Significance of Plasma Ultrafiltrable Ca45 and P32 in Milk Synthesis

Summary Three Jersey cows were injected intravenously with Ca 45 and P 32 simultaneously, and the ultrafiltrable as well as total Ca 45 and P 32 in milk and plasma were determined periodically to study the importance of these fractions in the formation of milk calcium and phosphorus. While most of the plasma P 32 (85%) was still ultrafiltrable 1 hr after dosing, the activity appeared in both fractions of milk, indicating that plasma ultrafiltrable phosphorus is the precursor of milk phosphorus. After the first 5 hr post-dosing, the specific activity of plasma nonultrafiltrable P 32 began to rise, whereas that of ultrafiltrable P 32 continued to decline, and 43 hr were required for the ratio of plasma ultrafiltrable P 32 /total P 32 to reach a value (0.40) close to that of stable phosphorus (0.34). One hour after dosing, 66% of the plasma total Ca 45 was nonultrafiltrable, presumably protein-bound. Partitioning of Ca 45 in milk took place also within 1 hr after dosing and the maximum concentration of milk ultrafiltrable Ca 45 coincided in time (3–5 hr) with that of nonultrafiltrable Ca 45 , suggesting that plasma ultrafiltrable calcium is the precursor of milk Ca. Milk ultrafiltrable/total Ca 45 and P 32 fell to values close to those of stable Ca and P between the third and fifth hours after dosing, which is the time required for the complete conversion of plasma Ca and P into all fractions of milk Ca and P.