The response of non-pregnant rat myometrium to oxytocin in Ca-free solution

The contractile response of the longitudinal muscle of non-pregnant rat myometrium to oxytocin (0.2-20 nM) consisted of a phasic and a tonic component. Ca-removal abolished the phasic component but a tonic contraction could be evoked without reduction of amplitude for 50 h. Exceptionally, the tonic contraction also disappeared gradually in Ca-free medium containing 2 mM EGTA. When oxytocin was repeatedly applied in the absence of Ca, the response became at first progressively larger before reaching a steady state. Transient addition of Ca to the medium reduced the size of the subsequent oxytocin contraction. In Ca-free medium, the tissue lost Ca slowly, but it still contained 40 mumol kg-1 after 6 h and roughly 1 mumol kg-1 wet weight after 24 h exposure. 45Ca efflux was marginally increased by oxytocin (20 nM). Caffeine (5-30 mM) produced no contraction, but slightly reduced the resting tension and strongly inhibited the oxytocin response both in the presence and in the absence of Ca. Caffeine also blocked the contraction induced by Ca added to Ca-free 40 mM K solution. However, pretreatment with caffeine (30 mM) had no effect on the following oxytocin response. A calmodulin antagonist, trifluoperazine (1-10 microM) suppressed strongly the Ca-induced contraction, but had only a weak effect on the oxytocin response in Ca-free medium. Chlorpromazine (10-100 microM) and fluphenazine (10-30 microM) had similar effects. A different type of antagonist, N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide (W-7) (0.1 mM) almost completely blocked responses to both oxytocin and to Ca, but recovery of the Ca-induced contraction was much better than that of the oxytocin response in Ca-free solution. 6 Since no evidence was found for intracellular Ca release by oxytocin, and as there were several differences between the effects of calmodulin antagonists on the oxytocin response and on the Ca-induced response of similar size, the possibility remains that some Ca-independent process is involved in the contractile response to oxytocin observed in Ca-free solution.