Strips of canine tracheal smooth muscle were studied in vitro to determine the effects of changes in the extracellular calcium (Ca(o)) concentration on tonic contractions induced by acetylcholine and 5-hdyroxytryptamine. Strips were contracted with graded concentrations of the above agents in 2.4 mM Ca, after which CaCl2 was administered to achieve final concentrations of 5.0, 10.0, and 20.0 mM. Increases in Ca(o) to 5 mM or above caused significant relaxation of muscles contracted with 5-hydroxytryptamine but did not significantly relax muscles contracted with acetylcholine. Increases in Ca(o) also caused significant relaxation of muscles contracted with low concentrations of K+ (20 or 30 mM). However, in 60 or 120 mM K+, increases in Ca(o), resulted predominantly in muscle contraction. Inhibition of the Na+-K+-ATPase by ouabain (10-5 M) or K+ depletion reversed the effects of Ca(o) from relaxation to contraction in tissues contracted with 5-hydroxytryptamine. Increases in Ca(o) also caused contraction rather than relaxation in the presence of verapamil (10-6 M). We conclude that calcium has both excitatory and inhibitory effects on the contractile responses of canine tracheal smooth muscle. The inhibitory effects of Ca2+ appear to be linked to the activity of the membrane N+-K+-ATPase.
ASJC Scopus subject areas
- Physiology (medical)