Altered Protein Kinase A-Dependent Phosphorylation of Cav1.2 in Left Ventricular Myocardium from Cacna1c Haploinsufficient Rat Hearts

<i>CACNA1C</i> encodes the α1c subunit of the L-type Ca²⁺ channel, Cav1.2. Ventricular myocytes from haploinsufficient <i>Cacna1c</i> (<i>Cacna1c^+/-</i>) rats exhibited reduced expression of Cav1.2 but an apparently normal sarcolemmal Ca²⁺ influx with an impaired response to sympathetic stress. We tested the hypothesis that the altered phosphorylation of Cav1.2 might underlie the sarcolemmal Ca²⁺ influx phenotype in <i>Cacna1c^+/-</i> myocytes using immunoblotting of the left ventricular (LV) tissue from <i>Cacna1c^+/-</i> versus wildtype (WT) hearts. Activation of cAMP-dependent protein kinase A (PKA) increases L-type Ca²⁺ 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 <i>Cacna1c^+/-</i> 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 <i>Cacna1c^+/-</i> 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 Ca²⁺ influx in <i>Cacna1c^+/-</i> myocytes under basal conditions as well as the impaired response to sympathetic stimulation.