Proximal tubular response to variations in extracellular sodium concentration

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Abstract
In vitro microperfusion was used to study the effect of low and high extracellular Na concentrations on the transport capacity of the proximal convoluted tubule (PCT). Tubules from rabbit kidney were perfused with luminal and peritubular solutions containing 80, 115, 190, and 225 mM Na. Control solutions contained 150 mM Na. No ionic substitution was made and the proximal convoluted tubules were studied under hypo- or hypertonic conditions after a 20 min equilibration period. Na concentration was measured at 80, 150, and 225 mM Na in the perfused and collected fluids and no significant difference was observed. Net Na transport (JNa) remained relatively constant between 115 and 150 mM Na. It decreased progressively at high Na concentrations. Depression in JNa was also observed at 80 mM Na. Fractional Na reabsorption increased to 125.0% of control at 115 mM Na and decreased to 69.4 and 40.1% at 190 and 225 mM Na, respectively. At 80 mM, the results were not different from control. At concentrations of 115-225 mM Na the proximal convoluted tubule has the intrinsic capacity to regulate Na transport.