Original Article
Marked Increase in White Adipose Tissue Blood
Perfusion in the Type 2 Diabetic GK Rat
Caroline Kampf,1,2 Birgitta Bodin,2 Örjan Källskog,2 Carina Carlsson,2 and Leif Jansson2
The aim of the present study was to evaluate and correlate islet to brown and white adipose tissue (WAT)
blood perfusion in one obese rat and one nonobese rat
with type 2 diabetes (obese Zucker [OZ] and GK rats,
respectively). We measured blood perfusion with a microsphere technique in anesthetized animals and subsequently estimated the blood flow to seven different
WAT depots and brown adipose tissue, in addition to the
whole pancreas and pancreatic islets. Both GK and OZ
rats had higher islet blood perfusion than their respective control strains. Adipose tissue blood flow (ATBF)
was similar to or lower than that of controls in the
normoglycemic OZ rats. GK rats, however, had 5–10
times higher blood perfusion than control Wistar rats in
most WAT depots. Vascular density and macrophage
numbers in WAT did not differ between the different
strains. The discrepancy in ATBF between the obesenormoglycemic and type 2 diabetic rats opens the intriguing possibility that changes in this blood perfusion
may influence and/or modulate the ␤-cell dysfunction in
type 2 diabetes. Diabetes 54:2620 –2627, 2005
O
besity is a major worldwide health problem
that is statistically associated with increased
risk for cardiovascular disease, type 2 diabetes,
and stroke (1,2). Obesity closely coexists with
insulin resistance and endothelial dysfunction during the
natural history of type 2 diabetes (3). It seems established
that substances released from white adipose tissue (WAT),
including free fatty acids (FFAs), leptin, interleukin 6,
tumor necrosis factor-␣ (TNF-␣), and adiponectin, may
contribute to both insulin resistance, in muscle and WAT,
and ␤-cell dysfunction (3– 6).
Previous studies have demonstrated that pancreatic
islet blood flow is increased during glucose intolerance
and, at least initially, during type 2 diabetes (7,8). In this
From the 1Departments of Genetics and Pathology, Uppsala University,
Uppsala, Sweden; and the 2Department of Medical Cell Biology, Uppsala
University, Uppsala, Sweden.
Address correspondence and reprint requests to Dr. Caroline Kampf,
Department of Genetics and Pathology, Rudbeck Laboratory, C5:3, SE-751 85
Uppsala, Sweden. E-mail: [email protected].
Received for publication 29 March 2005 and accepted in revised form 20
June 2005.
ATBF, adipose tissue blood flow; BS-1, Bandeiraea simplicifolia-1; FFA, free
fatty acid; TNF-␣, tumor necrosis factor-␣; WAT, white adipose tissue.
© 2005 by the American Diabetes Association.
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked “advertisement” in accordance
with 18 U.S.C. Section 1734 solely to indicate this fact.
2620
context, it is of interest that an impaired WAT blood flow
has been linked to obesity and insulin resistance in humans (9 –11). Furthermore, in the obese Zucker (OZ) rat, a
decrease in WAT blood flow has been observed (12,13),
while pancreatic islet blood flow is increased (14). The
extraction of plasma triglycerides is probably influenced
by WAT blood perfusion, which also facilitates signaling
between adipose tissue and other tissues to regulate
metabolism (15,16). In addition, the release of FFAs can be
modulated by WAT blood flow (16). Thus, change in WAT
blood flow can be expected to affect its own metabolic
functions. To what extent such changes in adipose tissue
function may affect islet blood flow and islet hormone
release in general is at present unknown.
In view of the considerations presented above, we
hypothesized that adiposity, especially in animal models of
type 2 diabetes, may affect the blood perfusion of both
WAT and the islets. The aim of the present study was
therefore to correlate WAT and islet blood flow in one
normoglycemic obese rat strain and one nonobese model
of type 2 diabetes (the OZ rat and the GK rat, respectively).
Both these rat strains have an increased islet blood flow
(14,17,18). Macrophages have been suggested to infiltrate
WAT; therefore, we investigated macrophage density.
RESEARCH DESIGN AND METHODS
Male Wistar or GK rats aged 3– 4 months were purchased from B&M (Ry,
Denmark), and similarly aged OZ or lean Zucker (LZ) rats (crl:ZUC-faBR)
were purchased from Charles River Laboratories (Hannover, Germany). The
animals had free access to food (Type R3; B&K, Sollentuna, Sweden) and
water throughout the experiments. All experiments were approved by the
local animal ethic committee at Uppsala University (Uppsala, Sweden).
