Carbachol-induced increase of Na+/H+ antiport and recruitment of Na+,K(+)-ATPase in rabbit lacrimal acini

Parallel arrays of Na+/H+ and Cl-/HCO3- antiporters are believed to catalyze the first step of transepithelial electrolyte secretion in lacrimal glands by coupling Na+ and Cl- influxes across acinar cell basolateral membranes. Tracer uptake methods were used to confirm the presence of Na+/H+ antiport activity in membrane vesicles isolated from rabbit lacrimal gland fragments. Outwardly-directed H+ gradients accelerated 22Na+ uptake, and amiloride inhibited 96% of the H+ gradient-dependent 22Na+ flux. Amiloride-sensitive 22Na+ influx was half-maximal at an extravesicular Na+ concentration of 14 mM. In vitro stimulation of isolated lacrimal acini with 10 microM carbachol for 30 min increased Na+/H+ antiport activity of a subsequently isolated basolateral membrane sample 2.5-fold, but it did not significantly affect Na+/H+ antiport activity measured in intracellular membrane samples. The same treatment increased basolateral membrane Na+,K(+)-ATPase activity 1.4-fold; this increase could be accounted for by decreases in the Na+,K(+)-ATPase activities of intracellular membranes. Thus, it appears that cholinergic stimulation causes recruitment of additional Na+,K(+)-ATPase pump units to the acinar cell basolateral plasma membrane. The mechanistic basis of the increase in basolateral membrane Na+/H+ antiport activity remains unclear.