AJP - Endocrinology and Metabolism, Vol 256, Issue 2 E215-E220, Copyright © 1989 by American Physiological Society
Myometrial alpha 1-adrenoceptors in pregnant guinea pig: their distribution and increased number
S. J. Arkinstall and C. T. Jones
Laboratory of Cellular and Developmental Physiology, University of Oxford, Headington, United Kingdom.
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.
