Na+-Ca 2§ exchange in cultured vascular smooth muscle cells. Circulation Research. 62:486--493

Vascular smooth muscle cells (VSMC) contract as intracellular free calcium ([Ca2+],) rises. While Na+-Ca2+ exchange has been proposed to contribute to transmembrane Ca2+ flux, its role in cultured VSMC is unknown. Accordingly, we have investigated the role of Na+-Ca2+ exchange in unidirectional and net transmembrane Ca2+ fluxes in cultured rat aortic VSMC under basal conditions and following agonist-mediated stimulation. Transmembrane Ca2+ uptake was significantly increased in response to a low external Na+ concentration ([Na+]J compared with 140 mM [Na+],,. Na+-dependent Ca2+ uptake in response to low [Na+]o was further increased by intracellular Na+ loading by preincubation of the VSMC with 1 mM ouabain. Under steady-state conditions, Ca2+ content varied inversely with [Na+]o, increasing from 1.0 nmol Ca2+/mg protein at 140 mM [Na+], to 4.0 nmol Ca2+/mg protein at 20 mM [Na +)0. Increasing [K+], to 55 mM also enhanced Na+-dependent Ca2+ influx. Augmentation of Ca2 + uptake with K+ depolarization was not significantly inhibited by the calcium channel antagonist verapamil. Transmembrane Ca2+ efflux was increased in response to 130 mM [Na+)o compared with zero [Na+]o (iso-osmotic substitution with choline+), and was further stimulated by the vasoconstrictor angiotensin II, which is known to elevate [Ca2+],. These changes in [Ca2*], were studied directly using fura-2 fluorescence measurements. Elevated [Ca2+], levels returned to baseline more rapidly in th