Metabolic support of Na+ transport by the rabbit CCD: analysis of the use of equivalent current.

The role of metabolism in the support of ion transport by the cortical collecting duct (CCD) is being increasingly recognized as a complex process involving energy supply to the Na+/K+ pump and maintenance of cellular conductive pathways. In order to assess both of these processes, we measured the metabolic support of Na+ transport using transepithelial electrical measurements and, in some cases, simultaneous determination of lumen-to-bath Na+ flux. Analysis of the calculated equivalent current (Ieq), the product of the transepithelial voltage and conductance, showed a predicted (and a measured) discrepancy between this value and the magnitude of active Na+ transport. Under conditions of this study, the change in Ieq in a single tubule was a reasonable index of the change in Na+ transport. The majority of the support of Na+ transport appears to come from oxidative metabolism. Glucose supports transport better than the other substrates tested, but lactate, pyruvate, and some acids provide near maximal support. We found some conditions where large changes in Na+ transport occurred without significant changes in conductance. Conductance could also be altered without producing major changes in transport. These results demonstrate complex and possibly independent influences of metabolism in the regulation of Na+ transport and cell conductive pathways.