Vol. 276, Issue 6, H1811-H1817, June 1999
Cardiac myofibrillar and sarcoplasmic reticulum function are
not depressed in insulin-resistant
JCR:LA-cp rats
Tarun
Misra1,3,
James S. C.
Gilchrist1,2,
James C.
Russell4, and
Grant N.
Pierce1,3
1 Division of Stroke and
Vascular Disease, St. Boniface General Hospital Research Centre,
and Departments of 2 Oral Biology and
3 Physiology, University of Manitoba,
Winnipeg, Manitoba R2H 2A6; and
4 Department of Surgery,
University of Alberta, Edmonton, Alberta, Canada T6G 2S2
 |
ABSTRACT |
Depressed
myofibrillar Ca2+-ATPase activity
and sarcoplasmic reticulum (SR)
Ca2+ uptake are important
mechanisms that are responsible for the cardiac dysfunction exhibited
by insulin-deficient (type I) diabetic animals. The
JCR:LA-cp rat is a model for type II
non-insulin-dependent diabetes mellitus (NIDDM). This rat
is insulin resistant, obese, and has high levels of circulating
glucose, cholesterol, insulin, and triglycerides. The purpose of this
study was to determine whether changes in cardiac myofibrillar, SR, and
cardiomyocyte function exist in this model of type II diabetes.
Myofibrils and SR were isolated from hearts by differential
centrifugation. Surprisingly, we found that myofibrillar
Ca2+-ATPase activities were
unaltered in these animals. Ca2+
uptake in isolated SR fractions was increased in diabetic
cp/cp rats, whereas Ca2+-ATPase activity
and ryanodine binding were unchanged. Cardiomyocytes isolated from
hearts of control and experimental animals had similar active cell
shortening and intracellular Ca2+
concentration under basal conditions and in response to caffeine. Our
data argue against the presence of a cardiomyopathy in this diabetic
model and suggest that insulin may be an important factor in the
cardiomyopathy observed in type I diabetic models.
contractile proteins; calcium; diabetic cardiomyopathy; excitation-contraction coupling; myosin; cardiomyocytes; non-insulin-dependent diabetes mellitus; insulin
| |
INTRODUCTION |
DIABETES MELLITUS induces cardiac dysfunction
independent of vascular complications (5, 10, 28, 33, 36).
The subcellular basis for the diabetic cardiomyopathy has received much
research attention (see Refs. 5, 28, 33, and 38 for reviews). One
subcellular organelle that has been implicated in the contractile dysfunction is the contractile protein. Myosin and myofibrillar ATPase
activities have been closely correlated with tension development in the
heart (44). Myofibrillar, actomyosin, and myosin ATPase activities are
depressed in hearts from diabetic animals (2, 6-8, 20, 22, 23, 29,
30, 32, 34, 39, 43, 45). The sarcoplasmic reticulum (SR) is another
subcellular organelle critical for cardiac performance and
excitation-contraction coupling. Defects in SR
Ca2+ uptake and release have been
identified in hearts from diabetic animals (11, 27, 39, 43, 45, 46).
The vast majority of work done on the diabetic cardiomyopathy in
animals has been carried out on insulin-deficient models of diabetes
(chemically induced or Bio-Breeding Worcester rats). The
animals typically have high circulating glucose and lipid levels, low
circulating insulin concentrations, and depressed body weights (23, 29,
30, 34, 37). These characteristics resemble those of the type I,
insulin-dependent diabetic state (24, 28). However, there is a relative
lack of data on the more common form of diabetes, the insulin-resistant
type II diabetes [non-insulin-dependent diabetes mellitus
(NIDDM)]. In type II diabetes, circulating levels of glucose are
higher than normal despite elevated insulin concentrations, reflecting
a peripheral insulin resistance. The syndrome is strongly associated
with abdominal or android obesity (24). Only a few studies (21, 32, 43) have examined cardiac integrity and function in a model of type II
diabetes, and there are no studies examining cardiac subcellular function in a genetic model of type II diabetes.