Chemicals were purchased from Sigma-Aldrich (Irvine, U.K.) unless otherwise
stated.
Blood flow measurements. The rats were anesthetized with an intraperitoneal injection of thiobutabarbital sodium (120 mg per kg body weight)
(Inactin; Research Biochemicals, Natick, MA). The animals were then placed
on a heated operating table to maintain body temperature at ⬃38°C. Polyethylene catheters were inserted into the ascending aorta via the right carotid
artery and into the left femoral artery and vein. The former catheter was
connected to a pressure transducer (PDCR 75/1; Druck, Groby, U.K.), whereas
the venous catheter was used to infuse Ringer solution (5 ml per kg body
weight per h) to substitute fluid. When the blood pressure had remained stable
for at least 20 min, blood flow measurements were performed with a
microsphere technique as previously described (19). Briefly, 2.0 ⫻ 105 black
nonradioactive microspheres (EZ-Trac; Triton Microspheres, San Diego, CA)
with a diameter of 10 or 15 ␮m were injected through the catheter with its tip
in the ascending aorta for 10 s. Starting 5 s before the microsphere injection,
and continuing for a total of 60 s, an arterial blood sample was collected by
free flow from the catheter in the femoral artery (⬃0.6 ml/min). The exact
withdrawal rate was confirmed in each experiment by weighing the sample.
Arterial blood was used for determinations of hematocrit, glucose concentraDIABETES, VOL. 54, SEPTEMBER 2005
C. KAMPF AND ASSOCIATES
TABLE 1
All measurements were performed in thiobutabarbital-anesthetized rats aged 3– 4 months
Strain of animals
Experiments (n)
Body weight (g)
Pancreas weight (mg)
Pancreas weight (% body weight)
Psoas fat pad weight (mg)
Psoas fat pad (% body weight)
Islet volume (% pancreas)
Islet mass (mg)
Blood glucose (mmol/l)
Serum insulin (ng/ml)
Mean arterial blood pressure (mmHg)
Hematocrit (%)
Pancreatic blood flow (ml/min ⫻ g)
Islet blood flow (% total pancreatic blood flow)
Duodenal blood flow (ml/min ⫻ g)
Colonic blood flow (ml/min ⫻ g)
Adrenal blood flow (ml/min ⫻ g)
Wistar
GK
LZ
OZ
8
322 ⫾ 3
922 ⫾ 19
0.286 ⫾ 0.006
1,297 ⫾ 77
0.401 ⫾ 0.020
1.14 ⫾ 0.09
10.5 ⫾ 0.8
5.6 ⫾ 0.1
3.30 ⫾ 0.32
108 ⫾ 5
45.1 ⫾ 0.2
0.86 ⫾ 0.13
5.8 ⫾ 0.5
1.95 ⫾ 0.29
1.12 ⫾ 0.13
6.50 ⫾ 1.08
8
303 ⫾ 3
817 ⫾ 13
0.274 ⫾ 0.006
2,148 ⫾ 60*
0.710 ⫾ 0.021*
0.96 ⫾ 0.08
7.7 ⫾ 0.7
10.2 ⫾ 0.7†
2.72 ⫾ 0.20
113 ⫾ 5
45.2 ⫾ 0.2
1.08 ⫾ 0.14
4.1 ⫾ 0.5
3.06 ⫾ 0.42
0.99 ⫾ 0.20
8.82 ⫾ 0.87
7
311 ⫾ 10
1,084 ⫾ 34
0.350 ⫾ 0.013
895 ⫾ 58‡
0.289 ⫾ 0.020‡
0.96 ⫾ 0.06
10.5 ⫾ 1.3
6.1 ⫾ 0.2
2.36 ⫾ 0.38
103 ⫾ 4
45.3 ⫾ 0.4
0.87 ⫾ 0.09
4.5 ⫾ 0.5
2.04 ⫾ 0.15
1.00 ⫾ 0.10
6.88 ⫾ 0.52
9
451 ⫾ 19†
987 ⫾ 61*
0.221 ⫾ 0.015†
3,893 ⫾ 214‡†
0.864 ⫾ 0.031†
2.64 ⫾ 0.14†
27.0 ⫾ 2.0†
6.9 ⫾ 0.5
8.02 ⫾ 2.84†
124 ⫾ 3*
45.4 ⫾ 0.3
1.08 ⫾ 0.14
18.7 ⫾ 1.5‡
2.46 ⫾ 0.37
1.05 ⫾ 0.18
4.50 ⫾ 0.59*
Data are means ⫾ SE. *P ⬍ 0.01 and †P ⬍ 0.001 when compared with the corresponding control strain (Student’s unpaired t test). ‡P ⬍ 0.05
when compared with all other strains (ANOVA).
