w e r e
preamplified, amplified, p a s s e d t h r o u g h a 5 0
H z filter and fed to an electronic a m p l i t u d e
discriminator w h i c h allowed selection of action
potentials of given amplitudes above the
baseline noise. T h e frequency of the resulting
standardized c h e m o s e n s o r y p u l s e s (f ) w a s
counted every s e c o n d b y m e a n s of a n elec­
tronic counter-printer s y s t e m and obtained as
i n s t a n t a n e o u s rate t h r o u g h a frequency m e t e r .
R e c o r d i n g s of t h e c a r o t i d afferent a n d
p h r e n i c efferent n e r v e s o n the s a m e side is
justified by the fact that ventilatory c h e m o reflexes are not lateralized (Berger and
M i t c h e l l , 1976).
x
Testing of chemosensory
chemoreflex
responses
drive
and
T h e c o n t r i b u t i o n of the p e r i p h e r a l c h e m o ­
sensory drive to the p h r e n i c n e r v e activity
w a s estimated from the c h a n g e s in a m p l i t u d e
of the I E N G , resulting from the w i t h d r a w a l
of the c h e m o s e n s o r y i m p u l s e s c a u s e d b y e x ­
p o s u r e to 1 0 0 % 0 (Dejours, 1963) for 1 m i n
t h r o u g h the ventilator, and b y iv injections
of d o p a m i n e - H C l ( 1 - 5 0 u g / k g ; p r e p a r e d in
ascorbic acid 1 m M in saline) ( Z a p a t a and
Z u a z o , 1980).
C h e m o r e f l e x e s sensitivity and reactivity
w e r e assessed b y s t u d y i n g the p h r e n i c n e u r a l
responses to increasing iv injections of N a C N
(0.5 to 100 u g / k g ) and b y a d m i n i s t r a t i o n
of 1 0 0 % N for 10-15 s t h r o u g h the ventilator.
T h e tests w e r e repeated at t h e three T ' s at
steady-state conditions.
p h
2
2
b
Data
analyses
Ventilatory variables were m e a s u r e d and
averaged o v e r p e r i o d s of 1 m i n d u r i n g steadystate t h e r m o r e g u l a t o r y and ventilatory c o n d i ­
tions. Respiratory frequency ( f ) w a s m e a s u r ­
ed from the p h r e n i c burst frequency. C o n s i d ­
ering that the I E N G is the neural equivalent
of tidal v o l u m e ( V ) , a n e u r a l i n s p i r a t o r y
m i n u t e v o l u m e i n d e x (V ) w a s estimated by
multiplying I E N G ampfitude by f .
Results are expressed as m e a n s ± S E M ' s
or as p e r c e n t a g e s of their respective values.
Statistical differences for m u l t i p l e d e p e n d e n t
R
h
T
h
p h
R
148
Biol Res 27: 145-157 (1994)
samples were assessed by either Quade's or
F r i e d m a n ' s test, w h i c h e v e r the m o s t a p p r o ­
priate, followed b y paired comparisons through
C o n o v e r ' s tests ( T h e o d o r s o n - N o r h e i m , 1987).
T o c o m p a r e d a t a from different e x p e r ­
i m e n t s , the c y a n i d e dose-response curves w e r e
fitted to s y m m e t r i c a l s i g m o i d a l functions (De
L e a n et al, 1978). This m e t h o d provides a basis
for p o o l i n g data from separate e x p e r i m e n t s ,
and allows testing the characteristics w h i c h
are shared b y v a r i o u s c u r v e s . T h e data points
w e r e adjusted a c c o r d i n g t o the f o l l o w i n g
logistic e x p r e s s i o n for responses expressed in
percentages:
S
R = max R + [100 - max R] / [1 + (D/ED ) ]
50
of 3 5 . 5 , 37.5 and 4 0 . 2 ± 0.2° C in t h e 7 cats
w i t h intact carotid n e r v e s . Before adjustments
in the frequency of the p u m p , the P C 0
increased slightly from 33.6 ± 2.2 T o r r at
35.5° C to 37.0 ± 2.5 T o r r at 37.5° C (p <
0.05), and t h e n c o n s i d e r a b l y to 56.8 ± 7.9
T o r r w h e n T w a s raised to 4 0 . 2 ° C (p < 0.01).
In o r d e r to m a i n t a i n P C 0 relatively l o w ,
the frequency of the p u m p v e n t i l a t o r (f ) w a s
raised from 2 2 . 0 ± 2.8 m i n " t o 2 8 . 7 ± 3.1
m i n " at the h i g h e r T l e v e l (fig I B ) , w h i c h
resulted in a r e d u c t i o n in P C 0 to 33.5 ± 1 . 3
T o r r , a v a l u e n o t significantly different from
those o b s e r v e d at l o w e r t e m p e r a t u r e s (fig 1 A ) .
In 3 out of the 7 e x p e r i m e n t s , w e avoided the
potentially n o x i o u s effects of the h y p e r c a p n i a
and acidosis p r o v o k e d b y raising T to 4 0 . 2 °
C b y progressively increasing the frequency
of the ventilator during the w a r m i n g procedure.
T h e m e a n a m p l i t u d e of the I E N G record­
ed at b a s a l c o n d i t i o n s w a s c o n s i d e r a b l y
reduced at the n o r m o t h e r m i c level in c o m p a r ­
ison with the h y p o t h e r m i c level, and a further
but small decrease occurred w h e n r e a c h i n g the
h y p e r t h e r m i c i s o c a p n i c c o n d i t i o n (fig 2 A ) . O n
the contrary, the respiratory n e u r a l frequency
(f ) increased significantly w h e n T and f
w e r e elevated (fig 2 B ) . A s a result of the pro­
gressive decreases in m a g n i t u d e of the phrenic
inspiratory bursts and the reciprocal increases
in f observed while increasing T , the neural
m i n u t e v o l u m e ( V , ) w a s n o t significantly
modified b y thermal c h a n g e s (fig 2 C ) .
