Rat Myometrial Adrenergic Receptors in Late Pregnancy1

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Abstract
The myometrium of the rat has been found to contain both .alpha.1- and .beta.-adrenergic receptors. To investigate the implication of these adrenergic receptors in uterine reactivity near term delivery, we have measured the number and the affinity of .alpha.1-adrenergic antagonist ([3H]prazosin: [3H]PRAZ)-binding sites and of very high affinity .beta.2-adrenergic agonist ([3H]hydroxybenzylisoproterenol: [3H]HBI)-binding sites in myometrial membranes throughout the last 5 days of pregnancy and at delivery. The number of specific binding sites was constant from Day 18 of pregnancy up to 6 h prior to birth. In the last 6 h of pregnancy, there was a sharp increase in the number of .alpha.1-receptors (+70%, p<0.05). Simultaneously, the number of .beta.2-receptors coupled to the adenylate cyclase system dropped (-75%; p<0.001). These results indicate that with the approach of parturition, there ia a regulation of uterine reactivity by a modulation of the concentrations of myometrial adrenergic receptors during the last 6 h of gestation.