Effects of diltiazem on transmembrane potential and current of right ventricular papillary muscle of ferrets.

We studied the effects of diltiazem on electrical properties of isolated ferret right ventricular papillary muscles. By using standard microelectrode recording techniques and current clamp and voltage clamp protocols (single sucrose gap method), we measured action potential variables, depolarization-induced automaticity, slow (or second) inward current (Isi) and time-dependent and isochronal (1 sec) outward current. Resting potential was unaffected at all concentrations studied (from 2 nM-11 microM). At concentrations below 2 microM and at slow rates of stimulation (0.5 Hz), a small reduction of overshoot and prolongation of the action potential duration at 80% of full repolarization were observed. At concentrations of 2.2 microM or greater, marked use-dependent reductions of overshoot and plateau duration were observed that reversed with rest. Depolarization-induced automaticity was selectively suppressed at less negative diastolic potentials. In voltage clamp studies, peak Isi was markedly diminished over the concentration range studied (50% inhibitory concentration, 0.5 microM), but the current-voltage relation for Isi was not shifted on the voltage axis. The diminution in Isi was strongly use-dependent and voltage-dependent. Diltiazem (1.1 microM) had small effects on outward currents. Steady-state (isochronal) outward current and the time-dependent outward current were both reduced by 10 to 20% over the entire voltage range. Diltiazem is a potent inhibitor of the slow inward current in ventricular muscle. Its interaction with slow channel receptors appears to be strongly modulated by the state of the channels.