Actions of Calcium Channel Blockers on the Beating Rhythm and Membrane Potential of Cultured Chick Embryonic Heart Cell
Actions of Ca channel blockers such as verapamil and nifedipine on the electrical and mechanical activities of cultured cells from chick embryonic ventricles were studied. The action potentials were divided into three types based on their shape and amplitude; a sino-atrial cell type, a Purkinje fiber type and a ventricular cell type. Some cells without spontaneous beating responded to intracellular stimulation with ventricular type action potential. Both verapamil and nifedipine decreased the rate and strength of spontaneous contractions depending on their concentration. These drugs also reduced the height of plateau of ventricular type action potential much more than that of Purkinje fiber type. Nifedipine, at high concentrations, blocked initiation of the upstroke of the action potential from pacemaker potential. The occasional omission of action potentials resulted in a slow beat rate and irregular rhythm. During the recovery phase from nifedipine treatment, the rate and contractions transiently increased. Positive chronotropic and inotropic interventions during recovery could be explained by the increased Ca influx resulting from the increased concentration gradient between inside and outside of the cells.
