(Hypertension. 1995;25:235-241.)
© 1995 American Heart Association, Inc.
Articles |
Cardiac Glucose and Fatty Acid Oxidation in the Streptozotocin-Induced Diabetic Spontaneously Hypertensive Rat
From the Department of Pharmacology and Toxicology, College of Pharmacy, University of Rhode Island, Kingston.
Correspondence to Robert L. Rodgers, Department of Pharmacology and Toxicology, College of Pharmacy, University of Rhode Island, Kingston, RI 02881.
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Abstract Hypertension intensifies the cardiac dysfunction of diabetes. We investigated the possible role of altered exogenous fuel oxidation in this phenomenon. Diabetes was induced by streptozotocin in spontaneously hypertensive rats and normotensive Sprague-Dawley rats. Two weeks later, mechanical performance and the oxidation of glucose and palmitate were quantified in working hearts ex vivo at intermediate and high workloads. The results showed that the nondiabetic spontaneously hypertensive rat hearts, compared with those of the normotensive controls, oxidized glucose at a higher rate but oxidized palmitate at a much lower rate, as reported previously. The effects of diabetes in the hypertensive rats, compared with its effects in the normotensive strain, were characterized by (1) a more pronounced decrease in heart performance, (2) either a similar or a less marked reduction in the rate of glucose oxidation, depending on the workload, and (3) a relatively greater increase in palmitate oxidation, particularly at the higher workload. These findings suggest that the exaggerated stimulation of fatty acid oxidation by diabetes in the hypertrophic left ventricle may be a more important contributor to the premature mechanical dysfunction than the inhibition of glucose oxidation. Possible mechanisms include antagonism of energetically favorable shifts in fuel oxidation or inhibition of accelerated membrane lipid biosynthesis in left ventricular hypertrophy.
Key Words: heart • metabolism • hypertrophy • myocardial diseases • carnitine • adenosine triphosphate • coenzyme A
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Introduction |
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Hypertension intensifies the adverse effect of diabetes mellitus on the myocardium. We and others have reported that the decline in mechanical performance effected by diabetes is more severe in the presence of either genetic or renovascular hypertension.1 2 3 The intensity of the mechanical dysfunction can be correlated with the degree of ultrastructural4 and biochemical3 5 derangements. These experimental studies support clinical findings that hypertension is a significant risk factor in the morbidity and mortality caused by cardiovascular disease in diabetics.6
Although the interaction between hypertension and diabetes on the myocardium is well described, its underlying mechanism is not clear. In studies of normotensive animals, it has been proposed that specific metabolic abnormalities effected by diabetes are important etiologic factors in the pathogenesis of the cardiomyopathy.7 A central defect in diabetes is an altered pattern of exogenous fuel use, away from glucose and toward fatty acids.8 These and related metabolic derangements are thought to underlie, to a significant extent, the development of characteristic changes in mitochondrial activity, calcium homeostasis, and membrane function, which in turn adversely affect mechanical performance.7 8
Recently, we reported that the oxidation of exogenous fuels is abnormal in the hearts of spontaneously hypertensive rats (SHR).9 Specifically, the pattern of fuel use in the SHR heart is shifted toward glucose and away from fatty acids. The significance of this change in fuel use may be that it represents an additional example of biochemical adaptations undergone by the hypertrophic left ventricle that are thought to promote energetic economy in the face of adverse hemodynamic conditions.10 Interestingly, this pattern is opposite to that which is imposed by diabetes.8 On this basis, we hypothesized that diabetes might have a more pronounced effect on exogenous fuel use in the SHR heart than it does in the nonhypertrophic heart, helping to explain its relatively severe effect on mechanical function in left ventricular hypertrophy (LVH).
Accordingly, the purpose of this study was to simultaneously quantify heart function and glucose and palmitate oxidation of perfused hearts from nondiabetic and diabetic SHR and normotensive Sprague-Dawley rats (SD). We selected a duration of diabetes of 2 weeks to determine whether metabolic abnormalities precede the functional derangement in either the SHR or SD strain and whether genetic hypertension not only exacerbates1 but also accelerates the progression of the mechanical dysfunction.
