Evidence that the Maternal Pituitary Suppresses the Secretion of Relaxin in the Pregnant Rat*

The role of the pituitary in regulating relaxin synthesis and secretion by the corpus luteum during the second half of pregnancy was investigated by hypophysectomizing rats carrying either one conceptus (1C) or a full complement (FC) of conceptuses (n greater than or equal to 8) on day 13 of pregnancy. Serum and luteal relaxin levels, serum progesterone levels, and luteal weights in 1C rats were markedly lower during the period from days 14 through 20 of pregnancy than those in FC rats, as has been previously reported. After hypophysectomy of 1C rats (H1C), serum relaxin levels, serum progesterone levels, and corpus luteum weights increased to values that were not significantly different from those of FC rats. Additionally, hypophysectomized FC rats (HFC) had higher serum relaxin levels than FC rats. Luteal relaxin content was unaffected by hypophysectomy in spite of increased relaxin secretion. Other workers have suggested that placental testosterone may support luteal function during the second half of pregnancy. Serum testosterone levels in 1C rats were markedly lower than those in FC rats, but did not increase after hypophysectomy in either H1C or HFC rats. It is concluded that the putuitary has a suppressive effect on relaxin secretion (and perhaps synthesis) as well as progesterone secretion and corpus luteum growth, and that increased luteal function after hypophysectomy is not due to increased placental testosterone secretion.