Relaxin inhibition of KCl-induced uterine contractions in vitro: an alternative bioassay.

The effect on the rat uterus of the ovarian peptide hormone, relaxin, has been studied in vitro. Partial (20%) replacement of the NaCl in physiological saline by KCl resulted in a strong contraction of the uterus which was reversed by relaxin. The preparation contracted by this level of KCl substitution was shown to be very sensitive to the action of extracts of pregnant sow ovaries containing relaxin in comparison with the ones contracted by higher and lower degrees of KCl substitution. The ovarian extracts induced a concentration-dependent reversal of the contraction of the uterus; this activity of relaxin increased with the degree of purification of the hormone. Relaxin induced a slowly developing relaxation whereas epinephrine relaxed the same preparation very rapidly until a plateau relaxation was reached. The concentration-response curves to relaxin at different states of purification were parallel and were also parallel and comparable in potency to the ones measured for similar preparations from other laboratories. These results suggest that rat uterus contracted in vitro by KCl-substituted saline should be a valuable and reproducible bioassay for relaxin.