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Am J Physiol Heart Circ Physiol 256: H1500-H1504, 1989;
0363-6135/89 $5.00
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AJP - Heart and Circulatory Physiology, Vol 256, Issue 5 1500-H1504, Copyright © 1989 by American Physiological Society

ARTICLES

A novel effect of norepinephrine on cardiac cells is mediated by alpha 1-adrenoceptors

D. Fedida, Y. Shimoni and W. R. Giles
Department of Medical Physiology, University of Calgary School of Medicine, Alberta, Canada.

In the heart, alpha-adrenergic agonists have long been known to produce a positive inotropic effect that is rate dependent and associated with action potential prolongation but is not accompanied by adenosine 3',5'-cyclic monophosphate (cAMP) elevation. The ionic mechanism of these effects is unknown. We report that a transient outward K+ current, a major determinant of plateau duration in rabbit and human atria, is strongly inhibited by norepinephrine and the alpha-adrenoceptor agonists methoxamine and phenylephrine. These effects of alpha-stimulation can be blocked by prazosin. The reduction in the transient outward current substantially slows action potential repolarization. These results can explain the regional and species-dependent positive inotropic effects of alpha-adrenergic stimulation in the heart and give important new insight into the autonomic regulation of cardiac function. In addition, reduction in this repolarizing current during the enhanced alpha-adrenergic responsiveness of myocardial ischemia may be a factor in the genesis of arrhythmias produced by catecholamines.


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