Cataracts and Hypertension in Salt-Sensitive Rats
A Possible Ion Transport Defect
CARMEN
RoDRfGUEZ-S ARGENT,
JOSE
L.
CANGIANO, GRACIA BERR/OS CABAN,
EVELIO MARRERO, AND MANUEL
MARrfNEZ-MALDONADO
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SUMMARY In previous unrelated studies, we observed a 35 to 50% incidence of cataract formation
in several groups of Dahl salt-sensitive hypertensive rats (DS) over a 4-year period. In the present
study we evaluated longitudinal changes in blood pressure In DS in which cataracts eventually
developed and those in which cataracts did not develop when all animals were maintained on a high
sodium diet. Lenses were evaluated by slit-lamp microscopy to determine if cataractous lesions were
similar among rats, to classify lesion types, and to define the age at which cataracts were detectable in
DS. The possible participation of several cataractogenic risk factors as major Influences on cataract
formation also was evaluated. Finally, aqueous humor concentrations and lenticular content of
sodium and potassium were determined to evaluate the possibility that a defect in ion transport at the
lens epithelium and ciliary body might play a role in cataractogenesis in DS, since ion transport defects
have been shown to lead to lens opaciflcatlon in other models of genetic and experimental cataracts.
Parallel studies were performed in Dahl salt-resistant control rats (DR). A high incidence of cataract
formation was found in DS. Although systolic blood pressure was not consistently greater in adult DS
with cataracts compared with values in age-matched DS without cataracts, the initial pressor response
to a high salt diet was greatest in weanling DS in which cataractous lesions later developed. Slit-lamp
analysis revealed that cataracts in this genetic model were cortical, with one mixed cortical, nuclear
lesion. Posterior subcapsular lesions were not observed, suggesting that lesions were not steroidinduced. Serum ultrafiltrable calcium concentration was similar among DS with and without cataracts and normotensive DR, indicating that hypocalcemia was not involved in cataractogenesis.
Analysis of aqueous humor showed high potassium concentration and low sodium concentration in DS
with cataracts, suggesting a possible ion transport defect in the ciliary processes or lens epithelium (or
both). In addition, increased lenticular water and sodium content, as well as decreased potassium
content, indicated that mature cataracts in DS were associated with altered sodium and potassium
transport. Although aqueous humor sodium concentration and lenticular sodium content were not
altered in DS without cataracts, lenticular potassium content was decreased while aqueous humor
potassium concentration was increased compared with values in control DR. These data suggesting a
decreased transmembrane potassium gradient in DS without cataracts also suggest that a specific
defect in potassium transport may precede the development of cataracts in this genetic model. The
finding of a potential genetic model of cataracts in which cataractogenesis is associated with hypertension suggests a possible link between hypertension and cataract formation.
(Hypertension 9: 304-308, 1987)
KEY WORDS
W
• hypertension • cataracts • lens • aqueous humor • sodium
E have noticed a high incidence of cataract
as indicated by the excessive production of 18-OH
formation in Dahl salt-sensitive rats (DS),
a finding that, to our knowledge, has not
been reported previously. Hypertension in DS apparently is associated with altered ionic transport in
13
several
tissues
alteredServices,
adrenalVeterans
steroidogenesis,
From the
Research and
and Medical
Administra-
deoxycorticosterone.4'5 Either of these factors might
contribute to the development of cataracts in DS. 6 - 7
Decreased lenticular ionic transport resulting from a
specific decrease in Na + ,K + -adenosine triphosphatase
Submitted by G. Berrfos-Cabin in partial fulfillment of the Master's degreerequirementsof the Department of Biology, University
of Puerto Rico, San Juan, Puerto Rico.
Address for reprints: Carmen Rodriguez-Sargent, Ph.D., Research Service, VA Hospital, GPO Box 4867, San Juan, Puerto
Rico 00936.
