Hyposmolality stimulates Na(+)/H(+) exchange and HCO(3)(-) absorption in thick ascending limb via PI 3-kinase.
The signal transduction mechanisms that mediate osmotic regulation of Na(+)/H(+) exchange are not understood. Recently we demonstrated that hyposmolality increases HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through stimulation of the apical membrane Na(+)/H(+) exchanger NHE3. To investigate the mechanism of this stimulation, MTALs from rats were isolated and perfused in vitro with 25 mM HCO(3)(-)-containing solutions. The phosphatidylinositol 3-kinase (PI 3-K) inhibitors wortmannin (100 nM) and LY-294002 (20 microM) blocked completely the stimulation of HCO(3)(-) absorption by hyposmolality. In tissue strips dissected from the inner stripe of the outer medulla, the region of the kidney highly enriched in MTALs, hyposmolality increased PI 3-K activity 2. 2-fold. Wortmannin blocked the hyposmolality-induced PI 3-K activation. Further studies examined the interaction between hyposmolality and vasopressin, which inhibits HCO(3)(-) absorption in the MTAL via cAMP and often is involved in the development of plasma hyposmolality in clinical disorders. Pretreatment with arginine vasopressin, forskolin, or 8-bromo-cAMP abolished hyposmotic stimulation of HCO(3)(-) absorption, due to an effect of cAMP to inhibit hyposmolality- induced activation of PI 3-K. In contrast to their effects to block stimulation by hyposmolality, PI 3-K inhibitors and vasopressin have no effect on inhibition of apical Na(+)/H(+) exchange (NHE3) and HCO(3)(-) absorption by hyperosmolality. These results indicate that hyposmolality increases NHE3 activity and HCO(3)(-) absorption in the MTAL through activation of a PI 3-K-dependent pathway that is inhibited by vasopressin and cAMP. Hyposmotic stimulation and hyperosmotic inhibition of NHE3 are mediated through different signal transduction mechanisms.
Good
D W
DW
Deparments of Medicine and Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77555, USA.
Di Mari
J F
JF
Watts
B A
BA
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000042:Absorption / Q000187:drug effects; D000818:Animals; D001127:Arginine Vasopressin / Q000494:pharmacology; D001639:Bicarbonates / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000242:Cyclic AMP / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008138:Loop of Henle / Q000378:metabolism*; D008297:Male; D009994:Osmolar Concentration; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology*; D011493:Protein Kinase C / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*
D001639:Bicarbonates; D004791:Enzyme Inhibitors; C000613387:Slc9a3 protein, rat; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D001127:Arginine Vasopressin; D015124:8-Bromo Cyclic Adenosine Monophosphate; D000242:Cyclic AMP; D019869:Phosphatidylinositol 3-Kinases; D011493:Protein Kinase C