tions (MediSense, Stockholm, Sweden), and serum insulin concentrations
(Rat Insulin ELISA; Mercodia, Uppsala, Sweden).
The animals were then killed and the pancreas and adrenal glands were
removed, blotted, and weighed. Samples (⬃100 mg) from the midregions of
the duodenum and colon were also removed, blotted, and weighed. Furthermore, the whole psoas fat pads were dissected from surrounding tissues and
weighed. Samples (100 –200 mg each) of WAT were then taken intra-abdominally from the psoas fat, pancreatic/mesenteric fat, epididymal fat, and the
posterior surface of the sternal xiphoid process. Subcutaneous WAT was
sampled from the lower part of the abdominal wall, the throat, and the back
(between the shoulder blades). Brown adipose tissue was also excised from
the latter region.
The number of microspheres in the samples referred to above, including
pancreatic islets, was counted as previously described (20). This enabled us to
estimate the volume of the islets within the pancreas by a point-counting
method (21) and thereby also estimate the islet mass. Organ blood flow values
were calculated (19) and, with regard to islet blood perfusion, were expressed
both per gram wet weight of the whole pancreas and the estimated wet weight
of the islets themselves. Blood flow values based on the microsphere contents
of the adrenal glands were used to confirm that the microspheres were
adequately mixed in the circulation. A difference of ⬍10% in the blood flow
values was taken to indicate sufficient mixing.
Staining for Bandeiraea simplicifolia-1 (BS-1). This method has previously been described in detail (22). Four-micrometer sections from formalinfixed pancreas, brown adipose tissue and WAT from the psoas fat pad,
pancreatic fat, and subcutaneous fat from the abdomen and back were
studied. Briefly, the sections were incubated in neuroaminidase type V in 37°C
for 2 h followed by washing in Tris-buffered saline. Biotinylated lectin against
BS-1 1:100 was incubated at 4°C overnight. The sections were washed and
incubated with Vectastain ABC kit (Vector Laboratories, Burlingame, CA) for
30 min. The reaction was developed using Alkaline Phosphatase Substrate Kit
I (Vector Laboratories) for 10 –30 min. Sections were counterstained with
hematoxylin and mounted in Pertex (HistoLab, Gothenburg, Sweden).
Staining for ED-1/CD68. Sections of 4 ␮m from formalin fixed brown
adipose tissue and WAT from the psoas fat pad, pancreatic fat, and abdominal
subcutaneous fat were studied. Incubation with peroxidase blocking reagent
for 10 min was followed by rinsing in wash buffer. The sections were
pretreated with proteinase K for 5 min. The CD68 antibody clone ED-1
(Serotec, Oxford, U.K.) diluted 1:1,000 was incubated for 30 min at room
temperature followed by washing. The ChemMate EnVision detection kit was
developed with diaminobenzidine before counterstaining with hematoxylin.
The sections were dehydrated and mounted (Pertex; HistoLab). All reagents
were from DAKO, Glostrup, Denmark.
Morphometrical evaluation. Twelve or more randomly chosen tissue sections from each adipose tissue depot and animal were stained with either BS-1
(to visualize blood vessels) or ED-1 (to visualize macrophages). The numbers
of stained blood vessels or macrophages were quantified in a light microscope
(40⫻ magnification). In each type of adipose tissue, ⬃8 fields were counted
(range 6 –9). The area of the investigated tissue was determined by using a
DIABETES, VOL. 54, SEPTEMBER 2005
computerized system for morphometry (Easy Image 300; Tekno Optik, Huddinge, Sweden). Vascular density and macrophage numbers (i.e., the number
of stained blood vessels or macrophages per measured adipose area [expressed as mm2] and/or per number of adipose cells) were then calculated.
