Specific 14C Research Articles

Glucose as a precursor of milk constituents in the intact dairy cow

Abstract 1. 1. Glucose uniformly labeled with 14C was injected into the jugular veins of two normal lactating dairy cows as single doses of 3 and 6 microcuries per kg body weight respectively. The injected glucose, about 1 gram, amounted to less than 10% of the plasma glucose. 2. 2. Between 30 and 40 minutes after injection the radioactivity of the respiratory CO2 reached a maximum of 9 microcuries per unit of the relative injected dose (μc injected per kg body weight). 3. 3. During the first 3 hours after injection of uniformly labeled glucose less 14C appeared in the respiratory CO2 than was expired during the corresponding period in earlier trials after injection of 14C labeled acetate, propionate, or butyrate. 4. 4. More than 50% of the 14C injected as uniformly labeled glucose appeared in the organic constituents of the milk within 10 hours after injection. Over 80% of this 14C in milk components was found in lactose. 5. 5. In the first milk sample, 3 hours after injection, the specific 14C activity in the components of the milk decreased in the following order: Lactose → Citrate → Casein → Milk fat. 6. 6. Only about 1% of the carbon transferred from plasma glucose to lactose passed the carbonate pool, 4 to 10% of the carbon transfer to casein followed this path. 7. 7. Three hours after injection the specific 14C activity in blood glucose had decreased to about 1% of its theoretical level at the time of injection, the latter calculated from injected dose, plasma volume and glucose level in plasma. 8. 8. About 4 5 of the lactose carbon came from carbon in plasma glucose or a pool in rapid exchange with plasma glucose, 1 5 of the carbon in citrate originated from this pool and about 5% of the carbon in casein and milk fat.