The actin-binding proteins dystrophin and alpha-actinin are members of a family of actin-binding proteins that may link the cytoskeleton to membrane proteins such as ion channels. Previous work demonstrated that the activity of Ca2+ channels can be regulated by agents that disrupt or stabilize the cytoskeleton. In the present study, we employed immunohistochemical and electrophysiological techniques to investigate the potential regulation of cardiac L-type Ca2+ channel activity by dystrophin and alpha-actinin in cardiac myocytes and in heterologous cells. Both actin-binding proteins were found to colocalize with the Ca2+ channel in mouse cardiac myocytes and to modulate channel function. Inactivation of the Ca2+ channel in cardiac myocytes from mice lacking dystrophin (mdx mice) was reduced compared with that in wild-type myocytes, voltage dependence of activation was shifted by 5 mV to more positive potentials, and stimulation by the beta-adrenergic pathway and the dihydropyridine agonist BAY K 8644 was increased. Furthermore, heterologous coexpression of the Ca2+ channel with muscle, but not nonmuscle, forms of alpha-actinin was also found to reduce inactivation. As might be predicted from a reduction of Ca2+ channel inactivation, a prolonging of the mouse electrocardiogram QT was observed in mdx mice. These results suggest a combined role for dystrophin and alpha-actinin in regulating the activity of the cardiac L-type Ca2+ channel and a potential mechanism for cardiac dysfunction in Duchenne and Becker muscular dystrophies.