Comparative roles of the renal apical sodium transport systems in blood pressure control.

Human genetic studies suggest that the genes encoding renal apical Na(+) transport proteins play an essential role in the control of extracellular fluid volume and BP. Mice with mutations in each of these genes provide the unique opportunity to directly assess their respective involvement in fluid homeostasis and BP control in vivo. Inactivation of either the epithelial Na(+) channel (ENaC) or the Na(+)-Cl(-) cotransporter decreases BP to the same extent in mice fed a low-salt diet, despite a more pronounced perturbation of fluid homeostasis in ENaC-deficient mice. In contrast, inactivation of Na(+)/H(+) exchanger 3 (NHE3) or the Na(+)-K(+)-2Cl(-) contransporter reduces BP with a normal-salt diet and renders mice unable to survive with a low-salt diet. Therefore, the general conception that ENaC in the collecting duct is the main renal controller of Na(+) balance and extracellular fluid volume should be tempered. For example, NHE3 in the proximal convoluted tubule seems to play a more substantial role in the control of fluid homeostasis. The overall effect of NHE3 inactivation on BP may also involve absorptive defects in the intestine and colon, where the exchanger normally reabsorbs significant amounts of Na(+) and water.