Expression of a sodium bicarbonate cotransporter in human parotid salivary glands

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+–HCO3− 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 HCO3− 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 HCO3−-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+–HCO3− 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.