Relaxin Immunoactivity in Plasma During the Reproductive Cycle of the Female Guinea Pig

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Abstract
Plasma relaxin immunoactivity was measured in guinea pigs during the estrous cycle from midpregnancy to parturition as well as during acute suckling using a homologous porcine relaxin radioimmunoassay. Samples were collected at frequent intervals from an indwelling jugular vein catheter. The peak levels found during pregnancy, ∼12 ng/ml, were higher than the peak levels during the estrous cycle and suckling (4-5 ng/ml). During the estrous cycle, relaxin levels were low at estrus and early diestrus and were high at late diestrus and proestrus preceeding the opening of the vaginal membrane. During late pregnancy, episodic secretion of immunoreactive relaxin was common and the timing of this secretion varied. In some guinea pigs, relaxin levels increased in mid to late pregnancy; others showed only a rise before parturition or only the mid to late pregnancy rise. Spectral analysis of the data revealed clear periodicities in the results from several animals but others showed no evidence of periodicity. The most common period was 18 h. In most animals, but not all, the onset of lengthening of the pubic ligament as determined by palpation was preceded by a period of relaxin production. There was no evidence for an acute effect of suckling to elevate relaxin secretion in this species; indeed suckling decreased relaxin immunoactivity with mean levels of control animals significantly higher than those of animals suckling their young.