Statistical calculations. All values are given as means ⫾ SE. Probabilities
(P) of chance differences were calculated with Student’s unpaired t test, or
with one-way repeated-measurement ANOVA with Tukey’s correction (SigmaStat; SSPD, Erfart, Germany) A value of P ⬍ 0.05 was considered to be
statistically significant. Unless otherwise stated, comparisons were made
between GK and Wistar rats or OZ and LZ rats, respectively.
RESULTS
General values (Table 1). The OZ rats were markedly
heavier than all other rats. The pancreas weight was
similar in all groups with the exception of LZ, which had
larger pancreata compared with the other groups. When
pancreas weight was expressed as a fraction of body
weight, this ratio was lower in OZ rats and higher in LZ rats
when compared with the Wistar and GK rats.
The weight of the psoas fat pads was markedly higher in
OZ rats when compared with all the other rats, whereas
the fat pad weight in GK rats was higher than in Wistar
rats. When the psoas fat pad weight was given as a fraction
of the body weight, GK and OZ rats had values that were
higher than for their corresponding control animals. No
differences in psoas fat pad weight between GK and OZ
rats were seen (ANOVA), whereas the value for LZ was
lower than that for Wistar rats (P ⬍ 0.05; ANOVA).
Anesthetized OZ rats had slightly higher mean arterial
blood pressures than LZ rats, while hematocrit was similar
in all animals. Both islet volume density and islet mass
were similar in Wistar, GK, and LZ rats, whereas the
corresponding values were higher in OZ rats (P ⬍ 0.05 for
all comparisons; ANOVA). The GK rats had the highest
blood glucose concentrations, whereas serum insulin concentrations were highest in OZ rats.
Blood flow measurements. Total pancreatic blood flow
was similar in all groups of animals when the measurements were performed with 15-␮m microspheres (Table
1). Islet blood flow expressed per gram pancreas was
markedly higher in the OZ rats (Fig. 1A). When islet blood
2621
WHITE ADIPOSE TISSUE BLOOD PERFUSION IN RATS
FIG. 1. Islet blood flow expressed per gram pancreas (A) or per
milligram islet (B) in Wistar (WF), GK, LZ, and OZ rats. Data are
means ⴞ SE for 7–9 experiments. §P < 0.05 when compared with LZ
rats (Student’s unpaired t test); ***P < 0.001 when compared with the
other three strains (ANOVA).
flow was expressed per milligram islet tissue to compensate for the differences in islet volume, the blood perfusion
of OZ rats was higher when compared with LZ rats (Fig.
1B). There were no differences between GK and Wistar
rats. However, GK rats had a higher total pancreatic and
islet blood flow compared with Wistar rats when the blood
flow measurements were performed with 10-␮m microspheres (online appendix [available at http://diabetes.
diabetesjournals.org]). When LZ and OZ rats were
compared, islet blood flow was higher in the latter group.
Pancreatic blood flow did not differ between LZ and OZ
rats (online appendix).
It was a consistent finding in all rat strains examined
that the WAT blood perfusion in general was low. The only
exception was the blood flow to the subcutaneous fat on
the throat, which was higher than that of other WAT
depots in Wistar, GK, and OZ rats (P ⬍ 0.01 for all
comparisons within these strains; ANOVA) (Fig. 2A) but
not in LZ rats. No other regional differences between the
studied sites were seen when comparisons were made
within the strains. Neither were any significant differences
between the blood perfusion of subcutaneous and intraabdominal fat depots seen when intrastrain values were
compared.
When the blood flow to WAT depots was compared
between strains, GK rats had markedly higher blood
2622
perfusion in all depots when compared with WF (Fig. 2B
and C and Fig. 2E and F), OZ, and LZ rats (P ⬍ 0.01 for all
comparisons), with the exception of epididymal fat (Fig.
2D). Also, the blood flow to brown adipose tissue was
higher in GK rats when compared with the other strains
(Fig. 2H). In epididymal and dorsal subcutaneous fat, the
blood perfusion was lower in OZ compared with LZ rats
(Fig. 2C and D).
Duodenal and colonic blood flow values were of the
same order of magnitude in the different strains. Adrenal
blood flow was lower in OZ rats when compared with LZ
rats (Table 1).
Morphological studies. Staining with BS-1 made it easy
to identify blood vessels in both pancreatic islets (Fig. 3A)
and WAT (Fig. 3B). Islet vascular density was similar in
Wistar and GK rats, while islets of OZ rats contained
slightly more capillaries than LZ rats (Fig. 4A). When OZ
rats were compared with WF or GK rats, no differences
were seen (P ⬎ 0.7; ANOVA).
