Ultra fine tip microelectrodes (300 MOhm) were used to study the electrical properties of the chromaffin cell membrane in situ in the intact toad adrenal gland. 2. In the presence of physiologic [K+]o (2 mM) the resting membrane potential (Vm) was -53 +/- 3.2 mV. Vm depended on [K+]o as predicted by the constant field equation with PNa/PK of 0.16. 3. A small fraction (20%) of the impaled cells exhibited spontaneous electrical activity, though in all the cells examined, the injection of depolarizing current pulses elicited repetitive spikes. 4. Our measurements of the chromaffin cell input resistance (326 +/- 35 MOhm) is substantially smaller than the values reported for bovine isolated chromaffin cells, suggesting that the toad adrenal chromaffin cells might be electrically coupled. 5. Tetraethylammonium (TEA) increased the amplitude and duration of spikes, probably inhibiting outward K+ current. In the presence of tetrodotoxin (TTX) action potentials were abolished, although they reappeared if TEA was added, suggesting the participation of both Na+ and Ca2+ currents in the genesis of spikes. 6. As expected, acetylcholine (ACh) and nicotine depolarized the cells, though they did not always elicit electrical activity. 7. Muscarine (10-100 microM) had no effect on both Vm and on the depolarization induced by ACh or nicotine. Since muscarine inhibits catecholamine (CA) secretion induced by ACh and nicotine, we concluded that the inhibition of CA release by muscarine in the toad probably occurs at a level other than the membrane.
