Ca-mediated action potentials in ventricular myocardium. Variation by various stimulation patterns and by K-channel blockers.

In rabbit papillary muscle, Ca-mediated action potentials (AP's) and maximum rate of rise (Vmax) were measured at 18 mM Ko. At steady state stimulation Vmax, overshoot and diastolic membrane potential were enhanced with increasing cycle length (CL) (from 8.7 to 14.1 V/s, from 25.1 to 34.1 mV, from -34.5 to -49.6 mV, respectively, between the CL of 1 and 60 s). AP-duration was only slightly prolonged (from 109 to 138 ms at 50% repolarization). 4-aminopyridine (5 mM) caused no or minor changes in the magnitude and CL-dependence of Vmax and overshoot. The AP-duration, however, was strongly prolonged due to a slower repolarization of the plateau phase with increasing CL. CsCl (20 mM) caused similar changes. Ca-mediated AP's elicited after a period of rest were found to be qualitatively similar to those which were obtained at increased steady state CL. Vmax of premature Ca-mediated AP's increased exponentially with the interval of stimulation, t, and was accurately described by: Vmax = A(1 - exp - (t-T)/tau). The K-channel blockers had no significant effect on the absolute refractory period, T, but increased the time constant, tau, from 123 to 166 ms. The restitution of AP-duration nicely matched this time course. The voltage dependence of membrane responsiveness showed a sigmoid shape, and was accurately described by: Vmax = A(1 + exp(V-Vs)/s)-1. The K-channel blockers increased s from 2.2 to 3.6 mV and shifted Vs from -26.8 to -22.1 mV.(ABSTRACT TRUNCATED AT 250 WORDS)