Abstract
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1.
Small sinoatrial-node preparations were voltage clamped by two micropipettes. Both the amplitude (mean current) and the fluctuation of acetylcholine-induced potassium current were recorded. The fluctuations were analyzed by both calculating their variance and establishing their power density spectrum.
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2.
It is assumed that acetylcholine receptors, when occupied, open momentarily ionic channels. Each channel is described by its conductance γ and the average open time τ.
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3.
The variance did not increase linearly with the mean current, but reached a saturation and even decayed with large mean currents, indicating that more than 50% of the total number of channels are open.
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4.
The average maximum mean current (all channels open) for a preparation approximately 300 μm ×200 μm×50 μm was 89±22 nA (n=7).
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5.
The average single channel conductance, calculated from the variance, was 3,71±0.48 pS (n=7).
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6.
The power density spectra of the fluctuations were fitted by single Lorentzian curves. The single channel conductance was thus determined to be γ= 3.79±1.25 pS and the corner frequencyf c=0.96±0.25 Hz (n=21). This corresponds to an average open time τ=166 ms.
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7.
TheQ 10 of the average open time (determined by the corner frequency (f c) was 2.83±0.38 (n=4).
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8.
The density of channels was approximately 1 per 1 μm2 of cell surface or roughly 2000 channels per cell.
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9.
The muscarinic acetylcholine receptor of S-A node cells may thus be characterized by a relatively long open time (160 ms), a low density of channels and a small elementary conductance of about 4 pS.
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This work was supported by the Deutsche Forschungsgemeinschaft SFB 38, Membranforschung, Projekt G.
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Noma, A., Peper, K. & Trautwein, W. Acetylcholine-induced potassium current fluctuations in the rabbit sino-atrial node. Pflugers Arch. 381, 255–262 (1979). https://doi.org/10.1007/BF00583257
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DOI: https://doi.org/10.1007/BF00583257