Tonic regulation of excitation-contraction coupling by basal protein kinase C activity in isolated cardiac myocytes

A high-speed imaging technique was used to investigate the effects of inhib itors and activators of protein kinase C (PKC) on the [Ca2+](i), transients and contraction of fura-2 loaded rat Ventricular cardiac myocytes. The amp litude of the [Ca2+](i) transient was reduced following treatment with 100 nM phorbol 12,13-dibutyrate (PDBu), whereas the PKC inhibitors staurosporin e (0.5 mu M) and calphostin C (10 mu M) increased [Ca2+](i) transient ampli tude, elevated basal [Ca2+](i) and slowed the decay of the [Ca2+](i) transi ent. These changes were paralleled by similar alterations in the rate and e xtent of cell shortening. The activity of nitrendipine-sensitive Ca2+ chann els was monitored indirectly as the rate of Mn2+ quench of cytosolic fura-2 in electrically-paced cells. PDBu reduced Mn2+ influx by six-fold, whereas staurosporine and calphostin C increased the influx rate by eightfold and seven-ford over basal quench, respectively. The caffeine releasable Ca2+ po ol was reduced in the presence of PDBu and increased transiently in presenc e of staurosporine. The effects of PKC activation and inhibition on sarcopl asmic reticulum Ca2+ content may be secondary to alterations of sarcolemmal Ca2+ influx. However, the PKC inhibitors also decreased the rate of sarcop lasmic reticulum Ca2+ uptake in permeabilized myocytes, suggesting that a d irect effect of PKC on the sarcoplasmic reticulum may contribute to the pro longation of the [Ca2+](i) transient under these conditions. The present wo rk demonstrates that basal PKC activity has a potent depressant effect, med iated primarily through inhibition of sarcolemmal Ca2+ influx, which may pl ay a key role in setting the basal tone of cardiac muscle. (C) 1998 Academi c Press.