Beta 2-adrenoreceptor binding sites in circular and longitudinal myometrial layers of the virgin guinea-pig: the influence of ovarian steroids.

Membranes prepared from both circular and longitudinal muscle layers of the uteri of two groups of virgin adult guinea-pigs were used to study the influence of ovarian steroids upon beta-adrenoreceptor binding sites. Animals were (i) treated for 14 days with oestradiol cypionate, beginning on day 9-10 of the oestrous cycle; or (ii) treated as in (i) and in addition, with progesterone for the last four days of oestradiol administration. (-)-[125I]-iodocyanopindolol ([125I]-CYP) was used to determine the numbers and characteristics of beta-adrenoreceptor binding sites in the four membrane preparations. In all cases, binding displayed characteristics of a saturable bimolecular reaction; the estimates of receptor site density (Bmax; 0.26-0.33 pmol g-1 wet weight) were similar in all four preparations, as were those of binding affinity KD; 18-31 pmol l-1). The mean negative logarithms of apparent dissociation constants (pKD) for the inhibition of specific [125I]-CYP binding by ICI 118, 551 (beta 2-adrenoreceptor selective antagonist) ranged from 8.4 to 8.6; and those for L 643, 717-01J10 (beta 1-adrenoreceptor selective antagonist) were 5.7-6.2. Thus the [125I]-CYP binding sites in all four membrane preparations displayed the characteristics expected of homogeneous populations of adrenoreceptors of the beta 2-subtype. The pKD values for isoprenaline were also similar in each type of membrane preparation (5.8-6.0). It is concluded that the clearcut differences in the contractile responsiveness, to adrenoreceptor agonists, of the circular and longitudinal myometrial preparations from oestrogen-treated guinea-pigs are not due to differences in the numbers, subtype or binding affinities of beta-adrenoreceptor binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)