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Partition of Orally Administered Radioactive Phosphorus in the Blood and Milk of the Dairy Cow¹

Florida Agricultural Experiment Station, Gainesville

ABSTRACT

When phosphorus isotope P₃₂ was given orally to a cow in mid-lactation, the blood showed a marked activity after 59 min., mainly due to the activity of blood plasma acid-soluble phosphorus fraction. Later, two activity maximums were noted in both whole blood and blood plasma; the first appeared about 5 to 6 hr. after the beginning of the test and the second one about 30 hr. later. During the first of these periods, only the acid-soluble phosphorus fraction in plasma was labeled. The blood plasma phospholipid phosphorus fraction did not show any activity until several hours later. The increase in the specific activity of phospholipid phosphorus fraction also was much slower than in the plasma acid-soluble phosphorus fraction. The comparison of the specific activity of phosphorus in different blood and milk fractions at different periods following administration of P₃₂ shows clearly that both the acid-soluble phosphorus in the milk serum and the casein phosphorus originate from the blood plasma acid-soluble phosphorus fraction and not from the phospholipid phosphorus fraction. On the basis of the proportionally high activity of both the casein phosphorus and the acid-soluble phosphorus in milk serum, it is considered that possibly only one fraction of the phosphates usually determined as blood plasma inorganic phosphates serves as the main precursor of the phosphorus in milk. There was some evidence to indicate that blood plasma phospholipids also may be removed from the blood by the mammary gland.

¹ Published with the permission of the Director of the Florida Agricultural Experiment Station.

² Permanent address: University of Helsinki, Finland.

³ Permanent address: UT-AEC Agricultural Research Program, Oak Ridge.