To determine the quantitative roles of the basolateral and luminal Na+- dicarboxylate (Na-DC) cotransporters in establishing and maintaining the α- ketoglutarate (αKG) gradient required for renal tubular secretion of organic anions, we measured net steady-state transepithelial secretion of fluorescein (FL) in real time in isolated, perfused S2 segments of rabbit renal proximal tubules. Net 'basal' FL secretion in the absence of exogenous αKG had a K(t) of ~4 μM and a maximal transepithelial secretion rate (J(max)) of ~380 fmol · min-1 · mm-1 (where K(t) is the FL concentration that produces one-half the J(max)). It could be almost completely inhibited by basolateral p-aminohippurate (PAH). Selective inhibition of the basolateral Na-DC cotransporter indicated that recycling via this transporter of αKG that had been exchanged for FL supports ~25% of the 'basal' FL secretion. Physiological αKG concentrations of 10 μM in the bath or 50 μM in the perfusate stimulated net secretion of FL by ~30 or ~20%, respectively. These data indicate that the basolateral Na-DC cotransporter supports ~42% of the net FL secretion. The luminal and basolateral effects of physiological concentrations of αKG were additive, indicating that the combined function of the luminal and basolateral Na-DC cotransporters can support ~50% of the net FL secretion. This apparently occurs by their establishing and maintaining ~50% of the outwardly directed αKG gradient that is responsible for driving basolateral FL/αKG exchange. The remaining ~50% would be maintained by metabolic production of αKG in the cells.
- Sodium-dicarboxylate cotransporters
- Transepithelial transport in real time
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