Acetylcholine concentration and its role in ionic transport by the corneal epithelium

Corneal epithelium is known to have a high acetylcholine (ACh) concentration, but its role remains uncertain. Furthermore, rabbit corneal epithelium is devoid of cholinergic receptors. ACh concentration in calf, rabbit, and frog corneal epithelium was determined with Fellman's fluorometric assay to be 16.9, 11.1, and 21.8 micrograms of ACh per gram of epithelium, respectively. The isolated frog cornea was used to examine a possible role of the cholinergic system on active ionic transport. A 2 mM concentration of exogenous ACh has a moderate inhibitory effect on Na transport but no effect on Cl transport. A 10(-4)M concentration of 4-(1-naphthylvinyl) pyridine (NVP), a choline acetyltransferase inhibitor, reversibly inhibited both Na and Cl transport by about 70%. NVP also reduced ACh content of frog corneal epithelium by 51%. Thus endogenous ACh, but not exogenous ACh, seems to be stimulatory of active ionic transport. Of several muscarinic or nicotinic blockers tested, 10(-3)M atropine inhibited Na transport by 55% and Cl transport by 83%; 10(-3)M nicotine inhibited Na transport by 33% and Cl transport by 17%. If frog cornea (like rabbit cornea) contains no cholinergic receptors, the effects of ACh, nicotine, and atropine on ionic transport may be mediated through a nonspecific pathway.