PAH transport by snake proximal renal tubules: differences from urate transport

Para aminohippurate (PAH) transport was studied in isolated, perfused snake (Thamnophis spp.) proximal renal tubules. PAH secretion from bath to lumen against a concentration gradient occurred primarily in distal proximal tubules. Rate of secretion was only one third as great in proximal proximal tubules and was not always against a concentration gradient. In distal proximal tubules, PAH tubular fluid to bath concentration ratio varied inversely with flow rate but averaged 6.0 at all flow rates with 2 x 10^-5 M PAH in bath. During secretion in this segment, cell PAH concentration was greater than that of tubular fluid or bath. This is compatible with active transport into cells on peritubular membrane and subsequent diffusion into lumen. Permeabilities of luminal and peritubular cell membranes were calculated on basis of this model. Passive efflux predicted from these permeabilities agreed well with measured value. Net secretion was not influenced significantly by perfusion rate. Urate (40 x 10^-5 M) in bath did not affect PAH secretion. This observation, together with differences in permeabilities, major transport sites, and saturation curves indicates that PAH and urate are secreted by separate mechanisms.