Proximal tubular transport accounts for about 70% of the reabsorptive work of the kidney (35, 42). The tubular cells use a variety of metabolic substrates to provide the energy needed for these transport processes (15), The actual substrates that are metabolized depend on the substrate mix present in the extracellular environment, on hormonal influences, and on the metabolic state (eg acid-base) of the animal.

Substrates may enter the proximal tubular cells either through the luminal side, from the glomerular filtrate, or through the basolateral side, from the blood. The proximal tubules reabsorb most of the metabolic substrates from the filtrate, and these are, therefore, potentially available to be metabolized for energy (I5). Transporters for glucose, lactate, Krebs cycle intermediates, and amino acids have been identified in brush border membranes (72). Some evidence indicates that basolateral transporters also exist for some of these (34, 56). Fatty acids,. which are mostly bound to albumin, are not filtered and therefore may enter through the basolateral side in vivo (15). Substrates entering the proximal tubular cells face three possibilities:(a) transepithelial transport;(b) oxidation to CO2; and (c) interconversion to another substrate.