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Experimental Animals and Induction of Diabetes
Thirteen-week-old male SHR and SD were obtained from Charles River Breeding Laboratories (Kingston, NY, or Wilmington, Mass), housed communally, and fed food and water ad libitum. The SD strain was selected in preference to the Wistar-Kyoto rat (WKY) strain as the normotensive control for a variety of reasons.9 The SD is unquestionably normotensive, and by far the largest fraction of studies of rat cardiac metabolism have been carried out using this strain. In contrast, the WKY can be moderately hypertensive and hypertrophic, can undergo premature heart failure, and may be endocrinologically and metabolically nonhomogeneous, depending on the commercial source. Systolic arterial pressures were determined biweekly by the standard tail-cuff technique. At 15 weeks of age, rats were weight- and pressure-matched, within strain, and assigned to either nondiabetic or diabetic groups. Diabetes was induced via a single tail vein injection, with rats under ether anesthesia, of streptozotocin (45 mg/kg in SHR, 60 mg/kg in SD) in 0.1 mol/L sodium citrate (pH 4.5).1 Nondiabetic rats were injected with a similar volume of citrate buffer. The severity of diabetes was verified at the time of death by serum measurements of glucose by the glucose oxidase method. Previous results from this laboratory had indicated that 45 mg/kg streptozotocin in the SHR caused an equivalent hyperglycemia and hypoinsulinemia as 50 mg/kg did in the WKY group5 and that the same degree of hyperglycemia could be achieved in SD with 60 mg/kg streptozotocin.1 The serum glucose levels in this study were as follows: SHR nondiabetic, 8.4±0.4 mmol/L; SHR diabetic, 32.2±2.4 mmol/L; SD nondiabetic, 8.7±0.9 mmol/L; and SD diabetic, 31.6±4.4 mmol/L. Systolic arterial pressures of the nondiabetic SHR were significantly higher than those of the nondiabetic SD group. Diabetes had no effect on systolic arterial pressure in either strain. The average systolic arterial pressure values were as follows: SHR nondiabetic, 25.5±1.3 kPa; SHR diabetic, 24.3±1.2 kPa; SD nondiabetic, 18.1±0.9 kPa; and SD diabetic, 18.7±1.2 kPa (20 kPa=150 mm Hg).
Tissue Levels of Carnitine and Coenzyme A Derivatives
We carried out preliminary studies to compare the effects of
diabetes on serum and ventricular levels of metabolic intermediates in
the SHR and SD groups in vivo. Animals were killed by decapitation
under ether anesthesia 2 weeks after the induction of diabetes,
according to the guidelines of the Institutional Animal Care and Use
Committee of the University of Rhode Island. Serum was isolated and the
hearts were freeze-clamped in situ. Serum and myocardial levels of
free and acylcarnitine derivatives were measured by the
radiometric method of McGarry and Foster.11 Total tissue
levels of carnitine derivatives were determined from acid-soluble
and acid-insoluble fractions after homogenization of frozen heart
powder in 6% perchloric acid containing 15 mmol/L
dithiothreitol.12 Total tissue levels of free
coenzyme A (CoASH) and acetyl CoA (ACoA) were also determined from
reneutralized perchloric acid homogenates of these hearts by gradient
high-performance liquid chromatographic (HPLC)
analysis.13 Metabolite content was normalized per gram
dry weight. Free carnitine was assayed directly from serum samples, and
acyl and total carnitine were determined after alkaline hydrolysis and
reneutralization of the serum samples.
