Contractions of rat uterine smooth muscle induced by acetylcholine and angiotensin II in Ca2+‐free medium

Abstract

1 The effects of acetylcholine (ACh, 10⁻⁴m) and angiotensin II (Ang II, 10⁻⁶ M) have been studied on the mechanical and electrical activities of rat myometrial strips perfused in Ca²⁺-free EGTA-containing solutions. 2 Both ACh and Ang II produced transient contractions, the amplitude of which can be taken as a measurement of the amount of Ca²⁺ present in a drug-sensitive Ca²⁺ store. The degree of filling of this store depended on the external Ca²⁺ concentration, and on the presence of contractile responses during the Ca²⁺ loading period. The existence of two pathways (either direct or transcytoplasmic) is suggested for Ca²⁺ uptake into the internal Ca²⁺ store. 3 The rate of filling of the Ca²⁺ store in 2.1 mm-Ca²⁺-containing solution was faster (time to half-maximal response, t_1/2 = 29 ± 2.2s, n = 4) than the rate of depletion in Ca²⁺-free solution (t_1/2 = 3 ± 0.3 min, n = 3). The gradual depletion of this store was much slower at 18°C than at 35°C, and in the presence of vanadate which is known to inhibit Ca²⁺-ATPases. 4 Methoxyverapamil (D600, 10⁻⁶-10⁻⁵m) had no appreciable effect on the direct Ca²⁺ uptake or on the release of Ca²⁺ from the store by ACh and Ang II. Mn²⁺ (10⁻³m) completely inhibited the direct pathway to the internal Ca²⁺ store and also reduced the release of Ca²⁺. 5 ACh and Ang II induced repetitive depolarizations close to zero potential which did not parallel the transient contractions as a function of the time of perfusion in Ca²⁺-free solution. Applications of 2 mm EGTA, 135 mm K⁺ or Ca²⁺ antagonists which suppressed or reduced the drug-induced depolarizations did not affect appreciably the drug-induced contractions. 6 These results suggest that myometrial cells have an intracellular Ca²⁺ store sensitive to different stimulus substances. This store is not affected by depolarization of the plasma membrane and is certainly different from that described in voltage-clamp experiments.