T h e large increase in f o b s e r v e d in 4 cats
w h e n a u g m e n t i n g f at 4 0 . 2 ° C (fig 2 B ) is
E T
2
b
E T
2
1
1
b
E T
2
b
where: R = response; m a x R = maximal
response; D = arithmetic dose; E D = m e d i a n
effective dose; S = slope factor determining the
steepness of e a c h curve. T h e curves w e r e fitted
t h r o u g h a c o m p u t e r p r o g r a m based o n a sim­
plex algorithm (Johnston, 1985). T h e goodness
of e a c h fit w a s tested using an A N O V A and
by the correlation coefficient ( r ) , calculated by
dividing the v a r i a n c e of the theoretical value
by that of the e x p e r i m e n t a l value.
5 0
RESULTS
Ejfect of thermal conditions
respiratory
variables
on
resting
70
R
b
R
F i g u r e 1A s h o w s a s u m m a r y of the m e a n
v a l u e s of P C 0 o b s e r v e d at steady-state T ' s
E T
p h
2
b
R
p
B
r
35.5
37.5
40.2
40.2°C
b
35
r
35.5
37.5
40.2
40.2"C
Fig 1. Means + SEM's of (A) end-tidal C 0 pressure ( P C 0 ) and (B) frequency of pump ventilator ( f ) at three different steadystate T 's in 7 cats with intact carotid nerves. Open bars, at 35.5° C; horizontally striped bars, al 37.5°C; hatched bars, at 40.2° C
before adjusting f to control P C 0 in 4 cats; cross-hatched bars, at 40.2° C after raising f to maintain P C 0 low. Statistical
significance of multiple comparisons (insets, overall p values from Friedman's tests, n = 7 ) , followed by paired comparisons
(asterisks, significantly different from all the rest al p < 0.01; between arrows, difference at p < 0.05). Quade's tests applied for
comparisons of hypercapnic hyperthermic cats with other conditions (n = 4).
2
E T
2
b
E T
2
E T
2
149
Biol Res 27: 145-157 (1994)
35.5
37.5
40.2
40.2*C
35.5
37.5
40.2
40.2'C
35.5
37.5
40.2
40.2-C
Fig 2. Means + SEM's of (A} phrenic integrated electroneurogram (IENG^), (B) neural respiratory frequency (f ) and (C) neural
inspiratory minute volume (V^) at 3 different steady-state T 's in 7 cats with intact carotid nerves. Bars and statistical analyses,
as in Figure 1.
R
b
clearly t h e result of the e n t r a i n m e n t of the
p h r e n i c inspiratory bursts b y the frequency of
the p u m p ventilator, since it is associated to
a fall in b a s a l P C 0 (fig 1A) and o c c u r s
w i t h o u t c h a n g e in T . T h e p h r e n i c bursts w e r e
also entrained to t h e ventilator rate at the other
T ' s , as illustrated in figure 3 . T h e r e , an abrupt
increase in f from 14 c y c l e s / m i n to 2 2 c y c l e s /
m i n p r o d u c e d an i m m e d i a t e c h a n g e in f from
14 " n e u r a l " b r e a t h s / m i n to 2 2 , w h i c h w a s also
abruptly reversed w h e n returning to the initial
c o n d i t i o n s . T h i s indicates that neural bursts
w e r e all t i m e entrained b y the l u n g inflations
i m p o s e d b y the p u m p . F u r t h e r m o r e , the abrupt
transition to a h i g h e r frequency of ventilatory
p u m p i n g a n d t h e n c e of p h r e n i c b u r s t s is
followed b y a slow p r o g r e s s i v e decrease in
the strength of the inspiratory p h r e n i c d i s ­
c h a r g e s estimated b y the I E N G (fig 3). Since
in all e x p e r i m e n t s f w a s increased, either
progressively or in steps, during the w a r m i n g
p r o c e d u r e , the e n t r a i n m e n t of p h r e n i c activity
m a y contribute to the observed increase in f
and decrease in I E N G w h e n passing from the
h y p o t h e r m i c to t h e n o r m o t h e r m i c and hyper­
t h e r m i c levels.
E T
35.5°C
2
b
b
R
p h
R
p h
In s o m e artificially ventilated cats w e o b ­
served the a p p e a r a n c e at r e g u l a r intervals of
b i p h a s i c bursts of p h r e n i c discharges within
a g i v e n c y c l e , the intensity and t i m i n g of the
initial p h a s e b e i n g s i m i l a r to t h o s e of the
r e m a i n i n g inspiratory cycles, b u t s u r m o u n t e d
by an extraordinary burst of h i g h e r instan­
t a n e o u s f r e q u e n c y . T h e s e t w o p h a s e s are
clearly d i s c e r n i b l e in the f
and R E N G
r e c o r d i n g s , b u t they add in the I E N G . to
exhibit a m o t o n e u r o n a l burst of m u c h larg­
er strength t h a n the rest. T h e s e a u g m e n t e d
inspiratory bursts w e r e m o r e c o m m o n l y seen
ph
p h
I min
A
A
Fig 3. Changes in tracheal pressure of CO, ( P C 0 ; first row),
phrenic integrated electroneurogram (IENG ; second row) and
phrenic rectified electroneurogram (RENG^; third row) in
response to switching the pump ventilator from 14 to 22
cycles/min (first arrow head) and then back to 14 cycles/min
(second arrow head). Cat with both carotid nerves intact and
thermostabilized at 35.5° C.
T
2
ft
at the h i g h e r T (see last r o w s in figs 7 and 8)
or they increased in frequency w h e n raising
T from 37.5 to 4 0 . 2 ° C.
b
b
Effect of thermal conditions
the response to NaCN
on
F i g u r e 4 illustrates the effects of steady levels
of T o n the p h r e n i c n e r v e r e s p o n s e s t o t w o
low-to-intermediate d o s e s of N a C N (5 and 10
ug/kg, iv) in o n e cat with b o t h carotid nerves
intact. T h e s e injections increased t h e m a g n i ­
t u d e of the I E N G and the duration of the
inspiratory bursts at 35.5 and 37.5° C, but did
not elicit any response at 40.2° C. T h e increase
in the I E N G in r e s p o n s e to 10 ug N a C N is
not associated to a n a u g m e n t e d instantaneous
rate of p h r e n i c bursts (f ) , thus i m p l y i n g that
b
p h
p h
h
150
Biol Res 27: 145-157 (1994)
NaCN (jugko)
r
TC0
(torr)
Is
J10
|5
jio
{5
A simpler and c o n t i n u o u s pattern of p r o l o n g e d
but paused phrenic discharge w a s initiated by
the last injection of this agent. Simulated gasps
(lung inflations along t w o cycles uninterrupt­
ed b y expirations, thus d o u b l i n g V ) , o n the
contrary, switched-off t h e a u g m e n t e d p h r e n i c
output, r e t u r n i n g t h e respiratory p a t t e r n to
basal conditions.
