SOURCES OF CALCIUM FOR ACETYLCHOLINE-INDUCED CONTRACTIONS OF DOG TRACHEAL SMOOTH-MUSCLE

Abstract

Acetylcholine (ACh)-induced contractions of dog tracheal smooth muscles were studied to determine the sources of Ca2+ for these contractions. In Ca2+-free medium the contractions become diminished with time at all drug doses and experiments with 45Ca indicate that this phenomenon is related to the efflux of Ca2+ from the muscle. This efflux of Ca2+ consists of 2 components, a loosely bound and a tightly bound Ca2+ source, and is dependent upon the dose of drug. At low doses of ACh (< 10-6 M) loosely bound Ca2+ plays a greater role and tightly bound Ca2+ a lesser role than at higher drug doses. Verapamil, a blocker of depolarization-dependent Ca2+ influx, inhibits ACh contractions at low drug doses but has no effect at higher doses. The sources of Ca2+ for ACh contractions of the trachea are dependent upon the drug dose. At low doses of ACh (< 10-6 M ACh) depolarization-dependent Ca2+ influx is involved in much of the contraction. At the higher drug doses a tightly bound Ca2+ source and a depolarization-independent, loosely bound Ca2+ source account for the contractions.