Effects of goniopora toxin on the action potential and membrane currents of guinea-pig single ventricular cells

Electrophysiological and mechanical effects of calcitonin gene-related peptide on guinea-pig atria

1. The effects of calcitonin gene-related peptide (CGRP) on mechanical and electrophysiological responses were studied in the guinea-pig atrial muscle preparations and in single cells. 2. CGRP (greater than 10(-9) M) enhanced the twitch contraction in a concentration-dependent manner in electrically driven left atria and increased heart rate in spontaneously beating right atria. The positive inotropic and chronotropic effects of CGRP were not inhibited by propranolol but were attenuated by reduction of the calcium concentration in the bathing medium. 3. In single left atrial cells, CGRP slightly hyperpolarized the resting potential but did not affect the other action potential parameters significantly. 4. Under whole-cell voltage-clamp conditions, CGRP increased the calcium inward current. The peptide also increased the steady inward current elicited by hyperpolarization and the late outward current by depolarization. 5. These results suggest that CGRP may produce the positive inotropic and presumably chronotropic effects by increasing calcium inward current. CGRP also increases the potassium permeability. Such effects on ionic currents may not produce any apparent change in the action potential conformation, due to their opposite directional actions and relatively weak potencies.

Effects of TYB-3823, a new antiarrhythmic drug, on the electrophysiological properties of guinea-pig ventricular muscles

1. The effects of TYB-3823 (B-GYKI 38233), a new antiarrhythmic drug, were electrophysiologically examined in the guinea-pig ventricular muscles. 2. TYB-3823 at concentrations of 1-3 x 10(-7) M significantly prolonged the action potential duration (APD) of the papillary muscle. However, the resting potential, action potential amplitude and maximum rate of depolarization (Vmax) were unaffected by the drug at such concentrations. At a higher concentration (1 x 10(-4) M) TYB-3823 reduced Vmax. 3. Voltage clamp experiments with single ventricular cells revealed that TYB-3823 at concentrations higher than 1 x 10(-7) M reduced the outward potassium currents, especially the time-dependent outward current, but that TYB-3823 failed to affect the calcium current. 4. These results suggested that TYB-3823 at low concentrations reduces the outward potassium current to give rise to a prolongation of APD and that at higher concentrations it additionally inhibits the sodium channels; both the effects may be related to the antiarrhythmic action of this drug.