110
2
T
IENGph
(u)
I ph
(Hz)
Fig 4. Phrenic neural responses to NaCN (5 and 10 Ug/kg,
iv) at three different steady-state T ' s in a cat with intact
carotid nerves. P C ,
tracheal C 0 pressure (first row);
IENG^, phrenic integrated electroneurogram, in arbitrary units
(second row); f , instantaneous frequency of phrenic
discharges (third row).
Effect of thermal
to 100% 0 and
2
conditions
dopamine
on the
responses
b
T
m
2
F i g u r e 7 illustrates the transient d e p r e s s i o n of
efferent p h r e n i c n e u r a l activity in response to
h
TABLE I
the e n h a n c e d p h r e n i c output w a s n o t d u e to an
increased m a x i m a l frequency of motoneuronal
discharges b u t to a p r o l o n g a t i o n of such dis­
c h a r g e s , as e v i d e n c e d b y t h e configuration of
f during t h e affected cycle.
A systematic study of the phrenic responses
elicited b y increasing doses of N a C N (0.5
to 100 ug/kg) w a s performed with t h e data
obtained from 7 cats with intact carotid nerves.
F o r this study, the P C 0 at 40.2° C w a s kept
at a similar v a l u e as those observed at 35.5
and 37.5° C. F i g u r e 5 s h o w s the fitting to a
s i g m o i d a l function of the m a x i m a l I E N G
r e s p o n s e s (in p e r c e n t a g e s of basal activity)
p r o d u c e d b y N a C N at the three steady-state
T ' s levels. Statistically significant differences
w e r e o b s e r v e d in only o n e of t h e three para­
meters defining the dose-response curves
(Table I). T h e m a x i m a l (theoretical) reactivity
and the slope factor did n o t differ b e t w e e n the
three t h e r m a l conditions, b u t t h e m e a n effec­
tive d o s e w a s displaced to the right by nearly
o n e o r d e r of m a g n i t u d e at 4 0 . 2 ° C.
T h e pattern of phrenic nerve discharges was
m u c h modified b y c y a n i d e in s o m e occasions.
F i g u r e 6 illustrates the effects of large doses
of N a C N ( 2 0 to 100 ug/kg iv) and those of
e v o k e d gasps ( s e e M e t h o d s ) o n the cycles
of p h r e n i c d i s c h a r g e s . N a C N prolonged the
duration of the inspiratory discharge of the
p h r e n i c n e r v e and thus a u g m e n t e d c o n s i d ­
erably the a m p l i t u d e of I E N G . A peculiar
pattern of interlocked bursts of phrenic cycles
(one very prolonged and thus m u c h enhanced
and o n e o r t w o w i t h n o r m a l duration b u t in­
creased m a x i m a l instantaneous frequency) w a s
triggered b y the t w o first injections of cyanide.
P a r a m e t e r s defining d o s e - r e s p o n s e
curves for c h a n g e s in p h r e n i c activity
e v o k e d by iv injections of N a C N in
7 cats w i t h intact carotid n e r v e s
h
E T
2
p h
T„°C
max R
ED
5 0
s
r
35.5
37.5
258.7
15.1
1.3
0.997
264.3
9.2
1.6
0.998
40.2
P
262.3
89.9 *
0.9
0.969
0.093
0.006
0.185
r,
correlation coefficients for fitting the curves to sigmoidal
functions.
p, overall significance, after Friedman's tests.
* p < 0.01, vs the rest, after Conover's tests.
b
p h
100*
1
10
100
N a C N (jug/kg) i.v.
Fig 5. Dose-response curves for changes in amplitude of
phrenic integrated electroneurogram ( I E N G ) elicited by
increasing doses of NaCN iv at 3 different steady-state T 's,
obtained from 7 cats with intact carotid nerves. P C 0 values:
31.5 ± 1.8 torr at 3 5 . 5 C (open squares), 32.6 ± 1.6 torr at
37.5° C (open circles) and 31.6 ± 1.2 torr at 40.2° C (filled
circles). Responses (means + or - SEM's) expressed as
percentages of basal values. SEM's are only depicted to
illustrate dispersion of values, since statistical analyses were
based on ranks (Friedman's tests).
pb
b
E T
D
2
151
Biol Res 27: 1 4 5 - 1 5 7 ( 1 9 9 4 )
Fig 6. Effects of large doses of NaCN (20-100 ug/kg iv, filled
arrows) and simulated gasps (G, open arrows) on phrenic
efferent activity in a cat with intact carotid nerves, at 35.5° C.
P C 0 , tracheal pressure of C 0 (first row); IENG^, phrenic
integrated electroneurogram (second row); f^, instantaneous
frequency of phrenic discharges (third row).
T
2
2
Figure 8 illustrates the phrenic neural
response to the iv administration of d o p a m i n e
(2-10 ug/kg). These doses were selected
b e c a u s e they did not elicit considerable pressor
r e s p o n s e s (less than 10 T o r r ) . L a r g e d o s e s of
d o p a m i n e ( 2 0 - 1 0 0 ug/kg) p r o d u c e d m a r k e d
h y p e r t e n s i v e reactions ( m e a n arterial p r e s s u r e
w e n t u p to 2 0 0 T o r r o r m o r e ) , w h i c h in turn
m a y d e p r e s s the neural p h r e n i c output since
ventilatory d e p r e s s a n t effects of l a r g e d o s e s
of d o p a m i n e in s p o n t a n e o u s l y b r e a t h i n g cats
are still o b s e r v e d after section of t h e carotid
and aortic n e r v e s (Zapata and Z u a z o , 1980),
b e c a u s e of rise in i n t r a c i s t e m a l c e r e b r o s p i n a l
fluid p r e s s u r e (Serani et al, 1981).
