In the preceding manuscript (Zhao, H., Xu, X., Diaz, J., and Muallem, S.(1995) J. Biol. Chem. 270, 19599-19605), we described a K-dependent H/HCO and Na influx pathway in the luminal membrane of salivary duct cells. In the present studies, we further characterized this pathway to show that the K-dependent Na influx was not mediated by the luminal amiloride-sensitive Na channel, the Na/H exchangers, or any electroneutral or conductive Cl-dependent transport pathway. Thus, K efflux probably maintained electroneutrality during Na influx induced by removal of K. Accordingly, Na influx was largely inhibited by 2.5 mM external Ba. The K site of the K-dependent Na influx showed the selectivity sequence Cs > K > NH > Li which is different from that of several known K channels. More importantly, Na influx is 50% inhibited at about 20 mM K, and significant Na influx occurred even at 80 mM K. This is a critical property for the pathway to play a role in Na reabsorption and K secretion by the duct. The large Na influx in resting duct cells is matched by high activity of the ductal Na pump which is about 8-fold faster than that of acinar cells. Stimulation of submandibular ducts with various agonists increased [Na] in an agonist-specific manner. The parasympathetic agonist epinephrine was more effective than isoproterenol and the sympathetic agonist carbachol. The use of various inhibitors of Na and K transporters suggests that different pathways mediate Na influx in stimulated acinar and duct cells of the gland. In duct cells, Na influx was inhibited only by extracellular Cs and Ba. The overall findings support a significant role for the K-dependent pathway(s) in Na reabsorption and K and HCO secretion and explain several features of transepithelial electrolyte transport by salivary ducts.

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