Acetylcholine‐induced membrane depolarization and potential fluctuations in the rat adrenal chromaffin cell

Abstract

1. The effect of acetylcholine (ACh) on the rat adrenal chromaffin cell membrane was examined in culture by using an intracellular recording technique. ACh was applied with gas pressure through a pipette which had an internal tip diameter of about 3 μm. 2. Upon application of ACh the chromaffin cell membrane depolarized with superimposed potential fluctuations. Occasionally repetitive spikes were observed during the entire period of ACh application up to 30 sec. In some cells a hyperpolarization with a slow time course followed the initial depolarization. 3. The depolarization was detectable with 0·1 μ m‐ACh; the amplitude increased with higher concentrations of ACh and saturated at 100 μ m. The half‐maximal concentration was approximately 10 μ m. 4. The amplitude of the ACh depolarization increased when the membrane was hyperpolarized by passing current through the recording electrode, and decreased upon depolarization. This technique gave an apparent reversal potential of ‐24±4 mV (mean±S.D., n = 6). 5. When the mean amplitude of the ACh depolarizations and the variance of the potential fluctuations in a given cell were plotted, a linear relation was found. The slope was 0·23±0·06 mV (n = 9) at the average membrane potential level of ‐60 mV. With a few assumptions this value approximates the amplitude of an elementary potential change. 6. Temporal characteristics of the potential fluctuations were obtained by calculating an auto‐correlation function. The averaged auto‐correlation function in a given cell declined exponentially with time. The mean time constant at 31 °C was found to be 40±19 msec (n = 9) at the average membrane potential level of ‐60 mV. With a few assumptions this value provides an estimate of the mean duration of an elementary potential change. 7. The ionic mechanism of the ACh depolarization was examined by changing the ion composition of the external solution. Na, K and Ca ions appear to be the current carriers during the ACh depolarization; Cl ions do not contribute. 8. Various ACh receptor agonists and antagonists were applied to determine the pharmacological properties of these ACh receptors. The receptors were found to be nicotinic. 9. The nature of the ACh‐induced membrane potential fluctuations is discussed.