Altered Protein Kinase A-Dependent Phosphorylation of Cav1.2 in Left Ventricular Myocardium from Haploinsufficient Rat Hearts.

encodes the α1c subunit of the L-type Ca channel, Cav1.2. Ventricular myocytes from haploinsufficient () rats exhibited reduced expression of Cav1.2 but an apparently normal sarcolemmal Ca influx with an impaired response to sympathetic stress. We tested the hypothesis that the altered phosphorylation of Cav1.2 might underlie the sarcolemmal Ca influx phenotype in myocytes using immunoblotting of the left ventricular (LV) tissue from versus wildtype (WT) hearts. Activation of cAMP-dependent protein kinase A (PKA) increases L-type Ca current and phosphorylates Cav1.2 at serine-1928. Using an antibody directed against this phosphorylation site, we observed elevated phosphorylation of Cav1.2 at serine-1928 in LV myocardium from rats under basal conditions (+110% versus WT). Sympathetic stress was simulated by isoprenaline (100 nM) in Langendorff-perfused hearts. Isoprenaline increased the phosphorylation of serine-1928 in LV myocardium by ≈410%, but the increase was significantly smaller than in WT myocardium (≈650%). In conclusion, our study reveals altered PKA-dependent phosphorylation of Cav1.2 with elevated phosphorylation of serine-1928 under basal conditions and a diminished phosphorylation reserve during β-adrenergic stimulation. These alterations in the phosphorylation of Cav1.2 may explain the apparently normal sarcolemmal Ca influx in myocytes under basal conditions as well as the impaired response to sympathetic stimulation.