Hyperosmolality inhibits bicarbonate absorption in rat medullary thick ascending limb via a protein-tyrosine kinase-dependent pathway.
In the rat medullary thick ascending limb (MTAL), hyperosmolality inhibits transepithelial HCO3- absorption (JHCO3-) by inhibiting apical membrane Na+/H+ exchange. To examine signaling mechanisms involved in this regulatory response, MTALs were isolated and perfused in vitro with 25 mM HCO3- solutions (290 mosmol/kg H2O). Osmolality was increased in lumen and bath solutions by addition of 300 mM mannitol or 75 mM NaCl. Addition of mannitol reduced JHCO3- by 60% and addition of NaCl reduced JHCO3- by 50%. With the protein tyrosine kinase (PTK) inhibitor genistein (7 microM) or herbimycin A (1 microM) in the bath, addition of mannitol reduced JHCO3- only by 11% and addition of NaCl reduced JHCO3- only by 15%. Staurosporine (10(-7) M) or forskolin (10(-6) M) in the bath had no effect on inhibition of JHCO3- by hypertonic NaCl. Genistein had no effect on inhibition of JHCO3- by vasopressin (a cyclic AMP-dependent process) or stimulation of JHCO3- by prostaglandin E2 (a protein kinase C-dependent process). Under isosmotic conditions, addition of genistein or herbimycin A to the bath increased JHCO3- by 30% through stimulation of apical membrane Na+/H+ exchange. Addition of the tyrosine phosphatase inhibitor molybdate (50 microM) to the bath reproduced the inhibition of JHCO3- observed with hyperosmolality. These data indicate that 1) the effect of hyperosmolality to inhibit MTAL HCO3- absorption through inhibition of apical membrane Na+/H+ exchange is mediated via a PTK-dependent pathway that functions independent of regulation by cyclic AMP and protein kinase C, and 2) a constitutive PTK activity inhibits apical membrane Na+/H+ exchange and HCO3- absorption under isosmotic conditions. Our results suggest that tyrosine phosphorylation is a critical step in inhibition of the apical Na+/H+ exchanger isoform NHE-3 by hyperosmolality.
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Department of Internal Medicine, University of Texas Medical Branch, Galveston 77555, USA.
D000042:Absorption; D000470:Alkaloids / Q000494:pharmacology; D000818:Animals; D001127:Arginine Vasopressin / Q000494:pharmacology; D016227:Benzoquinones; D001639:Bicarbonates / Q000378:metabolism*; D005576:Colforsin / Q000494:pharmacology; D015232:Dinoprostone / Q000494:pharmacology; D004347:Drug Interactions; D019833:Genistein; D007529:Isoflavones / Q000494:pharmacology; D007679:Kidney Medulla / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D047029:Lactams, Macrocyclic; D008138:Loop of Henle / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D008353:Mannitol / Q000494:pharmacology; D008982:Molybdenum / Q000494:pharmacology; D009994:Osmolar Concentration; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011809:Quinones / Q000494:pharmacology; D051381:Rats; D017828:Rifabutin / Q000031:analogs & derivatives; D012965:Sodium Chloride / Q000494:pharmacology; D019311:Staurosporine
D000470:Alkaloids; D016227:Benzoquinones; D001639:Bicarbonates; D007529:Isoflavones; D047029:Lactams, Macrocyclic; D011809:Quinones; D001127:Arginine Vasopressin; C044659:molybdate; D005576:Colforsin; D017828:Rifabutin; D008353:Mannitol; D012965:Sodium Chloride; C020754:herbimycin; D008982:Molybdenum; D019833:Genistein; D011505:Protein-Tyrosine Kinases; D019311:Staurosporine; D015232:Dinoprostone