Lactate metabolism in resting and exercising dogs

The effect of treadmill run on the turnover rates of glucose ([2 ³H]glucose) and lactate ([U ¹⁴C]lactate), on the rates of oxidation (R(ox)) of lactate, and its conversion to glucose (L→G) were measured with the primed constant infusion technique. Comparable lactate turnover rates were obtained at rest by infusing epinephrine, or Na L(+) lactate with or without norepinephrine. With increasing lactate levels (L) the rate of disappearance (R(dL)), R(ox), and L→G increase in a linear manner. At the same lactate level, R(dL), R(ox), and L→G are significantly higher in the running dog. Exercise increased the metabolic clearance rate of lactate threefold. At rest R(ox) and L→G represented about 50% and 18-19% of R(dL), respectively. The corresponding values in the running dogs were 55% and 25%, respectively. At rest about 9% of the hepatic glucose output arose from lactate while during exercise this varied from 7 to 26% depending on R(dL). It is concluded that a) the working muscle produces and utilizes lactate at the same time, and b) 'in vivo' the major factor which controls both R(ox) and gluconeogenesis is the substrate supply.