The Response to Endogenous Relaxin of Guinea Pigs Refractory to Porcine Relaxin

Summary

Guinea pigs made resistant to porcine relaxin by repeated injections were able to complete gestation and give birth to pups weighing the same as control animals. The progression of pelvic relaxation as gestation proceeded was retarded in the resistant animals although the optimum degree of relaxation was achieved just prior to parturition. Estrogen-primed, resistant animals responded to progesterone to the same extent as did controls, suggesting that the delay in pelvic relaxation during gestation is unlikely to be due to a reduction in secretion of relaxin or to a loss of tissue sensitivity to relaxin but rather to neutralization of pla-cental relaxin by circulating antibodies.

Note added in proof. Dr. B. G. Steinetz, of Ciba-Geigy Corporation, Ardsley, N.Y., has demonstrated the existence of anti-porcine relaxin antibodies in the sera of the relaxin-resistant animals used in these experiments. The antibody content was measured by the ability of the serum specifically to bind ¹²⁵I-poly-tyrosyl relaxin. Of the labeled relaxin added, 14 and 12%, respectively, was bound by 25 μl of serum from animals in groups II and III (Table I); the fraction bound by the same quantity of serum from control animals averaged less than 2%.