Metabolic clearance of rat prolactin in the lactating and non-lactating rat

The metabolic clearance of rat prolactin, assessed by the continuous infusion to equilibrium method, increased from 0-77 to 2-15 ml/min in lactating and from 0-65 to 1-75 ml/min in non-lactating rats as the infusion rate was increased from 100 to 472 ng/min. The clearance rate of rat prolactin did not increase when large doses of ovine prolactin were infused simultaneously with rat prolactin. The clearance rates computed from equilibrium levels of immunoreactive 131I-labelled rat prolactin in plasma were highly correlated with, but were considerably greater than, those computed from unlabelled rat prolactin which was infused simultaneously. In lactating and non-lactating rats, the clearance of each prolactin tested (RP-1, B-1, Nicoll's secreted and 131I-labelled) increased and had stabilized within the 30-35 min of infusion of each new higher dose of the hormone. The ability of the rat to clear quickly greater amounts of prolactin from the circulation as the infusion rate increases implies that plasma prolactin concentrations do not necessarily mirror the rate of prolactin secretion from the pituitary.