Lactate Uptake by Toadfish Hepatocytes: Passive Diffusion Is Sufficient

Mechanisms of hepatic lactate uptake were studied in the gulf toadfish, Opsanus beta by following the accumulation of [C]lactate by isolated hepatocytes in vitro. Uptake rates were linear for up to 5 s at external lactate concentrations of 1 and 20mmoll~'. 5-s uptake rates for several concentrations in the physiological range (i.e. 0-2-20mmoll") yielded a linear double reciprocal plot (l/v vs l/[lactate]) that passed through the origin. Lactate uptake was not inhibited by known inhibitors of carrier-mediated lactate transport (e.g. 5 mmolP a'-cyano-4-hydroxycinnamate; lOmmoll" pyruvate), and was not stereospecific for L-( + )-lactate. These observations indicate that lactate uptake is by passive diffusion in toadfish hepatocytes. Lactate uptake by toadfish hepatocytes further differed from lactate uptake by mammalian tissues in that rates were not altered by changes in either pH,. (65-8-0) or [Na]c (3-151 mmol I" ). Rates of lactate conversion to glucose and CO2 were measured and compared to uptake rates, and it appears that rates of lactate metabolism are not limited by passive diffusion. These results are discussed in relation to rates of lactate uptake and metabolism in mammalian hepatocytes.