T h e a m p l i t u d e and d u r a t i o n of t h e I E N G
depressant response to d o p a m i n e was markedly
a u g m e n t e d at 3 7 . 5 ° C as c o m p a r e d with that
observed at 35.5° C. At 37.5° C, the d o s e s of
5 and 10 ug/kg briefly silenced t h e p h r e n i c
n e r v e d i s c h a r g e s (fig 8). At 4 0 . 2 ° C, t h e
d e p r e s s a n t effects of d o p a m i n e u p o n p h r e n i c
output persisted, but they were not more
p r o n o u n c e d than t h o s e o b s e r v e d at 37.5° C.
h
r
TCO
(torr)
Z
IENGph
Effects of thermal conditions on basal
evoked chemosensory
activities
Fig 7. Transient effects of 1 or 1.5 min periods of 100% 0
inhalation on phrenic neural activity at three different T 's.
P C 0 , tracheal pressure of C 0 (first rows); I E N G , phrenic
integrated electroneurogram, in arbitrary units (second rows).
At 40.2°C, response tested at two levels of P C 0 ; note there
the presence of the neural equivalent of spontaneous gasps.
2
b
T
2
2
F i g u r e 9 s u m m a r i z e s the effects of c h a n g e s
in steadyrstate T ' s o n the m e a n frequencies
b
ph
E T
35.5»C
2
IENGp
b r e a t h i n g 1 0 0 % 0 at the three T ' s levels and
at t w o different P C 0 values. At 3 5 . 5 ° C ,
100% 0 inhalation r e d u c e d only m o d e r a t e l y
the I E N G (upper left p a n e l ) . A m o r e p r o ­
n o u n c e d d e c r e a s e of the integrated neural
output w a s observed at 37.5° C (upper right
panel). W h e n the f w a s raised at 4 0 . 2 ° C to
m a i n t a i n P C 0 l o w (lower left panel) an
intense transient reduction of the a m p l i t u d e of
the I E N G . w a s o b s e r v e d in response to hyperoxia. H o w e v e r , w h e n the frequency of the
ventilator w a s r e d u c e d and b o t h basal P C 0
and b a s a l a m p l i t u d e of I E N G
augmented,
1 0 0 % 0 inhalation w a s ineffective to reduce
the p h r e n i c output ( l o w e r m i d d l e panel). After
the return to the previous conditions, 1 0 0 %
0 inhalation a g a i n d i m i n i s h e d the amplitude
of phrenic n e r v e inspiratory bursts (lower right
panel).
2
b
E T
and
P
ETco,-
P
ET 0
3 6
-0
h
(u)
2
2
p h
E T
2
E T
40.2°C
C
-33.4 torr
2
2
p h
2
2
DA (ug/kg)
2
5
10
Fig 8. Phrenic neural responses to iv injections of dopamineHC1 (2, 5 and 10 ug/kg, at arrows) at three different T 's in a
cat with intact carotid nerves. IENG . , phrenic integrated
electroneurogram, in arbitrary units. Note the presence of
neural equivalent of spontaneous gasps, at 40.2°C.
b
152
Biol Res 27: 145-157 (1994)
120
P" 0.06
100
--o
•
80
b
O'
3QS f >
:
levels. M a x i m a l reactivity in absolute terms
w a s different from o n e animal to other b e c a u s e
of the different n u m b e r of c h e m o s e n s o r y fibers
recorded from their respective carotid n e r v e s .
H o w e v e r , n o significant c h a n g e s in d o s e response curves b e t w e e n different T ' s w e r e
also o b s e r v e d in three other cats. In o n l y 1 out
of the 5 cats, t h e m a x i m a l reactivity w a s con­
siderably reduced at 4 0 . 2 ° C, n e a r l y to o n e
third of that recorded at 35.5° C, w i t h an inter­
m e d i a t e v a l u e r e a c h e d at 37.5° C, b u t this
o b s e r v a t i o n m a y be a c o n s e q u e n c e of a p r o ­
gressive reduction in t h e n u m b e r of active
fibers within the dissected carotid n e r v e along
the several h o u r s of recording.
60
/
Hz)
- -A
40
20
35.5
37.5
40.2 °C
Relationship
and phrenic
Fig 9. Frequencies of basal mean chemosensory discharges
(bas f ) recorded from one cut carotid nerve in 5 cats studied
at steady-state T 's of 35.5, 37.5 and 40.2°C. Each symbol
is the mean of 6 0 consecutive counts of 1 -s periods in a
different cat. Rectangles, 99% normal distribution of the grand
means (horizontal lines at the middle of the rectangles), p at
inset, statistical significance of multiple comparisons by
Friedman's test.
b
between carotid
nerve
discharges
chemosensory
F i g u r e 11 illustrates t h e effects of several
increasing doses of N a C N (0.5 to 100 ug/kg,
iv) o n the carotid c h e m o s e n s o r y frequency
and the integrated p h r e n i c d i s c h a r g e s in o n e
cat (with o n e carotid n e r v e s e c t i o n e d ) studied
at three different T levels. N a C N increased
f and I E N G
a m p l i t u d e in a d o s e - r e l a t e d
manner, with a clear cut correlation in inten­
sities and timing, particularly e v i d e n t at 37.5°
C(figllB).
b
x
of basal c h e m o s e n s o r y activity in 5 cats in
w h i c h o n e carotid n e r v e w a s cut for recording.
T h e s e basal v a l u e s were obtained from 1 m i n
recordings under isocapnic conditions. Al­
t h o u g h in 5 e x p e r i m e n t s the m e a n f ' s in­
creased b e t w e e n 37.5 and 4 0 . 2 ° C and in 4 of
them decreased b e t w e e n 35.5 and 37.5° C,
the differences observed b e t w e e n the grand
m e a n s did n o t reach statistical significance
( F r i e d m a n ' s test; p = 0.06).
