Fluid absorption and active and passive ion flows in the rabbit superficial pars recta.

Isolated rabbit proximal straight tubules were perfused with Krebs-Ringer buffers containing 127.8 mM NaCl and 3.8 mM NaHCO 3 (pH 6.6), and bathed with Krebs-Ringer buffers containing 105 mM NaCl and 25 mM NaHCO 3 (pH 7.4). Na isethionate replaced Na acetate in both mediums; acetate omission had no effect on net volume absorption (J V , nl min -1 mm -1 ). In acetate-free mediums, J V was 0.41 ± 0.04 (SE) at 37°C and 0.16 ± 0.01 at 21°C. J V at 21°C was referable to passive driving forces. The transepithelial voltage was +2.7 mV (lumen positive) at 37°C and rose to +3.7 mV at 21°C. At 37°C, net Cl - absorption accounted for 85 ± 3% of net cation absorption. The data were analyzed in terms of model calculations with flow-diffusion equations for lateral intercellular spaces. Approximately one-third of the Na + absorption at 37°C could be accounted for by dissipative transport processes; the remainder was active. The computed ionic concentrations along the entire length of intercellular spaces were nearly identical to the ionic concentrations in the bathing solutions, and passive ion transport across the epithelium was accurately predicted from bulk phase ion concentrations. With intercellular spaces isosomotic to the external solutions, fluid absorption could be accounted for in terms of the oppositely directed gradients of Cl - and HCO 3 - across junctional complexes, because the junctional complexes were more permeable to Cl - than HCO 3 - .