Peripheral Circulatory and Metabolic Consequences of Thyrotoxicosis II. Uptake of free fatty acids by the gracilis muscle in normal and thyrotoxic dogs

AbstractFrey, H. M. M. Peripheral Circulatory and Metabolic Consequences of Thyrotoxicosis II: Uptake of free fatty acids by the gracilis muscle in normal and thyrotoxic dogs. Scand. J. clin. Lab. Invest. 19 15-28, 1967. The transport of free fatty acids (FFA) in normal and thyrotoxic gracilis muscle has been investigated in the anesthetized dog.Arterial FFA level was not increased in the thyrotoxic animals, but FFA turnover was higher. Intravenous glucose infusion reduced arterial FFA level and turnover to the same extent in both groups.Uptake of FFA by the gracilis muscle was greatly elevated in the thyrotoxic group. The main determinants of muscle uptake were arterial FFA level and muscle blood flow. Muscle oxygen consumption was not related to FFA uptake.The amount of FFA taken up by normal resting muscle was sufficiently large to cover the entire oxygen consumption alone, provided oxidation was immediate. In resting thyrotoxic muscle FFA uptake exceeded oxygen consumption by 100 per cent.In working muscle, both normal and thyrotoxic, uptake of FFA fell considerably short of covering the oxygen consumption, and this relationship became increasingly evident with greater work loads. Glucose infusion improved mechanical work efficiency equally in both groups.The importance of endogenous lipid stores as sources of muscle energy production is discussed.