Ion transport mechanisms in rat parotid intralobular striated ducts

The intracellular pH (pHi) indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and microfluorimetry were used to characterize several ion transport mechanisms in rat parotid striated ducts. The recovery of ductal pHi from an acute acid load was Na+ dependent and inhibited by the amiloride analogue ethylisopropylamiloride with 50% inhibitory concentration 4.7 +/- 0.8 microM, indicating the presence of a Na(+)-H+ exchanger of the amiloride-insensitive type. The rate of this recovery was stimulated approximately 20% in ducts pretreated with the muscarinic agonist carbachol (10(-5) M) and inhibited approximately 20% in ducts pretreated with the beta-adrenergic agonist isoproterenol (10(-6) M). Upon removal of extracellular K+, ductal pHi rapidly decreased (0.19 +/- 0.02 pH units/min), consistent with a coupling between K+ and H+ (or OH-) fluxes in this tissue. In HCO(3-)-containing medium, the acidification due to K+ removal was blunted, arguing against ductal K(+)-HCO3- cotransport. However, the effect of K+ removal was inhibited by the K+ channel blocker Ba2+ (1 mM) and by the H+ channel blocker Zn2+ (25 microM), consistent with the involvement of electrically coupled K+ and H+ channels. The effect of K+ removal was unaffected by pretreatment of ducts with isoproterenol (10(-6) M) but markedly inhibited (approximately 50%) by pretreatment with carbachol (10(-5) M). No evidence for a significant component of Cl(-)-HCO3- exchange was found in striated ducts. The properties of the Na(+)-H+ exchanger and K(+)-H+ exchange mechanism identified here are consistent with their involvement in ductal salt reabsorption and secretion.