T o ascertain t h e d e g r e e of carotid c h e m o ­
receptor excitation d e p e n d e n t on the n o r m o x i c
conditions at different T ' s , w e studied the falls
in f resulting from 1 0 0 % 0 inhalations in
the 5 animals in w h i c h carotid c h e m o s e n s o r y
discharges were recorded under isocapnic
conditions. T h e m a x i m a l transient reductions
in f i n d u c e d b y h y p e r o x i c tests w e r e n o t
significantly different at the three different
T ' s ( Q u a d e ' s test; p = 0.25).
F i g u r e 10 illustrates the changes in fx in­
d u c e d by increasing doses of N a C N given
iv at the three steady-state T ' s . In spite of the
fact that b a s f w a s enhanced at the hyper­
thermic level in this animal, no c h a n g e s in
m a x i m a l reactivity, slope o r m e a n effective
d o s e w e r e observed b e t w e e n t h e three thermal
p h
x
BOO
700
•
35.5°C
O
37.5°C
4 3 ± 3 Hz
28 ± 1 Hz
V
40.2°C
8 2 ± 2 Hz
600
500
400
b
x
2
H
Z
300
200
1 00
x
b
b
x
1
10
N a C N U/g/kg) i.v.
Fig 10. Changes in the frequency of chemosensory impulses
(f ) recorded from one cut carotid nerve in response to in­
creasing doses of NaCN given iv to a cat at 35.5°C (squares),
37.5°C (circles) and 40.2°C (triangles). Inset, means ± SEM's
of basal chemosensory frequency. For the three conditions,
frequencies evoked by NaCN 5 Ug/kg and higher doses were
significantly above the 95% normal distribution of their re­
spective basal means.
153
Biol Res 27: 1 4 5 - 1 5 7 ( 1 9 9 4 )
(see R E N G recording) and the a m p l i t u d e of
the I E N G . T h e c h e m o s e n s o r y excitation
declined rapidly, while phrenic efferent activity
r e m a i n e d e l e v a t e d for a p r o l o n g e d p e r i o d .
H e r e , the rectified E N G s h o w s that t h e early
and late increases in amplitude of the integrated
E N G were not d u e to h i g h e r d i s c h a r g e freq u e n c i e s of p h r e n i c m o t o n e u r o n s , b u t to p r o longations of their b u r s t s of inspiratory d i s c h a r g e s . C o n t r a r i w i s e , less intense increases
of p h r e n i c n e r v e bursts in r e s p o n s e to N a C N
injections observed at 4 0 . 2 ° C (fig 11C) w e r e
associated to p r o l o n g a t i o n s of e x p i r a t o r y silences.
p h
P C0
T
p h
2
dorr)
iiiii
lENGph
mÊÊÊÊÈÊÈÊÊà
(ul
IENG
(u)
Figure 13 s h o w s the relationships b e t w e e n
the carotid c h e m o s e n s o r y afferent and the
p h r e n i c efferent activities at the three steadystate T ' s levels, but at similar P C 0 levels.
These data were obtained during chemosensory
excitations i n d u c e d by i n c r e a s i n g d o s e s of
N a C N and 1 0 0 % N tests, and d u r i n g transient
d e p r e s s i o n s of c h e m o s e n s o r y d r i v e by d o p a m i n e (5-10 ug/kg) and 1 0 0 % 0 tests. W h e n
the I E N G
is e x p r e s s e d in arbitrary units
( b u t d e r i v e d from r e c o r d i n g s at t h e s a m e
amplification), a gross inspection gives the
b
i..n.,i H » .
.mill iin mi
i
E T
2
2
2
l^lMwiMWllliW*li»l«llll^*^Killl' *l*
l
( /kg)
u 5
t
t
•
t
t
I
2
5
10
20
p h
50
100
35.5'C
IENGph
RENGph
(u)
50
Fig 11. Carotid chemosensory and phrenic efferent responses
to several doses of NaCN (1 to 100 pg/kg, iv) at 35.5°C (A),
37.5°C (B) and 40.2°C (C), in a cat with one carotid nerve
sectioned. P C 0 , pressure of tracheal C 0 (first rows);
IENG ., phrenic integrated electroneurogram (second rows);
R E N G , phrenic rectified electroneurogram (third rows);
f , instantaneous frequency of chemosensory discharges (fourth
rows).
T
2
RENGph
500r
2
ph
In spite of the a b o v e , time correlations b e t w e e n the d i s c h a r g e s of c h e m o s e n s o r y
afferents and phrenic efferents were sometimes
distorted. T h u s , figure 12 displays at h i g h e r
speed the r e s p o n s e s to N a C N 100 ug/kg iv at
35.5° C to s h o w that the d r u g increased f , the
duration of the burst of phrenic discharges
f>
250 -
(Hz)
NaCN IOO/jg/kg
i min
Fig 12. Response of carotid afferents and phrenic efferents to
a large dose of NaCN (100 Ug/kg, iv) at 35.5°C in a cat with
one carotid nerve sectioned. P C 0 , tracheal pressure of C 0
(first row); IENG^, phrenic integrated electroneurogram
(second row); R E N G , phrenic rectified electroneurogram
(third row); f , instantaneous frequency of carotid chemosensory discharges (fourth row). Display at higher speed of
recording from upper right comer of Fig 11.
