Maintained contractions of rat uterine smooth muscle incubated in a Ca2+‐free solution

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Abstract
1 The effects of acetylcholine (10−4m), prostaglandin E2 (10−6m), vanadate (5 × 10−4m) and fluoride (10−2m) have been studied on the mechanical and electrical activities of rat myometrial strips perfused in Ca2+-free EGTA-containing solutions. 2 All four substances produced maintained contractions which could be initiated repeatedly after exposure to Ca2+-free solution for more than 1 h, without a significant decrease. The largest contractions were obtained with vanadate and the smallest ones with acetylcholine. The tension was usually 7–30% of the control contraction triggered by an action potential in Ca2+ containing solution. 3 Maintained contractions induced by fluoride were unaffected by isoprenaline while those induced by acetylcholine, prostaglandin E2 and vanadate were completely relaxed. 4 Prostaglandin E2 -and vanadate-induced contractions were slightly reduced by Na+ removal or by adding Ca2+ antagonists. In contrast, contractions induced by acetylcholine were suppressed in Na+-free solution and largely inhibited in the presence of Ca2+ antagonists. 5 The depolarization induced by acetylcholine in Ca2+-free solution was strongly dependent on the external Na+ concentration. The relationship between the size of the acetylcholine-induced depolarization and the membrane potential (shifted by constant currents) was linear, giving an apparent reversal potential for acetylcholine close to zero potential. 6 In Ca-free solutions and in the presence of atropine, Na+ action potentials of long duration can be evoked which produced contractions of the same order of magnitude as those initiated by acetylcholine-induced depolarizations. 7 These results are consistent with the hypothesis that the maintained contractions in Ca2+-free solutions induced by several stimulants could be related to Ca2+-independent mechanisms (fluoride) or Ca2+ release from an intracellular store. This latter mechanism would include both pharmacomechanical (prostaglandin E2, vanadate) and electromechanical (acetylcholine) coupling.