Journal of Neurophysiology, Vol 63, Issue 5 1052-1059, Copyright © 1990 by APS
Acetylcholine-activated ionic currents in parasympathetic neurons of bullfrog heart
N. Tateishi, D. K. Kim and N. Akaike
Department of Neurophysiology, Tohoku University School of Medicine, Sendai, Japan.
1. The electrical and pharmacologic properties of acetylcholine
(ACh)-induced current (IACh) were studied in the parasympathetic neurons
isolated from bullfrog heart with the use of the concentration-clamp
technique, which allows intracellular perfusion and rapid change of
external solution within 2 ms under the single-electrode voltage-clamp
condition. 2. The IACh consisted of an initial transient peak component and
a successive steady-state plateau component. Both currents increased in a
sigmoidal fashion with increasing ACh concentration. The dissociation
constant (Kd value) and the Hill coefficient for each component were 2.2 X
10(-5) M and 1.6, respectively. 3. In the K(+)-free solution, the reversal
potential (EACh) of IACh was close to the Na+ equilibrium potential (ENa).
The current-voltage (I-V) relation showed inward rectification at positive
potentials. 4. Nicotine mimicked only the peak component of IACh. However
both peak and steady-state components were blocked nonselectively by the
nicotinic blockers d-tubocurarine and hexamethonium. 5. Carbamylcholine
(CCh) mimicked the steady-state component of IACh. The steady-state
component was selectively inhibited by atropine at concentrations 1,000
times lower than that required for inhibition of the peak component. The
steady state was blocked equally by either pirenzepine (M1 blocker) or
AF-DX-116 (M2 blocker). 6. It was concluded that the IACh consisted of a
peak component having double exponential activation and inactivation,
mediated through the nicotinic actions, and a steady-state component having
no inactivation, mediated through the muscarinic action.
