THE ELIMINATION OF ADMINISTERED ZINC IN PANCREATIC JUICE, DUODENAL JUICE, AND BILE OF THE DOG AS MEASURED BY ITS RADIOACTIVE ISOTOPE (Zn)

The excretion of intravenously injected labeled zinc into pancreatic juice, duodenal juice, and bile of the dog was investigated. The use of the radioactive isotope permitted the injection of minute amounts of zinc, amounts that were negligible when compared with the total amount of zinc already present in the animal. 1. A large proportion of the injected Zn⁶⁵ was eliminated by way of the external secretion of the pancreas. As much as 11 per cent was excreted in the pancreatic juice in 14 days. 2. Very little of the administered radiozinc appeared in the bile. The maximum excretion observed for this pathway was 0.4 per cent in 8 days. 3. Radiozinc was also found in large amounts in the juice obtained from an isolated loop of the duodenum.

THE TURNOVER RATE OF PHOSPHOLIPIDS IN THE PLASMA OF THE DOG AS MEASURED WITH RADIOACTIVE PHOSPHORUS

1. A method for the determination of turnover time and turnover rate of plasma phospholipid is presented. 2. During the postabsorptive state 5.2 to 8.0 mg. of phospholipid phosphorus are turned over per hour in the plasma of dogs weighing 6–9 kilos. 3. The amount of phospholipid in an organ that is supplied by plasma phospholipid per hour is calculated.

Origin of milk cholesterol in the rat: dietary versus endogenous sources

Slices of mammary gland from lactating rats, incubated with acetate-1-(14)C or mevalonate-2-(14)C, synthesized cholesterol-(14)C. Within 2 min of an intravenous injection of 1 ml of a suspension of chylomicrons containing cholesterol-4-(14)C, mammary glands of lactating rats removed as much as 29% of the labeled cholesterol; those of 2-day-postlactating rats removed none. Rats were fed a diet containing 0.05% cholesterol-4-(14)C from 7 days prepartum to 20 days postpartum. At isotopic equilibrium, the relative specific activities of milk and dietary cholesterol indicated a dietary origin for 11% of the milk cholesterol. The extent to which endogenous sources-liver and mammary gland-contributed cholesterol to milk proved entirely dependent on whether dietary cholesterol, in the form of chyle lipo-proteins, was first processed by liver or taken up directly by mammary gland. Lack of information regarding the extent to which chyle cholesterol is removed from blood under physiological conditions by mammary gland and other tissues precludes precise assessment of the endogenous contributions to milk cholesterol and, moreover, casts doubt on the quantitative interpretability of cholesterol-(14)C-feeding experiments reported in the literature.