EFFECTS OF FASTING AND REFEEDING AND INTRAVENOUS GLUCOSE-INFUSION ON INVITRO LIPOGENESIS IN BOVINE ADIPOSE-TISSUE

Experiments were conducted with growing-finishing steers to determine: 1) the effects of fasting and refeeding on the rates of acetate and lactate utilization in vitro for fatty acid synthesis in bovine adipose tissue, 2) the effect of intravenous D-glucose infusion during refeeding on acetate, lactate and glucose incorporation into fatty acids in vitro and 3) the effects of fasting on the quantities of lactate dehydrogenase (LDH) isoenzymes in bovine adipose tissue. Fasting caused a decline in fatty acid synthesis from acetate, lactate and glucose of 91.7, 79.1 and 84.0%, respectively. By days 10 to 17 of refeeding, the rates of fatty acid synthesis from acetate, lactate and glucose exceeding the respective prefasting rates of fatty acid synthesis. Intravenous infusion of glucose (500 g/day) tended to increase the rate of fatty acid synthesis from all three substrates at day 3 of refeeding but not at days 10, 17 and 24 of refeeding. Glucose metabolism in vitro to CO2 was not altered significantly by glucose infusion or the presence or absence of insulin in the incubation media. Feeding regime did not affect the LDH isoenzyme profile or total activity. Heart-type LDH was the predominant isoenzyme (95 ± 2 and 78 ± 2% of total LDH activity as determined by kinetic analysis or by electrophoresis, respectively). Total LDH activity was not altered significantly by fasting. The results of these experiments show that ruminant adipose tissue can use lactate as a substrate for in vitro fatty acid synthesis; this process responds to fasting analogous to the response observed for acetate incorporation into fatty acids. Providing glucose intravenously during refeeding did not stimulate in vitro fatty acid synthesis above the stimulation observed by refeeding alone.