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

The effect of exogenous α-ketoglutarate (αKG) 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 μM fluorescein (FL), a model OA, and epifluorescence microscopy. The effect of different transmembrane distributions of 10 μM αKG on peritubular FL uptake was measured at 37°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 αKG, preloading tubules with αKG (trans-configuration) or acute exposure to αKG (cis-configuration) increased FL uptake 62% and 54%, respectively, whereas a cis-transconfiguration of αKG increased FL uptake by 76%. The cis-stimulation of FL uptake by αKG 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 αKG. In the absence of exogenous αKG, 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 αKG that has left the cell in exchange for FL and that this activity supports ~25% of baseline activity of the OA/DC exchanger. With recycling of αKG accounting for ~25% of FL uptake and with accumulation of exogenous αKG accounting for another ~75% increase in FL uptake, Na-DC cotransport appears to directly support (25% + 75%)/175%, or ~57%, of total FL transport.