β-Adrenergic Regulation of the L-type Ca2+ Channel Does Not Require Phosphorylation of α1C Ser1700

Rationale: Sympathetic nervous system triggered activation of protein kinase A, which phosphorylates several targets within cardiomyocytes, augments inotropy, chronotropy, and lusitropy. An important target of -adrenergic stimulation is the sarcolemmal L-type Ca2+ channel, Ca(V)1.2, which plays a key role in cardiac excitation-contraction coupling. The molecular mechanisms of -adrenergic regulation of Ca(V)1.2 in cardiomyocytes, however, are incompletely known. Recently, it has been postulated that proteolytic cleavage at Ala(1800) and protein kinase A phosphorylation of Ser(1700) are required for -adrenergic modulation of Ca(V)1.2. Objective: To assess the role of Ala(1800) in the cleavage of (1C) and the role of Ser(1700) and Thr(1704) in mediating the adrenergic regulation of Ca(V)1.2 in the heart. Methods and Results: Using a transgenic approach that enables selective and inducible expression in mice of FLAG-epitope-tagged, dihydropyridine-resistant Ca(V)1.2 channels harboring mutations at key regulatory sites, we show that adrenergic regulation of Ca(V)1.2 current and fractional shortening of cardiomyocytes do not require phosphorylation of either Ser(1700) or Thr(1704) of the (1C) subunit. The presence of Ala(1800) and the (1798)NNAN(1801) motif in (1C) is not required for proteolytic cleavage of the (1C) C-terminus, and deletion of these residues did not perturb adrenergic modulation of Ca(V)1.2 current. Conclusions: These results show that protein kinase A phosphorylation of (1C) Ser(1700) does not have a major role in the sympathetic stimulation of Ca2+ current and contraction in the adult murine heart. Moreover, this new transgenic approach enables functional and reproducible screening of (1C) mutants in freshly isolated adult cardiomyocytes in a reliable, timely, cost-effective manner.