Carbachol-induced [Ca2+](i) oscillations in single smooth muscle cells of guinea-pig ileum

1. Changes in cytosolic Ca2+ concentration ([Ca2+](i)) produced by carbachol (CCh) were measured in single smooth muscle cells of guinea-pig ileum using a Ca2+-sensitive fluorescent dye, fura-2, to clarify the underlying mechanisms of muscarinic [Ca2+](i) oscillations. 2. Half of the cells, when exposed to 0.2 mu m CCh, exhibited repeated changes in [Ca2+](i) giving a serrated appearance. The oscillatory changes [Ca2+](i) were very similar to those evoked by increasing extracellular K+ concentration ([K+](o)) to 30 nm, which were abolished by removal of extracellular Ca2+, nifedipine and La3+, but remained unchanged after depletion of internal Ca2+ store with cyclopiazonic acid, thapsigargin and ryanodine. 3. Every individual [Ca2+](i) oscillation was just like a [Ca2+](i) increase generated spontaneously in about 8% of cells of triggered by an action potential evoked by a current pulse in current-clamped cells. 4. In the remaining half of the cells exposed to 0.2 mu m CCh, slower [Ca2+](i) oscillations were elicited and every individual [Ca2+](i) oscillation was always preceded by the fast brief increase in [Ca2+](i). 5. [Ca2+](i) oscillations elicited by 2 mu m CCh were temporally and functionally distinct from those induced by high [K+](o). They were more or less regular in the periodicity and pattern, comprised pacemaker potential-like [Ca2+](i) increases or sinusoidal types of [Ca2+](i) increases, and could be elicited even in 100 mM K-o(+). 6. Removal of extracellular Ca2+ or application of nifedipine, methoxyverapamil (D600), diltiazem or La3+ during CCh (2 mu m)-induced [Ca2+](i) oscillations caused them to disappear. In cells in which internal Ca2+ stores were depleted, 2 mu m CCh did not evoke [Ca2+](i) oscillations but occasionally induced single or repeated generation of the increase in [Ca2+](i) with a serrated appearance. 7. The results indicate that CCh can induce two types of [Ca2+](i) oscillation in guinea-pig ileal smooth muscle cells; one arises from Ca2+ influx associated with action potential discharges and the other from periodic release of Ca2+ from internal stores. The latter [Ca2+](i) oscillation requires extracellular Ca2+ to sustain it.