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Synergistic effects of insulin-like growth factor I and gonadotrophins on relaxin and progesterone secretion by ageing corpora lutea of pigs

Summary. Insulin-like growth factor I (IGF-I) is involved in paracrine/autocrine regulation of gonadal steroidogenesis and peptide hormone biosynthesis. This study was designed to determine whether IGF-I alone, or an interaction of IGF-I, is involved in augmenting the actions of luteinizing hormone (LH) and prolactin in controlling relaxin and progesterone secretion from ageing corpora lutea of hysterectomized gilts at days 110, 113 and 116 after oestrus. Luteal tissue slices were incubated for 8 h with IGF-I (0, 50, 300 ng ml^–1), LH (0, 100, 1000 ng ml^–1), and prolactin (0, 100, 1000 ng ml^–1) alone or in combination. Progesterone and relaxin concentrations were determined by radioimmunoassay of spent medium and of homogenates from luteal tissue slices before and after incubation. Porcine luteal tissue from day 110 had a net output of 25 ng progesterone and 26 ng relaxin in the control and of 65 ng progesterone and 2125 ng relaxin in the combined IGF-I, LH and prolactin treatment mg^–1 of luteal tissue, respectively. IGF-I, LH and prolactin alone or in combination significantly increased (P<0·01) progesterone production by luteal tissue from day 110, but they were partially effective at day 113 and ineffective at day 116. By contrast, the same hormone treatments increased relaxin production by luteal tissue from days 110 and 113. Even at day 116, prolactin alone or with LH or IGF-I continued to stimulate relaxin production. In conclusion, IGF-I augments the ability of prolactin and LH to increase relaxin production by ageing corpora lutea; however, a decrease in progesterone secretion and an increase in relaxin secretion at day 113 indicate that different mechanisms control progesterone and relaxin secretion in pigs.

Keywords: IGF-I; gonadotrophins; relaxin; progesterone; ageing luteal cells; pig

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