Analysis of reabsorbate concentrations in the proximal tubules of dog kidneys during osmotic diuresis.
To examine to what extent the reabsorbate concentrations, calculated as the flux ratios between solutes and water, represent the fluid composition in the lateral intercellular space (LIS) in the proximal tubules, reabsorption was stimulated by elevating PCO2 from 5 to 13 kPa before and during infusion of mannitol to a plasma concentration of 70 mM in volume-expanded dogs receiving ethacrynic acid. The reabsorbate concentration of NaHCO3 increased by 50 mM during mannitol infusion. The real concentration of NaHCO3 in LIS could not, however, be elevated by this amount, since the driving forces for fluid reabsorption then would have increased during osmotic diuresis due to diffusion of mannitol into LIS from plasma. A model analysis of diffusion in LIS showed that transcellular transport can only lead to trivial increases of LIS concentrations compared to plasma, whereas diffusion across tight junctions can increase LIS concentrations by several mM. NaCl diffusion and coupled transcellular water transport may therefore represent a significant contribution to total bicarbonate-dependent NaCl and water reabsorption in the proximal tubules.
Ostensen
J
J
Institute for Experimental Medical Research, University of Oslo, Ullevaal Hospital, Norway.
D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D001834:Body Water / Q000378:metabolism; D004234:Diuretics, Osmotic / Q000494:pharmacology*; D004285:Dogs; D005110:Extracellular Space / Q000032:analysis; D005919:Glomerular Filtration Rate / Q000187:drug effects; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D008353:Mannitol / Q000008:administration & dosage; D008954:Models, Biological; D012965:Sodium Chloride / Q000493:pharmacokinetics; D014882:Water-Electrolyte Balance / Q000187:drug effects
D001639:Bicarbonates; D004234:Diuretics, Osmotic; D008353:Mannitol; D012965:Sodium Chloride