Heart Perfusion and Assessment of Heart Performance
Separate groups of rats were injected with heparin (1000 U/kg)
and killed by decapitation under ether anesthesia 10 minutes later. The
hearts were rapidly excised and the aorta isolated. Hearts were
initially perfused in the Langendorff mode with Krebs-Henseleit buffer
containing (mmol/L) NaCl 120, KCl 5.6, MgSO4 0.65,
NaH2PO4 1.21, CaCl2 2.4, EDTA 0.2,
glucose 10, and NaHCO3 25 (pH 7.4 when gassed with 95%
O2/5% CO2 at 37°C). Perfusion was
then switched to the recirculating working mode after the left atrium
was cannulated14 and the heart placed into the jacketed
heart chamber. Aortic and coronary outflows were recirculated through a
closed-system oxygenator by means of a peristaltic pump. The
recirculating perfusate (100-mL volume) was a modified Krebs-Henseleit
buffer containing either (1) 0.4 mmol/L palmitate, 10 mmol/L glucose,
and 75 µU/mL bovine insulin (Sigma Chemical Co) for nondiabetic rat
hearts or (2) 1.2 mmol/L palmitate, 30 mmol/L glucose, and 15 µU/mL
insulin for diabetic rat hearts.15 These concentrations
were selected to mimic the in vivo physiological environment of the
respective groups as much as possible. The selection of insulin
concentrations was based on serum analyses of 8-week diabetic
animals.5 14 Palmitate was added to the perfusate after
binding to 3% bovine serum albumin.12 Left ventricular
and aortic pressures were monitored continuously by indwelling cannulas
connected to Statham P23 pressure transducers linked to a Narco
physiograph. All hearts were paced at 265 beats per minute. Before data
collection, left ventricular pressure was monitored for 5 to 10 minutes
to ensure stability of each heart. During this period, left atrial
filling pressure was set at 10 cm H2O and aortic resistance
at 0.95 kPa/cm3 per minute.
Fuel Oxidation and Tissue High-Energy Phosphate Levels
Glucose and palmitate oxidation rates were determined
simultaneously by the addition of U-[14C]glucose
(approximately 28 000 cpm/µmol glucose) and
9,10-[3H]palmitate (approximately 150 000 cpm/µmol
palmitate) to the recirculating perfusate (both isotopes from NEN–Du
Pont). Glucose oxidation was measured by quantifying the rate of
14CO2 appearance in the effluent gas and
buffer.16 Aliquots from the 10 mol/L KOH gas traps were
placed directly into scintillant and counted. The
14CO2 present as bicarbonate was determined
by syringe removal of an aliquot of the perfusate buffer without
exposure to air. Aliquots were placed in 25-mL stoppered Erlenmeyer
flasks with a suspended center well containing 1 mol/L KOH. The
perfusate samples were acidified by syringe addition of 1 mL of 6N HCl
through the stopper. The flask was shaken for 1 hour to evolve all of
the 14CO2. The center well was then transferred
to scintillation vials for quantification of radioactivity. Palmitate
oxidation was measured simultaneously as the rate of appearance of
3H2O in the perfusate.17 In
characterizing the system, we had established that simultaneous
perfusion with [3H]palmitate and
1-[14C]palmitate yielded identical oxidation rate values.
Separation of 3H2O was initiated by the
addition of 0.5 mL of 1 mol/L HClO4 to a 0.5-mL perfusate
aliquot and centrifugation at 2500g for 15 minutes, which
caused the evolution of 14CO2 and the removal
of unreacted 9,10-[3H]palmitic acid in the pellet. The
3H2O content of the supernatant was determined
and effectively separated from U-[14C]glucose by
adjusting the scintillation counting windows. After addition of
radioactive substrates, perfusion was continued under conditions of
moderate cardiac work for 20 minutes, at which time steady oxidation
rates had been attained. Cardiac work was then increased by changing
the resistance to aortic outflow from 0.95 to 2.10 kPa/cm3
per minute, while maintaining volume loading of 10 cm H2O,
and oxidation rate measurements were continued for an additional 20
minutes. Oxidation rates were determined at 5-minute intervals and are
expressed as micromoles of fuel oxidized per minute per gram dry heart
weight. At the end of the perfusion, hearts were quickly frozen with
tongs cooled to the temperature of liquid nitrogen, weighed, and then
shattered into small pieces under liquid nitrogen. A dry weight was
obtained after one piece of tissue was placed into a drying oven at
80°C for 48 hours.
The remaining tissue was powdered under liquid nitrogen and stored at -80°C for subsequent analysis of adenine nucleotides by gradient HPLC using a Supelcosil LC-18 column (4.6 mmx15 cm) with a 3-µm packing size.18 Levels were expressed as nanomoles per milligram soluble protein in the perchloric acid extract.19
Statistics
All data are reported as mean±SD unless otherwise indicated.
