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Binding of [³H]oxytocin to purified myometrial plasma membranes was unaffected by continuous infusion of bradykinin over 5 days in rats pretreated with oestradiol 2 days before collection of tissue. In contrast, oxytocin treatment resulted in a 76% decrease in maximal binding of [³H]oxytocin and thereby in oxytocin receptor concentration without affecting the dissociation constant. The K_M value (molar concentration giving half maximal contraction) of isolated uterine strips stimulated with oxytocin was increased and maximal contractile responses were reduced following oxytocin infusions.

The binding of [³H]bradykinin to purified plasma membranes was influenced by treatment with both oxytocin and bradykinin. Bradykinin infusions down-regulated the bradykinin receptor concentration by 19%, while the receptor affinity remained unchanged. Maximal contraction (E_max) values of isolated strips stimulated with bradykinin exhibited a slightly attenuated response and K_M values were significantly enhanced. Long-term treatment with oxytocin down-regulated myometrial bradykinin receptors by 31%. In addition, oxytocin infusions caused E_max to decrease and K_M to increase in experiments with isolated uterine strips stimulated with bradykinin.

It is concluded that the down-regulation of oxytocin and bradykinin receptors following prolonged exposure to oxytocin may result from changes in a common pathway for intracellular peptide receptor processing. Likewise, the increased K_M values of isolated myometrial strips (despite unchanged dissociation constants) suggest that prolonged oxytocin treatment affects the coupling between receptor activation and contractile response.

J. Endocr. (1988) 118, 81–85

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