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Diabetes-induced changes in cardiac sarcoplasmic reticulum function Lopaschuk, Gary David


A prominent finding in the diabetic rat heart is a decrease in the rate at which the ventricular muscle can contract and relax. Since cardiac sarcoplasmic reticulum is thought to be intimately involved in muscle contraction and relaxation we studied the ability of diabetic rat cardiac sarcoplasmic reticulum to transport Ca²⁺ . Hearts were obtained from female Wistar rats 7, 30, 42, and 120 days after the induction of diabetes by a single i.v. injection of either alloxan (65 mg/kg) or streptozotocin (60 mg/kg). At all Ca²⁺ concentrations tested (0.2 μM-5.0 μM free Ca²⁺) cardiac sarcoplasmic reticulum obtained from 42 and 120 day diabetic rats showed a significant decrease in the rate of ATP-dependent tns-oxalate facilitated ²⁺ransport. This was accompanied by a decrease in Ca²⁺ -ATPase activity. The levels of long chain acylcarnitines associated with the microsomal sarcoplasmic reticulum preparation from 42 and 120 day diabetic rats were significantly higher than those present in sarcoplasmic reticulum from control rats. Palmitylcarnitine, the most abundant of the long chain acylcarnitines, in concentrations < 7 μM was found to be a potent time-dependent inhibitor of both Ca²⁺ transport and Ca²⁺ -ATPase in both control and diabetic rat sarcoplasmic reticulum preparations; inhibition of Ca²⁺ transport was found to be more marked in the control preparations. This would indicate that a degree of inhibition produced by the high endogenous levels of palmitylcarnitine may already be present in the diabetic rat heart preparations. Cardiac sarcoplasmic reticulum prepared from acutely diabetic rats (7 days) did not show any decrease in Ca²⁺ transport ability. Levels of long chain acylcarnitines associated with the microsomal preparation enriched in sarcoplasmic reticulum were also unchanged. Insulin treatment of diabetic rats could significantly increase the ability of cardiac sarcoplasmic reticulum to transport Ca²⁺, although at the time period obtested (30 days) the SR Ca²⁺ transport activity was only slightly depressed as compared to control. Insulin treatment also resulted in a slight, but non-significant, lowering of the levels of long chain acylcarnitines associated with the sarcoplasmic reticulum microsomal preparations. These findings suggest that the alteration in sarcoplasmic reticulum function in chronically diabetic rats may be due to the buildup of cellular long chain acylcarnitines which inhibit sarcoplasmic reticulum Ca²⁺ transport. The absence of any significant change in Ca²⁺ transport activity or levels of long chain acylcarnitines at 7 and 30 days suggests that the alterations in 42 and 120 day diabetic rats must be of gradual onset. Cardiac sarcoplasmic reticulum is known to be regulated by a number of factors, among them calmodulin, cAMP-dependent protein kinase, and K⁺. Since Ca²⁺ transport activity in cardiac sarcoplasmic reticulum from chronically diabetic rats is depressed, the role that these regulators play was investigated. Calmodulin (0.61 μM), cAMP (10 μM) plus cAMP-dependent protein kinase (0.2 mg/0.5 ml), and K⁺ (0-110 mM) all stimulated Ca transport in both control and streptozotocin-treated diabetic rats to the same degree. This suggests that the depression observed in sarcoplasmic reticulum function from diabetic rats is not due to altered regulation by these putative mediators of Ca²⁺ uptake. A number of studies suggest that carnitine administration may lower myocardial levels of long chain acylcarnitines in the diabetic rat. Therefore, D,L-carnitine (1 g/kg/day, orally) was administered to 120 day diabetic rats for a 30 day period. The elevated levels of long chain acylcarnitines normally seen in diabetic rats were significantly reduced in the diabetic rats administered carnitine. Carnitine administration, however, could not reverse the previously noted depression in diabetic rat heart function, as measured on an isolated working heart apparatus. In an effort to prevent the onset of the diabetic cardiomyopathy D,L-carnitine was administered (3 g/kg/day, orally) 3 days after the induction of diabetes for a 42 day period. As previously mentioned, sarcoplasmic reticulum Ca²⁺ transport activity was depressed in diabetic rats, as compared to control rats, at all free Ca²⁺ concentrations tested (0.1 μM-3.5 μM). Similarly, sarcoplasmic reticulum levels of long chain acylcarnitines were significantly elevated in these diabetic rats. The diabetic rats treated with carnitine did not show any depression in Ca²⁺ transport activity; long chain acylcarnitine levels were also similar to control. The carnitine-treated diabetic rats, however, showed no improvement in heart function compared to untreated-diabetic rats. These data suggest that although the long chain acylcarnitines are inhibiting cardiac sarcoplasmic reticulum function in chronically diabetic rats other factors must also be contributing to the depression in heart function.

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