Role of extracellular and intracellular calcium on proctolin-induced contractions in an insect visceral muscle.

The pentapeptide proctolin requires extracellular calcium to produce an increase in frequency and amplitude of myogenic contractions and an increase in basal tonus of the oviducts of the locust, Locusta migratoria. Decreasing saline calcium concentrations reduced myogenic contractions, the delay to reach maximum basal tonus and the maximum basal tonus achieved. Proctolin (5.10(-9) M) resulted in an immediate influx of extracellular calcium into locust oviduct muscle during the first 15 s of stimulation. This was followed by an increase in calcium efflux which was maintained over the time of proctolin stimulation. A Na+/Ca2+ exchanger is suggested to occur in the oviducts and may play a role in Ca2+ extrusion following proctolin stimulation.