AJP - Heart and Circulatory Physiology, Vol 259, Issue 2 409-H423, Copyright © 1990 by American Physiological Society

ARTICLES

Influence of Na+ gradient on Ca2+ transients and contraction in vascular smooth muscle

S. Bova, W. F. Goldman, X. J. Yauan and M. P. Blaustein
Department of Physiology, University of Maryland School of Medicine, Baltimore 21201.

The effects of reducing external Na+ ([Na+]o) and increasing internal Na+ ([Na+]i) on evoked tension in arterial rings were correlated with intracellular Ca2+ transients evoked in cultured A7r5 cells. Contractions were elicited in rings of rat aorta and a small mesenteric artery branch by serotonin (5-HT), arginine vasopressin (AVP), or a high K+ concentration ([K+]o). Contractions were augmented by reduction of [Na+]o (replaced by Li+ or N-methylglucamine) or inhibition of the Na+ pump (to raise [Na+]i). These effects were dependent on external Ca2+. Ca2+ transients were measured by digital imaging of fura-2/AM-loaded A7r5 cells. The apparent free Ca2+ concentration [( Ca2+]app) in eight unstimulated cells was 110 +/- 10 nM in the peripheral cytoplasm. Activation of A7r5 cells by 5-HT, AVP, or high [K+]o evoked transient increases in [Ca2+]app. Lowering [Na+]o or raising [Na+]i elevated [Ca2+]app significantly in unstimulated cells and increased the amplitude and duration of the Ca2+ transients. These effects required external Ca2+. Caffeine (5-10 mM) reduced [Ca2+]app in resting cells and attenuated the vasoconstrictor-evoked Ca2+ transients in low [Na+]o. The data imply that Na+ gradient reduction promotes Ca2+ entry and slows Ca2+ extrusion via Na(+)-Ca2+ exchange. This increases cytosolic free Ca2+ and the intracellular Ca2+ stores so that more Ca2+ is delivered to the contractile apparatus during cell activation. Such augmented Ca2+ transients could induce the amplified vasoconstrictor-evoked contractions observed in arterial rings under reduced Na+ gradient conditions.

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