Acetylcholine-induced Na+ influx in the mouse lacrimal gland acinar cells: demonstration of multiple Na+ transport mechanisms by intracellular Na+ activity measurements.

In the isolated, superfused mouse lacrimal gland, intracellular Na+ activities (aNai) of the acinar cells were directly measured with double-barreled Na+-selective microelectrodes. In the nonstimulated condition aNai was 6.5 +/- 0.5 mM and membrane potential (Vm) was -38.9 +/- 0.4 mV. Addition of 1 mM ouabain or superfusion with a K+-free solution slightly depolarized the membrane and caused a gradual increase in aNai. Stimulation with acetylcholine (ACh, 1 microM) caused a membrane hyperpolarization by about 20 mV and an increase in aNai by about 9 mM in 5 min. The presence of amiloride (0.1 mM) reduced the ACh-induced increase in aNai by approximately 50%, without affecting Vm and input resistance in both nonstimulated and ACh-stimulated conditions. Acid loading the acinar cells by an addition/withdrawal of 20 mM NH4Cl or by replacement of Tris+-buffer saline solution with HCO3-/CO2-buffered solution increased aNai by a few mM. Superfusion with a Cl(-)-free NO3- solution or 1 mM furosemide or 0.5 mM bumetanide-containing solution had little effect on the resting aNai levels, however, it reduced the ACh-induced increase in aNai by about 30%. Elimination of metabolite anions (glutamate, fumarate and pyruvate) from the superfusate reduced both the resting aNai and the ACh-induced increase in aNai. The present results suggest the presence of multiple Na+ entry mechanisms activated by ACh, namely, Na+/H+ exchange, Na-K-Cl cotransport and organic substrate-coupled Na+ transport mechanisms.