PAH/α-KG countertransport stimulates PAH uptake and net secretion in isolated rabbit renal tubules

Possible stimulation of both basolateral uptake and net transepithelial secretion of p-aminohippurate (PAH) by PAH/α-ketoglutarate (α-KG) countertransport was examined in intact perfused and nonperfused rabbit proximal S2 renal tubules. Preloading tubules with α-KG (100 μM) for 30 min increased [¹⁴C]-PAH rate of uptake by nonperfused tubules and rate of net secretion by perfused tubules approximately three- to sixfold. During stimulation of net secretion, intracellular [¹⁴C]PAH concentration increased to about the same extent as net secretion. Presence of Li⁺ (2 mM) or absence of Na⁺ (inhibitors of Na⁺-dicarboxylate cotransport) in bathing medium during α-KG preloading eliminated stimulation of PAH transport. Addition of unlabeled α-KG (1 mM) to bathing medium stimulated efflux of [¹⁴C]PAH across the basolateral membrane of tubules with oil-filled lumina, further supporting the concept of PAH/ α-KG countertransport. Preloading with succinate (100 μM) also stimulated rates of [¹⁴C]PAH uptake by nonperfused tubules and net secretion by perfused tubules, but stimulation was only ∼1.5-fold. Moreover, preloading with methyl succinate, a slowly metabolized derivative of succinate, did not stimulate [¹⁴C]PAH uptake by nonperfused tubules or net secretion by perfused tubules. Thus it seems most likely that succinate preloading stimulates PAH transport via metabolism, possibly by conversion to α-KG, not by direct countertransport for PAH. This study indicates for the first time in intact mammalian proximal S2 renal tubules that PAH/α-KG countertransport can stimulate net PAH secretion by generating an increased intracellular PAH concentration.