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Turnover of Plasma Palmitate in Fed and Fasted Lactating Cows¹

Department of Physiological Sciences, School of Veterinary Medicine, University of California, Davis

ABSTRACT

Palmitate-9,10-T was injected intravenously as a single dose into three normal, fed, lactating cows and later into the same cows after they had been carried through a four-day fast. The tritium concentration was measured at frequent intervals in plasma nonesterified fatty acids (NEFA) and in body water. The time course of NEFA specific activity followed a log linear rate during the first 5 to 6 min, then followed a more complex pattern. The intercept on the Y axis slope of the initial change in specific activity with time provided the basis for calculating several kinetic parameters for the NEFA pool. The first component had a space somewhat larger than the plasma volume and a turnover time of 1.9 min in fed cows, showing that this component is very active metabolically. The turnover time of plasma NEFA was increased to 2.9 min in fasted cows. Fasting caused a five- to sixfold increase in the concentration of plasma NEFA and a five- to ninefold increase in the pool size. The flux or exchange rate of the NEFA pool increased from 9.2 to 38.5 µEq/kg body weight per minute when the lactating cows were deprived of food for four days.

Tritium from the palmitate-9,10-T appeared very rapidly in the plasma water, being present in significant amounts within 1 min after intravenous injection. Calculations indicated that within 6 to 10 min after injection of the labeled palmitate, the total body water contained from 9–24% of the tritium as T₂O. Subsequently, the concentration of tritium in body water did not change greatly during several days. The standardized specific activity of the plasma water was greater in fasted cows, reflecting greater palmitate oxidation which supports the conclusion that there was a more rapid utilization of NEFA palmitate due to the energy stress of fasting. Thus, between 9 and 24% of the plasma palmitate underwent rapid oxidation. The remaining tritiated palmitate was oxidized more slowly, resulting in the appearance of tritium in body water at about the same rate it was being lost due to water turnover in the cows.

¹ Supported in part by a grant from the National Institutes of Health GM-08183.

² On sabbatical leave from Purdue University. NIH Post-doctoral trainee, G.M. 646. Training in Animal Physiology.

³ Present address: Department of Biochemistry, Queen's University, Kingston, Ontario, Canada. Post-doctoral Fellow of the National Institutes of Arthritis and Metabolic Diseases (F-F2-AM-24, 702-02).