Abstract
The mechanism underlying the increase in the potassium conductance of the acetylcholine (ACh)-induced channel on increasing the extracellular potassium concentration (|K|0) was studied. The relaxation as well as the current fluctuations of the drug-induced current were measured at 3 and 12 mM |K|0 by conducting voltage clamp experiments in the rabbit sinoatrial node. The following results were obtained:
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1.
The time constant of relaxation (τrelax) was not affected by changing |K|0. In both cases τrelax was about 130 ms at −80 mV, 100 ms at −40 mV and 60 ms at + 20 mV. It is, therefore, unlikely that the increase of the ACh-induced potassium conductance is due to a longer average open time of the drug-operated potassium channels.
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2.
The chord conductance of the ACh-induced K current was increased by a factor of 1.7 at −40 mV and by 1.5 at +10 mV on elevation of |K|0 from 3 to 12 mM.
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3.
From the relation between the variance of the current fluctuations and the mean amplitude of the current the single channel conductance was determined to be 3.3 pS at 3 mM |K|0 and 5.9 pS at 12 mM |K|0, thus γ was increased by a factor of 1.7.
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4.
From the power density spectrum an increase in the single channel conductance by a factor of 1.8 (at −40 mV) could be calculated. The corner frequency was not affected.
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5.
The increase in the potassium conductance, expected from the constant field equation when |K|0 is increased, agrees well with the experimental results.
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This work was supported by the Deutsche Forschungsgemeinschaft SFB 38, project G.
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Noma, A., Osterrieder, W. & Trautwein, W. The effect of external potassium on the elementary conductance of the ACh-induced potassium channel in the sino-atrial node. Pflugers Arch. 381, 263–269 (1979). https://doi.org/10.1007/BF00583258
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DOI: https://doi.org/10.1007/BF00583258