Responses of heart function and intracellular free Ca2+ to phosphatidic acid in chronic diabetes

In view of the crucial role of phosphatidic acid (PA) in signal transduction and Ca(2+)-handling in myocardium, it was the objective of this study to examine the effects of PA on cardiac contractile force and intracellular free Ca2+ in control and chronic diabetic rats.

Diabetes was induced in rats by a single intravenous injection of streptozotocin (65 mg/kg.) and the animals were used for experiments eight weeks after the injection. Heart function was measured by using 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 cardiomyocytes was estimated by employing Fura-2/AM method.

PA (5 x 10(-8) to 1 x 10(-5) M) produced a concentration-dependent increase in +dP/dt and -dP/dt in the isolated heart; however, these responses were significantly attenuated in diabetic hearts. ATP also caused a positive inotropic effect at concentrations of 1 x 10(-5) to 1 x 10(-4) M but the magnitude of these responses was similar in both control and diabetic groups. Using freshly isolated cardiomyocytes and Fura-2 technique, PA (1 x 10(-6) to 1 x 10(-4) M) was observed to evoke a concentration-dependent increase in [Ca2+]i in both control and diabetic groups. The EC50 and EC95 values for PA were not different but the maximum increase of [Ca2+]i in diabetic hearts was significantly lower in comparison to the control group (152 +/- 41 versus 304 +/- 56 nM). On the other hand, no difference in the increase of [Ca2+]i due to ATP or potassium chloride was seen between control and diabetic cardiomyocytes. Adrenaline pretreatment enhanced [Ca2+]i responses to ATP and PA in both groups; however, the PA-induced increase in [Ca2+]i, unlike the ATP-induced increase, was lower in the diabetic group compared to the control cells with similar pretreatment with adrenaline. The diminished increase in [Ca2+]i due to PA was also observed in cardiomyocytes obtained from rats in which diabetes was induced by intravenous alloxan (65 mg/kg).

PA induced [Ca2+]i mobilization and positive inotropic response were depressed in diabetic heart; this defect in the signal transduction mechanism may contribute to the lower tonic responses of certain inotropic agents in chronic diabetes.