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, L}) 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 −50mV and maximum amplitude at −10mV, whereas I_{Ca, L} threshold was about −30mV and maximum amplitude was at +10mV. The half-maximal activation voltages of I_{Ca, T} was −31.4±0.2mV and I_{Ca, T} was −6.2±2.0mV. 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 ±0.4pA/pF and 12.5 ±1.3pA/pF, respectively. I_{Ca, T} current density in working atrial muscle cells was 0.73 ±0.31pA/pF. Both I_{Ca, L} and I_{Ca, T} were blocked by 2mM cobalt. I_{Ca, L} but not I_{Ca, T} was blocked by 1 μM nifedipine. Nickel (Ni²⁺: 40 μM) 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 primarypacemaker activity.