Received March 10, 1986; accepted October 15, 1986.
tion Hospital, and Department of Biology, University of Puerto
Rico, San Juan, Puerto Rico.
Supported by Merit Review Funds of the Veterans Administration, a grant-in-aid of the Puerto Rico Chapter of the American
Heart Association, and National Institutes of Health Grants AM
30201 and RR 8021.
304
CATARACTS IN GENETICALLY HYPERTENSIVE KAlSIRodrlguez-Sargent et al.
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(ATPase) activity at the lens epithelium leads to cataract formation in the Nakano mouse. 8 ' 9 In this genetic
model the only identified defect is the cataract.9 Furthermore, in vitro incubation of lenses with the
Na + ,K + -ATPase inhibitor ouabain also results in lens
opacification.10 Low Na + ,K + -ATPase activity has
been described in renal microsomal preparations from
hypertensive DS compared with values in control Dahl
salt-resistant rats (DR), 1 ' 3 and if decreased activity of
this transport enzyme is a generalized epithelial defect,
the formation of a metabolic cataract analogous to that
observed in the Nakano mouse might occur. Alternatively, since chronic steroid therapy has been shown to
result in an increased incidence of cataract formation
secondary to the development of glaucoma," the altered steroidogenesis described in hypertensive DS
could be responsible for cataract formation.
In the present study we evaluated the incidence of
cataract formation in DS and DR. Slit-lamp analysis
was performed to verify the presence of cataracts as
well as to determine whether opacities were cortical,
nuclear, or posterior subcapsular. In addition, aqueous
humor electrolyte composition was determined, and
lenticular sodium and potassium content and state of
hydration were evaluated to further characterize cataractous lesions. Some factors that might contribute to
cataract formation were also studied. Serum ultrafiltrable calcium concentration was measured, for example,
since hypocalcemic states are associated with a high
incidence of cataract formation12 and since hypocalcemia has been proposed to contribute to the development of hypertension.13 Finally, because of the
high incidence of cataract formation during diabetes
mellitus, fasting serum glucose concentration was
measured.14
Materials and Methods
The study included 40 female DS and 40 female DR
(Brookhaven National Laboratory, Upton, NY, USA).
All rats were maintained on a high sodium diet (0.9%
saline in place of drinking water) beginning at the age
of 4 weeks. Systolic blood pressure was measured
before and after the first 2 weeks on a high salt diet,
and additional measurements were made weekly in
each rat. Each measurement was based on 10 consecutive determinations in conscious, restrained rats by tail
cuff plethysmography. Cataract formation was assessed by visual inspection. At the age of 20 weeks,
the rats were divided into three groups: DS in which
moderate to severe cataracts had developed (DSc), DS
in which cataracts had not developed (DSnc), and DR.
DS in which mild cataractous lesions developed were
not evaluated. A total of 12 rats from each group were
selected at random for further study. Slit-lamp evaluation was performed in half of the rats from each group.
The animals then were subjected to an overnight fast.
The following morning, the rats were decapitated and
blood samples (2.0 ml) were collected for the determination of fasting serum glucose concentration and serum ultrafiltrable calcium concentration. Blood urea
305
nitrogen was measured in samples obtained from separate groups of DSc, DSnc, and DR that were similarly
studied to evaluate the possibility of chronic renal failure in DSc. This measurement was performed in a total
of six rats selected at random from each group.
Immediately upon decapitation, hand-drawn glass
pipettes were quickly inserted tangentially into the
anterior chamber of the eye for collection of aqueous
humor. The lenses were then dissected from the eyeballs, blotted with filter paper that had been moistened
with a sodium-free buffer solution, and transferred to
tared weighing vials. The vials were placed in an oven
at 100°C, and lenses were dried to constant weight (72
hours). Water content was expressed as the difference
between wet and dry weights and was corrected for dry
weight. Lenses were then digested by addition of 50 /nl
of concentrated hydrochloric acid to each vial and
again placed in the oven for 24 hours.
