REGULATION OF THE CALCIUM SLOW CHANNEL BY CYCLIC-GMP DEPENDENT PROTEIN-KINASE IN CHICK HEART-CELLS

In order to assess the interaction between the cAMP-dependent and the cGMP-dependent phosphorylation pathways on the slow Ca2+ current (I-Ca(L)), whole-cell voltage-clamp experiments were conducted on embryonic chick heart cells. Addition of 8Br-cGMP to the bath solution reduced the basal (unstimulated) I-Ca(L), Intracellular application of the catalytic subunit of PK-A (PK-A(cat); 1.5 mu M) via the patch pipette rapidly potentiated I-Ca(L) by 215 +/- 16% (n = 4); subsequent addition of 1 mM 8Br-cGMP to the bath reduced the amplitude of I-Ca(L) towards the initial control values (123 +/- 29%). Intracellular application of PK-G (25 nM pre-activated by 10(-7) M cGMP), rapidly inhibited the basal I-Ca(L) by 64 +/- 6% (n = 8). Heat-denatured PK-G was ineffective. Subsequent additions of relatively high concentrations of 8Br-cAMP (1 mM) or isoproterenol (ISO, 1-10 mu M) did not significantly remove the PK-G blockade of I-Ca(L). The results of the present study suggest that: (a) 8Br-cGMp can inhibit the basal or stimulated (by PK-A(cat)) I-Ca(L) in embryonic chick myocardial cells. (b) PK-G applied intracellularly inhibits the basal I-Ca(L).