Effect of extracellular K+ on saliva secretion by isolated,perfused rat submandibular glands

Changes in perfusate K⁺ concentration altered the secretory response of the glands to 10^?6 M acetylcholine. Lack of extracellular K⁺ caused a transient fluid secretory response lasting less than 10 min, and a 91 per cent reduction in the overall volume of saliva secreted in 60 min; it inhibited the response to acetylcholine even when the perfusate was changed to K⁺-containing solutions after 30 min. Absence of K⁺ in the perfusate resulted in increased Na⁺ and decreased K⁺ and Cl^? concentrations in saliva. An increase in the perfusate K⁺ concentration to 50 mM/l caused a reduced but more sustained secretory response, although the volume of saliva secreted in 60 min was still reduced by 76 per cent compared to that obtained when the perfusate contained 4.6 m-equiv./l K⁺. Acetylcholine release induced by the high K seemed mostly responsible as the response was inhibited by atropine. However, in the presence of excess of exogenous acetylcholine, perfusion with high K⁺ medium resulted in reduced Na⁺ and elevated K⁺ and Cl^? concentrations in saliva. It seems that a physiological (4–5 m-equiv./l) extracellular K⁺ concentration is required for acinar fluid secretion and for transductal electrolyte transport in the rat glands; lack of external K⁺ hyperpolarizes salivary cells and, although allowing an initial increase in Na⁺ conductance capable of causing secretion, prevents the further influx of this ion required to sustain saliva secretion. It also inhibits K⁺secretion and Na⁺ re-absorption in salivary ducts, probably by inhibiting the Na⁺, K⁺ pump in duct cells; high external K⁺ depolarizes acinar cells and may reduce Na⁺ conductance. It also enhances K⁺ secretion and Na⁺ re-absorption in salivary ducts; the effects of K⁺ omission and of high K⁺ on fluid secretion are likely to be the result of changes in the transmembrane K⁺ gradient in acinar cells and of the way they affect the opening of Na⁺ conductance pathways; perfusion with K⁺-free or high K⁺ solutions reveals a dissociation in ductal reabsorption of Na⁺ and Cl^? and unmasks the presence of independent transport mechanisms for these two ions in salivary ducts.