The JCR:LA-cp rat is an excellent
alternative model of NIDDM that has advantages for the study of heart
disease (9, 16, 19, 21, 40-42). This unique strain exhibits a
genetically determined development of insulin resistance and
cardiovascular disease. Rats homozygous for the autosomal recessive
cp gene
(cp/cp)
exhibit insulin resistance, glucose intolerance, elevated circulating lipid levels, hyperinsulinemia, and obesity, whereas heterozygous (+/cp) or homozygous normal (+/+)
rats are unaffected (9, 19, 21, 40-42). The
cp/cp
rats are unusually sensitive to ischemic challenge to the heart (21).
The
cp/cp
male rats have atherosclerosis and exhibit spontaneous myocardial
lesions (40-42). Of particular interest is the inability of hearts
from these animals to function in the presence of high circulating
Ca2+ concentrations
([Ca2+]) (19). This
raises the possibility that one or more of the subcellular organelles
that respond to Ca2+ or that
regulate intracellular
[Ca2+] may be
defective within the myocardium of these rats. Changes in the
Ca2+ sensitivity of the
myofibrillar ATPase activity and/or the SR Ca2+ uptake in these hearts may
explain the unusual response of these hearts to circulating
Ca2+. Furthermore, a change in
Ca2+-related function of these
organelles would be entirely consistent with previous work in
insulin-deficient models of diabetes (2, 6-8, 20, 22, 23, 29, 30,
34, 39, 45, 46). It is also unclear from previous studies whether the
hearts from these animals exhibit defects in contractile performance
under basal conditions. Conflicting data exist on this issue, with one study (19) demonstrating normal performance in the JCR:LA
cp/cp rat hearts and another study (21) demonstrating a transient depression
in contractile performance in these rat hearts. The purpose of the
present study, therefore, was to examine the status of myofibrillar and
SR function in hearts from JCR:LA-cp rats.
| |
METHODS |
Experimental animals.
Male JCR:LA rats were bred and maintained until they reached 3 or 6 mo
of age in the established breeding colony at the University of Alberta
(9, 40-42). JCR:LA rats that are homozygous for the cp gene are obese, hyperphagic,
insulin resistant, hyperinsulinemic, glucose intolerant, and
hyperlipidemic (9, 40-42). JCR:LA animals that are heterozygous
for the cp gene (+/cp) or homozygous
normal (+/+) have a normal metabolic profile. Thus we have used
heterozygous or homozygous normal animals as the lean control group
(+/?). All care and treatment of the animals were in
conformity with the Guidelines of the Canadian Council on Animal Care
and subject to prior review by the appropriate institutional animal
welfare committees. The
cp/cp
rats are infertile and difficult to obtain; therefore, experiments were
carefully designed and kept to an absolute minimum. For this reason,
the majority of experiments were carried out on 3-mo-old animals, and
only in selected cases were 6-mo-old animals tested. By 3 mo of age,
cp/cp
rats had already exhibited insulin resistance and glucose intolerance
for ~2 mo.
Biochemical measurements.
Plasma cholesterol, glucose, and triglyceride were measured in the
postprandial state as previously described (29, 30, 32). Insulin levels
were also measured in response to a defined meal. Rats were deprived of
food overnight and bled from the tip of the tail for an initial sample.
The rats were then returned to their cages and presented with a test
meal consisting of a 5-g pellet of rat chow that was consistently eaten
immediately. Further samples of blood were taken from the tail. Plasma
was separated and analyzed for insulin by radioimmunoassay (Kahi
Pharmacia, Uppsala, Sweden).