The vascular density was similar in WAT in all depots
when Wistar and GK rats were compared (Fig. 4B). When
LZ and OZ rats were compared, the density was markedly
lower in the psoas fat pad of OZ rats (Fig. 4B). LZ and OZ
rats had similar vascular density of pancreatic (P ⫽ 0.097),
abdominal subcutaneous (P ⫽ 0.056), and dorsal subcutaneous fat (P ⬎ 0.25).
The size of individual fat cells in the different WAT
depots varied. The number of fat cells per millimeter
squared was similar in Wistar and GK rats (Fig. 5A),
whereas the number of fat cells was much lower in OZ rats
than in LZ rats in all depots investigated (Fig. 5A).
When calculating the number of microvessels per fat
cell to compensate for strain-dependent differences in size
of adipocytes, a somewhat different picture appeared (Fig.
5B). Thus, there were once again no differences between
Wistar and GK rats. However, the blood perfusion per fat
cell was much higher in all depots of OZ rats when
compared with LZ rats except for dorsal subcutaneous
WAT (Fig. 5B).
The CD68 antibody clone ED-1 made macrophages
within the WAT easily distinguishable (Fig. 3C). The
number of macrophages per millimeter squared was similar when the WAT depots were compared between Wistar
and GK rats and LZ and OZ rats, respectively, with the
exception of the psoas fat pad in GK rats, which contained
fewer macrophages than Wistar rats (Fig. 6A). This value
in GK rats was also lower than that observed in LZ rats
(P ⫽ 0.01; ANOVA).
When the number of macrophages were calculated per
adipocyte, the general finding was that there were minor
differences when Wistar and GK rats were compared (Fig.
6B). However, the WAT depots of OZ rats contained more
macrophages than those of LZ rats (Fig. 6B).
DISCUSSION
The major finding in the present study was that GK rats, a
type 2 diabetes model, have higher adipose tissue blood
perfusion in most depots, both when compared with
control rats and normoglycemic markedly OZ rats. This
opens the intriguing possibility that changes in adipose
tissue blood flow (ATBF) may influence and/or modulate
DIABETES, VOL. 54, SEPTEMBER 2005
C. KAMPF AND ASSOCIATES
FIG. 2. A–H: WAT blood flow
in different depots and brown
adipose tissue blood flow in
Wistar, GK, LZ, and OZ rats.
Data are means ⴞ SE for 7–9
experiments. *P < 0.02, **P <
0.01, and ***P < 0.001 when
compared with the corresponding control animals with
Student’s unpaired t test.
the ␤-cell dysfunction in type 2 diabetes, perhaps by
altering the storage and release of lipids.
In the present study we could, as expected, verify the
massive obesity of the OZ rat. These animals are leptin
resistant due to a receptor point mutation (23). The insulin
resistance in OZ rats can, at least initially, be compensated
for by hyperinsulinemia, so the animals remain normoglyDIABETES, VOL. 54, SEPTEMBER 2005
cemic. This was reflected in a higher islet mass in the OZ
rats when compared with the other strains. Thus, OZ rats
are an obese animal model, which through adaptive processes is able to maintain normoglycemia.
The GK rat, on the other hand, was hyperglycemic, had
normal serum insulin concentrations, and had an islet
mass which was similar to that of WF rats, even though
2623
WHITE ADIPOSE TISSUE BLOOD PERFUSION IN RATS
FIG. 4. Vascular density per area in endogenous pancreatic islets (A) or
different depots of WAT (B) in Wistar, GK, LZ, and OZ rats. Data are
means ⴞ SE for six experiments. **P < 0.01 when compared with the
corresponding value for LZ rats (Student’s unpaired t test).
FIG. 3. Sections from a pancreas (A) and subcutaneous WAT (B and C)
from a Wistar rat stained for the lectin BS-1 (arrows, A and B) or for
the CD68 antibody clone ED-1 (arrows, C). Background staining
hematoxylin. Scale bar ⴝ 100 ␮m.
there is a tendency (P ⫽ 0.08) toward a decrease in the
latter. Somewhat surprisingly, the weight of the psoas fat
pad was almost doubled in the GK rats when compared
with WF rats. This suggests, since the weight of this depot
fairly accurately mirrors total body fat (24), that the GK rat
2624
is also an obese rat model, albeit not to the same degree as
OZ rats. Thus, the GK rat mirrors several traits of type 2
diabetes in humans, including obesity and increased insulin resistance in both liver (25) and WAT (26), but lack
hyperinsulinemia. In view of this it should be possible to
elucidate the roles of insulin resistance associated with
obesity and hyperglycemia on islet and WAT blood perfusion.
