Effect of alpha-ketoglutarate on organic anion transport in single rabbit renal proximal tubules.

The effect of exogenous alpha-ketoglutarate (alpha KG) and the peritubular Na(+)-dicarboxylate (Na-DC) cotransporter on organic anion/dicarboxylate (OA/DC) exchange in S2 segments of single, nonperfused rabbit proximal tubules was measured using 1 microM fluorescein (FL), a model OA, and epifluorescence microscopy. The effect of different transmembrane distributions of 10 microM alpha KG on peritubular FL uptake was measured at 37 degrees C using bicarbonate-buffered, nutrient-containing buffers, which are conditions similar to those found in vivo. Compared with FL uptake in the absence of exogenous alpha KG, preloading tubules with alpha KG (trans-configuration) or acute exposure to alpha KG (cis-configuration) increased FL uptake 62% and 54%, respectively, whereas a cis-trans-configuration of alpha KG increased FL uptake by 76%. The cis-stimulation of FL uptake by alpha KG was rapid, within 5-7 s. This stimulation was blocked 96% by simultaneous exposure to 2 mM Li+, indicating that stimulation of transport was secondary to the uptake of exogenous alpha KG. In the absence of exogenous alpha KG, selective inhibition of Na-DC cotransport using 2 mM Li+ or 1 mM methylsuccinate decreased FL uptake by 25% (effects that were reversible but not additive), suggesting that the Na-DC cotransporter recycles endogenous alpha KG that has left the cell in exchange for FL and that this activity supports approximately 25% of baseline activity of the OA/DC exchanger. With recycling of alpha KG accounting for approximately 25% of FL uptake and with accumulation of exogenous alpha KG accounting for another approximately 75% increase in FL uptake, Na-DC cotransport appears to directly support (25% + 75%)/175%, or approximately 57%, of total FL transport.