Cyclic GMP-dependent protein kinase regulates the L-type calcium current in rat ventricular myocytes

This study aimed to elucidate the mechanism of cyclic granosine monophosphate (cGMP)-dependent protein kinase (PK-G) regulation of the L-type Ca2+ channel current (ICa(L) in young rat ventricular myocytes using whole-cell voltage clamp with internal perfusion. ICa(L) was usually elicited from a holding potential (HP) of -40 mV. Stimulated ICa(L) (by 2 microM isoproterenol) was inhibited to the basal level by internal perfusion with 50 nM PK-G (activated by 8Br-cGMP, 0.1 microM). Basal ICa(L) (non-stimulated through the cyclic adenosine monophosphate (cAMP)/PK-A pathway) was also inhibited to various degrees (large, medium, or small) by internal application of PK-G (25 nM). The average inhibition was 42.1%, and there were no differences in the inhibition during development. The inhibition by PK-G was blocked by the PK-G substrate peptide (300 microM) and by heart-inactivation of PK-G. The inhibited ICa(L) was reversed by ISO (seven of eight cells). When an HP of -80 mV was used, the inhibition produced by PK-G was much less. The inhibitory effects of PK-G were not mediated by activating phosphodiesterases or protein phosphatases, 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 the activities of PK-A and PK-G in regulating the slow Ca2+ channels at two separate sites.