Lipid rafts and caveolae are cholesterol-rich membrane microdomains that play an important role in membrane signalling and smooth muscle contraction. These microdomains depend on cholesterol for stability and depletion of cholesterol results in disruption of caveolae and rafts. Dreja et al. (2002) demonstrated in rat tail artery that contractions elicited by 5-hydroxytryptamine were inhibited by depletion of cholesterol whereas contractions to high potassium were unaffected, highlighting the importance of cholesterol in membrane signalling. We have used methyl-β-cyclodextrin (MCD), cholesterol oxidase and cholesterol esterase to reduce membrane cholesterol and investigated their effect on force and [Ca²⁺]_i in the rat myometrium.

Pregnant Wistar rats or neonates (10-14 day) were humanely killed with CO₂ and longitudinal strips of myometrium loaded with the Ca²⁺-sensitive indicator indo-1 (10µM for 3 h) to enable simultaneous measurement of calcium and force. MCD, cholesterol oxidase and esterase were added to the superfusion solution as required. All experiments were carried out at 35 °C; all values represent mean ± S.E.M.; n is the number of animals, and statistical differences were tested for by Student’s paired t test (P < 0.05).

Strips from pregnant rats were spontaneously active and contacted at a rate of around 1 min^-1. Addition of 2 % MCD resulted in rapid inhibition of spontaneous contractions (n = 4), abolition of associated Ca²⁺ transients and a transient decrease in the basal [Ca²⁺]_i which then rose to above the resting level. Removal of MCD after 30 min produced a transient increase in force which returned towards the basal level. MCD also siginificantly decreased spontaneous contractions in neonatal strips but at a much slower rate than in the adult tissue (n = 4). In strips from pregnant rats, application of either cholesterol oxidase (2 U ml^-1) or esterase (0.5 U ml^-1) resulted in rapid abolition of spontaneous contractions and Ca²⁺ transients similar to that seen with MCD. Upon their removal the spontaneous contractions were restored.

We have shown that MCD, cholesterol oxidase and esterase inhibit spontaneous contractions in myometrial strips. In the case of cholesterol oxidase and esterase, removal resulted in rapid restoration of spontaneous activity whereas with MCD the inhibition was long lasting. We suggest that the inhibitory effects of these agents were produced via depletion of cholesterol from the cell membrane and the disruption of lipid rafts and caveolae. In turn these data indicate that aspects of Ca²⁺ signalling in myometrial cells is localized to membrane rafts.