Hearts were removed from animals at death, immediately immersed in
liquid N2, and then stored at
80°C for subsequent analysis. Cardiac
myofibrils were isolated by differential centrifugation as previously
described (29, 30). All isolation solutions contained 1 mM
dithiothreitol, 1 mM phenylmethylsulfonyl fluoride, and 1 µM
leupeptin. The final fraction was suspended in 0.1 M KCl, 20 mM
imidazole (pH 7.0), and 1 mM dithiothreitol. Fractions were
stored at 5°C for a maximum of 48 h after isolation. Relative purity of the fractions was determined by assaying for
Na+-K+-ATPase,
succinic dehydrogenase, and mannose-6-phosphatase activities as
previously described (4, 13). Protein was measured as previously
described (4, 13).
Myofibrillar ATPase activity was measured at 37°C for 5 min in a
medium containing (in mM) 20 imidazole (pH 7.0), 2 MgCl2, 2 Na2ATP, 10 NaN3, 1 EGTA, 50 KCl, and varying
total CaCl2 to generate different
[Ca2+] according to
the MaxChelator computer program (26). The
Ca2+ association constants of Bers
(3) were used in this program. The reaction was initiated with the
addition of 50 µg of myofibrillar protein and stopped with the
addition of 1 ml of ice-cold 12% trichloroacetic acid. Liberation of
inorganic phosphate was measured as previously described (30). When
enzyme activity was examined as a function of reaction time, the
Ca2+-independent activity was
carried out in the medium described above containing 0.1 µM
Ca2+, whereas the medium for
determining Ca2+-ATPase activity
contained 100 µM Ca2+. The
reaction was linear over 20 min.
Cardiac SR vesicles were obtained by differential centrifugation as
described in detail elsewhere (14). The final SR fraction was
resuspended in 30% sucrose and 20 mM Tris (pH 7.0) and
stored under a liquid N2
atmosphere before use. Oxalate-supported
Ca2+ uptake,
Ca2+-ATPase activity, and
ryanodine binding in the SR vesicles were measured as described in
detail previously (14).
Cardiomyocytes were isolated by standard collagenase digestion
methodology as described elsewhere (18). The yield of rod-shaped, Ca2+-tolerant cardiomyocytes was
~70-80% in both groups. Myocyte contractile activity was
monitored in an unloaded setting with a video edge-detection system
(Crescent Electronics, Sandy, UT) coupled to a Pulnix monochrome charge-coupled device camera. Cell shortening was monitored during electrical stimulation with platinum electrodes at a rate of 0.5 Hz
with 200-ms duration. A microscope stage micrometer was used to
calibrate the cell length. Cells were perfused in a Leiden chamber on
the microscope stage at a rate of 1 ml/min with a perfusate heated to
37°C and bubbled extensively with 100%
O2. The perfusate contained 140 mM
NaCl, 6 mM KCl, 1 mM MgCl2, 1.25 mM CaCl2, 6 mM HEPES (pH 7.4), 10 mM dextrose, and 0.02% bovine serum albumin. Cells were equilibrated
in this solution for a period of time before the start of any
experimental interventions.
Intracellular [Ca2+]
was measured spectrofluorometrically using the
Ca2+ indicator dye fura 2 (Molecular Probes, Eugene, OR) as previously described (18). In the
system described above, a SPEX Fluorolog spectrofluorometer was
attached to a Nikon Diaphot epifluorescent microscope. Photomultiplier
tubes were coupled to a Pentium computer for quantitation of the
fluorescent signal. Calibration of the signal was carried out with a
microscope stage micrometer.
Statistical analyses.
A two-tailed Student's t-test was
used to measure statistical significance
(P < 0.05).
| |
RESULTS |
Animal characteristics and plasma metabolite levels.
Male
cp/cp
rats exhibited higher body, heart, and liver weights than their
corresponding lean +/+ control counterparts at 3 mo of age (Table
1). Similar trends exist for older animals (data not shown). The heart-to-body weight ratio was lower in the
cp/cp
rats compared with those in the age-matched, lean control animals. A
similar trend was observed in another model of NIDDM (32). Conversely,
despite the large increase in body mass, the liver-to-body weight ratio
was maintained in the
cp/cp
rats.
