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

The possibility that p-aminohippurate (PAH)-α-ketoglutarate (α-KG) countertransport could stimulate basolateral uptake and net transepithelial secretion of PAH was examined in intact isolated nonperfused and perfused snake (Thamnophis spp.) distal proximal 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 fivefold without increasing cellular ATP content. During stimulation of net secretion, intracellular [¹⁴C]PAH concentration increased to same extent as net secretion. Presence of Li⁺ (2 mM) or absence of Na⁺ (inhibitors of Na⁺-dicarboxylate cotransport) in bath during α-KG preloading eliminated stimulation of PAH transport. Addition of unlabeled α-KG to bathing medium stimulated efflux of [¹⁴C]PAH across basolateral membrane of tubules with oil-filled lumina, further supporting the concept of PAH-α-KG countertransport. Preloading with succinate (100 μM) also stimulated [¹⁴C]PAH uptake by nonperfused tubules and net secretion by perfused tubules, but stimulation was only 1.5- to 2-fold and cellular ATP content increased. Moreover, preloading with methyl succinate, a slowly metabolized derivative of succinate, did not stimulate PAH uptake by nonperfused tubules or secretion by perfused tubules. Thus succinate appears to stimulate PAH transport via metabolism, possibly by conversion to α-KG, not by direct countertransport. This study indicates for the first time in intact renal tubules that PAH-α-KG countertransport can stimulate net PAH secretion by generating an increased intracellular PAH concentration.