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SUMMARY1. Electrophysiological properties of the rat chromaffin cell were studied using intracellular recording techniques.2. The resting potential in the chromaffin cell was -49+6 mV (mean + S.D., n = 14) in standard saline containing 10 mm-Ca whereas that in Na-free saline was -63 + 9 mV (n = 17). At rest, the membrane has a substantial Na permeability.3. Action potentials were evoked by passing current through the recording electrode. In standard saline the major fraction of the action potential disappeared either upon omission of external Na ions from standard saline or addition of 1 /lM tetrodotoxin (TTX). We conclude that action potentials in the chromaffin cell are due mainly to an increase in the permeability of the membrane to Na ions.4. Small but significant regenerative action potentials were observed in Na-free saline, and when Ca in Na-free saline was replaced by Ba, prolonged action potentials occurred. We conclude that action potentials in the chromaffin cell also have a Ca component.5. Iontophoretic application of acetylcholine (ACh) produced a transient membrane depolarization in standard saline.6. Spontaneous action potentials were recorded extracellularly by microsuction electrodes. They occurred at a rate of 0 05-0-1/sec in almost all cells.7. When the perfusion fluid contained 3 x 10-7 M to 104 M ACh the spike frequency increased up to about 2/sec. This stimulatory effect of ACh was blocked by 10-7 M atropine but not by 10-3 M hexamethonium nor by 10-5 M-d-tubocurarine.8. The importance of Ca entry during action potentials for catecholamine secretion is discussed.

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