Prepulse-induced mode 2 gating behavior with and without β-adrenergic stimulation in cardiac L-type Ca channels

Mode 2 gating of L-type Ca channels is characterized by high channel open probability (NPo) and long openings. In cardiac myocytes, this mode is evoked physiologically in two apparently different circumstances: membrane depolarization (prepulse facilitation) and activation of protein kinase A. To examine whether the phosphorylation mechanism is involved during prepulse-induced facilitation of cardiac L-type Ca channels, we used isolated guinea pig ventricular myocytes to analyze depolarization-induced modal gating behavior under different basal levels of phosphorylation. In control, NPo measured at 0 mV was augmented as the duration of prepulse to +100 mV was prolonged from 50 to 400 ms. This was due to the induction of mode 2 gating behavior clustered at the beginning of test pulses. Analysis of open time distribution revealed that the prepulse evoked an extra component, the time constant of which is not dependent on prepulse duration. When isoproterenol (1 microM) was applied to keep Ca channels at an enhanced level of phosphorylation, basal NPo without prepulse was increased by a factor of 3.6 +/- 2.2 (n = 6). Under these conditions, prepulse further increased NPo by promoting long openings with the same kinetics of transition to mode 2 gating (tau congruent with 200 ms at +100 mV). Likewise, recovery from mode 2 gating, as estimated by the decay of averaged unitary current, was not affected after beta-stimulation (tau congruent with 25 ms at 0 mV). The kinetic behavior independent from the basal level of phosphorylation or activity of cAMP-dependent protein kinase suggests that prepulse facilitation of the cardiac Ca channel involves a mechanism directly related to voltage-dependent conformational change rather than voltage-dependent phosphorylation.