T
pb
x
2
2
154
Biol Res 27: 145-157 (1994)
i m p r e s s i o n that t h e s a m e c h e m o s e n s o r y input
p r o d u c e d a smaller p h r e n i c efferent activity
at t h e h i g h e r T (fig 1 3 A ) . H o w e v e r , w h e n
I E N G w a s expressed as a p e r c e n t a g e of t h e
b a s a l v a l u e s , t h e difference disappeared (fig
1 3 B ) . T h u s , t h e s a m e c h e m o s e n s o r y input
p r o d u c e d a p r o p o r t i o n a l p h r e n i c output a b o v e
b a s e l i n e . I n fact, t h e amplitude of t h e basal
IENG
w a s s t r o n g l y r e d u c e d at 4 0 . 2 ° C,
explaining t h e maintenance of the ratio e v o k e d /
b a s a l I E N G activities at different t e m p e r ­
atures.
b
p h
p h
p h
DISCUSSION
O b s e r v a t i o n s o n t h e s e n s i t i v i t y o f carotid
bodies t o local t h e r m a l c h a n g e s , originated
from different laboratories and referred to in
the Introduction, indicate that these o r g a n s
are e x t r e m e l y s e n s i t i v e d e t e c t o r s o f fast
c h a n g e s in t e m p e r a t u r e and that their basal
levels of neural afferent discharges are
positively correlated with t h e steady levels of
local t e m p e r a t u r e . T h e h i g h energies of a p ­
parent activation (u), h i g h thermal coefficients
( Q ) and high thermal gains (df /dT ) ex1 0
x
b
hibited b y t h e rates of afferent d i s c h a r g e s o f
the carotid b o d i e s indicate that t h e y fulfill
the criteria for b e i n g considered as potential
t h e r m o s e n s o r s ( A l c a y a g a et al, 1 9 9 3 ) .
Furthermore, their profuse vascularization
and intimate relation to t h e b l o o d m a k e t h e m
suitable t o s e r v e as a d e q u a t e d e t e c t o r s for
c h a n g e s of t h e b l o o d t e m p e r a t u r e . B e i n g t h e
t e m p e r a t u r e of the blood at d e e p structures t h e
best representative o f b o d y c o r e t e m p e r a t u r e
( T ) , the functioning and location of the carotid
b o d i e s m a k e t h e m ideally suited to serve as
sensors for c h a n g e s i n t h e steady-state levels
ofT„.
Since t h e carotid b o d i e s p l a y a p r e d o m i n a n t
role in t h e regulation of respiration at resting
conditions and d u r i n g adaptation t o c h a n g e s
in t h e c h e m i c a l c o m p o s i t i o n o f t h e b l o o d
(Fitzgerald and Lahiri, 1986), it w a s pertinent
to ask if these receptors should participate in
the important adjustments in p u l m o n a r y and
alveolar ventilation produced by thermal
challenges.
In a w a k e cats, t h e m e a n rectal t e m p e r a ­
ture of 38.8° C is m a i n t a i n e d at e n v i r o n m e n t a l
t e m p e r a t u r e s of 2 0 to 3 0 ° C; i n c r e a s i n g the
ambient temperature above 30° C induces
progressively h i g h e r rectal t e m p e r a t u r e s of u p
to 4 0 . 5 ° C, a n d i n c r e a s e d r e s p i r a t o r y fre­
q u e n c y , p a n t i n g a p p e a r i n g w h e n t h e environ­
m e n t surpasses 4 1 ° C ( A d a m s et al, 1970;
B o n o r a and Gautier, 1989). A l s o p e a k rectal
t e m p e r a t u r e s of 4 0 . 7 ± 0.1° C h a d b e e n re­
corded in u n a n e s t h e t i z e d cats injected with
synthetic p y r o g e n s (Morilak etal, 1987). Since
ventilation is i m p o r t a n t for h e a t dissipation in
cats, because of their furred skin and restriction
of s w e a t g l a n d s to toes and footpads, this
s p e c i e s is w e l l suited t o study v e n t i l a t o r y
reactions to variations in b o d y t e m p e r a t u r e ,
and t h e possible contribution o f carotid b o d y
c h e m o r e c e p t o r s as t h e r m a l afferents.
P r e v i o u s o b s e r v a t i o n s from this laboratory
(Fadic et al, 1991) indicate that increasing T
in t h e p e n t o b a r b i t o n e a n e s t h e t i z e d c a t , by
m o d e r a t e l y heating t h e skin surface, p r o v o k e d
p u l m o n a r y and alveolar hyperventilation. T h i s
was mostly mediated b y increasing frequencies
of respiration a n d s p o n t a n e o u s g a s p s , with
a m i n o r contribution of V . T h e last o n e w a s
clearly d e p e n d e n t o n t h e i n d e m n i t y of t h e
carotid n e r v e s , while p r o n o u n c e d increases in
f w e r e still o b s e r v e d after bilateral section of
b
b
O
f
x
(Hz)
Fig 13. Relationships between afferent carotid chemosensory
and efferent phrenic activities in a cat at three different T ' s .
Abscissae, frequency of chemosensory discharges (f ).
Ordinates, phrenic integrated electroneurogram (IENG^),
expressed in arbitrary units at the same amplification (A) and
as percentages of basal values (B). Increases in both variables
were produced by N a C N injections at different doses;
decreases (below 100%, basal values), by 100% 0 tests and
dopamine injections.
b
2
T
R
155
Biol Res 27: 1 4 5 - 1 5 7 ( 1 9 9 4 )
the carotid n e r v e s . Similarly, v o n E u l e r et al
(1970) o b s e r v e d increases in b o t h V and f
w h e n raising T from 3 7 . 1 ° to 39.6° C in pen­
t o b a r b i t o n e anesthetized cats, t h e t a c h y p n e a
b e i n g p r e s e r v e d after b i l a t e r a l v a g o t o m y ,
w h i c h i n c l u d e d interruption o f t h e c h e m o ­
s e n s o r y afferences from t h e a o r t i c b o d i e s .
Studies o n p e n t o b a r b i t o n e anesthetized cats
( G r u n s t e i n et al, 1 9 7 3 ) a n d i n u r e t h a n e
anesthetized
rabbits
(Kobayasi
and
Y a m a g u c h i , 1981) h a d s h o w n that f w a s
e x t r e m e l y d e p e n d e n t o n T in t h e r a n g e of 3 5 °
C to 4 1 ° C, resulting i n fairly h i g h Q v a l u e s
(6.1 and 5 . 3 , respectively), persisting after b i ­
lateral v a g o t o m y .
