ONSET OF CALCIUM SECRETION DURING LACTOGENESIS IN THE GOAT

This communication describes, for the first time to the authors' knowl edge, the onset of milk calcium secretion during lactogenesis in the g oat. In addition, the effect of injecting fragment 1-34 of parathyroid hormone-related protein (PTHrP) into an experimental gland a few days before parturition has also been studied. This fraction of PTHrP is n aturally produced by the mammary gland during lactation, beginning on the day of parturition (Ratcliffe et al. 1992). It has been suggested that it may function as an autocrine or paracrine factor stimulating c alcium transport from blood to milk in the mammary gland because the c oncentrations of PTHrP in milk from cows with established lactation co uld be correlated with milk calcium concentrations (Law et al. 1991) a nd intravenous infusion of PTHrP(1-34) caused increases in the concent rations of calcium and magnesium in the milk of goats (Barlet et al. 1 992). However, injecting an antiserum to PTHrP(1-34) into lactating mi ce had no detectable effect on the concentration of calcium in their m ilk (Melton et al. 1990). We have given a single injection of this hor mone fragment into the sinus of the mammary gland of the goat a few da ys before the natural onset of calcium secretion, at a time when the t ight junctions of the secretory epithelium are 'loose' making it possi ble for injectate to spread into the extracellular fluid of the gland. Small volumes of fluid were then taken from experimental and control glands daily until 2 d post partum, and analysed in order to detect an y effect of this injection on the onset of calcium secretion.