/HCO 3 Ϫ exchange at the brushborder membrane, tubular carbonic anhydrase (CA) activity, CAII protein, and a proportion of tubular SO 4 2Ϫ secretion that is CA dependent. CA activity is required for about one-half of this net SO 4 2Ϫ secretion but is also required for about one-half of the net reabsorption in bird proximal epithelium. A CA-SO 4 2Ϫ /anion exchanger metabolon arrangement is proposed that may speed both the secretory and reabsorptive processes. renal proximal tubule; transport metabolon INORGANIC SO 4 2Ϫ IS THE SECOND most abundant anion in the sea and one of the most abundant anions in vertebrate plasma, with concentrations varying among species from 0.3 to 1.8 mM. This large tetrahedral divalent anion can be obtained from the diet (83) or oxidation of the sulfur-containing amino acids cysteine and methionine (77). Sulfoconjugation reactions are the initial detoxification process for many endogenous (e.g., steroid hormones) and xenobiotic compounds and are required for the activation of numerous biological molecules (e.g., heparin, cholecystokinin, and gastrin) (59). SO 4 2Ϫ is a major component of glycosaminoglycans (dermatan sulfate, chondroitin sulfate, keratan sulfate, and heparan sulfate), which are structural components of numerous tissues, including cartilage, bone, myelin, and basal lamina (40 ]) in the renal tubule lumen reduces Ca 2ϩ and Mg 2ϩ reabsorption (66).
RENAL SULFATE CLEARANCEThe kidneys are the major avenue for SO 4 2Ϫ excretion in vertebrates. Nearly all of plasma SO 4 2Ϫ is ultrafilterable in mammals (4), whereas 80% and 47% of plasma SO 4 2Ϫ are ultrafilterable in birds (71) and fish (75), respectively. Variation in the estimates of the ultrafilterable fraction of plasma SO 4 2Ϫ in vertebrates may arise from species differences or variations in method of determination. Urine content is determined by glomerular filtration, tubular reabsorption, and tubular secretion. This review will focus on the latter and will provide only a brief overview of tubular reabsorption because it has been described in detail elsewhere (2,51,52,57).Reabsorption of filtered SO 4 2Ϫ occurs primarily in the proximal tubule (35) secretion does not occur in these animals. SO 4 2Ϫ loading apparently has no effect on the saturable reabsorptive transport maximum for SO 4 2Ϫ (Tm SO 4 2Ϫ ) in the renal tubule of the human, dog, and frog (3,28,48). In contrast, Tm SO 4 2Ϫ in rats is reduced ϳ50% after plasma SO 4 2Ϫ loading, indicating that adaptive mechanisms are in place to depress reabsorption, thus increasing excretion (27). Similarly, in domestic fowl, Tm SO 4 2Ϫ decreases with plasma SO 4 2Ϫ loading, leading to the conclusion that a fraction of the excreted SO 4 2Ϫ is the result of tubular secretion (30). Thiosulfate, an inhibitor of SO 4 2Ϫ transporters,