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

ABSTRACT

The effects of acetylcholine (ACh, 10(-4)M) 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. 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. 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 +/- 2.2 s, 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 degrees C than at 35 degrees C, and in the presence of vanadate which is known to inhibit Ca2+-ATPases. Methoxyverapamil (D600, 10(-6)-10(-5) M) 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(-3) M) completely inhibited the direct pathway to the internal Ca2+ store and also reduced the release of Ca2+. 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. 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.