Further studies on coupling between sodium transport and respiration in toad urinary bladder

When sodium transport (JNa) and CO2 production (JCO2) were measured simultaneously in the toad urinary bladder in the absence of electrochemical gradients under conditions of spontaneous variation of JNa, the curve of JNa on JCO2, phiJNa/phiJCO2, was found to be highly linear in an individual epithelium. However, no unique value appeared to characterize a population of bladders. In an effort to investigate the nature of this variation, phiJNa/phiJCO2 was examined under a variety of conditions. It was found that agents that affect sodium entry into the active transport pool, e.g. vasopressin and amiloride, or those that influence the energy-linked exit step, e.g. ouabain and insulin, have no effect on the phiJNa/phiJCO2. To investigate the possibility of the presence of a significant backleak from the serosal side into the cell, the sodium concentration was changed to 75 and 160 mM. Neither of these maneuvers influenced phiJNa/phiJCO2 or (JCO2)JNa=O. Furthermore, in the absence of mucosal sodium, ouabain had no effect on JCO2, suggesting that no significant recirculation is occurring. It is concluded that for each individual epithelium JNa is coupled to JCO2, and their ratios appear, within experimental error, to be invariant. It is suggested that sodium traverses the active transport pathway largely or entirely in one direction.