Maintenance of secretion of calcium and magnesium during infusions of parathyroid hormone to lactating goats.

Hourly secretion of calcium and magnesium by goat mammary glands was studied following infusions of bovine parathyroid hormone (PTH) into the mammary artery in amounts (6-15 ng/kg per min) giving rise to levels of PTH in mammary venous blood commonly observed during spontaneous hypocalcaemia (2.4-13 micrograms/l). In two series of experiments a 3 h infusion of PTH was given. The first series was carried out at a normal level of plasma calcium, while the availability of calcium was increased by a 20 h infusion of CaCl2 in the other series. No changes in mammary secretion of calcium or magnesium could be observed during the hormone infusions. A decrease in the urinary excretion of calcium occurred, thus demonstrating the activity of the infused PTH. In a third experiment an 18.5 h infusion of PTH (6 ng/kg per min) into the mammary artery was given to study possible effects of a more prolonged increase of PTH concentrations. This infusion caused a gradual increase in plasma levels of calcium. Average concentrations of calcium in milk increased slightly (about 5%) in both glands (P less than 0.05). The hourly yield of milk was not significantly changed in the infused gland after 18.5 h of PTH infusion. A small but significant increase occurred in the non-infused gland. Yields of magnesium were not influenced by the infusion of PTH. It was concluded that losses of calcium into milk are independent of the PTH concentration in the circulation.