White ATBF was ⬃3 ml 䡠 100 g⫺1 䡠 min⫺1 in the control
strains, thereby confirming previous results in humans and
rodents (15). One striking difference, however, was the
five- to tenfold higher blood perfusion found in the subcutaneous fat on the anterior throat in all studied rat strains.
The reason for this marked difference is unknown.
ATBF distribution in different sites did not differ either
when different strains or fat depots were compared. One
exception from this general finding was the subcutaneous
fat on the anterior throat just described above. This is in
accord with a previous study in female Wistar rats, where
no regional differences were detected when retroperitoneal, parametral, mesenteric, and subcutaneous fat blood
flow was compared in either basal or trained rats (27). A
heterogeneity has also been seen in humans, where subcutaneous ATBF was higher in the upper than the lower
parts of the abdomimal wall. It should be noted, however,
that the blood perfusion is very low in the present study,
DIABETES, VOL. 54, SEPTEMBER 2005
C. KAMPF AND ASSOCIATES
FIG. 5. Fat cell density (A) and number of blood vessels per fat cell in
different depots of WAT (B) in Wistar, GK, LZ, and OZ rats. Data are
means ⴞ SE for six experiments. ****All values in the OZ rats are
different from (P < 0.001) the corresponding value in LZ rats (Student’s unpaired t test).
FIG. 6. Macrophage density per area (A) and number of macrophages
per fat cell (B) in different depots of WAT in Wistar, GK, LZ, and OZ
rats. Data are means ⴞ SE for six experiments. *P < 0.05, **P < 0.01,
and ***P < 0.001 when compared with the corresponding control rats
(Student’s unpaired t test).
since the animals have not been recently fed (16), and
minor differences are likely to be difficult to detect.
The ATBF in the obese, normoglycemic OZ rat was
similar to or lower than that in the control LZ animals. This
supports previous findings in this model (12,13,28,29). We
also found that the individual WAT cells were larger in OZ
rats, whereas the vascular density was approximately
similar. Thus, when the number of microvessels was
expressed per fat cell, the vascular density was higher in
OZ rats when compared with LZ rats. This means that the
unchanged or slightly impaired ATBF most likely reflects
changes in the regulation of the blood perfusion rather
than a decreased vascularity. It may be that the decreased
ATBF reflects the hypercellular-hypertrophic obesity in
this animal model, which has been suggested to also
encompass a functionally important blood flow–mediated
decrease in delivery and removal of nutrients and substrates to and from WAT (13).
The most surprising finding in the present study was
that the GK rat had markedly increased ATBF values
(ranging from 5 to 10 times higher) in all examined depots
besides the epididymal fat pad. This flow increase was not
due to any increase in vascularity, since the vascular
density was similar to that seen in WF rats. Thus, the
regulation of ATBF is likely to be affected. Before trying to
interpret possible responsible mechanisms, we would like
to shortly recapitulate how normal ATBF regulation occurs. Major factors affecting the blood perfusion of WAT
include feeding, which in humans can increase subcutaneous ATBF up to fourfold (30). This increases substrate
delivery for triglyceride clearance (15,31), and when ATBF
was increased by adrenaline it was reported that triglyceride extraction increased in parallel with increased blood
flow (31). Also, exercise and other types of stress increase
ATBF in humans, although less markedly (32,33). Furthermore, ATBF increases during fasting overnight (34). During both these conditions, WAT releases nonesterified
fatty acids and thereby requires a supply of albumin for
transportation purposes.
In humans, adrenergic influences on ATBF are predominant with ␤-mediated vasodilation and ␣2-mediated vasoconstriction (15,16). This may explain the influences of
exercise or fasting. It seems as if circulating cat-
DIABETES, VOL. 54, SEPTEMBER 2005
2625
WHITE ADIPOSE TISSUE BLOOD PERFUSION IN RATS
echolamines are more important in exercise-induced hyperemia as seen after studies in patients with spinal cord
lesions (35). The increased ATBF seen after feeding is not
fully explained. It correlates with insulin secretion and
suppression of nonesterified fatty acids. Even though
insulin is not the local signal responsible it can stimulate
ATBF through sympathetic activation (9). It can, however,
be blocked by propranolol, completely in some depots and
partially in others (36), suggesting that adrenergic receptors are involved. Thus, increased sympathetic nervous
activity could be one explanation for the increased ATBF
in GK rats (see further below).
