RESPONSES OF HEART FUNCTION AND INTRACELLULAR FREE CA2-ACID IN CHRONIC DIABETES( TO PHOSPHATIDIC)

OBJECTIVE: In view of the crucial role of phosphatidic acid (PA) in si gnal transduction and Ca2+-handling in myocardium, it was the objectiv e of this study to examine the effects of PA on cardiac contractile fo rce and intracellular free Ca2+ in control and chronic diabetic rats. METHODS: Diabetes was induced in rats by a single intravenous injectio n of streptozotocin (65 mg/kg.) and the animals were used for experime nts eight weeks after the injection. Heart function was measured by us ing the isolated perfused heart preparations, and values for systolic pressure, diastolic pressure, rate of contraction (+dP/dt) and rate of relaxation (-dP/dt) were monitored. Intracellular free Ca2+ in cardio myocytes was estimated by employing Fura-2/AM method. RESULTS: PA (5x1 0(-8) to 1x10(-5) M) produced a concentration-dependent increase in +d P/dt and -dP/dt in the isolated heart; however, these responses were s ignificantly attenuated in diabetic hearts. ATP also caused a positive inotropic effect at concentrations of 1x10(-5) to 1x10(-4) M but the magnitude of these responses was similar in bath control and diabetic groups. Using freshly isolated cardiomyocytes and Fura-2 technique, PA (1x10(-6) to 1x10(-4) M) was observed to evoke a concentration-depend ent increase in [Ca2+](i) in both control and diabetic groups. The EC( 50) and EC(95) values for PA were not different but the maximum increa se of [Ca2+](i) in diabetic hearts was significantly lower in comparis on to the control group (152+/-41 versus 304+/-56 nM). On the other ha nd, no difference in the increase of [Ca2+](i) due to ATP or potassium chloride was seen between control and diabetic cardiomyocytes. Adrena line pretreatment enhanced [Ca2+](i) responses to ATP and PA in both g roups; however, the PA induced increase in [Ca2+](i), unlike the ATP-i nduced increase, was lower in the diabetic group compared to the contr ol cells with similar pretreatment with adrenaline. The diminished inc rease in [Ca2+](i) due to PA was also observed in cardiomyocytes obtai ned from rats in which diabetes was induced by intravenous alloxan (65 mg/kg). CONCLUSIONS: PA induced [Ca2+](i) mobilization and positive i notropic response were depressed in diabetic heart; this defect in the signal transduction mechanism may contribute to the lower tonic respo nses of certain inotropic agents in chronic diabetes.