Abstract
The mechanism underlying the regulation of the K-channel by the muscarinic receptor was examined with patch-clamp experiments in atrial cells isolated enzymatically from the rabbit heart. The patch-electrode and the recording chamber were perfused with various solutions while the activity of the K-channels in the membrane-patch was recorded continuously.
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1)
In the absence of muscarinic agonists, opening of K-channels occurred at a low frequency (basal activity). Application of ACh to the bath did not affect the basal activity, but perfusion of the patch electrode with ACh markedly increased the channel activity in the “cell-attached” patch. Application of oxotremorine, i.e. a specific muscarinic agonist, via the pipette also opened K-channels.
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2)
When the membrane patch was isolated from the cell body (“inside-out” patch), ACh-induced single K-channel currents were still observed, but the frequency was reduced.
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3)
Perfusion of atropine or scopolamine, two muscarinic antagonists, through the patch-electrode depressed the basal activity. In the case of scopolamine, channel-activity recovered after washing out the drug.
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The current voltage relationship determined from the basal activity was similar to that of ACh-induced single K-channel currents.
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The mean open time was 0.49 ms at basal activity and 1.35 ms during the application of 0.1 μM ACh via the patch electrode. Application of oxotremorine via the pipette hardly affected the open-time, it remained at 99±45 (n=7) of the control.
These results suggest that the ACh-induced K current is generated by a class of K-channels having open and close kinetics and that the ACh-muscarinic receptor-complex directly increases the probability of this K-channel being open by coupling with the channel. The increase in openstate probability is mainly due to shortening of the mean closed-time. Basal activity is most probably generated by this K-channel. The muscarinic antagonists depress the ACh-induced K-current not only by blocking the binding of the agonist to the receptor, but also through a depressing influence of the antagonist-receptor-complex on the K-channel.
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Soejima, M., Noma, A. Mode of regulation of the ACh-sensitive K-channel by the muscarinic receptor in rabbit atrial cells. Pflugers Arch. 400, 424–431 (1984). https://doi.org/10.1007/BF00587544
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DOI: https://doi.org/10.1007/BF00587544