Effects of Isoprenaline and Dibutyryl‐C‐AMP on the Electrical and Mechanical Activity of the Rabbit Myometrium

Mechanical and electrical responses of spontaneously contracting muscle strips from oestrogen dominated rabbit uterus were investigated by the sucrose gap method. The effects of isoprenaline and dibutyryl‐c‐AMP in the presence and absence of propranolol were tested. It has been shown previously that the β‐adrenoceptor agonist isoprenaline inhibits the spontaneously contracting uterus even in the presence of the β‐blocker propranolol. The rise in c‐AMP which is caused by isoprenaline, is, however, blocked by propranolol. The aim of this investigation was to study further the role of c‐AMP in the isoprenaline‐induced relaxation of the uterus. Isoprenaline (10‐6 M) consistently abolished spikes and mechanical contractions, usually accompanied by hyperpolarization. Propranolol (3 × 10‐6 M) markedly reduced the incidence of hyperpolarization, but did not affect the other actions of isoprenaline. Hyperpolarization does not seem to be a prerequisite for inhibition of uterine contraction. Dibutyryl‐c‐AMP mimicked the actions of isoprenaline. No positive evidence to support a hypothesis of a c‐AMP‐independent mechanism was found. Therefore a possible explanation of the results could be that isoprenaline increases c‐AMP in a small intracellular pool, mediating electrical and mechanical responses but not influenced by propranolol.