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To measure simultaneously the concentration profiles of bicarbonate, chloride and inulin along the length of the superficial proximal convoluted tubule, free-flow micropuncture measurements were made sequentially from the end-proximal tubule to Bowman's space in 10 tubules of hydropenic Munich-Wistar rats. Bicarbonate and volume reabsorption were 354 +/- 21 pmol X mm-1 X min-1 and 5.9 +/- 0.4 nl X mm-1 X min-1 in the first millimeter and fell progressively in the remaining 3.8 mm of tubule, averaging 83 +/- 4 pmol X mm-1 X min-1 and 2.3 +/- 0.5 nl X mm-1 X min-1, respectively. The values in the initial millimeter represents a high transport capacity since they exceed rates that have been observed when comparable or even higher mean luminal substrate concentrations were presented to the late proximal tubule. In contrast, chloride reabsorption was only 206 +/- 55 peq X mm-1 X min-1 in the first millimeter compared with a mean of 306 +/- 22 peq X mm-1 X min-1 in the rest of the tubule. In conclusion, there is substantial axial transport heterogeneity, with bicarbonate and water reabsorption higher but chloride reabsorption lower in the early compared with the late superficial proximal convoluted tubule.

Rabbit proximal straight tubules rapidly swell to a maximum volume when abruptly immersed into hypotonic medium. However, in a second, slower phase, termed volume regulatory decrease (VRD), tubules shrink toward their basal volume due to the efflux of K, accompanying anion and water. In the present study, we investigated the nature of the anion during hypotonic volume regulation. We removed Cl and/or the HCO3 buffer to assess their relative importance in VRD, and we used furosemide, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), and barium to investigate the nature of the transmembrane anion pathway in VRD. Isosmotic replacement of peritubular Cl with gluconate had no effect on either tubule volume in isotonic medium or the initial osmometric swelling response in hypotonic medium. However, the VRD in such Cl-depleted tubules was significantly inhibited. Control tubules regulated 81% below their maximal volume in dilute medium. By contrast, Cl-depleted tubules regulated only 39%. This inhibitory effect could not be attributed to the absence of peritubular Cl or to the presence of gluconate. The absence of the HCO3 buffer or the presence of SITS (0.5 mM) had no inhibitory effect on the rate or extent of VRD. Furosemide alone (1 mM) also had no inhibitory effect on VRD. However, whereas barium alone delays VRD, addition of furosemide to barium-treated tubules further slowed their maximal rate of fluid efflux and delayed VRD even more.(ABSTRACT TRUNCATED AT 250 WORDS)

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 (Jv) and sodium (JNa) fluxes in rat proximal tubules. 2. In a double blind manner, the shrinking drop technique of Gertz was used to measure Jv; JNa 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 Jv decreased significantly by 15 and 64%, respectively. Net JNa 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 Jv nor JNa changed. Thus, at zero tubular flow, differential Cl/HCO3 concentrations do not provide significant driving forces for net Jv or JNa. 6. When only intratubular but not peritubular Na was elevated to 279 mmol/l, Jv and JNa 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.(ABSTRACT TRUNCATED AT 250 WORDS)