Metabolic Clearance Rate, Production Rate, and Mammary Uptake and Metabolism of Progesterone in Cows

Tracer kinetic techniques have been used to measure the production rate, metabolic clearance rate and mammary uptake of progesterone in six experiments on two Jersey cowsmthe cows were surgically prepared so that the carotid artery, jugular vein and mammary vein concentrations of progesterone, and udder blood flow, could be determined in conscious animals without anaesthesia or stress. The mean production rate of progesterone was 173 +/- 23-3 (S.ET) mug/min, with values ranging from 80 to 276 mug/min in pregnancy. The metabolic clearance rate was 22-5 +/- 2-0 1/min, or 0-21 +/- 0-025 1/min/kg metabolic body weight. The mammary uptake of progesterone was low, 3-1 +/- mug/min, and udder uptake accounted for about 3% of progesterone production rate. During [3H]A1progesterone infusion, radioactivity was transferred from blood to milk, probably by diffusion down a concentration gradient. Progesterone accounted for more than 88% of the ether-soluble radioactivity recovered from milk.