Actions of forskolin and isoprenaline on the separated myometrial layers of the guinea-pig uterus: influence of ovarian steroids and pregnancy.

Responses to forskolin and isoprenaline of electrically stimulated preparations of longitudinal and circular myometrium were compared. The guinea-pigs used were: untreated (dioestrous), ovarian steroid-treated, pregnant and post-partum. Forskolin inhibited electrically evoked contractions of both myometrial layers obtained from all groups; there was no change in its potency or its maximum effect (90-100% inhibition). The inhibition of contractions of longitudinal myometrium by isoprenaline was enhanced with steroid treatment, attaining a maximum in late pregnancy and immediately post-partum. In circular myometrium, there was no significant change in the potency of isoprenaline on tissues from ovarian steroid-treated animals. However, maximum potency as well as maximum response was observed in late pregnancy. Since the effects of isoprenaline but not of forskolin were modulated, we suggest that ovarian steroids and pregnancy may alter the coupling of beta-adrenoceptors to the adenylate cyclase system.