Like in many other vascular beds in the body, that of
WAT is also sensitive to nitric oxide (NO). This substance
determines the absolute level of ATBF, but a major
proportion of the postprandial enhancement of ATBF is
under ␤-adrenergic control (37). This means that NO has a
permissive effect for other blood flow regulatory systems
to exercise their effects, similar to what has been observed
in pancreatic islets (38). In humans, one previous study
demonstrated no effects on ATBF of the NO synthase
inhibitor monomethyl-arginine (39), but they used ethanol
outflow-to-inflow ratio in a microdialysis system to evaluate blood flow, which has a low sensitivity (40). It should
be noted that the increased islet blood flow seen in GK rats
can be normalized by NO synthase inhibition (38). Furthermore, an increased NO synthase activity has been observed in the retina of GK rats (41). Thus, there is some
evidence that NO levels may be increased in GK rats, and
if this is the case this may well explain the hyperperfusion
of blood in GK WAT.
The degree of insulin sensitivity in humans and experimental animals is closely related to ATBF responsiveness
(9,15). Thus, impaired regulation of ATBF seems to be
another facet of the insulin resistance syndrome (9).
Substances released from WAT may contribute to these
changes in blood perfusion as well as insulin resistance
and ␤-cell dysfunction. Such factors include TNF-␣, FFAs,
adiponectin, resistin, and leptin (3,42). In OZ rats, TNF-␣
has been suggested to be important, mainly by inducing
hepatic insulin resistance (43), whereas the role of this
substance in GK rats is unknown. Both resistin and
adiponectin expression are decreased in OZ rats (44), and
once again, their concentrations in GK rats are unknown.
Since both resistin and adiponectin increase blood flow
(45), their participation in the increased ATBF remains a
possibility. Leptin, however, is a general vasodilator acting
on receptors of endothelial cells, which then probably
mediate the effects through NO (46). In OZ rats the leptin
receptor is deficient, whereas no such changes are known
or likely to exist in GK rats. Thus, a reasonable working
hypothesis for further studies is that hyperleptinemia in
GK rats may mediate an increased ATBF through increased local synthesis of NO. However, the findings on
islet blood flow referred to below shed some doubt also on
this possibility.
Pancreatic islet blood perfusion is also regulated by the
factors referred to above. When 10-␮m microspheres were
used for blood flow measurements, islet blood perfusion
of both GK and OZ rats was increased when compared
with the control strains (online appendix), thereby confirming previous studies (17,18,47). For a further discus2626
sion on islet blood perfusion in these animals, see the
online appendix.
It seems as if the GK rat is unique in the sense that both
islet blood perfusion and ATBF are increased simultaneously, whereas only islet blood flow is increased in the
OZ rat. It has been suggested that macrophages may
infiltrate WAT in obesity (48,49). These cells can, in
addition to adipocytes, also express cytokines and may by
themselves affect the production of adipocytokines (50).
However, there were no major differences in macrophage
content of WAT when comparing GK or OZ rats when they
were expressed per area. At the moment, the coupling
between type 2 diabetes in GK rats and their increased
ATBF is unknown. It can be speculated that the ability to
increase ATBF to such a large extent may enable GK rats
to continuously mobilize FFAs and thereby prevent accumulation of WAT depots (i.e., prevent any marked degree
of obesity). The increased blood perfusion of brown
adipose tissue may also increase caloric expenditure.
Some support from that stems from the increased serum
FFAs and cholesterol of GK rats. In support of that, we
have recently observed that both fatty acids and triglycerides may by themselves increase islet blood flow (L.J.,
Ö.K., A. Delgado Verdugo, T. Alsgård, C.K., unpublished
observation).
ACKNOWLEDGMENTS
Financial support was received from the Swedish Research Council (72X-109), the Swedish Diabetes Association, the Juvenile Diabetes Research Foundation, the
EFSD/Novo Nordisk for Type 2 Diabetes Research Grant,
and the Family Ernfors Fund.
The skilled technical assistance of Astrid Nordin and
Eva Törnelius is gratefully acknowledged.
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