Plasma metabolite characteristics for these animals are similar to
those reported elsewhere (9, 19, 21, 40-42). Plasma cholesterol,
glucose, and triglyceride levels were elevated in the
cp/cp
rats compared with those in the corresponding age-matched control
animals (Table 2).
Resting serum insulin levels were elevated in corpulent rats compared
with those in control animals (Fig.
1). The
cp/cp
rats also exhibited markedly higher insulin levels in immediate
response to the ingestion of a defined meal than the corresponding
control animals. These levels remained elevated for the entire 3-h
period of study after ingestion of the meal.
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Fig. 1.
Serum insulin levels in JCR: LA-cp
lean control (+/?;  ) and corpulent
(cp/cp;
 ) rats before and after ingestion of a test meal of 5 g of rat chow
at time 0. Values represent means ± SE for 7 animals in each group.
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Myofibrillar characteristics and activities.
Myofibrillar yield for the +/+ control rats was 73 ± 8 and 59 ± 6 mg/g wet tissue weight for the 3- and 6-mo-old rats, respectively, and 63 ± 2 and 63 ± 3 mg/g wet tissue weight for the 3- and
6-mo-old cp/cp
rats, respectively (n = 8). There were
no differences between the genotypes
(P > 0.05).
Purity of the myofibrillar protein fractions was estimated by measuring
typical marker enzyme activities from sarcolemmal membranes,
mitochondria, and SR.
Na+-K+-ATPase,
succinic dehydrogenase, and mannose-6-phosphatase activities were
undetectable in the myofibrillar fractions from all groups. Furthermore, SDS-polyacrylamide gel analysis revealed no detectable differences in the protein profile between the myofibrillar fractions.
Myofibrillar Ca2+-ATPase
activities were measured as a function of reaction time in cardiac
myofibrils isolated from 3-mo-old cp/cp
and +/+ rats (Fig.
2A).
There were no significant differences in activities between the two
groups (P > 0.05). It is possible that the duration of diabetes was insufficient to induce changes in the
function of the cardiac subcellular organelles. For example, hearts of
JCR:LA-cp rats respond very
differently to an ischemic insult, depending on the age of the animals
(3 or 6 mo old) (21). Insulin and glucose abnormalities are apparent at
~1 mo of age in the JCR:LA-cp rats
(data not shown). To investigate the possibility that the duration of
diabetes is an important factor, cardiac myofibrillar
Ca2+-ATPase activities were
studied in rats that were 6 mo old. There were no significant
differences in activity over 1-20 min of reaction time (Fig.
2B).
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Fig. 2.
Myofibrillar Ca2+-ATPase activity
as a function of reaction time in hearts from 3- (A) and 6-mo-old
(B) JCR:LA lean control () and
corpulent () rats. Values represent means ± SE for 4-6
samples. There were no significant differences between genotypes
(P > 0.05).
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Cardiac myofibrillar ATPase activity was measured as a function of
varying [Ca2+] (Fig.
3). Increasing
[Ca2+] stimulated
ATPase activity. Maximal ATPase activity was observed at ~8 × 10
6 M
Ca2+. Half-maximal activation was
observed at ~2.5 × 106 M
Ca2+. There were no significant
differences in the ATPase activity as a function of
[Ca2+] between groups
in 3-mo-old animals (Fig. 3A).
Furthermore, we could not detect any significant differences in
Ca2+-ATPase activity as a function
of [Ca2+] in 6-mo-old
rats (Fig. 3B). The ATPase activity did exhibit a modest but statistically insignificant shift to the right in Ca2+ sensitivity. The Hill
coefficients from plots of these data for the lean and corpulent groups
are 1.22 and 1.44, respectively. The
EC50 for the two sets of data are
2.2 and 2.5 µM Ca2+ for the lean
and corpulent groups, respectively. Thus myofibrillar ATPase activities
were not altered in JCR:LA-cp rats
that were 3 or 6 mo old.
