Membrane potentials recorded from the mucosa of the toad's tongue during chemical stimulation.

An isolated stretch of tongue mucosa was obtained from the Chilean toad (Callyptocephalella gayi). The preparation was observed under transmitted illumination through a binocular microscope. The surface cells were impaled with micro-electrodes and different chemical agents were applied to the area. The following results were obtained.1. The surface cells had resting potentials of -6 to -40 mV (mean of -17.6 mV) with the preparation bathed in Ringer solution.2. The cells underwent depolarization by application of different salts (NaCl, NaF, KCl, Na(2)SO(4), CaCl(2) and MgCl(2)) in concentrations of 0.25-1.0 M. The potentials evoked by the salt solutions often overshot the zero membrane potential level by several millivolts. The positive-going potential change produced by application of salts was increased during hyperpolarization of the membrane by inward current and was decreased by current of the opposite sign. Application of salts during depolarization of the membrane to a certain positive level produced a negative-going potential change. The potentials evoked by different salts were about the same, qualitatively, when recordings were made from different areas of the tongue, i.e. top of the fungiform and filiform papillae, doughnut-shaped folds at the base of the fungiform papillae, areas between papillae and from the side of the tongue totally devoid of papillary structures.3. Quinine applied in concentrations of 2 x 10(-2)M did not change the resting polarization of the surface epithelial cells. However, quinine applied to cells already depolarized by outward currents induced further depolarization. When it was delivered to cells hyperpolarized by inward current the substance induced further hyperpolarization.4. Sucrose (0.5-1.0 M) did not change the membrane potential of lingual cells regardless of the level of cell polarization induced by injected currents.5. Hydrochloric, sulphuric, nitric and acetic acids produced minimal biphasic effects: a brief hyperpolarization followed by a slower secondary depolarization.6. Water increased the membrane potential of the surface cells. Salts applied at the peak of the water-evoked hyperpolarization induced cell depolarization which was much larger than that evoked during application of salts to cells bathed in Ringer solution.