Comparisons of the effects of rat strain, diabetes, and increased
workload were made using two- or three-factor ANOVA with repeated
measures where appropriate. Evaluation of significant differences was
accomplished by simple effects tests and the Tukey post hoc test for
multiple comparisons. The mean square error terms from the parent ANOVA
were used to calculate all follow-up statistics. A level of
significance of P<.05 was considered sufficient. Lower
probabilities are not reported.
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Tissue Levels of Carnitine and CoA Derivatives
A comparison of nondiabetic SHR and SD groups revealed some differences in ventricular tissue and serum carnitine levels and in tissue CoA derivatives obtained in vivo (Tables 1 and 2). With regard to carnitine, the SHR exhibited decreased tissue free, long-chain, and total carnitine levels but no differences in tissue short-chain carnitine (Table 1) or in serum free, acyl, or total carnitine (Table 2). The SHR and normotensive controls were also not significantly different with respect to myocardial ACoA or CoASH levels, although the ratio of ACoA to CoASH was approximately 50% lower in the SHR ventricle (Table 2). Although these were crude tissue measurements, the mean values for tissue long-chain acylcarnitine and ACoA-CoASH ratios were generally consistent with suppressed myocardial oxidation of fatty acids relative to glucose in the SHR strain.
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The effects of 2 weeks of diabetes on these variables were not identical in the normotensive and hypertensive groups (Tables 1 and 2). In the former, diabetes had no effect on myocardial carnitine derivatives, despite lowering serum free and total carnitine (Table 1), nor did it influence myocardial CoA derivatives (Table 2). In contrast, diabetes elevated long-chain acylcarnitine and reduced short-chain acylcarnitine in SHR myocardium but did not affect free or total carnitine in the serum (Table 1). Also, diabetes significantly decreased free CoASH and increased ACoA-CoASH ratios in SHR myocardium (Table 2). Overall, these results might suggest that diabetes enhanced fatty acid oxidation and suppressed glucose oxidation to a greater extent in the SHR than in the SD strain, in part because it significantly increased long-chain acylcarnitine and the ACoA-CoASH ratio only in the SHR ventricle.
Heart Performance
The performance ex vivo of nondiabetic SHR hearts, as indexed by
maximal left ventricular pressure development at fixed external volume
loads, was not different from that of the nondiabetic SD rat hearts at
either the moderate or higher pressure load (Fig 1).
These results confirm earlier observations showing that the function of
SHR hearts at this age is normal with respect to a variety of indexes
of mechanical performance.1 After 2 weeks of diabetes, the
function of SHR hearts was depressed at both workloads, whereas that of
the SD group was not significantly affected (Fig 1). Previously, we had
shown that an 8-week period of diabetes had caused a more profound
suppression of heart function in the SHR compared with its adverse
effects in normotensive control groups.1 Thus, the results
of the present study show that diabetes not only intensifies but
also accelerates the development of mechanical dysfunction in the
SHR.
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Fuel Oxidation and Tissue High-Energy Phosphate Levels
The pattern of exogenous fuel oxidation by the nondiabetic SHR
hearts was distinctly different from that of the SD group (Figs 2 through 4). The SHR
myocardium was characterized by higher rates of glucose oxidation (Fig 2), depressed rates of palmitate oxidation (Fig 3), and consequently
marked increases in the ratio of glucose to palmitate oxidation (Fig 4). Thus, these data were consistent with the preliminary indications
obtained from crude tissue levels of carnitine and CoA derivatives for
the nondiabetic rats, as described above.
