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Effect of Fasting on the Oxidation of C¹⁴-Labeled Glucose, Palmitate and Valine in Growing Pigs¹

Canada Department of Agriculture, Nappan, Nova Scotia

Abstract

The rate of oxidation of C¹⁴-labeled glucose, palmitate and valine injected into pigs at 1, 6, 12 and 24 hr. after feeding was determined by placing the pigs in a respiration chamber and measuring the C¹⁴ activity of expired CO₂. The peak rate of expired C¹⁴O₂ occurred earliest with C¹⁴-palmitate (15 min.) and latest with C¹⁴-glucose (45 to 105 min.). The time at which the peak oxidation of C¹⁴-glucose occurred was delayed up to 1 hr. with longer periods of fasting. The length of fasting interval before injections had little effect on the total C¹⁴O₂ expired over the following 24 hr. period when C¹⁴-glucose (29–43% of the dose recovered) or C¹⁴-valine (11–13%) were given, but six times as much C¹⁴-palmitate (36%) was oxidized when injected after a 24-hr, fast than after a 1-hr. fast. Once a peak in C¹⁴O₂ activity had been attained, the rate of decline was found to be exponential for a period of 3 to 4 hr. with a half-life of 1 to 3 hr. When C¹⁴-glucose was given orally with the feed, the peak in C¹⁴O₂ activity was lower and broader than when the tracer was injected 1–24 hr. after feeding. There was very little C¹⁴O₂ expired by pigs given oral C¹⁴-palmitate and no peak in C¹⁴O₂ was detected during the next 24 hours. Peak oxidation of oral C¹⁴ valine occurred 1 hr. later than with injections but 40% more of the dose was expired as C¹⁴O₂ in the 24 hr. following administration. Tissue analysis 24 hr. after the pigs received the tracers orally indicated that most of the unoxidized C¹⁴ of C¹⁴-palmitate could be accounted for in carcass fat with very little found in muscle protein precipitated with trichloroacetic acid. The C¹⁴-activity of muscle protein was highest when C¹⁴-valine was injected but much of the unoxidized C¹⁴-valine could not be accounted for in the fat and protein of the carcass, and it appeared that most of it was still present in the trichloroacetic acid-soluble portion of the tissues.

¹ Contribution No. 210, Division of Animal Science, Research Branch, Canada Department of Agriculture, Nappan, Nova Scotia

² The author gratefully acknowledges the technical assistance of R. T. Ripley, J. G. Allen, H. A. Harrison and J. D. Nelson.