Interrelationships between Insulin and Lipid Metabolism in Normal and Alloxan-Diabetic Cattle

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Abstract
Three experiments were performed to elucidate the role of insulin in regulating adipose tissue lipogenesis and lipolysis in the bovine. First, 4-day fasted steers were injected with 110 mg/kg alloxan on the day of refeeding; half the group received sufficient insulin to maintain near-normal blood glucose levels. Biopsy samples of subcutaneous adipose tissue were obtained throughout this and subsequent experiments. Insulin therapy increased the incorporation of 14C-labeled substrates into fatty acids, but was not necessary to increase lipogenic enzyme activities to control values. Steers in the second experiment were not fasted prior to alloxan injection (60 mg/kg). Seven days after alloxan treatment, the animals were injected with insulin, while a control group was pair-fed to the level of intake observed for the alloxan-treated steers during the first 7-day period. Whereas alloxan treatment decreased, and insulin therapy increased substrate incorporation and lipogenic enzyme activities, these effects were more closely correlated with levels of feed intake than with insulin levels. Alloxan treatment increased the basal but not the stimulated lipolytic rate in adipose tissue, and insulin treatment depressed the increased rate of lipolysis, as reflected in the plasma levels of free fatty acids and triglycerides. In the third experiment, one group of steers was injected with 1–6 U/kg of insulin, while a second group received insulin injections plus glucose infusions (125.3 mmol/hour). Increases in substrate incorporation and the activity of acetyl CoA carboxylase were observed in the insulin plus glucose-infused steers, but not in those animals receiving insulin alone. There were no effects of insulin or glucose plus insulin treatment on basal or stimulated lipolytic rates. The results of this study suggest that circulating insulin levels do not influence in vivo rates of lipogenesis in the bovine animal to the degree observed in the laboratory rat.