At this time, 200 fxl of concentrated sulfuric acid
was added to each vial and the lenses were maintained
at room temperature (25 °C) for 12 hours to complete
digestion of lenticular tissue. Samples were diluted for
electrolyte analysis. Sodium and potassium concentrations in lenticular preparations and aqueous humor
samples were measured by flame photometry. Serum
ultrafiltrates for the measurement of calcium were obtained by filtration using a micropartition system
(MPS-1; Amicon Corp, Lexington, MA, USA). Calcium concentration was determined colorimetrically,
and glucose concentration was measured with a glucose analyzer (Beckman Instruments, Palo Alto, CA,
USA). Blood urea nitrogen was measured by autoanalyzer technique (Technicon; Tarrytown, NY, USA).
Statistical analysis of the data was done using Student's t test between groups, paired t evaluation within
groups, and analysis of variance among the three
groups. A/7 value less than 0.05 was considered significant. Values were expressed as the arithmetic mean
± SEM. All experimental procedures were approved
by the local institutional review board for animal care
and use, and procedures were performed in accordance
with the guidelines of the National Institutes of Health
and the American Physiological Society.
Results
A 35% incidence of cataract formation was observed among 20-week-old DS maintained on a high
sodium diet. Slit-lamp analysis confirmed cataracts in
DSc and lens transparency in DSnc and DR. All cataracts studied were anterior cortical, and nuclear opacification was also present in one severe cataract. Fasting serum glucose concentration was similar among
DSc (96 ± 3 mg/dl), DSnc (102 ± 5 mg/dl), DR
(90 ± 4 mg/dl). Serum ultrafiltrable calcium concentration was also similar among the three groups (DSc:
4.1 ± 0.2; DSnc: 4.4 ± 0.5; DR: 3.7 ± 0.2 mg/dl). In
addition, no differences in blood urea nitrogen (DSc:
33 ± 1; DSnc: 31 ± 2; DR: 31 ± 2 mg/dl) were observed among the three groups.
Under these conditions, aqueous humor sodium
306
VOL
HYPERTENSION
9, No 3, MARCH
1987
TABLE 1. Concentration of Sodium and Potassium in Aqueous
Humor
Group
DS
With cataracts
Without cataracts
DR
Na + (mEq/L)
K+ (mEq/L)
13.6+1.7*-t
139+1*4
10.2±0.4*
149 ± 2
148±4
8.4±0.5
Values are means ± SEM. Significant variations in aqueous
humor sodium concentration (p < 0.05) and potassium concentration (p < 0.001) were observed among the three groups.
*p < 0.05, compared with corresponding values in DR.
t p < 0.05, compared with corresponding values in DS without
cataracts.
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concentration was lower in DSc than in DSnc and DR
(Table 1). Potassium concentration in the same sample
was higher in DSc than in DSnc and DR (see Table 1).
In addition, the aqueous humor potassium concentration was greater in DSnc than in DR, despite similar
sodium concentrations in these two groups.
Lenticular water content was increased in DSc as
compared with values in DSnc and DR (Figure 1). No
difference was observed between the lens water content in DSnc and DR. Increased water content in DSc
was associated with a marked rise in lens sodium content (Figure 2) and a decrease in lens potassium content
(Figure 3) as compared with values in DSnc and DR.
Although lenticular sodium content was similar in
DSnc and DR, a modest but significant decrease in
lens potassium content was observed in DSnc as compared with values in DR. Systolic blood pressure was
elevated in all DS between the ages of 18 and 21 weeks
independently of the presence of cataracts, as compared with values in DR (Table 2). At this age systolic
blood pressure was greater in DSc than in DSnc, although the difference between the two groups was not
consistently significant from week to week. In weanling rats, however, the rise in systolic blood pressure in
response to the first 2 weeks on a high sodium diet was
greater in DS in which cataracts later developed
(152 ± 4 mm Hg) than in DS in which cataracts did not
develop (133 ± 4 mm Hg; p < 0 . 0 1 ) and in DR
(107 ± 2 mm Hg;/?<0.01).
