cAMP-dependent modulation of L-type calcium currents in mouse diaphragmatic cells

The regulation of calcium channels by cAMP-dependent phosphorylation was investigated in the diaphragm muscle. Experiments were performed on dissociated costal diaphragmatic cells from 16- to 17-day-old fetal mice. The ionic current through calcium channels was measured using the whole cell clamp technique with barium as the charge carrier. A depolarizing pulse delivered from a holding potential of -80 mV elicited a low-threshold dihydropyridine (DHP)-insensitive T-type current and a high-threshold DHP-sensitive L-type current. Agents that either increase intracellular cAMP levels (forskolin, 10(-4) M, and dibutyryladenosine 3'-5' cyclic monophosphate, 10(-4) M) or inhibit cAMP degradation (theophylline, 10(-4) M) produced relative increases in L-type current amplitude of 24.4 +/- 13.8%, 13.4 +/- 4.6%, and 15.9 +/- 2.8% (p < 0.05), respectively. Current intensity increased after application of the beta-adrenergic agonist isoproterenol (10(-5) M, 16.5 +/- 3.6%, P < 0.005). None of these agents affected the T-type current. These results suggest that L-type calcium channel activities of the diaphragm muscle are regulated by cAMP-dependent phosphorylation.