Archives of Oral Biology 47 (2002) 1–9 Expression of a sodium bicarbonate cotransporter in human parotid salivary glands K. Park a,1 , P.T. Hurley a , E. Roussa a,b , G.J. Cooper a , C.P. Smith a , F. Th´ evenod a,c , M.C. Steward a,∗ , R.M. Case a a School of Biological Sciences, University of Manchester, G.38 Stopford Building, Manchester M13 9PT, UK b Anatomisches Institut, Universit¨ at des Saarlandes, Medizinische Fakult¨ at, D-66421 Homburg (Saar), Germany c Physiologisches Institut, Universit¨ at des Saarlandes, Medizinische Fakult¨ at, D-66421 Homburg (Saar), Germany Accepted 11 September 2001 Abstract The human parotid gland secretes much of the bicarbonate that enters the mouth. Prompted by studies of animal models, this study sought evidence for the expression of a functional Na + –HCO 3 - cotransporter (NBC) in human parotid acinar cells. Microfluorometric measurements of intracellular pH in isolated acini showed that the recovery from an acid load was achieved in part by HCO 3 - uptake via a Na + -dependent, DIDS-sensitive mechanism. By reverse transcriptase–polymerase chain reaction, a full-length NBC1 clone was obtained showing more than 99% homology with the human pancreatic isoform hpNBC1. Expressed in Xenopus oocytes, the electrogenicity of the transporter was detected as an inwardly directed, Na + - and HCO 3 - -dependent flux of negative charge. Immunohistochemistry using antibodies raised to NBC1 showed strong staining of the basolateral membrane of the acinar cells. Therefore, it was concluded that a functional electrogenic Na + –HCO 3 - cotransporter is expressed in the human parotid gland, and that it contributes to pH regulation in the acinar cells and could play a significant part in salivary secretion. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Bicarbonate secretion; Intracellular pH; NBC 1. Introduction Bicarbonate has an important protective role in saliva in buffering the acid produced from dietary sugars by oral bacteria, and in providing an optimal pH for salivary amy- lase activity. The concentration of HCO 3 - ions in the saliva is, however, variable and shows a marked dependence on Abbreviations: BCECF, 2 ′ 7 ′ -bis(2-carboxyethyl)-5(6)-carboxyfluo- rescein; BSA, bovine serum albumin; MBP, maltose-binding protein; NBC, sodium/bicarbonate cotransporter; NMDG + , N- methyl-d-glucamine; PBS, phosphate-buffered saline; RT–PCR, re- verse transcriptase–polymerase chain reaction ∗ Corresponding author. Tel.: +44-161-275-5455; fax: +44-161-275-5600. E-mail address: martin.steward@man.ac.uk (M.C. Steward). 1 Present address: Department of Physiology, College of Dentistry, Seoul National University, 28 Yongondong, Chongnoku, Seoul 110-749, South Korea. salivary flow rate. In resting human saliva, the HCO 3 - con- centration can be as low as 1 or 2 mM, but the concentration increases with flow rate and there is a substantial contribu- tion from the parotid glands. During stimulated secretion, the parotids contribute more than 50% of the total saliva, and the concentration of HCO 3 - in the parotid saliva can reach up to 60 mM at high flow rates (Dawes, 1996). As micropuncture data are not available for this gland, it is not known whether the HCO 3 - ions are secreted predominantly by the acinar cells or by the duct cells. According to the classical, two-stage model of salivary secretion, the rise in HCO 3 - concentration with flow rate would suggest that the acinar cells secrete a HCO 3 - -rich primary fluid and that the duct cells reabsorb HCO 3 - ions, with most noticeable effect at lower secretory rates (Cook et al., 1994). This possibility was first proposed in a study of the flow dependence of Cl - and HCO 3 - concentra- tions in the saliva of the rhesus monkey parotid, which 0003-9969/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S0003-9969(01)00098-X