Na⁺-dependent transport of tricarboxylic acid cycle intermediates by renal brush border membranes. Effects on fluorescence of a potential-sensitive cyanine dye

The effect of the transport of tricarboxylic acid cycle intermediates on the membrane potential of renal brush border vesicles was studied using fluorescence of the cyanine dye, 3,3′-dipropylthiadicarbocyanine iodide. The behavior of the dye in the preparation was established with valinomycin-induced K⁺-diffusion potentials; increases in fluorescence were associated with depolarizing conditions. Addition of 1 mM succinate or citrate to membrane/dye suspensions produced transient increases in fluorescence, indicative of a depolarizing event(s) associated with the transport of these substrates. The transient response in fluorescence was Na⁺ dependent, of greater magnitude under Na⁺-gradient as compared to Na⁺-equilibrium conditions, and was a saturable function of substrate concentration. The specificity of the fluorescence response was identical to that obtained from studies of the competitive inhibition of succinate transport by tricarboxylic acid cycle intermediates and analogs. We conclude that the major tricarboxylic acid cycle intermediates are transported via a common Na⁺-dependent transport system in renal brush border membranes.