Coupling of sodium transport to respiration in the toad bladder

Summary

Energy expenditure and transepithelial sodium transport were measured continuously and simultaneously from isolated urinary bladders of the Dominican toad,Bufo marinus. Sodium transport was measured as the short-circuit current and CO₂ produced by the bladder was measured conductometrically by the method of Maffly. The rates of sodium transport and CO₂ production were linearly related. The slope of the regression of sodium transport on CO₂ production,dJ _Na/dJ _{CO 2}, was found to be quite similar in paired half bladders but to differ significantly between bladders from different toads. Thus, in this preparation there appears to be no unique stoichiometric ratio characterizing sodium transport and metabolism and past efforts to arrive at such a value by averaging results obtained from different animals do not seem warranted. The CO₂ production by the isolated bladder which is unrelated to sodium transport was determined by two means: 1) extrapolating the regression ofJ _Na onJ _{CO 2} toJ _Na=0, and 2) measuring CO₂ production with sodium transport suppressed by removal of all sodium from the mucosal bathing medium. The two methods gave values which were in close agreement in each preparation. This suggests that metabolism which supports nontransport activities in this tissue cannot be recruited to support the energy requirement of sodium transport and vice versa.