Different effects of intracellular Ca and protein kinase C on cardiac T and L Ca currents

We studied the effects of changing the intracellular Ca level ([Ca]i) and activating protein kinase C on the cardiac T and L Ca channels in single canine ventricular and Purkinje cells. Lowering [Ca]i increased the L current but decreased the T current, whereas elevating [Ca]i caused opposite changes. In ventricular cells, isoproterenol (1 microM) increased the amplitude of not only the L but also the T currents; the latter effect probably was secondary to a rise in [Ca]i following the augmentation of the L current. 12-O-tetradecanoylphorbol-13-acetate (TPA, 100 nM) decreased the T current but first increased and then decreased the L current. The TPA effects on the T and L currents were not mimicked by a phorbol ester that does not activate PKC (4 alpha-phorbol 12,13-didecanoate) and were prevented by a protein kinase inhibitor (H-8), confirming the involvement of PKC activity in these modulatory processes. We conclude that elevating [Ca]i and activating PKC have opposite effects on the T and L Ca currents in canine cardiac cells. The extent and time course of the changes in these two intracellular messengers will most likely determine the effects on the two cardiac Ca currents of neurotransmitters and hormones that can activate phospholipase C.