The effects of l-propranolol and practolol on atrial and nodal transmembrane potentials

Effects of l-propranolol, racemic propranolol and practolol have been determined on transmembrane potentials recorded in guinea-pig left atrium and in sinoatrial and atrioventricular nodes of the rabbit heart. In addition to the voltage -time recording of the action potential, its first time derivative was displayed as a function of membrane voltage, forming a phase-plane trajectory. A number of parameters of the action potential were determined from this trajectory. At the concentrations used the beta adrenoceptor blocking agents reduced the maximum rate of rise of the atrial action potential and slowed repolarization. The velocity of the propagated spike was reduced, and the maximum ionic conductance was also reduced. The excitation potential of the propagated spike was unaffected. The membrane effects were markedly dependent on the frequency of stimulation. The beta adrenoceptor blocking agents were without effect on either the sinoatrial or atrioventricular nodes. Since the spontaneously active right atrium beats more slowly than the heart of the intact rabbit, the atrioventricular node was electrically stimulated at 5 to 6 Hz. Neither drug affected the ability of the node to follow these stimulation frequencies. The importance of these effects in the control of cardiac arrhythmias is discussed.