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Metabolism of Uniformly Labeled Glucose-C¹⁴ Introduced into the Rumen of a Lactating Cow. I. Transfer of C¹⁴ to Respired Air, Volatile Fatty Acids, and Major Milk Constituents^{1, 2,}

School of Veterinary Medicine and The College of Agriculture, University of California, Davis

ABSTRACT

To study the metabolic fate of dietary glucose in the lactating cow, uniformly C¹⁴-labeled glucose (G-U-C¹⁴) was introduced through a fistula into the rumen. Continuous samples of respired CO₂ were collected and assayed for carbon-14; in addition, samples of rumen and duodenal contents, jugular and ruminal vein blood, and milk were collected at frequent intervals and assayed for Carbon-14.

Following the administration of G-U-C¹⁴, the specific activity reached a maximum in respired CO₂ at 45 min and in blood volatile fatty acids (VFA) at 15 min, but the maximum was not reached in plasma glucose until 2 hr had elapsed.

The first sample of rumen VFA, collected 1 hr after G-U-C¹⁴ administration, had the highest specific activity and later samples decreased in specific activity by approximately one-half each 4–5 hr. Carbon-14 was not detected in rumen VFA after 24 hr.

The specific activity of milk constituents after rumen administration of G-U-C¹⁴ was different from that observed after intravenous injection of G-U-C¹⁴, but was similar to results obtained after intravenous injection of C¹⁴-labeled short-chain fatty acids.

These results indicate that glucose was rapidly converted to short-chain fatty acids in the rumen before its absorption and utilization in the tissues of the cow for synthesis of milk components and oxidation to carbon dioxide.

This study presents new information on the difference in metabolic fate of dietary glucose compared to intravenously administered glucose in the ruminant and shows the rapid transfer of carbon from dietary glucose to blood VFA, which had not been previously demonstrated.

¹ This investigation was supported, in part, by a research grant RG-8183 from the Division of General Medical Science, P.H.S.

² Approved by the Director of the Hawaii Agricultural Experiment Station as Technical Paper No. 607.

³ On leave from the University of Hawaii, Honolulu, during 1961–1962 academic year in the Department of Physiological Sciences, School of Veterinary Medicine. Held a Post-Doctoral Traineeship under PHS Training Grant 2G-646-PYTC from the Division of General Medical Sciences.

⁴ Supported by PHS Post-Doctoral Fellowship, AF-11, 212 from the National Institute of Arthritis and Metabolic Diseases.

⁵ Fellow of the Kellogg Foundation.