T h e m o d e s t role played b y carotid b o d i e s
afferents i n t h e ventilatory adaptation to slow
c h a n g e s in T , o b s e r v e d in s p o n t a n e o u s l y
b r e a t h i n g cats ( F a d i c et al, 1991), m a y be
ascribed to t h e alveolar hyperventilation itself
caused b y h y p e r t h e r m i a , t h e resulting c h a n g e s
in blood g a s e s c o u n t e r a c t i n g t h e stimulating
local effects of the increased blood temperature
u p o n carotid b o d i e s . N e v e r t h e l e s s , t h e present
results, obtained in artificially ventilated
cats in w h i c h t h e s e c h a n g e s in blood m a y
b e controlled and s u p p r e s s e d , indicate that
the reflex effects elicited by c h e m o s e n s o r y
stimulants o f t h e carotid and aortic b o d i e s are
not potentiated b y increasing T a b o v e n o r m a l
levels. R e s u l t s indicate that t h e correlation
b e t w e e n c h a n g e s in t h e frequency of c h e m o ­
sensory afferents and t h o s e in t h e strength of
the ventilatory o u t p u t (phrenic efferents) is
well m a i n t a i n e d at t h e three levels of tempera­
ture studied: m o d e r a t e h y p o t h e r m i a , n o r m o t h e r m i a and mild h y p e r t h e r m i a .
C o n s i d e r i n g I E N G as t h e neural output
e q u i v a l e n t o f V , t h e d e c r e a s e in I E N G
o b s e r v e d at 3 7 . 5 ° C w h e n c o m p a r e d to that
r e c o r d e d at 35.5° C (fig 2 A ) m a y b e related to
the d e c r e a s e i n V occurring in spontaneously
b r e a t h i n g p e n t o b a r b i t o n e - a n e s t h e t i z e d cats
w h e n r e w a r m i n g from 2 8 to 38° C (Gautier
and G a u d y , 1986). Similarly, t h e further d e ­
c r e a s e in I E N G
observed when reaching
4 0 . 2 ° C u n d e r isocapnic conditions (fig 2 A )
m a y b e c o m p a r e d to t h e d e c r e a s e in V
occurring in unanesthetized cats w h e n T is
displaced from 3 8 . 6 to 39.4° C b y external
w a r m i n g ( B o n o r a and Gautier, 1990). B o t h
decreases i n V w e r e associated to c o n c o m i ­
tant increases in f ( B o n o r a and Gautier, 1990;
T
R
b
R
b
1 0
b
b
p h
T
p h
T
p h
T
b
T
R
G a u t i e r and G a u d y , 1986), a s also o b s e r v e d in
o u r artificially ventilated cats (fig 2 B ) .
W i t h regard to resting ventilation, t h e basal
l e v e l s o f carotid c h e m o s e n s o r y d i s c h a r g e s
m a i n t a i n a certain d e g r e e of tonic reflex e x ­
citation of ventilatory centers ( " c h e m o s e n s o r y
drive"), demonstrated by the immediate,
transient decreases in tidal v o l u m e in r e s p o n s e
to b r e a t h i n g 1 0 0 % 0 f o r a f e w s e c o n d s
(Dejours, 1963) o r iv injections o f d o p a m i n e
(Zapata and Z u a z o , 1980), as c o n s e q u e n c e of
the sudden withdrawal of carotid c h e m o ­
sensory d i s c h a r g e s , briefly silenced b y t h e s e
m a n e u v e r s (Leitner et al, 1 9 6 5 ; L i a d o s and
Zapata, 1978). T h e r e f o r e , if a rise in blood
t e m p e r a t u r e s h o u l d i n c r e a s e t h e b a s a l fre­
q u e n c y of c h e m o s e n s o r y d i s c h a r g e s , the
c h e m o s e n s o r y drive u p o n resting ventilation
should also b e e n h a n c e d . I n h u m a n s u s i n g
heated flying suits, an increase in rectal
t e m p e r a t u r e of 1.6° C e n h a n c e d t h e c h e m o ­
sensory drive u p o n ventilation, as r e v e a l e d by
the m o r e p r o n o u n c e d t r a n s i e n t v e n t i l a t o r y
depression in r e s p o n s e to 2 b r e a t h s o f p u r e 0
( J e n s e n et al, 1 9 7 8 ) . I n p e n t o b a r b i t o n e
anesthetized, s p o n t a n e o u s l y b r e a t h i n g c a t s ,
brief hyperoxic tests induced transient
decreases in tidal v o l u m e a n d increases in
P C 0 w h i c h w e r e significantly l a r g e r at 4 0 °
C than at 3 7 ° C (Fadic et al, 1991). N e v ­
ertheless, p r e s e n t results indicate that in the
artificially ventilated cat in w h i c h P C 0 is
kept within e u c a p n i c r a n g e , 1 0 0 % 0 tests and
d o p a m i n e injections applied i n t h e h y p e r t h e r ­
mic and normothermic conditions decreased
the I E N G amplitude m o r e p r o f o u n d l y t h a n
in h y p o t h e r m i a (figs 7 and 8 ) , in spite o f t h e
similar depressant effects u p o n c h e m o s e n s o r y
d i s c h a r g e s exerted by t h e s e m a n e u v e r s at t h e
three levels of T studied. It is n o t e w o r t h y that
the ventilatory drive exerted b y arterial c h e ­
m o r e c e p t o r s w a s n o t proportionally r e d u c e d
in h y p e r t h e r m i a , as o n e w o u l d e x p e c t from
the increased t h e r m a l drive exerted directly
u p o n central respiratory n e u r o n s . Similarly, in
unanesthetized cats d o p a m i n e injections p r o ­
d u c e s i g n i f i c a n t d e c r e a s e s i n V in b o t h
normothermic and hypothermic conditions
( B o n o r a and Gautier, 1990).
