Guanine nucleotide regulation and cation sensitivity of agonist binding to rat brain oxytocin receptors

The inhibitory effects of oxytocin on distal colonic contractile activity in rabbits are enhanced by ovarian steroids

AIMS

To study the effects of oxytocin on isolated rabbit distal colon and the regulation of ovarian steroids by its action.

METHODS

Muscle strips parallel to either the circular or the longitudinal fibres were excised and suspended in tissue chambers containing 5 mL Krebs solution (37 degrees C) and bubbled continuously with 95% O(2) and 5% CO(2). The effects of oxytocin on isometric spontaneous contractile responses were recorded. The effects of atosiban, tetrodotoxin, Mg(2+), progesterone and oestradiol on the oxytocin-induced response were also examined.

RESULTS

Oxytocin (1, 10 and 100 nmol L(-1)) dose dependently decreased the area under the contraction curve of distal colonic smooth muscle strips. The oxytocin receptor antagonist atosiban blocked the oxytocin (10 nmol L(-1))-caused responses in a dose-dependent manner. Tetrodotoxin (10 micromol L(-1)) had no effect on the oxytocin-induced response. Mg(2+)-free Krebs solution attenuated the oxytocin-induced response, but oestradiol (0.1 micromol L(-1)) or progesterone (0.1 micromol L(-1)) increased the oxytocin-induced response.

CONCLUSION

These results suggest that oxytocin inhibits the contractile motility of the distal colon, which is regulated by Mg(2+) and ovarian steroids.

Solubilization of oxytocin receptors in porcine renal LLC-PK1 cell membranes

The present studies were undertaken to better characterize the pharmacological properties of oxytocin receptors (OTRs) of the porcine kidney cell line, LLC-PK1, in their natural membranous environment and after solubilization. In intact membranes, binding of a selective radioligand was rapid, reversible, saturable, and of high affinity. High-affinity agonist binding was reduced by a GTP analogue, suggesting that these OTRs are associated with G-protein(s). After solubilization with the zwitterionic detergent CHAPSO, OTRs retained their high affinity for the radioligand and rank order potency for oxytocin analogues, and agonist binding remained biphasic and GTP sensitive.