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Fig. 3.
Myofibrillar Ca2+-ATPase activity
in hearts as a function of varying
Ca2+ concentration
([Ca2+]) in 3- (A) and 6-mo-old
(B)
JCR:LA-cp lean control ( ) and
corpulent () rats. Values represent means ± SE for 4-6
samples. There were no significant differences between genotypes
(P > 0.05).
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Cardiac SR function and characteristics.
The SR is another subcellular organelle that is critical for cardiac
contractile performance. SR Ca2+
uptake contributes to force generation and relaxation (15, 28) and is
defective in hearts from insulin-dependent diabetic animal models (11,
27, 39, 43, 45, 46). SR vesicles were isolated from hearts from
3-mo-old JCR:LA-cp rats and examined for oxalate-supported Ca2+ uptake
as a function of assay reaction time (Fig.
4). Surprisingly, there was a modest but
significant increase in SR Ca2+
uptake in the diabetic
cp/cp
rats compared with that in the +/+ control group.
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Fig. 4.
SR Ca2+ uptake as a function of
reaction time in hearts from JCR:LA-cp
lean control () and corpulent () rats. Values represent means ± SE for 7 samples. * P < 0.05 vs. lean control.
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SR Ca2+ uptake occurs through the
activity of the SR Ca2+-ATPase. SR
Ca2+-ATPase activity was also
examined in hearts from lean and corpulent rats in the absence or
presence of varying
[Ca2+]. No significant
change in SR Ca2+-ATPase activity
was observed under any of the experimental conditions (Fig.
5).
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Fig. 5.
SR ATPase activity in hearts from
JCR:LA-cp lean control (open bars) and
corpulent (filled bars) rats. Values represent means ± SE for
6-7 samples. There were no significant differences between
genotypes (P > 0.05).
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SR Ca2+ release is regulated
through ryanodine-sensitive
Ca2+-release channels (14, 46).
These channels can be quantitated with radioisotopic assays for
[3H]ryanodine binding. Ryanodine
binding sites are depressed in myocardial homogenates obtained from
insulin-dependent diabetic rats (46). However, in SR isolated from +/?
and
cp/cp
rat hearts, specific ryanodine binding was unchanged (Fig.
6).
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Fig. 6.
Ryanodine binding in cardiac SR isolated from hearts from
JCR:LA-cp lean control (+/?) and
corpulent
(cp/cp)
rats. Values represent means ± SE for 7 samples. There were no
significant differences between genotypes
(P > 0.05).
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It is possible that the results obtained on subcellular organelles may
not accurately reflect activities at a cellular level; therefore,
single cardiomyocytes were isolated from
JCR:LA-cp rats and monitored for
active cell shortening. There were no significant differences in active
cell shortening between the two groups in response to electrical
stimulation (data not shown). Even if the electrical stimulation was
varied from 0.5 to 1.0 Hz, no differences in active cell shortening
were observed between the two groups (data not shown). To confirm the
results obtained with the isolated SR vesicles, we also examined the
response of the cells when perfused with 10 mM caffeine. Caffeine is
capable of increasing contractile activity by releasing
Ca2+ from SR stores. As shown in
Fig. 7, there were no significant differences in active cell shortening in response to 10 mM caffeine between cells from the two groups. Significant differences in resting
cell length were detected only at two early time points after exposure
to caffeine (Fig. 8). However, all other
time points examined were similar in the two groups.
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Fig. 7.
Effects of 10 mM caffeine on active cell shortening in isolated
cardiomyocytes from JCR:LA-cp lean
control () and corpulent () rats. Values represent means ± SE
for 9-22 cells. There were no significant differences between
genotypes (P > 0.05).
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Fig. 8.
Effects of 10 mM caffeine on resting cell length in isolated
cardiomyocytes from JCR:LA-cp lean
control () and corpulent () rats. Values represent means ± SE
for 9-22 cells. * P < 0.05 vs. lean control.
