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Two Ca²⁺ entry pathways mediate InsP₃-sensitive store refilling in guinea-pig colonic smooth muscle

John G. McCarron, Elaine R. M. Flynn, Karen N. Bradley and Thomas C. Muir

1. Sarcolemma Ca²⁺ influx, necessary for store refilling, was well maintained, over a wide range (-70 to + 40 mV) of membrane voltages, in guinea-pig single circular colonic smooth muscle cells, as indicated by the magnitude of InsP₃-evoked Ca²⁺ transients.

2. This apparent voltage independence of store refilling was achieved by the activity of sarcolemma Ca²⁺ channels some of which were voltage gated while others were not. At negative membrane potentials (e.g. -70 mV), Ca²⁺ influx through channels which lacked voltage gating provided for store refilling while at positive membrane potentials (e.g. +40 mV) voltage-gated Ca²⁺ channels were largely responsible.

3. Sarcolemma voltage-gated Ca²⁺ currents were not activated following store depletion.

4. Removal of external Ca²⁺ or the addition of the Ca²⁺ channel blocker nimodipine (1 µM) inhibited store refilling, as assessed by the magnitude of InsP₃-evoked Ca²⁺ transients, with little or no change in bulk average cytoplasmic Ca²⁺ concentration. One hypothesis for these results is that the store may refill from a high subsarcolemma Ca²⁺ gradient.

5. Influx via channels, some of which are voltage gated and others which lack voltage gating, may permit the establishment of a subsarcolemma Ca²⁺ gradient. Store access to the gradient allows InsP₃-evoked Ca²⁺ signalling to be maintained over a wide voltage range in colonic smooth muscle.

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