Local control of parathyroid hormone-related protein secretion by the mammary gland of the goat.

In late-pregnant goats, daily removal of secretion from one mammary gland stimulated an early secretion of fluid by that gland in five of the six animals studied. This early secretion of fluid was accompanied by an early increase of parathyroid hormone-related protein (PTHrP) 1-86 concentration in the secretion of that gland alone. When lactation was established, glands emptied thrice daily secreted PTHrP (1-86) in greater quantities and concentrations than glands emptied once daily concurrently in the same animals. Toward the end of lactation, abrupt cessation of emptying one gland caused levels of PTHrP (1-86) to decline in the fluid in that gland but not the contralateral, emptied gland. We conclude that, in the goat, PTHrP (1-86) secretion into milk is linked to autocrine factor(s) that stimulate milk secretion when fluid is removed from the gland.