Fig. 3. Model of net transepithelial urate transport in reptilian (snake) proximal tubules. Circle with solid arrow indicates either primary or secondary active transport. Broken arrows indicate transport down electrochemical gradient. For countertransport, solid arrow indicates movement against electrochemical gradient and broken arrow, movement down electrochemical gradient. Broken arrows with question marks indicate possible passive movements. A^- indicates anion of unspecified nature. Apparent permeabilities of luminal P_L and peritubular membranes (P_P) are shown (Dantzler, 1996).