Increased mammary blood flow in the lactating goat induced by parathyroid hormone-related protein.

Human synthetic parathyroid hormone-related protein (PTHrP) increased mammary blood flow (MBF) following close-arterial infusion via the external pudic artery in goats during mid-lactation. MBF increased 74 +/- 8% within 30 min of the start of continuous infusion of PTHrP compared with 10 +/- 3% in controls. MBF decreased by 90 min, however, and was not different from control values for the remainder of the infusion. The increase in plasma concentrations of calcium and decrease in phosphate during PTHrP suggests that this was not due to altered activity of PTHrP, but may relate to downregulation of response or production of counter-regulatory vasoconstrictive agents within the gland. This problem was alleviated when PTHrP was infused in a pulsatile fashion. An average 14-40% increase in MBF was achieved over 6 h, but this did not alter the rate of milk secretion, suggesting that mammary hyperaemia is not sufficient by itself to increase milk yield in the normally lactating goat. MBF increased in a dose-dependent fashion, although the lowest dose used to give a detectable response was approximately 40-fold higher than the concentration normally present in the mammary venous circulation. Thus, endogenous PTHrP may not be an important regulator of MBF during lactation in the goat.