ION-TRANSPORT MECHANISMS IN RAT PAROTID INTRALOBULAR STRIATED DUCTS

The intracellular pH (pH(i)) 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 pH(i) from an acute acid load was Na+ dependent and inhibited by the amiloride analogue ethylisopropylamiloride with 50% inhibitory concentration 4.7 +/- 0.8 mu M, indicating the presence of a Na+-H+ exchanger of the amiloride-insensitive type. The rate of this recovery was stimulated similar to 20% in ducts pretreated with the muscarinic agonist carbachol (10(-5) M) and inhibited similar to 20% in ducts pretreated with the beta-adrenergic agonist isoproterenol (10(-6) M). Upon removal of extracellular K+, ductal pH(i) rapidly decreased (0.19 +/- 0.02 pH units/min), consistent with a coupling between K+ and H+ (or OH-) fluxes in this tissue. In HCO3--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 mu M), 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 (similar to 50%) by pretreatment with carbachol (10(-5) M). No evidence for a significant component of Cl--HCO3- exchange was found in striated ducts. The proper ties of the Na+-H+ exchanger and K+-H+ exchange mechanism identified here are consistent with their involvement in ductal salt reabsorption and secretion.