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)

Electrophysiological effects of a new antiarrhythmic substance Bonnecor (AWD 19-166, GS 015) on mammalian myocardium

A new antiarrhythmic acting drug (Bonnecor, AWD 19-166, GS 015, 3- Carbethoxyamino-5-dimethyl-amino-acetyl-iminodibenzyl -hydrochloride) was tested electrophysiologically on isolated sinus node preparations and right ventricular papillary muscle of the rabbit. Transmembrane potentials were measured using a standard microelectrode technique. The substance was tested within a concentration range between 0.1 and 10 mg/l (2.5 X 10(-7) to 2.5 X 10(-5) mol/l). AWD 19-166 exerted negative chronotropic effects due to decreasing of the slope of the diastolic depolarization in sinus node cells. In ventricular myocardium AWD 19-166 increased the threshold strength of stimulation, prolonged the duration of action potentials at 90% but shortened it at 25% repolarization. The maximum upstroke velocity was found to be depressed. The maximum overshoot potential was diminished inspite of a drug-induced hyperpolarization of the resting transmembrane potential. Analysis of membrane responsiveness due to premature stimulation showed a concentration-dependent delay in the restitution of the maximum upstroke velocity of the premature action potentials and both a flattening and a shift towards more negative potentials of the potential-responsiveness relationship. AWD 19-166 decreased both the maximum upstroke velocity and the duration of Ca-mediated action potentials in concentrations between 2 and 8 mg/l. It is concluded that AWD 19-166 is able to exert potent antiarrhythmic effects in influencing both the fast and slow channel activity.