β-Adrenergic stimulation of L-type Ca²⁺ channels in cardiac myocytes requires the distal carboxyl terminus of α_1C but not serine 1928

β-Adrenoceptor stimulation robustly increases cardiac L-type Ca ²⁺ current (I_CaL); yet the molecular mechanism of this effect is still not well understood. Previous reports have shown in vitro phosphorylation of a consensus protein kinase A site at serine 1928 on the carboxyl terminus of the α_1C subunit; however, the functional role of this site has not been investigated in cardiac myocytes. Here, we examine the effects of truncating the distal carboxyl terminus of the α_1C subunit at amino acid residue 1905 or mutating the putative protein kinase A site at serine 1928 to alanine in adult guinea pig myocytes, using novel dihydropyridine-insensitive α_1C adenoviruses, coexpressed with β₂ subunits. Expression of α_1C truncated at 1905 dramatically attenuated the increase of peak I_CaL induced by isoproterenol. However, the point mutation S1928A did not significantly attenuate the β-adrenergic response. The findings indicate that the distal carboxyl-terminus of α_1C plays an important role in β-adrenergic upregulation of cardiac L-type Ca²⁺ channels, but that phosphorylation of serine 1928 is not required for this effect.