β2‐ADRENORECEPTOR BINDING SITES IN CIRCULAR AND LONGITUDINAL MYOMETRIAL LAYERS OF THE VIRGIN GUINEA‐PIG: THE INFLUENCE OF OVARIAN STEROIDS

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Abstract
1 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. 2 (-)-[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). 3 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). 4 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. Moreover, enhancement of the inhibitory potencies of .beta.-adrenoreceptor agonists, in preparations of longitudinal myometrium from oestrogen-primed guinea-pigs treated with progesterone, is not due to an effect of progesterone upon .beta.-adrenoreceptor binding sites.