Rapid penetration of potassium and other salts into the frog tongue papilla

The permeabilities of the frog tongue epithelium for potassium and other ions during a short time span were investigated electrophysiologically. The fungiform papilla of the bullfrog tongue was suctioned into a U-shaped glass suction electrode, through which Ringer solution was circulated. Compound nerve action potentials were recorded antidromically from the electrode following electrical stimulation of the glossopharyngeal nerve. When more than 5-10 mM potassium salts, 30 mM RbCl, 30 mM CsCl, 0.025 g in dl solution tetrodotoxin, 0.1 g in dl solution lidocaine hydrochloride or 3 g in dl solution ethanol, each of which was dissolved in Ringer solution containing 1.9 mM KCl, were flowed through the suction electrode, only the negative components of action potentials were gradually reduced and finally disappeared. The time needed for 50% reduction of negative components was about 10 sec for 0.1 M potassium salts and longer for the nonelectrolytes. A single suctioned papilla, which was flowed with various test solutions, was stimulated electrically and the change in current threshold of the papillary nerve was measured by recording orthodromic action potentials from the glossopharyngeal nerve. The threshold decreased within 10 sec after 0.05 M BaCl2 was flowed, but increased within 10 sec after 0.1 M KCl was flowed. The reduction of negative components of nerve action potentials may be due to the conduction block induced by potassium and other ions invading to the space around axons terminals. The threshold change also may be induced by the ions reaching the axon. These results suggest that chemical substances can rapidly penetrate the tongue epithelium of the frog, reach the papillary nerve fibers and contribute or modify gustatory informations.