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The effects of diabetes on fuel oxidation were not identical in the two strains (Figs 2 through 4). First, diabetes depressed glucose oxidation to a relatively greater extent in the SD hearts, particularly at the elevated workload (Fig 2). Second, the absolute level of palmitate oxidation was elevated and not different in the two diabetic groups (Fig 3). However, because palmitate oxidation had been initially suppressed in the SHR, the relative effect of diabetes was greater in that group, and the effect was most evident at the higher workload. This is reflected by the marked suppression of the ratio of glucose to palmitate oxidation in the SHR (Fig 4). Thus, these data partially confirm the results of the tissue assays; the relatively greater enhancement of palmitate oxidation in the SHR (Fig 3) is consistent with the increased tissue long-chain acylcarnitine levels (Table 1). However, the relatively smaller suppression of glucose oxidation in the diabetic SHR group (Fig 2) is not consistent with the elevated ACoA-CoASH ratio for that group obtained in vivo (Table 2). One possible interpretation of this inconsistency might be that effects of diabetes on the control of pyruvate dehydrogenase20 may be different in the SHR versus the SD myocardium. None of the differences in fuel oxidation in any group was associated with any change in total tissue levels of high-energy phosphates (Table 3).
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It should also be noted that the coupling of workload to fuel oxidation of both nondiabetic and diabetic hearts was also dissimilar in the SHR and SD groups (Figs 2 and 3). Increasing the workload stimulated glucose oxidation of nondiabetic SHR and SD hearts to about the same extent (Fig 2). This positive coupling of workload to glucose oxidation was abolished by diabetes in the SD strain, confirming previous observations by Kobayashi and Neely.20 However, it was maintained in the diabetic SHR, providing additional indirect evidence of altered control of pyruvate dehydrogenase in that group.20 Strain-dependent differences in workload coupling to palmitate oxidation are also evident. In the absence of diabetes, increasing the workload had no effect on palmitate oxidation in the SD but further suppressed it in the SHR (Fig 3), as reported previously.9 After the imposition of diabetes, however, the coupling of workload to palmitate oxidation was positive in both strains.
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Discussion |
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As we had reported recently,9 the nondiabetic SHR heart oxidized exogenous glucose at a higher rate and palmitate at a lower rate than did the nondiabetic, normotensive SD heart ex vivo (Figs 2 and 3). These findings were largely consistent with the preliminary measurements of crude tissue levels of carnitine and CoA derivatives obtained from biopsies in situ (Tables 1 and 2), which had revealed reduced levels of free, long-chain acyl, and total carnitine and somewhat diminished ACoA-CoASH ratios in nondiabetic SHR ventricle. Reibel and coworkers21 had reported that myocardial total carnitine and serum free, acyl, and total carnitine were lower in the normotensive WKY compared with the SHR. We have unpublished data generally confirming their findings. Thus, the results in Table 2 indicate that the apparent carnitine status of the SHR depends on the normotensive strain used. Although these strain-dependent differences are difficult to explain, they do seem to lend support to the concept that the WKY is an atypical normotensive group and that the SD strain is more representative, because decreased cardiac tissue carnitine is the general pattern in various models of LVH.21
The mechanism for the altered fuel use by the hypertrophic SHR heart is not clear but may involve several sites. The alterations in fuel oxidation rates were not associated with differences in either heart performance (Fig 1) or tissue high-energy phosphate levels (Table 3) between the nondiabetic SHR and SD groups. The enhanced glucose oxidation might be related to increased glucose uptake,22 23 glycolytic flux,24 25 or flux through pyruvate dehydrogenase26 brought about in part by altered intramitochondrial ratios of ACoA to CoASH, as suggested by the results in Table 2. The suppressed fatty acid oxidation might be secondary to decreased fatty acid uptake or binding to fatty acid binding protein,22 23 reduced intracellular total carnitine levels (Table 1), or abnormal activity or regulation of carnitine palmitoyl transferase.27
Regardless of its mechanism, the altered fuel use in nondiabetic SHR ventricular tissue may represent an adjustment in stable LVH to relatively hypoxic or ischemic conditions. Concentric LVH is characterized by reduced capillary densities, increased oxygen diffusion distance, and elevated myocardial oxygen demand,28 29 creating a condition of either mild hypoxia or ischemia or both, particularly under conditions of acute stress. A shift away from fatty acids toward glucose use would increase the amount of ATP formed per mole of oxygen consumed. This metabolic pattern in LVH, together with other biochemical changes such as shifts in isozymes of lactate dehydrogenase, creatine kinase, and myosin ATPase,30 31 32 is reminiscent of myocardial metabolism and biochemistry in the relatively hypoxic fetal environment.33 Alternatively, the heart undergoing hypertrophy, as well as the fetal/neonatal heart exhibiting rapid growth, may have an enhanced requirement for lipids to be shunted away from oxidation toward membrane biosynthesis.34
When the normotensive SD control is used as the standard, the deficit in calculated ATP production represented by the decline in palmitate oxidation in the SHR heart is incompletely offset by the attendant increase in glucose oxidation. Total calculated ATP production from exogenous glucose and palmitate in the SHR, at the moderate and high workloads, is 87% and 77%, respectively, that of the SD hearts. Whether the remaining apparent gap can be accounted for by enhanced glycolysis, glycogenolysis, or triglyceride turnover remains to be determined. However, the gap may not need to be closed completely. The proposed energetic economy of the concentrically hypertrophic ventricle, discussed above, presumably involves reduced rates of ATP consumption and turnover at a given level of mechanical stress.10 The developed stress on individual myocytes appears to be normal in the SHR ventricle when calculated per unit of myofibrillar cross-sectional area.35 In that sense, the nonhypertrophic heart may be an inappropriate reference when determining total ATP production requirements from exogenous and endogenous fuels in LVH.
