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Regulation of cardiac lipoprotein lipase during hypertension and diabetes

University of British Columbia

Hypertension and diabetes often co-exist in humans and the cardiac dysfunction greatly exceeds that observed with either condition alone. Induction of streptozotocin (STZ) diabetes in the spontaneously hypertensive rat (SHR) produces a more extensive cardiomyopathy relative to normotensive rats. Both diabetes and hypertension are associated with early metabolic alterations that could underlie the pathogenesis of cardiac dysfunction. In diabetes, the heart is exclusively dependent on free fatty acid (FFA) oxidation for its energy production. On the other hand, during hypertension, the heart has been shown to utilize glucose predominantly. Interestingly, when diabetes was induced, SHR hearts showed a remarkable shift towards FFA oxidation that strongly correlated with severe cardiac dysfunction observed in SHR-diabetic hearts. To date, the underlying mechanism(s) for these metabolic lesions is not understood. Lipoprotein lipase (LPL), a "gate-keeping" enzyme plays an important role in supplying free fatty acid (FFA) to the heart and therefore could play a potential role in cardiac metabolism. We hypothesized that changes in cardiac metabolism that occur during hypertension and diabetes are a consequence of alterations in cardiac LPL activity. Clinical and experimental evidence suggest that hypertension and hypertriglyceridemia co-exist, and may be causally related to each other, arguably as a result of defective LPL action. In fact, LPL activity is decreased in skeletal muscle and adipose tissues in hypertensive patients, Dahl-hypertensive rats and in stroke-prone SHR rats. We investigated the effect of hypertension on cardiac LPL in two hypertensive animal models: a) the Spontaneously hypertensive (SHR) rat that has a genetic propensity to develop hypertension, and b) the fructose hypertensive rat which is an acquired model of hypertension. Hearts from spontaneously hypertensive (SHR) rats were examined before or after the development of severe hypertension in SHR rats. Age matched Wistar Kyoto (WKY) rats were used as normotensive controls. With the development of hypertension in SHR rats, there was a concomitant and progressive reduction in the heparin-releasable coronary endothelial LPL activity. Neither insulin action nor cell-associated enzyme activity could explain this low LPL 2+ activity in coronary blood vessels. However, acute vasodilation with nifedipine (a Ca influx blocker) or CGS-21680 (A2-purinergic receptor agonist) increased the peak heparin-releasable LPL activity in hearts isolated from SHR rats. Fructose feeding induced hypertension, hypertriglyceridemia and hyperinsulinemia in male Wistar rats. Acute fructose treatment did not alter cardiac heparin-releasable LPL activity, whereas a significant decrease in LPL activity was seen in the chronic treated group. Withdrawal of fructose treatment normalized blood pressure and cardiac heparin-releasable LPL activity. Similar to the SHR study, acute vasodilation by in vitro perfusion of coronary vasodilators like nifedipine and CGS-21680 increased cardiac heparin-releasable LPL activity in the chronic group to control levels. These studies demonstrate that hypertension may play a significant role in regulating cardiac LPL activity and that the decrease in enzyme could be a result of poor perfusion through the cardiac vasculature. The diabetic heart has elevated levels of FFA and TG, being supplied from various sources. The relative contribution of LPL to this supply of FFA during diabetes is not clear. We previously demonstrated that moderate diabetes (induced by 55 mg/kg STZ) augments heparin releasable LPL and that this augmentation is possibly due to an increased translocation of the enzyme from its site of synthesis (i.e. cardiomyocytes). To determine the precise location of the augmented LPL, a modified Langendorff retrograde perfusion was used to isolate the enzyme at the coronary lumen from that in the interstitial effluent. In response to heparin, a 4-fold increase in LPL activity and protein mass was observed in the coronary perfusate after 2 weeks of STZdiabetes. Release of LPL activity into the interstitial fluid of control hearts was slow but progressive, whereas in diabetic hearts, peak enzyme activity was observed within 1 to 2 min after heparin, followed by a gradual decline. Immunohistochemical studies of myocardial sections confirmed that the augmented LPL in diabetic hearts was mainly localized at the capillary endothelium. To study the acute effects of insulin on endothelial LPL activity, we examined rat hearts at various times after the onset of hyperglycemia. An increased heparinreleasable LPL activity in diabetic rats was demonstrated shortly (6 to 24 hours) after STZ injection or after withdrawal from exogenous insulin. Heparin-releasable coronary LPL activity was also increased after an overnight fast. These studies indicate that the intravascular heparinreleasable fraction of cardiac LPL activity is acutely regulated by short-term changes in insulin rather than glucose. Thus, during short periods (hours) of hypoinsulinemia, increased LPL activity at the capillary endothelium can increase the delivery of FFAs to the heart. To study the effect of this enlarged LPL pool on triglyceride (TG) rich lipoproteins, we examined the metabolism of very low-density lipoprotein (VLDL) perfused through control and diabetic hearts. Diabetic rats had elevated lipoprotein TG levels compared to control. However, fasting for 16 hours abolished this difference. When the plasma lipoprotein fraction of density < 1.006 g/ml from fasted-control and -diabetic rats (and thus containing mainly VLDL) were incubated in vitro with purified bovine or rat LPL, diabetic VLDL was hydrolyzed as efficiently as VLDL obtained from control animals. Moreover, VLDLs derived from diabetic rats were found to have a similar apolipoprotein pattern when compared to control VLDL. Post-heparin plasma lipolytic activity was comparable between control and diabetic animals. [3H]VLDL obtained from control rats was metabolized at a significantly faster rate by perfused diabetic hearts than by control rat hearts. This increased VLDL-TG hydrolysis was essentially abolished by prior perfusion of the diabetic heart with heparin, implicating LPL in this process. These findings suggest that the enlarged LPL pool in the diabetic heart is present at a functionally relevant location (at the capillary lumen), and is capable of hydrolyzing VLDL. In summary, hypertension per se regulates cardiac LPL in SHR and fructose hypertensive rats. The decreased LPL at its functional location could possibly restrict the supply of FFA (derived from circulating TGs) to the cardiac tissue leading to an increased glucose oxidation. On the other hand, cardiac LPL is enhanced within hours after induction of diabetes, inducing a corresponding increase in TG breakdown. These changes complement the observed increases in FFA oxidation in the diabetic heart. Interestingly, previous studies in our lab indicate that induction of diabetes in SHR rats counteracted the hypertension-induced decrease in cardiac LPL activity and significantly increased functional LPL in the heart. This may serve to increase the delivery of FFA to the heart and the resultant metabolic changes may lead to the severe cardiomyopathy observed in the hypertensive-diabetic heart.

Thesis/Dissertation

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2009-07-03

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http://hdl.handle.net/2429/10091

Pharmaceutical Sciences

University of British Columbia

UBCV

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