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Intracellular [Ca2+]
was studied in response to electrical stimulation in cardiomyocytes
isolated from the two groups of rats. Diastolic cell
[Ca2+] was 202 ± 17 and 211 ± 36 nM in lean and corpulent rat cells, respectively
(P > 0.05). Systolic cell
[Ca2+] was 335 ± 28 and 428 ± 46 nM in lean and corpulent rat cardiomyocytes, respectively (P > 0.05, n = 9-22). The effect of caffeine
on cell [Ca2+] was
also studied. As shown in Fig. 9, caffeine
induced a decrease in the intracellular
Ca2+ transient. This was expected
from previous work (15). This effect is due to an ability of caffeine
to increase myofilament sensitivity to
Ca2+ (15). Consistent with the
cell shortening data, there were essentially no significant differences
between the two groups in the response of cellular
Ca2+ to caffeine. Only one of the
data points was significantly different between the two groups.
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Fig. 9.
Effects of 10 mM caffeine on intracellular
[Ca2+] in response to
caffeine in isolated cardiomyocytes from
JCR:LA-cp lean systolic (), lean
diastolic ( ), corpulent systolic (), or corpulent diastolic ( )
rats. Values represent means ± SE for 9-22 cells.
* P < 0.05 vs. corresponding
lean control.
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| |
DISCUSSION |
The present study demonstrates a lack of depression in myocardial
myofibrillar and SR function in the
cp/cp
rats compared with the +/+ control
JCR:LA-cp rats despite the presence of
diabetes. This result is surprising in view of the number of studies
that have reported a depression in contractile protein ATPase activity (2, 6-8, 20, 22, 23, 29, 30, 32, 34, 39, 43, 45) and SR
Ca2+ uptake (11, 27, 39, 45, 46)
in insulin-deficient, type I diabetic rat hearts. We have confidence in
the validity of our results from several perspectives. Our results were
not confounded by differential contamination of the myofibrils or SR by
other subcellular fractions. The activities for myofibrillar ATPase and
SR Ca2+ uptake in control animals
are similar to those reported previously (25, 29, 30, 32). The number
of different measurements undertaken (ATPase activity and
Ca2+ uptake as a function of
varying [Ca2+] and
reaction time; ryanodine binding; cell shortening and
[Ca2+] as a function
of caffeine) and the types of activities measured (myofibril, SR,
isolated cells) produced a consistent pattern of results. Indeed, in
one case there was actually an increase in SR
Ca2+ uptake in hearts from
cp/cp
rats (Fig. 5). This increase was not large but may have been
responsible for some of the isolated changes in cell shortening and
[Ca2+] in response to
caffeine as shown in Figs. 8 and 9. Furthermore, the age of the
JCR:LA-cp rats did not influence the
results. This is particularly important because the cardiac response to
ischemic insult in JCR:LA-cp rats has
been shown to change with age (21). All of these experiments
consistently demonstrate that myofibrillar and SR functions were not
depressed in the hearts from diabetic cp/cp
rats compared with these functions in the control +/? rats.
Our results appear to be difficult to reconcile with the fact that so
many previous studies have reported a significant depression in
myocardial contractile protein ATPase activity (2, 6-8, 20, 22,
23, 29, 30, 32, 34, 39, 43, 45) and SR function (11, 27, 39, 43, 45,
46) during diabetes. The most obvious difference between our work and
the majority of these previous studies is that the animals in the
present study exhibited an insulin-resistant, hyperinsulinemic, type II
diabetic state, not an insulin-deficient, type I diabetic state. Our
data would suggest that myocardial contractile protein defects and SR
Ca2+ transport lesions do not
necessarily accompany diabetes where it is defined by glucose
intolerance and hyperinsulinemia. Instead, the type of diabetes
mellitus (IDDM vs. NIDDM) may be critical for the generation of the
myocardial dysfunction. Several studies have reported depressions in
contractile protein ATPase activity, SR
Ca2+ transport (32, 43), and cell
[Ca2+] (1) in NIDDM
models. However, the animal models used in these studies have important
limitations that may restrict the conclusions. Neither of the other two
models of NIDDM (1, 32, 43) had high circulating basal insulin levels.
