The source and secretion of immunoactive relaxin in rat milk

The milk of many mammalian species contains hormones and growth factors in addition to nutrients and immunocompetent substances. These factors can be absorbed into the circulation of suckling neonates to exert important effects on metabolism and promote tissue and organ growth. Frequently, there is uncertainty as to whether such substances are gene products of the mammary glands themselves or are produced elsewhere and concentrated from the systemic circulation. The 6 kD polypeptide, relaxin, appears in milk of several mammalian species, including that of the rat, but proof of its source of secretion (corpus luteum vs. mammary gland) is so far lacking. The specific monoclonal anti-rat relaxin antibody MCA1 has previously been utilized successfully to investigate many of relaxin's actions in the rat, including those affecting the development of the mammary apparatus. In this report, MCA1 was utilized to aid in the identification of the source of relaxin in rat milk. Treatment of lactating rats with MCA1 completely neutralized the luteal relaxin circulating in serum but did not decrease the concentration of immunoactive relaxin secreted in milk. Moreover, the antibody did not appear to reach the mammary epithelium. The evidence thus supports the view that in the rat, the relaxin secreted in milk is primarily a product of the mammary glands and not concentrated from the systemic circulation.