The effects of adrenaline, isoprenaline and forskolin upon the evoked contractions of field-stimulated preparations of human, pregnant, isolated myometrium have been examined. Specimens were obtained at lower segment Caesarean section from patients at 31 (n = 1) and 36-40 (n = 10) weeks of gestation. 2. Adrenaline enhanced the electrically evoked contractions of all preparations studied, indicating that its predominant action on these pregnant myometrial tissues was at alpha- and not beta-adrenoceptors. 3. Isoprenaline in concentrations at and below 10 mumol/l produced inhibitory effects in eight of 11 experiments. In the remaining three experiments, tissues were not responsive to the inhibitory effects of isoprenaline. 4. In all preparations exposed to higher concentrations of isoprenaline (30 or 100 mumol/l), its effects were excitatory. 5. Forskolin produced inhibitory effects on preparations from all uteri, including those from which tissues unresponsive to isoprenaline had been obtained. 6. It is suggested that forskolin in the concentrations which were effective in this study produced its inhibitory effects largely through activation of adenylate cyclase. This implies that the lack of an inhibitory response of some preparations to isoprenaline was not due to reduced activity of the adenylate cyclase system, but that the failure of isoprenaline to produce an inhibitory effect could be due to diminished numbers of beta-adrenoceptors and/or increased numbers of alpha-adrenoceptors, or to a defect in the coupling of the receptors to the adenylate cyclase system. Alternatively, the presence of an endogenous antagonist of the effects of isoprenaline (for example, an eicosanoid), could mask its inhibitory effects. 7. The absence of an inhibitory effect of isoprenaline on some specimens of human gravid myometrium could have clinical implications, in view of the widespread use of beta 2-adrenoceptor agonists as uterine relaxants.
