Activation of the Na+/H+ exchanger by cellular pH and extracellular Na+ in rat adipocytes; inhibition by isoproterenol.
The activity of the Na+/H+ exchanger was studied by measuring the effects of intracellular pH (pHi) and extracellular Na+ [(Na+)o] on pHi recovery and 22Na uptake in rat adipocytes. The resting pHi was acidified from 7.30 +/- 0.02 to 6.99 +/- 0.01 with nigericin in the absence of (Na+)o. pHi recovery induced by 30 mM NaCl was blocked by 100 microM amiloride. The reversibility of the exchanger was studied by Na+ loading, which raised the pHi from 7.30 +/- 0.02 to 7.50 +/- 0.01, and by removing (Na+)o, which decreased pHi to 6.97 +/- 0.01. Both functions of the exchanger, forward and backward, were inhibited by amiloride. The Na+/H+ exchanger was inactive at pHi higher than 7.1 and became increasingly active as pHi decreased to 6.2 (22Na+ uptake, 0.029 +/- 0.003 vs. 0.155 +/- 0.009 nmol/10(5) cells.2.5 min; P < 0.001); this 5-fold stimulation was largely abolished by amiloride (0.025 +/- 0.002; P < 0.001). Na+ influx was also increased as a function of (Na+)o, with an apparent Km of 35 mM. Respective 5- and 44-fold stimulations at 5 mM (0.135 +/- 0.007) and 140 mM (Na+)o (1.228 +/- 0.046 nmol/10(5) cells.2.5 min; P < 0.001) were inhibited by ethylisopropylamiloride. Isoproterenol (Iso; 100 nM) and agents that stimulate cAMP production, such as forskolin (10 microM) and theophyline (1 mM), inhibited the activity of amiloride-sensitive 22Na+ uptake by 85%. Iso inhibited the Na+/H+ exchanger, without affecting the Na+/K(+)-adenosine triphosphatase-dependent and the Na+/K+/Cl- cotransport mechanisms. (Bu)2cAMP (1 mM), a membrane-permeant cAMP analog, mimicked the effects of Iso on the exchanger. The inhibitory effect of Iso was blocked by propranolol, but not by metoprolol, a beta 1-antagonist. In addition, the alpha-adrenergic agonists, phenylephrine (alpha 1) and clonidine (alpha 2), and the alpha-antagonists, prazocin (alpha 1) and yohimbine (alpha 2), did not prevent Iso-induced inhibition of the exchanger. In conclusion, rat adipocytes possess a reversible Na+/H+ exchange mechanism, which is activated by low pHi and normal (Na+)o and is inhibited by Iso via a beta 2-adrenergic receptor stimulation and a cAMP-dependent mechanism.
Arsenis
G
G
Endocrine Section, Veterans Administration Medical Center, Bay Pines, Florida 33504, USA.
D017667:Adipocytes / Q000187:drug effects / Q000378:metabolism*; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007545:Isoproterenol / Q000494:pharmacology*; D008297:Male; D009550:Nigericin / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology*; D012979:Sodium Radioisotopes; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors / Q000378:metabolism*; D028021:Sodium-Potassium-Chloride Symporters; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013806:Theophylline / Q000494:pharmacology
D002352:Carrier Proteins; D012979:Sodium Radioisotopes; D017923:Sodium-Hydrogen Exchangers; D028021:Sodium-Potassium-Chloride Symporters; D005576:Colforsin; D000584:Amiloride; D012964:Sodium; D013806:Theophylline; D000242:Cyclic AMP; D000254:Sodium-Potassium-Exchanging ATPase; D007545:Isoproterenol; D009550:Nigericin