AJP - Cell Physiology, Vol 263, Issue 3 C628-C634, Copyright © 1992 by American Physiological Society
Ca2+ influx via Na(+)-Ca2+ exchange in immortalized aortic myocytes. I. Dependence on [Na+]i and inhibition by external Na+
R. M. Lyu, L. Smith and J. B. Smith
Department of Pharmacology, School of Medicine, University of Alabama, Birmingham 35294.
We have found that immortalized aortic myocytes have high Na(+)-Ca2+
exchange activity that is similar in amount to that of low-passage myocyte
cultures. Replacing all external Na+ with K+ slightly increased 45Ca2+
uptake in cells with basal Na+ and greatly increased 45Ca2+ uptake in cells
with increased intracellular Na+. External Na+ competitively inhibited
45Ca2+ uptake (inhibition constant approximately 10 mM). The dependence of
exchange activity on intracellular Na+ was sigmoidal. The 50% maximum
concentration (K0.5) for Na+ was 33 +/- 1 mmol/l cell water space. The Hill
coefficient was 2.9 +/- 0.2, which is consistent with a stoichiometry of 3
Na+/1 Ca2+. Cytosolic free Na+ was estimated from the ratio of
sodium-binding benzofuran isophthalate (SBFI) fluorescence and an in vivo
calibration curve. Sulfinpyrazone, an inhibitor of anion transport,
markedly increased the SBFI content of the cells without affecting cell
Na+. Cytosolic free Na+ was 9 mM under basal conditions. Because free Na+
did not differ significantly from total Na+, little or no Na+ is bound or
compartmentalized in these cells. The complete replacement of extracellular
Na+ with K+ fails to evoke appreciable Ca2+ influx, at least in part,
because of the low cell Na+ concentration relative to the K0.5 of the
exchanger and its sigmoid dependence on Na+.
