title: Diluting segment in kidney of dogfish shark. I. Localization and characterization of chloride absorption.
abstract: Single tubules, dissected from the peritubular sheath of the dorsal bundle zone of kidney of the dogfish shark, Squalus acanthias, were perfused in vitro at 17-18 degrees C. This segment is the largest of the five in the peritubular sheath and had average inner and outer diameters of 46.9 +/- 1.2 and 74.4 +/- 2.1 microns, respectively (n = 32). These values suggest that this is the intermediate IV segment. When perfused with symmetrical buffered elasmobranch saline, intermediate IV segments exhibited high rates of Cl- absorption (JCl, pmol.s-1.cm-2): 1,696 at an average perfusion rate (Vo) of 8.2 nl/min. Cl- absorption was highly flow dependent [1/JCl = 57.95(1/Vo) + 1.75; r = 0.71, P less than 0.01]. Maximal rates of Cl-absorption, calculated from reciprocal transformation of the flow dependence of JCl, yielded a value of 5,714 pmol.s-1.cm-2. In the presence of a 200-mosmol/kg transepithelial osmotic gradient, fluid absorption was negligible. The spontaneous transepithelial voltage (Vte, mucosal with respect to serosal compartment) averaged 8.0 +/- 1.0 mV (n = 26). Such active transport of Cl- in the absence of fluid movement and in the presence of a lumen-positive transepithelial voltage is characteristic of amphibian and mammalian diluting segments. Na(+)-to-Cl- permeability ratios (PNa/PCl) averaged 2.5 +/- 0.5, indicating that, as in mammalian thick ascending limbs, this segment is Na+ (cation) permselective. Vte was dependent on the presence of Na+ and Cl- in the external solutions and was reversibly abolished by isosmotic replacement with N-methyl-D-glucamine or with isethionate, respectively. Ouabain inhibited Vte but was not reversible within the time course of these experiments. Furosemide (10(-4) M), but not equimolar concentrations of amiloride or hydrochlorothiazide, added to the luminal perfusate inhibited both Vte and JCl. These results suggest that apical membrane Na+ entry in intermediate IV segments is mediated by Na(+)-K(+)-Cl- cotransport and is consistent with the existence of a functional role of urinary dilution in the reabsorption of urea in the elasmobranch kidney.
authors.affiliation: Mt. Desert Island Biological Laboratory, Salsbury Cove, Maine 04672.
authors.forename: P A
authors.initials: PA
authors.lastname: Friedman
authors.forename: S C
authors.initials: SC
authors.lastname: Hebert
mesh_terms: D000042:Absorption; D000818:Animals; D001826:Body Fluids / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism*; D000242:Cyclic AMP / Q000494:pharmacology; D004232:Diuretics / Q000494:pharmacology; D004284:Dogfish / Q000378:metabolism*; D004594:Electrophysiology; D066298:In Vitro Techniques; D007684:Kidney Tubules / Q000033:anatomy & histology / Q000378:metabolism* / Q000502:physiology; D009995:Osmosis; D010042:Ouabain / Q000494:pharmacology; D010539:Permeability; D012754:Sharks / Q000378:metabolism*; D014018:Tissue Distribution; D014867:Water / Q000378:metabolism
chemical_list: D002712:Chlorides; D004232:Diuretics; D014867:Water; D010042:Ouabain; D000242:Cyclic AMP