ACETYLCHOLINE‐EVOKED POTASSIUM AND SODIUM TRANSPORT IN RAT LACRIMAL SEGMENTS: EVIDENCE FOR A SODIUM‐CHLORIDE CO‐TRANSPORT SYSTEM

An investigation was made of the effects of acetylcholine (ACh) on transmembrane movements of potassium (K + ) and sodium (Na + ) in isolated segments of rat lacrimal glands. ACh elicited dose‐dependent and transient increases in K + concentration in the effluent (K + release). The ACh‐induced K + outflow was unaffected by either pre‐treatment of lacrimal segments with loop diuretics (frusemide, piretanide and bumetanide, all 10 −4 M) or replacement of chloride (Cl − ) in the physiological salt solution with nitrate (NO 3 − ). In contrast, ACh caused a significant ( P 〈 0·001) reduction in Na + concentration in the effluent (Na + uptake). The ACh‐evoked Na + uptake was sensitive to loop diuretics and Cl − removal. Pre‐treatment of tissue with ouabain (10 −3 M) resulted in a marked sustained K + release. ACh produced a further increase in K + efflux in the continuing presence of ouabain. Incubation of the tissue with 10 mM‐tetraethylammonium (TEA) resulted in an uptake of K + . The results suggest the presence of a diuretic‐sensitive Na + ‐Cl − co‐transport system in lacrimal acinar cell membranes.