Pathways for carboxylic acid transport by rabbit renal brush border membrane vesicles

Brush border membrane vesicles were purified from rabbit renal cortex using a calcium-precipitation procedure, and the uptake of carboxylic acids was determined by a rapid-filtration method. L-Lactate, pyruvate (monocarboxylic acids), and succinate (dicarboxylic acid) demonstrate features of Na⁺ cotransport: enhanced initial rate (1 s) of uptake with an inward Na⁺ gradient compared with the Na⁺-free control and transient accumulation of substrate within the vesicles. Kinetic parameters derived for L-lactate and succinate show that each substrate is transported via a single pathway and that the two substrates exhibit marginal cross-inhibition. A range of monocarboxylic acids including pyruvate and ketone bodies appear to interact with the monocarboxylic acid carrier. The kinetic of Na⁺-dependent pyruvate uptake suggest at least two transport pathways - namely, that this monocarboxylic shares both the mono- and dicarboxylic acid carriers. We conclude that isolated rabbit renal microvillus membranes possess independent transport systems for mono- and polycarboxylic acids.