Na-Ca Exchange in Cultured Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMC) contract as intracellular free calcium ([Ca],) rises. While Na-Ca exchange has been proposed to contribute to transmembrane Ca flux, its role in cultured VSMC is unknown. Accordingly, we have investigated the role of Na-Ca exchange in unidirectional and net transmembrane Ca fluxes in cultured rat aortic VSMC under basal conditions and following agonist-mediated stimulation. Transmembrane Ca uptake was significantly increased in response to a low external Na concentration ([Na]J compared with 140 mM [Na],,. Na-dependent Ca 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, Ca content varied inversely with [Na]o, increasing from 1.0 nmol Ca /mg protein at 140 mM [Na], to 4.0 nmol Ca/mg protein at 20 mM [Na + )0. Increasing [K ], to 55 mM also enhanced Na-dependent Ca influx. Augmentation of Ca + uptake with K depolarization was not significantly inhibited by the calcium channel antagonist verapamil. Transmembrane Ca 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 [Ca],. These changes in [Ca*], were studied directly using fura-2 fluorescence measurements. Elevated [Ca], levels returned to baseline more rapidly in the presence of normal (130 mM) [Na]0 compared with zero [Na ]o (iso-osmotic substitution with choline). These findings suggest that a bidirectional Na*-Ca exchange mechanism is present in cultured rat aortic VSMC. Na-Ca exchange appears to play a part in Ca homeostasis, particularly under conditions of altered intracellular Na or increased [Ca], following agonist stimulation. (Circulation Research 1988;62:486-493)