To what extent does acetazolamide inhibit renal bicarbonate reabsorption?

A polymorphic variant of human erythrocyte carbonic anhydrase I with a widespread distribution in Australian aborigines, CAI Australia-9 (8 Asp leads to Gly): purification, properties, amino acid substitution, and possible physiological significance of the variant enzyme

Carbonic anhydrase I (EC 4.2.1.1) purified from the pooled packed red blood cells of 100 individuals typed as heterozygous for the common Australian Aboriginal carbonic anhydrase I variant CAI Australia-9 had a slightly higher specific CO2 hydratase or esterase (toward p-nitrophenyl acetate) activity than the normal component and a higher Km and Vmax using the esterase substrate. The variant enzyme was slightly more resistant to heat inactivation. The extent of inhibition of both enzymes by the specific inhibitor acetazolamide was identical, as was their immunological behavior and the lability of the active-site zinc ion. The variant enzyme was more resistant to chloride inhibition. The physiological importance of this observation is discussed in the context of a proposed adaptive advantage of the variant gene in the arid western and central regions of Australia. The amino acid substitution in the Aboriginal variant of a glycine for an aspartic acid residue has been located at residue 8 from the N terminus (i.e., 8 Asp leads to Gly), by proteolytic and partial acid hydrolyses. The possible effects of this substitution on the structure and function of the molecule are discussed.

Abolished relationship between pancreatic HCO-3 secretion and arterial pH during carbonic anhydrase inhibition

After acetazolamide administration, CO2 hydration in pancreatic cells would be slow and might become a rate-limiting factor to pancreatic HCO-3 secretion. Correspondingly, pancreatic HCO-3 secretion-normally pH dependent-would become slow and pH-independent. However, acetazolamide would not be expected to interfere with the capacity of the secretory mechanism to generate a proton potential gradient between pancreatic cells and interstitial fluid. These predictions were examined in 5 anesthetized, secretion infused (2.7 C. U./kg b.wt. h-1) pigs. Pancreatic juice was collected from a catheter in the pancreatic duct. Arterial pH was varied through i.v. HCl and NaHCO3 infusions and CO2 addition to inspired air. Before acetazolamide, HCO-3 secretion varied with plasma pH and averaged 298 +/- 30 mumol/min at control arterial pH. Acetazolamide (150 mg/kg, i.v.) reduced HCO3 secretion to 84 +/- 12 mumol/min and rendered secretion independent of arterial pH between pH 7.6 and pH 7.0. It is concluded that acetazolamide imposes a pH-independent transport maximum on pancreatic HCO-3 secretion, but does not reduce the capacity of the secretory mechanism to sustain a proton potential gradient between cells and interstitial fluid.