THE EFFECTS OF HIGH Na AND Cl CONCENTRATIONS ON RAT PROXIMAL VOLUME AND Na FLUXES AT ZERO TUBULAR FLOW

1. In vivo micropuncture techniques, with and without peritubular capillary perfusion, were used to study the effects of high extracellular Na and Cl concentrations on transepithelial volume (J_v) and sodium (J_Na) fluxes in rat proximal tubules. 2. In a double blind manner, the shrinking drop technique of Gertz was used to measure J_v; J_Na was calculated from this and the tubular fluid Na concentration. 3. At both 184 and 279 mmol/l pericellular Na concentrations (both inside and outside the tubular epithelium), net J_v decreased significantly by 15 and 64%, respectively. Net J_Na remained constant at 184 but decreased by 29% at 279 mmol/l Na concentration. 4. Thus, at both Na concentrations, when translated to free flow conditions, fractional Na reabsorption must have decreased. These findings, also supported by previous results at these Na concentrations, indicate that active Na transport was inhibited by high pericellular Na concentrations. 5. When intratubular Cl concentration was varied between 108 and 138 mmol/l while peritubular Cl was maintained constant (blood perfusing the capillaries), neither J_v nor J_Na changed. Thus, at zero tubular flow, differential Cl/HCO₃ concentrations do not provide significant driving forces for net J_v or J_Na. 6. When only intratubular but not peritubular Na was elevated to 279 mmol/l, J_v and J_Na increased markedly by 50 and 187%, providing evidence that a true solvent drag (solute drag) effect does exist in rat proximal tubules. 7. These findings offer a mechanism to explain why Na reabsorption is not increased when the filtered load of Na is increased with an elevation of plasma Na. That is, the high Na, which surrounds the tubular epithelium, inhibits Na and volume flux at the cellular level by mechanisms as yet unknown. The results also showed that differential Cl/HCO₃ concentrations made no difference to Na or volume fluxes at zero tubular flow.