Hepatic and extrahepatic lactate metabolism in sheep: effects of lactate loading and pH

Hepatic lactate metabolism was studied in five sheep receiving infusions of either lactic acid or sodium lactate using an arteriovenous difference technique. Hepatic uptake of lactate was a saturable process with second order (Michaelis-Menten) kinetics. Although lactic acid infusion decreased blood pH, hepatic saturation of lactate uptake occurred before changes in pH could influence hepatic lactate metabolism. The Vmax for hepatic lactate metabolism is 5.72 mmol X kg-0.75 X h-1 and the Km 3.06 mmol/l. These findings have therapeutic relevance. In acidosis, the hepatic response to therapeutic administration of sodium lactate during fluid therapy will be variable. If plasma lactate is low, lactated fluids may increase hepatic uptake and produce an alkalizing effect. If plasma lactate is already high, hepatic lactate metabolism is nearly saturated, and additional lactate will not produce a hepatic alkalizing response. Extrahepatic tissues switch from lactate production to lactate utilization under conditions of lactate loading. They remove more lactate than liver at high plasma lactate concentrations. Muscle may be important in lactate removal in certain types of exercise.