title: The role of HCO3- and Na+/H+ exchange in the response of rat parotid acinar cells to muscarinic stimulation.
abstract: The intracellular pH (pHi) of a rat parotid acinar preparation was monitored using the pH-sensitive fluorescent dye, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. Under resting (unstimulated) conditions both Na+/H+ exchange and CO2/HCO3- buffering contribute to the regulation of pHi. Muscarinic stimulation (carbachol) of the acini produced a gradual rise in pHi (approximately 0.1 unit by 10 min) possibly due to activation of the Na+/H+ exchanger. When the exchanger was blocked by amiloride or sodium removal, carbachol induced a dramatic (atropine inhibitable) decrease in pHi (approximately 0.4 pH unit with t1/2 approximately 0.5 min at 1 mM carbachol). The rate of this acidification was reduced by removal of exogenous HCO3- and by the carbonic anhydrase inhibitor methazolamide. Also, acini stimulated with carbachol in Cl- -free solutions showed a more pronounced acidification than in the corresponding Cl- -replete media. Taken together, these data indicate that the carbachol-induced acidification of rat parotid acinar cells unmasked by inhibition of the Na+/H+ exchanger is due to a rapid loss of intracellular HCO3-. Carbachol induced acidification was inhibited by the Cl- channel blocker diphenylamine 2-carboxylate but not by 4-acetomido-4'-isothiocyanostilbene-2,2'-disulfonic acid, an inhibitor of Cl-/HCO3- exchange. In addition, this acidification could not be sustained in Ca2+-free media and was totally blocked by chelation of intracellular Ca2+. Interpreted in terms of HCO3- loss, these results closely parallel the pattern of carbachol-induced Cl- release from this same preparation and indicate that HCO3- is secreted in response to muscarinic stimulation via the same or a very similar exit pathway, presumably an apical anion channel. Under normal physiological conditions the intracellular acidification resulting from HCO3- secretion is buffered by the Na+/H+ exchanger.
authors.affiliation: Clinical Investigations and Patient Care Branch, National Institute of Dental Research, Bethesda, Maryland 20892.
authors.forename: J E
authors.initials: JE
authors.lastname: Melvin
authors.forename: A
authors.initials: A
authors.lastname: Moran
authors.forename: R J
authors.initials: RJ
authors.lastname: Turner
mesh_terms: D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology*; D001639:Bicarbonates / Q000378:metabolism* / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology*; D002352:Carrier Proteins / Q000378:metabolism*; D002712:Chlorides / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008962:Models, Theoretical; D010306:Parotid Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D017923:Sodium-Hydrogen Exchangers
chemical_list: D001639:Bicarbonates; D002352:Carrier Proteins; D002712:Chlorides; D017923:Sodium-Hydrogen Exchangers; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; D001285:Atropine; D002217:Carbachol