Effects of pyruvate on intracellular Ca2+ regulation in cardiac myocytes from normal and diabetic rats

1, Pyruvate has been shown to enhance the contractile performance of cardia c muscle when provided as an alternative substrate to glucose. The aims of the present study were to determine whether the inotropic effects of pyruva te are due to increased mobilization of intracellular Ca2+ ([Ca2+](i)) and to compare the effects of pyruvate on [Ca2+](i) levels in myocytes from nor mal and diabetic animals, 2, Fura-2 was used to monitor [Ca2+](i) in ventricular myocytes isolated fr om control and streptozotocin-treated male Wistar rats. The experiments wer e performed at 25 degrees C, with an estracellular [Ca2+] of 1.5 mmol/L and either 10 mmol/L glucose or 10 mmol/L pyruvate as the substrate. 3, In myocytes from both control and diabetic rats, increasing the stimulus frequency from 0.33 to 2,0 Hz resulted in significant increases in resting and peak [Ca2+](i) as well as in the amplitude of the [Ca2+]i transient, i rrespective of substrate. Compared with glucose, pyruvate significantly inc reased resting and peak [Ca2+](i) and the amplitude of the [Ca2+](i) transi ent at each stimulus frequency in myocytes from both control and diabetic a nimals. However, the extent of potentiation of the [Ca2+](i) transient ampl itude produced by pyruvate was significantly less in myocytes from the diab etic rats. 4. The rate of restitution of the [Ca2+](i) transient was used a s an index of the rate of Ca2+ cycling by the sarcoplasmic reticulum (SR), Pyruvate enhanced the rate of restitution in control but not diabetic rat c ells,, 5. The time course of decay of the [Ca2+]i transient was analysed as a meas ure of the rate of removal of Ca2+ from the cytosol, Pyruvate tended to inc rease the rate of decay in cells from control but not diabetic animals. The rate of decay was slower in cells from diabetic animals compared with cont rols. 6, The data reveal that pyruvate increases SR Ca2+ cycling, leading to grea ter Ca2+ release and an increase in the amplitude of the [Ca2+](i) transien t. Therefore, it seems highly likely that increased [Ca2+](i) mobilization is responsible for the previously reported positive inotropic actions of py ruvate. These effects of pyruvate are attenuated in diabetic rat cells, whi ch may reflect an impaired capacity of mitochondria in diabetic hearts to o xidize pyruvate, thus limiting potential energetic benefits.