ACETYLCHOLINE-INDUCED POTASSIUM CURRENT FLUCTUATIONS IN THE RABBIT SINO-ATRIAL NODE

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. 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. 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. 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). 5. The average single channel conductance, calculated from the variance, was 3,71±0.48 pS (n=7). 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 frequency fc=0.96±0.25 Hz (n=21). This corresponds to an average open time τ=166 ms. 7. The Q10 of the average open time (determined by the corner frequency (fc) was 2.83±0.38 (n=4). 8. The density of channels was approximately 1 per 1 μm2 of cell surface or roughly 2000 channels per cell. 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. © 1979 Springer-Verlag.