As expected, the 2-week period of diabetes depressed glucose oxidation and elevated fatty acid oxidation in the normotensive SD strain (Figs 2 and 3). However, the magnitude of the decline in glucose oxidation was not nearly as great and the increase in fatty acid oxidation was much more pronounced than had been previously reported.16 36 37 The apparent discrepancy can be explained by our use of "diabetic buffer" to perfuse hearts from diabetic rats. To our knowledge, this is the first report of the oxidation of exogenous glucose and fatty acids by diabetic rat hearts under more pathophysiologically relevant conditions of high fatty acids, high glucose, and low insulin (rather than a complete absence of insulin) ex vivo. Most often in previous studies, the concentrations of either glucose or palmitate or both were closer to those found in normal serum, and insulin was rarely added to the perfusate.16 36 37 It would appear, therefore, that the quantification of the effects of diabetes on myocardial fuel use is influenced substantially by the perfusate concentrations of both insulin and oxidizable substrates.
Under these conditions, the influence of diabetes was found to be somewhat different in the SHR and normotensive SD strains. Although diabetes suppressed glucose oxidation in both groups, its effect was more pronounced in the SD hearts at either workload (Fig 2). The relatively weak effect in the SHR, together with the elevated rates of glucose oxidation in that group, suggests that a greater fraction of glucose oxidation in the hypertrophic ventricle is insulin independent. Abnormalities in the relation between serum insulin and glucose in the SHR have been reported previously.38 39 In contrast, the effect of diabetes on fatty acid oxidation was to increase it to the same absolute level in both the SHR and SD hearts (Fig 3). However, because fatty acid oxidation had been initially suppressed, diabetes caused a relatively greater increase in fatty acid oxidation in the SHR (Fig 3), resulting in a much more pronounced decline in the ratio of glucose to fatty acid oxidation, particularly at the higher workload (Fig 4). These results do not, of course, establish a direct causal link between altered fuel use and the exaggerated mechanical dysfunction in the diabetic SHR. However, they do demonstrate a marked difference in the pattern of myocardial fuel use between the hypertensive and normotensive strains, which precedes the accelerated onset of contractile impairment in the SHR (Fig 1). If there is a causal relation between diabetes-induced derangements of fuel use and premature contractile dysfunction in the SHR, then the most important abnormality would appear to be the relatively greater enhancement of fatty acid oxidation in that model. Indeed, the absolute rate of glucose oxidation was actually higher in the diabetic SHR when compared with the diabetic SD group at either workload (Figs 2 and 3).
