Mobilization of free Ca2+ measured during contraction-relaxation cycles in smooth muscle cells of the porcine coronary artery using quin2

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Abstract
Ca2+ mobilization in dispersed smooth muscle cells of the porcine coronary artery was investigated using the fluorescent Ca2+ indicator, quin2. The resting [Ca2+]i was 113±8 nM (a mean±SE), and was independent of intracellular quin2 concentrations. Acetylcholine (ACh; over 10 nM) or caffeine (over 3 mM) transiently increased the intensity of fluorescence, thereby reflecting the elevation of intracellular free Ca2+ (Ca2+ transient), while excess K+ gradually increased and maintained the intensity of fluorescence. Application of EGTA reduced the resting intensity of the fluorescence and blocked the K+-induced Ca2+ transient, but did not supress the Ach-or caffeine-induced ones. Nisoldipine (0.1 μM) did not affect the resting intensity of the fluorescence. This agent blocked the K+ induced but not the ACh-or caffeine-induced Ca2+ transient. Thus, sources of Ca2+ contributing to the K+-induced Ca2+ transient differ from those evoked by other agents. The amount of Ca2+, as estimated from the increased Ca2+ transient by caffeine or ACh, was increased in proportion to the excess K+-induced influx of Ca2+.