Myometrial alpha 1-adrenoceptors in pregnant guinea pig: their distribution and increased number

There is evidence for an excitatory influence in pregnant and parturient myometrium mediated by alpha 1-adrenoceptors. Despite this there are no reports of direct measurement of changes in the density of these receptors throughout pregnancy. Indirect evidence could suggest a fall early in gestation followed by a sharp rise near term. Hence, to follow pregnancy-related changes, [3H]prazosin was employed in this study to measure alpha 1-adrenoceptors in guinea pig myometrium. High-affinity [3H]prazosin binding [dissociation constant (Kd) = 0.51 +/- 0.05 nM (n = 29)] displayed the pharmacological characteristics of alpha 1-adrenoceptors and yielded linear Scatchard plots, which in nonpregnant myometrium indicated a maximum binding density (Bmax) of 48.6 +/- 6.1 fmol/mg protein (n = 5). At all times during pregnancy alpha 1-adrenoceptor density was approximately 40% higher than the nonpregnant value. This, together with uterine growth, resulted in a progressive increase in total alpha 1-adrenoceptor population of up to 34-fold with a mean value of 7.54 +/- 2.26 pmol/uterus (n = 6) at term (67 +/- 2 days). This slow rise in receptor number does not suggest a major role in the processes triggering labour in this species. The distribution of alpha 1-adrenoceptors in tubal, perifetal, and cervical regions was similar in the 60-day pregnant uterus. As much of the uterus is not innervated at this time, a considerable proportion of these receptors are probably extrasynaptic.