Discussion
Diabetes mellitus,14 hypoparathyroidism," renal
failure,16 and hypocalcemic states12 are associated with
an increased incidence of cataract formation. Hypertensive disease, however, has not been evaluated as
a cataractogenic risk factor in either experimental
or clinical studies. Although several models of cataract formation have been described previously in
mice, 8 ' "• l 8 blood pressure has not been reported in
these models. Yet, early clinical studies of cataract
formation in diabetes mellitus noted a high prevalence
of arterial hypertension in those diabetic subjects in
whom cataractous lesions developed." In the present
study we found a high incidence of cataract formation
in genetically hypertensive rats. Although cataracto-
O 25-
FIGURE 1. Lenticular water content in DS with cataracts
(DSc) and without cataracts (DSnc) and in control DR on a high
sodium diet. Asterisk denotes a value significantly different
(p<0.01) from the value observed in DR. Dagger denotes a
value significantly different (p<0.01) from that in DSnc. Lenticular water content varied significantly (p < 0.001) among the
three groups.
I
E
175-
15O
125"
3
100
Ul
73
5O-
FIGURE 2. Lenticular sodium content in DS with cataracts
(DSc) and without cataracts (DSnc) and in control DR on a high
sodium diet. Asterisk denotes a value significantly different
(p<0.001)from the value observed in DR. Dagger denotes a
value significantly different (p<0.001) from that in DSnc. Lenticular sodium content varied significantly (p<0.001) among
the three groups.
genesis in DS may be a genetic defect resulting from
successive inbreeding of rats from this strain, the increased pressor response to the first 2 weeks on a high
salt diet in DS in which cataracts later developed suggests a possible link with hypertension. The possibility
CATARACTS IN GENETICALLY HYPERTENSIVE RATS/Rodr(guez-Sargent et al.
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FIOURE 3. Lenticular potassium content in DS with cataracts
(DSc) and without cataracts (DSnc) and in control DR on a high
sodium diet. Asterisks denote values significantly different
(p<0.05)from the value observed in DR. Dagger denotes a
value significantly different (p<0.05)from that in DSnc. Lenticular potassium content varied significantly (p<0.001)
among the three groups.
of a relationship between sodium intake and cataractogenesis cannot be evaluated on the basis of the present
data. Nevertheless, the accelerated development of hypertension in DSc supports our hypothesis that cataract
formation is related to the degree of salt-sensitivity in
this model.
The precise mechanism of cataractogenesis in DSc
cannot be defined from the data presented here. Hypertonic saline loading has been reported to induce
cataracts as a consequence of lenticular dehydration
resulting from high extracellular fluid osmolality.20 Although the administration of isotonic saline in addition
to sodium in standard rat chow (0.45% sodium) constitutes mildly hypertonic sodium loading, the increased
lenticular water content in DSc is clear evidence that
cataracts did not result from lens dehydration. It is also
unlikely that cataract formation in DSc was the consequence of altered steroidogenesis, since steroids induce posterior subcapsular cataracts,21 which were not
observed in DSc. This finding, however, does not rule
out a possible contributing role of high adrenocortical
steroid levels to cataractogenesis in DSc.
Other factors that could have resulted in cataract
formation were also considered in the present study.
Hypocalcemia does not appear to be the cause of lens
opacification, since the presence of cataracts in DSc
was not associated with low serum ultrafiltxable calcium concentration. Neither was cataract formation
associated with renal failure, since blood urea nitrogen
was similar among DSc, DSnc, and DR. Equally unlikely is the possibility that cataracts resulted from
hyperglycemia, since fasting serum glucose levels
were similar among DSc, DSnc, and DR. In addition,
diabetes is not a factor in DS or DR.
