β-Adrenoceptor agonists increase membrane K+ conductance in cardiac Purkinje fibres

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Abstract
Hormonal modulation of the ionic conductance of cell membranes is a topic of considerable current interest; it has a major role, for example, in the improved performance of the vertebrate heart elicited by sympathetic nerve stimulation or by circulating catecholamines, an effect involving enhanced calcium influx. beta-Agonist catecholamines also abbreviate the action potential of cardiac Purkinje fibres, and increase the resting potential in a variety of cells, including cardiac cells, a hyperpolarization usually attributed to stimulation of the electrogenic Na+/K+ pump. We show here that nanomolar concentrations of beta-catecholamines cause hyperpolarization of cardiac Purkinje fibres, not by increasing Na+/K+ pump current, but by increasing resting membrane K+ conductance. The hyperpolarization and shortening of the action potential should increase availability of Na+ channels and reduce the refractory period, effects tending to safeguard impulse propagation through the ventricular conducting system despite the increased heart rate caused by beta-catecholamine action on the sinus node pacemaker.