Clearance of oxytocin and its potentially enzyme resistant analogues in the OXT‐receptor compartment of the potassium depolarized rat myometrium

The time–response behaviour of a group of oxytocin analogues structurally modified on potential sites of oxytocin splitting by tissue inactivation enzymes (“enzyme probes”) was investigated ex vivo on the potassium depolarized rat myometrium (at 30°C) and compared with the data obtained in the in vivo experiments. The modified oil‐immersion method by Kalsner and Nickerson was used to record time profiles after cessation of a steady state myometrium contraction triggered by analogues in a high potassium tissue medium. An exchange of the aqueous medium for mineral oil enables to suppress return diffusion of the peptide and to record its irreversible clearance near the corresponding receptor compartment. Response records were analysed by a nonlinear numeric procedure based on combination of steady state and kinetic terms that allows concomitant estimations of affinities from time–response measurements, in the given case for analogues on depolarized myometrium. Potential inactivation‐sensitive sites in the oxytocin chain are the Ν‐terminal peptide bond Cys(1)‐Tyr(2) (aminopeptidase splitting), the intramolecular disulphide bridge (reduction and formation of the practically inactive linear peptide) and the C‐terminal Leu(8)‐GlyNH(2) (9) or the Pro(7)‐Leu(8) (postprolin cleaving enzyme) bond, respectively. Clearance rate constants of single peptides in the OXT‐receptor compartment were in an interval of 0.025 to 0.28 min(−1). The fragment contribution analysis reveals a significant linear additivity of individual structural changes and thus a predictivity of irreversible inactivation rate in the receptor compartment. The most potent inactivation of oxytocin is associated with aminopeptidase splitting; other enzymes may play some though nondecisive role. Less significant differences within the peptide group were found for rate constants for peptide transport between receptor compartment and its external aqueous medium. Besides rate constants, the evaluation of time–response data yields affinity values of the tested peptides and indicates a 25‐times desensitation of depolarized compared with a native state.