Caffeine- and ryanodine-sensitive Ca2+ stores in cultured guinea pig myenteric neurons

In single fura 2-loaded myenteric neurons, caffeine caused concentration-dependent increases in intracellular Ca2+ concentration ([Ca2+](i)) that were quantal, saturable, and reversible. Inhibition of caffeine-induced Ca2+ release was demonstrated by ryanodine (1 mu M), dantrolene (10 mu M), and procaine (5 mM). Caffeine and cyclopiazonic acid (30 mu M) released overlapping Ca2+ stores, whereas the caffeine-releasable pool was a subset of Ca2+ released by the Ca2+ ionophore ionomycin (4 mu M). Both mild depolarization (7.5 mM KCl) and a submaximal concentration of caffeine (1 mM) produced neuronal [Ca2+](i) oscillations in one-third of cells examined, which could be abolished by ryanodine (1 mu M) or removal of extracellular Ca2+. Release of caffeine-sensitive Ca2+ stores induced influx of extracellular Ca2+. Immunolocalization using confocal microscopy revealed ryanodine receptor-like staining within the cytosol of cultured myenteric neurons.