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Department of Medicine and Medical Physiology, University of Calgary, Alberta, Canada.

1. Na+ channel mRNA levels in the heart can be modulated by changes in intracellular Ca2+ ([Ca2+]i). We have investigated whether this regulation of Na+ channel biosynthesis by cytosolic Ca2+ translates into functional Na+ channels that can be detected electrophysiologically. 2. Whole-cell Na+ currents (INa) were recorded using patch-clamp techniques from single ventricular myocytes isolated from neonatal rats and maintained in tissue culture for 24 h. Na+ current density, measured at a membrane potential of -10 mV, was significantly decreased in the cells which were exposed for 24 h to culture medium containing 10 mM of both external Ca2+ and K+ in order to raise [Ca2+]i compared with control cells which were maintained in culture medium containing 2 and 5 mM of Ca2+ and K+, respectively. In contrast, Na+ current density (at -10 mV) was significantly increased in cells exposed for 24 h to 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetraacetoxymethyl ester (BAPTA AM; a cell membrane-permeable Ca2+ chelator) which lowered the average [Ca2+]i compared with control. 3. Changes in current density were not associated with changes in the voltage dependence of activation and inactivation of INa. There were no changes in single-channel conductances. 4. It is concluded that Na+ current density in neonatal rat cardiac myocytes is modulated by [Ca2+]i. The findings suggest that the differences in current density are attributable to a change in Na+ channel numbers rather than to changes in single-channel conductance or gating. These changes are consistent with the previously documented modulation of Na+ channel biosynthesis by cytosolic Ca2+.

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