Contribution of the na-ca exchange to the action potential of human atrial myocytes. B[acute]enardeau, Agn[grave]es, St[acute]ephane N. Hatem, Catherine R[umlaut]ucker-Martin, Bruno Le Grand, Lo[diaeresis]ic Mac[acute]e, Patrice Dervanian, Jean-Jacques Mercadier, and Edouard Coraboeuf. Laboratoire de Cardiologie Mol[acute]eculaire et Cellulaire, Universit[acute]e de Paris XI-CNRS URA 1159, H[circumflex]opital Marie Lannelongue, Le Plessis Robinson, France, Centre de Recherche Pierre Fabre, Division des Maladies Cardiovasculaires, Castres, France
APStracts 3:0109H, 1996.
The role of the Na-Ca exchange (INa-Ca) was investigated in whole-cell patch clamped human atrial myocytes using Ca2+ dye Indo-1 to record Cai-transients. Following the activation of the calcium current, ICaL, by test pulses (20 msec) at +20 mV, a tail current (Itail) activated at a holding potential of -80 mV with a density of -1.29 +/- 0.06 pA/pF. The time course of Itail followed that of Cai -transients. Itail was suppressed by dialysing cells with EGTA, application of caffeine (5 mM) or substitution of [Na+]o by Li+, indicating that this current was mainly generated by INa-Ca. Two types of action potential were recorded: type A characterized by a narrow early plateau followed by a late low plateau phase, and type B characterized by a small initial peak followed by a prolonged high plateau phase. Type B action potentials were found in larger cells than type A action potentials (membrane capacitance: 81.8 +/- 4.5 pF in type A, vs 122.4 +/- 7.0 pF, in type B; p&LT0.001). [Na+]o substitution by Li+ shortened the late plateau of type A action potential and the prolonged plateau of type B. Suppression of Cai -transients by caffeine shortens the late part of both types of APs whereas its lengthening effect on the initial phase of AP can result from several different mechanisms. The beat-to-beat dependent relation between Cai-transients and action potentials could be mediated by INa-Ca. Delayed afterdepolarizations, DADs, were present in a significant proportion of atrial myocytes in our experimental conditions. They were reversibly suppressed by Li+ substitution for Na+ suggesting that they are generated by INa-Ca. We conclude that INa-Ca plays a major role in the development of action potentials and DADs in isolated human atrial myocytes. 

Received 28 June 1995; accepted in final form 9 February 1996.
APS Manuscript Number H592-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 27 March 96