Abstract
Responses of isolated preparations of longitudinal myometrium from the guinea-pig to fenoterol, salbutamol and isoprenaline were examined. Virgin adult guinea-pigs were treated (i) with oestradiol cypionate 20 μg/kg s.c. thrice weekly for two weeks; or (ii) as in (i) plus progesterone 3 mg/animal for the last four days. Other animals (dioestrous; cycle days 6–10) were untreated. The order of potency of the agonists in inhibiting field stimulation-induced contractions of the preparations (isoprenaline ⩾ fenoterol > salbutamol) indicated interaction with β 2-adrenoceptors. Oestrogen treatment enhanced the potency of isoprenaline 5-fold. Treatment with oestrogen plus progesterone enhanced the potency of all agonists 10- to 30-fold, and increased the mean maximum response to salbutamol. The mean pK D values for displacement by fenoterol of binding of (−)-[ 125I]cyanopindolol to membrane preparations from animals in both treatment groups were 5.57–5.92. These results raise the possibility that progesterone may enhance the coupling of β 2-adrenoceptors to transduction mechanisms in the longitudinal myometrium from oestrogen-primed guinea-pigs.
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