The most plausible explanation for our findings of
altered aqueous humor and lenticular electrolyte composition is that cataractogenesis in DSc may have resulted from altered ionic transport at the lens epithelium or ciliary body (or both). In the epithelium lining
the anterior lenticular surface, Na + ,K + -ATPase transports potassium from aqueous humor into the lens in
exchange for sodium. In this manner, Na + ,K + ATPase counterbalances the passive leak of sodium
and potassium across the posterior lens surface, as
proposed in the classic pump-leak hypothesis.7 Thus,
inhibition of the enzyme in the lens epithelium would
result in decreased lenticular chemical gradients of
sodium and potassium, which could explain the results
of the present study. Since aqueous humor formation is
regulated largely by transport processes at the ciliary
ridge epithelium,22 however, inhibition of Na + ,K + ATPase activity exclusively at the anterior lens surface
may not be sufficient to explain altered aqueous humor
electrolyte composition in DS. Decreased Na + ,K + ATPase activity at the ciliary ridge epithelium, on the
other hand, might partially account for our observations. At the ciliary processes where aqueous humor is
secreted, Na + ,K + -ATPase moves potassium from
aqueous humor across basolateral membranes of the
nonpigmented epithelial cell layer in exchange for sodium. Although sodium movement from blood to
aqueous humor appears to be mediated largely by
transport mechanisms other than Na + ,K + -ATPase, 23
selective inhibition of Na + ,K + -ATPase activity might
lead to measurable changes in aqueous humor electrolyte composition. Thus, inhibition of Na + ,K + -ATPase
at the lens epithelium or ciliary ridges (or both) might
account for our findings.
TABLE 2. Systolic Blood Pressure in DS With and Without Cataracts and in DR Between 18 and 21 Weeks of Age
Systolic blood pressure (mm Hg)
Group
21 wk
18 wk
19 wk
20 wk
DS
With cataracts
Without cataracts
DR
198±2*t
151±13*
98±2
307
188 + 6*
16O±17*
93+4
Values are means + SEM.
*p < 0.001, compared with corresponding values in DR.
t p < 0.05, compared with corresponding values in DS without cataracts.
185 + 9*
159±16*
102±2
192 + 8*t
157±11*
95 ± 4
308
HYPERTENSION
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Nevertheless, on the basis of the data presented
here, alternative explanations for the aqueous and lenticular electrolyte imbalance in DSc and DSnc cannot
be ruled out. The transport disturbance might result
from changes in membrane ionic permeabilities, for
example, rather than inhibition of Na+,K+-ATPase
activity. Furthermore, data in DS without cataracts
suggest a decreased transmembrane potassium gradient with no change in the sodium gradient, raising the
possibility that a specific potassium transport defect
may precede cataract formation in this genetic model.
Whatever the case, our data may reflect a generalized
disturbance in passive or active ion transport in DS.
The increased aqueous humor potassium concentration
and the decreased lenticular potassium content observed in DS without cataracts suggest that altered
ionic transport may be a characteristic of this genetic
model but that cataract formation mayresultonly when
the transport defect is severe.
In conclusion, we have described a potential genetic
model of cataracts in which cataractogenesis is associated with hypertension. This observation raises the
question of a possible link between hypertension and
cataract formation.
Acknowledgments
We are grateful to Dr. Jaime E. Irizarry for his expert advice and
for performing slit-lamp evaluations. We also acknowledge the
excellent secretarial assistance of Velia M. Ortiz and Claribel
Alvarez.
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Cataracts and hypertension in salt-sensitive rats. A possible ion transport defect.
C Rodríguez-Sargent, J L Cangiano, G Berríos Cabán, E Marrero and M Martínez-Maldonado
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Hypertension. 1987;9:304-308
doi: 10.1161/01.HYP.9.3.304
Hypertension is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1987 American Heart Association, Inc. All rights reserved.
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