Furthermore, the chemically induced model of NIDDM exhibits normal body
weight, which contrasts with the majority of patients with type II
diabetes, who are obese (24). These two factors may represent important
metabolic differences. Clearly, the strength of the present study lies
in the animal model employed. The abdominal obesity, insulin
resistance, and high risk of cardiovascular disease observed in the
JCR:LA-cp rat closely mimics the
clinical state seen in humans. The rats develop atherosclerotic lesions
in the heart, suffer from spontaneous small infarcts, and have a
metabolic profile (glucose, lipid, and hormonal) remarkably similar to
that of humans with NIDDM (9, 19, 21, 40-42). Our results do not
explain the contractile dysfunction exhibited by
JCR:LA-cp rats in response to high
circulating [Ca2+]
(11). It is possible that thin filament sensitivity to
Ca2+ may be altered. We did not
measure myosin isozyme changes. Furthermore, other
Ca2+ regulatory systems such as
the sarcolemmal membrane may play a role in this defect. It is also
important to recognize that the cell-shortening data are limited
because the cells were measured in an unloaded state. However, previous
studies (19) in Langendorff perfused hearts also demonstrated no
significant changes in contractile performance in the JCR:LA
cp/cp rats.
Our data suggest that lipid abnormalities and hyperglycemia per se are
not mechanistic factors likely to explain the presence of a
cardiomyopathy in the insulin-deficient diabetic animals. The
JCR:LA-cp rat exhibits both of these
metabolic abnormalities and still does not display the myocardial
dysfunction typically observed in insulin-deficient diabetic animals.
Indeed, the
cp/cp rat exhibits even higher circulating lipid levels than
insulin-deficient diabetic animals. Instead, chronic insulin deficiency
may play an important role in depressing cardiac contractile
function in insulin-deficient diabetes. It is well known that insulin
treatment can reverse the defects in myofibrillar and SR function in
insulin-deficient diabetic rats (11, 12, 20). Insulin can directly
increase SR Ca2+ uptake (31) and
can alter ATPase sensitivity to
Ca2+ (17), but it has no direct
effect on myofibrillar function (31). It is important to note that
compounds similar to insulin such as insulin-like growth factor-1 have
the capacity to induce significant improvements in the mechanical
characteristics of ventricular myocytes when overexpressed in these
cells (35). These changes were associated with alterations in
contractile proteins (35). It is possible, therefore, that a chronic
deficiency of insulin may induce a cardiodepressive state, whereas
chronic hyperinsulinemia may protect against this metabolic lesion.
However, this remains to be proven, and our results are only suggestive on this matter at the present time.
Our data reinforce the contention (33, 42) that cardiovascular disease
in the JCR:LA-cp diabetic model is a
vascular problem, not a cardiac one. The opposite holds for
insulin-deficient diabetic rats. It is tempting to speculate that type
I and type II diabetes in humans may also be very different with
respect to their expression of primary cardiac lesions.
| |
ACKNOWLEDGEMENTS |
The technical assistance of Thane G. Maddaford is greatly appreciated.
| |
FOOTNOTES |
This work was supported by a grant from the Canadian Diabetes
Association in honor of Agnes Dorothy Knudson and by grants from the
Heart and Stroke Foundations of Manitoba and Alberta and the Northwest
Territories. T. Misra was supported by a studentship provided by the
Manitoba Medical Services Foundation.
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: G. N. Pierce,
Div. of Stroke and Vascular Disease, St. Boniface General Hospital
Research Centre, 351 Tache Ave., Winnipeg, Manitoba, Canada R2H 2A6
(E-mail: pierce{at}sbrc.umanitoba.ca).
Received 16 September 1998; accepted in final form 1 February
1999.
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