T-TYPE CALCIUM CURRENT IN LATENT PACEMAKER CELLS ISOLATED FROM CAT RIGHT ATRIUM

Whole-cell voltage clamp techniques were used to study Ca2+ currents in latent pacemaker cells isolated from cat right atrium. T-type (I-Ca.T> and L-type (I?(Ca.T) Ca2+ currents were distinguished by their voltage dependence of activation, and sensitivity to channel blocking agents. In 2.7 mM [Ca](o), I-Ca.T activation exhibited a voltage threshold of about -50 mV and maximum amplitude at -10 mV, whereas I-Ca.L threshold was about -30 mV and maximum amplitude was at +10 mV. The half-maximal activation voltages of I-Ca.T was - 31.4 +/- 0.2 mV and I-Ca.L was -6.2 +/- 2.0 mV. Overlap of the steady-state activation-inactivation curves for I-Ca.T showed a ''window'' current at voltages compatible with the late phase of diastolic depolarization. Maximum I-Ca.T and I-Ca.L current densities were 3.3 +/- 0.4 pA/pF and 12.5 +/- 1.3 pA/pF, respectively. I-CA.T current density in working atrial muscle cells was 0.73 +/- 0.31 pA/pF. Both I-Ca.L and I-Ca.T were blocked by 2 mM cobalt. I-Ca.L but not I-Ca.T was blocked by 1 mu M nifedipine. Nickel (Ni2+; 40 mu M) inhibited I-Ca.T primarily at more negative voltages. In addition, Ni2+ decreased the late phase of diastolic depolarization and significantly increased pacemaker cycle length. These results indicate that latent atrial pacemaker cells exhibit I-Ca.T that is significantly larger in amplitude than in working atrial muscle cells. I-Ca.T may contribute current during the late phase of diastolic depolarization. Because latent pacemakers exhibit a more negative maximum diastolic potential, I-Ca.T may contribute more to latent than to primary pacemaker activity.