Parathyroid hormone-related peptide and milking-induced phosphaturia in dairy cows.

Five intact and four thyroparathyroidectomized (TPTX) lactating dairy cows were fitted with polyvinyl catheters implanted into the left carotid artery and the left mammary vein (for blood sampling) and into the right external jugular vein (for injections), and with a Foley catheter inserted into the urinary bladder. In these nine cows, daily morning and evening milking induced a prompt and transient rise in mammary venous blood plasma parathyroid hormone-related peptide (PTHrP)(1-34) concentrations, followed by an increase in both urinary phosphate concentration and phosphate renal clearance. Such a milking-induced phosphaturia was not observed when (tyr)34-bPTH(7-34)-NH2 (5 pmol/kg body wt) was infused during milking into the TPTX cows. These results indicate that PTHrP released systematically by the mammary gland during milking might be responsible for the observed phosphaturia.