Calcium Mobilization and Muscle Contraction Induced by Acetylcholine in Swine Trachealis

The roles of Ca(2+) mobilization in development of tension induced by acetylcholine (ACh, 0.1-100 &mgr;M) in swine tracheal smooth muscle strips were studied. Under control conditions, ACh induced a transient increase in free cytosolic calcium concentration ([Ca(2+)](i)) that declined to a steady-state level. The peak increase in [Ca(2+)](i) correlated with the magnitude of tension at each [ACh] after a single exposure to ACh, while the steady-state [Ca(2+)](i) did not. Removal of extracellular Ca(2+) had little effect on peak [Ca(2+)](i) but greatly reduced steady-state increases in [Ca(2+)](i) and tension. Verapamil inhibited steady-state [Ca(2+)](i) only at [ACh] <1 &mgr;M. After depletion of internal Ca(2+) stores by 10 min exposure to ACh in Ca(2+)-free solution and then washout of ACh for 5 min in Ca(2+)-free solution, simultaneous re-exposure to ACh in the presence of 2.5 mM Ca(2+) increased [Ca(2+)](i) to the control steady-state level without overshoot. The tension attained was the same as control for each [ACh] used. Continuous exposure to successively increasing [ACh] (0.1-100 &mgr;M) also reduced the overshoot of [Ca(2+)](i) at 10 and 100 &mgr;M ACh, yet tension reached control levels at each [ACh] used. We conclude that the steady-state increase in [Ca(2+)](i) is necessary for tension maintenance and is dependent on Ca(2+) influx through voltage-gated calcium channels at 0.1 &mgr;M ACh and through a verapamil-intensitive pathway at 10 and 100 &mgr;M. The initial transient increase in calcium arises from intracellular stores and is correlated with the magnitude of tension only in muscles that have completely recovered from previous exposure to agonists. Copyright 2001 S. Karger AG, Basel