Significance of Plasma Ultrafiltrable Ca45 and P32 in Milk Synthesis

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Abstract
Three Jersey cows were injected intravenously with Ca45 and P32 simultaneously, and the ultrafiltrable as well as total Ca45 and P32 in milk and plasma were determined periodically to study the importance of these fractions in the formation of milk Ca and P. While most of the plasma P32 (85%) was still ultrafiltrable 1 hr after dosing, the activity appeared in both fractions of milk. After the first 5 hr post-dosing, the specific activity of plasma nonultrafiltrable P32 began to rise; and that of ultrafiltrable P32 continued to decline. After 43 hr, the ratio of plasma ultrafiltrable p32/total P32 reached a value (0.40)-close to that of stable P (0.34). The ratios of UF/total P32 and Ca45 in milk were high for the 1st 3 hr after dosing and fell to values close to those of stable P (0.35 vs. 0.35) and Ca (0.22 vs. 0.24) in 5 hr after dosing. These results which indicate the complete conversion of plasma, Ca and P to those of milk require from 3 to 5 hr. One hr after dosing, 66% of the plasma total Ca45 was nonultrafiltrable (presumably protein-bound). Partitioning of Ca45 in milk took place within 1 hr after dosing.