In normotensive animals, altered processing of fatty acids has been implicated more extensively than abnormal glucose metabolism in the progression of diabetic cardiomyopathy.7 Free fatty acids and long-chain acyl esters of carnitine (Table 1) and CoA can accumulate in diabetic myocardium.16 These compounds have been proposed to disrupt membrane integrity40 and membrane-bound enzyme activities.41 However, causative links between amphiphilic metabolite accumulations and mechanical dysfunction have been questioned.16 In addition, free fatty acids can uncouple oxidative phosphorylation in isolated mitochondria,40 but in the present study, total tissue ATP levels were not affected (Table 3). Alternatively, high concentrations of serum and tissue lipids in diabetes may affect membrane-bound, ATP-dependent enzyme activities by altering ratios of phospholipid to cholesterol.33 34 42 43 Yu and McNeill44 have reported that blood triglycerides, and to a lesser extent cholesterol, are increased more markedly in diabetic SHR than in diabetic normotensive rat strains. If, in the hypertrophic nondiabetic SHR heart, fatty acid oxidation is suppressed to favor accelerated membrane phospholipid biosynthesis and turnover, then it is possible that a given increase in fatty acid oxidation induced by diabetes might have a more pronounced effect on membrane phospholipid profiles,34 activities of membrane-bound, ATP-consuming enzymes, and associated mechanical performance in that strain. In support of this hypothesis, diabetes has more marked detrimental effects on sarcoplasmic reticular ATP-dependent Ca2+ uptake, as well as mechanical contraction and relaxation, in the SHR than it does in normotensive strains.5
There were interesting differences in the coupling of workload to fuel oxidation between experimental groups. Glucose oxidation was similarly stimulated by increases in workload in both the nondiabetic SHR and SD groups, but at either workload it was elevated in the SHR (Fig 2). After the imposition of diabetes, the positive coupling of workload to glucose oxidation was suppressed in the normotensive SD, as previously reported,20 but preserved in the SHR (Fig 2). Because the effect of diabetes in the normotensive strain had been attributed to diabetes-induced abnormalities in the pyruvate dehydrogenase complex,20 these results (Table 2 and Fig 2) are consistent with the view that pyruvate dehydrogenase activity in the hypertrophic SHR heart is relatively more active and less insulin dependent. The pattern of palmitate oxidation was much different. At either workload, palmitate oxidation was lower in the SHR, but moving from the moderate to higher workload further suppressed palmitate oxidation in the SHR while having no effect in the SD strain (Fig 3). After induction of diabetes, however, the coupling of workload to palmitate oxidation was positive in both groups. The mechanism of this effect is not clear. However, in a volume-overload model of LVH, suppressed fatty acid oxidation was associated with diminished carnitine palmitoyl transferase activity of isolated mitochondria.27 If carnitine palmitoyl transferase synthesis and activity are stimulated by diabetes in the myocardium, as they are in the liver,45 then a proliferation of this enzyme might help to explain both the enhanced palmitate oxidation and the altered coupling of palmitate oxidation to workload in the diabetic state.
The relations between workload and fuel use ex vivo may have pathophysiological relevance. The more illustrative comparisons may be between the SHR groups at the higher pressure load and the SD groups at the moderate pressure load, conditions that more closely approximate the respective in vivo hemodynamics. When viewed in this manner, the differences discussed above become even more dramatic. Both the enhanced glucose oxidation and the suppressed palmitate oxidation of nondiabetic SHR hearts are intensified (Figs 2 and 3). The effect of diabetes on palmitate oxidation now appears to be not only relatively greater but also absolutely greater in the SHR group (Fig 3). However, the comparative pattern of glucose oxidation remains largely unaffected (Fig 2). These considerations thus strengthen the view that, in vivo, diabetes suppresses glucose oxidation to about the same extent but profoundly stimulates fatty acid oxidation in the SHR compared with its effects in the normotensive SD strain.
In summary, this study has shown that exogenous fuel oxidation is affected by diabetes differently in the normotensive SD and SHR model of hypertension. It would appear that the abnormality that most likely contributes to the premature cardiac dysfunction in SHR is a relatively marked increase in exogenous fatty acid oxidation. This effect would directly oppose metabolic shifts that are thought to promote energetic economy in LVH and may impede accelerated membrane biosynthesis, resulting in a more pronounced impact on the activities of membrane-bound enzymes and thus on mechanical performance.
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Acknowledgments |
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This work was supported in part by the American Heart Association, Rhode Island Affiliate.
Received July 5, 1994; first decision August 16, 1994; accepted October 17, 1994.
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