Relationship between the Rate of H+ Transport and Pathways of Glucose Metabolism by Turtle Urinary Bladder

Abstract

The urinary bladder of the fresh-water turtle acidifies its contents by actively transporting H⁺ ions across the luminal membrane. It is known that the H⁺ transport system is dependent upon oxidative metabolism and the substrate glucose; however, the specific biochemical events resulting in H⁺ translocation have not been identified. This study examines the relationship between active H⁺ transport and a specific oxidative pathway of glucose metabolism, the pentose phosphate shunt. To investigate this relationship the metabolic and transport rates were simultaneously measured under several well-defined conditions. When H⁺ transport was inhibited by either the application of an opposing pH gradient or by acetazolamide, glucose metabolism by the pentose phosphate shunt declined. Conversely, stimulation of H⁺ transport by either imposing a more favorable pH gradient or by CO₂ addition resulted in an increase in pentose phosphate shunt metabolism. Glycolytic activity, in contrast, was invariant with the maneuvers which altered the rate of H⁺ transport. Additional experiments localized pentose phosphate shunt enzyme activity to the mucosal fraction of the bladder which is the cell layer responsible for acid secretion. The finding that the rate of glucose metabolism by the pentose phosphate shunt is related to the rate of H⁺ transport suggests but does not prove that the pentose phosphate shunt may be an important metabolic pathway for H⁺ transport by the turtle urinary bladder.