Kinetics of sodium-dependent solute transport by rabbit renal and jejunal brush-border vesicles using a fluorescent dye

The kinetics of Na-coupled solute transport by renal and jejunal brush-border vesicles in the rabbit were examined using the potential-sensitive fluorescent dye diS-C3-(5). All organic solutes known to be transported across these membranes by Na-coupled mechanisms increase the fluorescence of the dye in the presence of Na, but not K. An increase in fluorescence (delta F) corresponds to a depolarization of the electrical potential difference (5-60 mV) across the brush-border membrane in the intact cell. delta F was independent of the valency of the transported solute. The fluorescence response was saturable, and for twelve solutes the Kf, i.e. the concentration of the substrate generating 50% of the maximal response, agreed quite closely with the Kt values reported from tracer studies. For six solutes increasing the Na concentration decreased Kf, and this agrees with the effect of Na on the kinetics of succinate transport in renal vesicles. We conclude that D-glucose, neutral amino acids and imino acids are co-transported with Na across both renal and jejunal brush-border membranes, and that carboxylic acids, beta-amino acids, and dibasic amino acids are co-transported with Na across the renal, but not jejunal, membranes.