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The relationship between lactation and the release of prolactin and growth hormone in the goat.

The ability of pituitary hormones to cause udder growth and to initiate milk secretion in the goat was clearly demonstrated when it was shown that both mammary growth and lactation were induced in the ovariectomized virgin goat by twice-daily application of the milking stimulus (Cowie, Knaggs, Tindal & Turvey, 1968). It was further demonstrated that both of these responses were completely abolished in animals in which the pituitary stalk was transected before 'milking' was started. As earlier work had shown that suckling often resulted in a fall in pituitary hormone concentration (see Cowie & Tindal, 1971) it seemed probable that both mammary growth and lactation were initiated by elevated blood levels of pituitary hormones. Concerning the maintenance of milk secretion, partly successful hormonal replacement studies in the hypophysectomized lactating goat were first described by Cowie & Tindal (1961) followed by Gale & Larson (1963). Complete restoration and maintenance of the milk yield was subsequently achieved with a combination of ovine prolactin, bovine GH, tri-iodothyronine, protamine zinc insulin and dexamethasone (Cowie, Knaggs & Tindal, 1964). The presence of prolactin was important for the restoration of the milk yield to the pre-hypophysectomy level but it was not always necessary for the maintenance of lactation at that level (Cowie, 1969). The anterior pituitary hormones prolactin and GH would thus appear to play an essential rôle in the establishment and maintenance of lactation in the goat. Using radioimmunoassays for ovine/caprine prolactin (Hart, and caprine GH (I. C. Hart, S. Andrews A. S. McNeilly, in preparation), it has now been possible to measure the circulating levels of these hormones under a variety of conditions in the goat and thereby attempt