AbstractCalcium channels are important targets for therapeutics, but their molecular diversity complicates characterization of these channels in native heart cells. In this study, we identify a new splice variant of a low-voltage activated, or T-type Ca2+, channel in murine atrial myocytes. To date, &agr;1G and &agr;1H are the only 2 T-type Ca2+channel isoforms found in cardiovascular tissue. We compared &agr;1G and &agr;1H channel current heterologously expressed in HEK 293 cells with T-type current from the murine atrial tumor cell, AT-1. AT-1 cell T-type current (IT) has the same voltage dependence of activation and inactivation as &agr;1G and &agr;1H. The cloned T-type channels and AT-1 T-type current share similar kinetics of macroscopic inactivation and deactivation. The kinetics of recovery from inactivation of T-type currents serves as an electrophysiological signature for T-channel isoform. &agr;1G and AT-1IThave a similar recovery from inactivation time course that is faster than that for &agr;1H. In all cases, T-type current recovers with a biexponential time course, and the relative amplitude of fast and slow time courses explains the slower &agr;1H recovery kinetics, rather than differences in the time constants of the individual transitions. Thus, the T-type channels may be an important contributor to automaticity in heart cells, and molecular diversity is reflected in the pathway of recovery from inactivation.