In vivo simultaneous comparison of pressor and uterine responses to a single agonist (oxypressin) in estrous rats.

To try to compare receptor compartment kinetics, receptor binding, and binding-response coupling for two smooth muscle types in vivo, pressor and uterine responses to oxypressin, an equipotent analog of oxytocin and vasopressin, were studied simultaneously in urethane-anesthetized, pentolinium-indomethacin treated rats. Access of peptide to the pressor and uterine receptor compartments and peptide-receptor dissociation rate had a negligible effect on the two responses. During both injections and infusions, the blood pressure response seemed to be determined largely by plasma levels of oxypressin. The uterine response to oxypressin, however, was paradoxical. The heights of individual uterine contractions seemed to be determined throughout by plasma oxypressin concentrations. The interval between contractions also seemed to be determined by plasma peptide concentrations during injections. However, as plasma peptide increased and reached steady state during infusion, contraction interval behaved as if plasma peptide concentrations were decreasing. This effect was more marked at the beginning of infusion. The implication of these results is that binding determines the pressor response to oxypressin and binding-response coupling does not change. In contrast, although binding determines the uterine response to oxypressin during injection and binding-response coupling appears constant, some factor in addition to binding affects the contraction interval portion of the uterine response and modifies the apparent binding-response coupling of this parameter during infusion.