A comparison of the electrophysiological actions of phentolamine with those of some other antiarrhythmic drugs on tissues isolated from the rat heart

Abstract

Glass microelectrodes were used to record transmembrane electrical activity from cells located just beneath the endocardial surface of segments from the atrial and right ventricular free walls of rat hearts during superfusion and electrical stimulation in vitro at 37.degree. C. Availability of the fast Na+ channels for current flow was inferred from the maximum rate of rise of membrane potential during phase 0 of the action potential. Phentolamine mesylate (2-20 .mu.M) caused a concentration-dependent block of the fast Na+ channel. This was reflected in prolongation of the refractory period and slowing of recovery of excitability following the action potential, without significant change in action potential duration or resting membrane potential. Increase in the concentration of KCl in the superfusate from 5-10 mM depolarized the muscle and potentiated the blocking action of phentolamine. Both the depolarizing and the phentolamine-potentiating actions of KCl were counteracted by simultaneous elevation of the concentration of CaCl2 in the superfusate from 2 to 10 mM.