Present results also indicate that s t r o n g e r
v e n t i l a t o r y effects w e r e e v o k e d b y m i l d e r
d e g r e e s of hypoxic stimulation i n c a t s
thermostabilized at 3 7 . 5 ° C t h a n i n t h o s e k e p t
2
2
E T
2
E X
2
p h
b
T
2
156
Biol Res 27: 145-157 (1994)
at either 35.5 or 40.2° C (figs 5 and 13A).
not only a reflex concomitant increase in
T h u s , t a k i n g t o g e t h e r the depth of the c h e m o ­
sensory d r i v e and the efficiency of the c h e m o reflexes, it appears that the m a x i m a l efficiency
of the arterial c h e m o r e c e p t o r s as controllers
of ventilation is attained at t h e n o r m a l T of
the animal. T h i s is the k i n d of operation that
o n e w o u l d e x p e c t for a thermostatic system
that c o r r e c t s rapidly a n d efficiently m i n o r
deviations from the t h e r m a l set point.
T h e pattern of ventilation is profoundly
affected b y T . T h e b r a d y p n e a of h y p o t h e r m i a
and the t a c h y p n e a of h y p e r t h e r m i a result
from changes in the durations of the inspiratory
bursts and expiratory silences exhibited by
those n e r v e s controlling the m a i n respiratory
m u s c l e s . P r e s e n t results indicate that the d u ­
ration of p h r e n i c inspiratory bursts decreases
m o d e r a t e l y while T is increased from 35.5°
C to 37.5° C, and that b o t h inspiratory duration
and total d i s c h a r g e s p e r cycle of the phrenic
n e r v e are r e d u c e d at 4 0 . 2 C. F u r t h e r m o r e ,
different patterns of ventilation w e r e observed
in r e s p o n s e to h y p o x i c stimulation at different
steady levels of T .
ventilatory output, but also resets the t i m i n g
c o m p o n e n t s (off-switch) of the central
generator of respiration.
A n interesting o b s e r v a t i o n w a s the appear­
ance at regular intervals in s o m e of t h e s e
artificially ventilated cats of b i p h a s i c bursts
of phrenic neural d i s c h a r g e s . T h e regularity
of these periodic cycles and their biphasic
pattern reveal that they correspond to the neural
equivalent of s p o n t a n e o u s g a s p s , also n a m e d
inspiratory a u g m e n t i n g r e s p o n s e s , sighs or
periodic d e e p breaths (Bartlett, 1971). T h e
e l e c t r o m y o g r a p h i c c o r r e l a t e of t h e s e a u g ­
mented breaths is an increased post-inspiratory
i n s p i r a t o r y activity in the d i a p h r a g m (van
L u n t e r e n et al, 1986), w h i c h is in turn e v o k e d
by an a u g m e n t a t i o n p h a s e in p h r e n i c nerve
activity, initiated close to the crest of the nor­
m a l inspiratory burst ( C h e m i a c k et al, 1991),
w h e n these variables are recorded in sponta­
neously breathing anesthetized cats. H o w e v e r ,
to o u r k n o w l e d g e , this is the first description
of these p h r e n i c E N G equivalents to sponta­
n e o u s g a s p s in artificially ventilated animals
and the first e v i d e n c e of their periodic o c ­
currence in this controlled condition. T h e i r
m o s t c o m m o n i n c i d e n c e o r h i g h e r frequency
at h y p e r t h e r m i c levels (see records in figs 7
and 8) is c o i n c i d e n t w i t h t h e i n c r e a s e in
s p o n t a n e o u s g a s p s observed in s p o n t a n e o u s l y
breathing cats in h y p e r t h e r m i a (Fadic et al,
1991) and m a y be explained by the modulatory
role exerted by carotid c h e m o s e n s o r y efferents
u p o n the p u l m o n a r y vagal reflex responsible
for these a u g m e n t e d breaths ( Z u a z o and Za­
pata, 1980).
b
b
b
b
It w a s interesting to observe that an intense
c h e m o r e c e p t o r stimulation -induced by the
largest d o s e s of N a C N - p r o v o k e d early and
d e l a y e d increases in the rate of the discharges
recorded from p h r e n i c m o t o n e u r o n s (fig 12).
T h e e a r l y e n h a n c e m e n t s of m o t o n e u r o n a l
d i s c h a r g e cycles w e r e correlated in time and
m a g n i t u d e with the c o n c o m i t a n t increases in
the frequency of carotid c h e m o s e n s o r y dis­
charges. H o w e v e r , the simultaneous analysis
of rectified and integrated phrenic discharges
s h o w e d l o n g e r durations of inspiratory bursts
along several cycles, well beyond the transient
early increases in c h e m o s e n s o r y discharges;
the potentiation of these m o t o n e u r o n a l dis­
c h a r g e s w a s n o t c o r r e l a t e d w i t h the mild
delayed increase in c h e m o s e n s o r y discharges.
T h i s is similar to an observation of Lahiri et al
(1991) in w h i c h an intracarotid injection of
2 0 ug of N a C N p r o d u c e d a brief increase of f
b u t a l o n g - l a s t i n g i n c r e a s e in v e n t i l a t i o n
(see fig 2 B of that report). This p h e n o m e n o n
r e m i n d s u s t h e p r o l o n g e d s t i m u l a t i o n of
respiration evoked b y several types of brief
intense stimuli, including the electrical stim­
ulation of the carotid nerves (Millhorn et al,
1980). T h u s , it is possible that carotid body
stimulation - u n d e r certain c o n d i t i o n s - e v o k e s
In s u m m a r y , o b s e r v a t i o n s here reported
indicate that in the n o r m o x i c e u c a p n i c animal,
the most p r o n o u n c e d influence of arterial c h e ­
m o r e c e p t o r s o p e r a t i n g as t h e r m o s e n s o r s
controlling ventilation is o b s e r v e d within the
n o r m o t h e r m i c range.
x
ACKNOWLEDGEMENTS
T h i s work was supported b y grant 9 2 - 0 6 5 2
from the N a t i o n a l F u n d for Scientific and
Technological Development ( F O N D E C Y T ) .
This laboratory also receives continuous
support from the R e s e a r c h Division of the
University ( D I U C ) .
Biol Res 27: 145-157 (1994)
R I received in 1992 a grant-in-aid from the
A n d e s F o u n d a t i o n , for t h e r e i n s e r t i o n of
Chilean scientists into the a c a d e m i c activities
of Chilean universities ( P r o g r a m C 12021-7).
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