FUNCTIONAL DETERMINATION OF OXYTOCIN AFFINITY IN NEAR-TERM PREGNANT RAT MYOMETRIUM - EFFECT OF CHRONIC HYPOXIA

We designed the present study to determine: (1) if phenoxybenzamine ca n be used as an irreversible blocker for oxytocin receptors, and as su ch to determine oxytocin affinity, (2) if prolonged hypoxic exposure a lters oxytocin receptor coupling efficacy of oxytocin receptors to pos t-receptor mediated mechanisms in the rat myometrium. Rats were expose d to room air (control), or to continuous hypoxia (10.5% O-2) from day 19 through day 21 (2-day exposure). On day 21, one uterine horn was r emoved and used for in vitro study of myometrial contractile responses to oxytocin, while the other was used for oxytocin receptor analysis. In normoxic tissues, phenoxybenzamine (20 mu M) decreased the maximum contractile response (E-MAX) to oxytocin (155 +/- 17 vs. 66 +/- 19 g s/cm(2)) and oxytocin binding sites (B-MAX 253 +/- 35 vs. 114.9 +/- 21 .3 fmol/mg protein). A similar degree of reduction in E-MAX and B-MAX were observed in hypoxic tissues. The oxytocin dissociation constant ( K-A) in the normoxic rat was 2.8 +/- 0.7 nM, which was not different f rom the chronic hypoxic rat (3.3 +/- 0.9 nM). Analysis of receptor occ upancy-response curves indicated no oxytocin receptor reserve in both normoxic and hypoxic myometrium. However, for a given fraction of the total oxytocin receptors occupied, hypoxic tissue elicited a lower con tractile response to oxytocin. We conclude that: (1) phenoxybenzamine is a useful tool to functionally study oxytocin receptor kinetics, (2) prolonged hypoxic exposure does not affect the oxytocin affinity, (3) no spare receptors for oxytocin are present in the rat myometrium, an d (4) prolonged exposure to hypoxia decreases oxytocin receptor-effect or coupling efficiency in rat myometrium. (C) 1998 Elsevier Science B. V. All rights reserved.