Transport of organic molecules by reptilian nephrons

This paper considers transport of glucose, amino acids, organic anions, urate, and organic cations by reptilian renal tubules. Glucose undergoes net reabsorption involving entry across the luminal membrane by a Na⁺-dependent, carrier-mediated process. However, entry may occur down an electrochemical gradient and exit at the basolateral membrane, against an electrochemical gradient. Amino acids may undergo both net reabsorption and net secretion. Taurine reabsorption involves entry across the luminal membrane against an electrochemical gradient via two independent electrogenic, Na⁺ and Cl^--dependent co-transport systems. Organic anions (e.g., PAH) undergo net secretion. This process involves entry into the cells at the basolateral membrane against an electrochemical gradient via a tertiary active process, the final step involving countertransport for α-ketoglutarate. Exit at the luminal membrane is down an electrochemical gradient, probably via anion exchange or carrier-mediated diffusion. Urate undergoes net secretion. This process involves transport into the cells against an electrochemical gradient via an unknown, Na⁺-independent, K⁺-sensitive, mechanism that may involve anion exchange and exit across the luminal membrane down an electrochemical gradient via what appears to be simple diffusion. Some organic cations, such as tetraethylammonium (TEA), undergo net secretion and some, such as N¹-methylnicotinamide (NMN), undergo net reabsorption. Net TEA secretion involves entry across the basolateral membrane, down an electrochemical gradient via carrier-mediated diffusion or organic anion exchange, and exit at the luminal membrane against an electrochemical gradient via countertransport for H⁺. Net NMN reabsorption involves entry across the luminal membrane, down an electrochemical gradient via Na⁺-dependent, carrier-mediated diffusion, with exit across the basolateral membrane against an electrochemical gradient via an unknown process that probably involves countertransport for an organic cation.