Na+/Ca2+ exchange in isolated smooth muscle cells demonstrated by the fluorescent calcium indicator fura-2.

Fura-2, a novel fluorescent indicator of cytoplasmic calcium concentrations ([Ca2+i]), was 'loaded' into smooth muscle cells isolated from guinea pig taenia coli. Resting cells maintained a stable [Ca2+i] of 107 +/- 26 nM (n = 13), which could be perturbed with ionomycin. [Ca2+i] was elevated by stimulation of the cells with carbachol or 50 mM KCl. Reduction of the plasmalemmal Na+ concentration gradient by inhibition of the Na+/K+-ATPase with ouabain markedly elevated [Ca2+i]; this elevation was dependent on extracellular Ca2+. [Ca2+i] was also increased by replacement of the extracellular Na+ with an organic cation.