Adenylyl Cyclase/cAMP-PKA-mediated Phosphorylation Of Basal L-type Ca²⁺ Channels In Mouse Embryonic Ventricular Myocytes

In fetal mammalian heart, constitutive adenylyl cyclase/cyclic AMP-dependent protein kinase A (cAMP-PKA)-mediated phosphorylation, independent ofβ-adrenergic receptor stimulation, could under such circumstances play an important role in sustaining the L-type calcium channel current (Ica,L) and regulating other PKA dependent phosphorylation targets. In this study, we investigated the regulation of L-type Ca2+ channel (LTCC) in murine embryonic heart. The data indicated, a higher phosphorylation state of LTCC in early developmental stage (EDS, E9.5-11.5) versus late developmental stage (LDS, E16.5-18.5). An intrinsic adenylyl cyclase (AC) activity, PKA activity and basal cAMP concentration were obviously higher in EDS versus LDS. The cAMP increase in the presence of isobutylmethylxanthine (IBMX, nonselective phosphodiesterase inhibitor) was further augmented at LDS but not at EDS by chelation of intracellular Ca2+ with 1,2-bis(2-Aminophenoxy)-ethane-N,N,N9,N9-Tetraacetic Acid (BAPTA). Furthermore, ICa,L increased with time after patch rupture in LDS cardiomyocytes dialyzed with pipette solution containing BAPTA wheras not in EDS. Thus we conclude that the high basal level of LTCC phosphorylation is due to the high intrinsic PKA activity and the high intrinsic AC activity at EDS. The latter is possibly owing to the little or no effect of Ca2+ influx via LTCCs on AC activity, leading to inability to inhibit AC.