CGMP-DEPENDENT PROTEIN-KINASE REGULATION OF THE L-TYPE CA2+ CURRENT IN RAT VENTRICULAR MYOCYTES

Regulation of L-type Ca2+ channel current [I-Ca(L)] by cGMP-dependent protein kinase (PK-G) was investigated in ventricular myocytes from 2- to 21-day-old rats using whole-cell voltage clamp with internal perfusion. I-Ca(L) was elicited by a depolarizing pulse to +10 mV from a holding potential of -40 mV. Stimulated I-Ca(L) (by 2 mu mol/L isoproterenol) was inhibited to the basal level by internal perfusion with 50 nmol/L PK-G (activated by 8Br-cGMP, 0.1 mu mol/L). When I-Ca(L) was enhanced by Bay K 8644 (1 mu mol/L), the enhanced basal I-Ca(L) was also reduced by PK-G. Basal I-Ca(L) (nonstimulated through the cAMP/cAMP-dependent protein kinase [PK-A] pathway) was also inhibited to various degrees (large, medium, or small) by internal application of PK-G (25 nmol/L). The average inhibition was 42.1% (n=36), and there were no differences in the inhibition during development. The inhibition by PK-G was blocked by the PK-G substrate peptide (cG-PKI, 300 mu mol/L) and by heat inactivation of the PK-G. Relatively specific PK-G inhibitors leg; (eg, cG-PKI and H-8) sometimes reversed the inhibition (5 of 25 cells), whereas isoproterenol stimulated I-Ca(L) (7 of 8 cells). When a holding potential of -80 mV was used, the inhibition produced by PK-G was much less. The inhibitory effects of PK-C were not mediated by activating phosphodiesterase or protein phosphatase but most likely by a direct phosphorylation of the Ca2+ channel or associated regulatory protein. The inhibitory effect of PK-G may be explained by a balance between activities of PK-A and PK-G in regulating the slow Ca2+ channels at two separate sites.