Contractile properties of chemically skinned fibers from pregnant rat myometrium: existence of an internal Ca-store

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Abstract
Smooth muscle fibers isolated from pregnant rat myometrium were skinned by saponin treatment. Properties of the contractile system and the involvement of an intracellular source of activator calcium were studied. (1) In the presence of 4 mM total EGTA, skinned fibers contracted in a concentration-dependent manner to micromolar applications of calcium ions. Rat myometrium exhibited a high calcium sensitivity (80% of the maximum contraction was achieved in the presence of 10−6 M Ca2+). (2) Several divalent cations induced concentration-dependent contractile responses in skinned uterine muscle. The rank order of potency was: Ca2+>Mn2+>Sr2+>Ba2+. All these divalent cation-activated contractions were antagonized by trifluoperazine in a concentration-dependent manner. (3) pretreatment of skinned fibers with cAMP (5×10−5–5×10−4 M) depressed cation-activated contractions. This effect was dependent on the free cation concentration. (4) In the presence of a low EGTA concentration (0.1 mM) the pCa-tension curve was shifted to the right with a 2.5-fold increase in the Ca-concentration required to induce half-maximum contraction. (5) After Ca-loading (10−6 M Ca2+ for 3 min in low EGTA-containing solution), total replacement of K+ ions by choline induced a small and tonic contraction. In these conditions, the ionophore A23287 (5×10−8 M –5×10−5 M) and inositol 1,4,5-triphosphate (InsP3; 2×10−6 M – 2×10−5 M) also produced contractions of skinned uterine fibers even in the presence of NaN3 (5 mM) and of NaCN (5 mM). Repeated K+-choline substitution or InsP3 addition induced repeated contractile responses. (6) Ryanodine (5×10−5 M), procaine (15 mM) and vanadate (2.5×10−4 M) strongly depressed the Insp3-induced contractile response. It is concluded that in physiological conditions, a Ca-calmodulin-myosin light chain kinase complex is implicated in the contractile process of rat uterine smooth muscle, and that a part of the activator calcium can be supplied by an intracellular store (presumably sarcoplasmic reticulum).

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