Contribution of Na+/Ca2+ exchange to action potential of human atrial myocytes

The Ca2+ dye indo 1 was used to record internal Ca2+ (Ca-i) transients in order to investigate the role of the Na+/Ca2+ exchange current (I-Na/Ca) in whole cell patch-clamped human atrial myocytes. After the activation of the L-type Ca2+ current by test pulses (20 ms) at +20 mV, a tail current (I-tail) was activated at a holding potential of -80 mV with a density of -1.29 +/- 0.06 pA/pF. The time course of I-tail followed that of Ca-i transients. I-tail was suppressed by dialyzing cells with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, applying 5 mM caffeine, or substituting external Na+ with Li+, indicating that this current was mainly generated by I-Na/Ca. Two types of action potential were recorded: type A, which is characterized by a narrow early plateau followed by a late low plateau phase, and type B, which is 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 and 122.4 +/- 7.0 pF in types A and B, respectively, P < 0.001). Substitution of external Na+ with Li+ shortened the late plateau of the type A action potential and the prolonged plateau of the type B action potential. Suppression of Ca-i transients by caffeine shortens the late part of both types of action potentials, whereas its lengthening effect on the initial phase of action potentials can result from several different mechanisms. The beat-to-beat dependent relationship between Ca-i transients and action potentials could be mediated by I-Na/Ca. Delayed afterdepolarizations 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 I-Na/Ca. We conclude that I-Na/Ca plays a major role in the development of action potentials and delayed afterdepolarizations in isolated human atrial myocytes.