Permeability properties of chick myotube acetylcholine-activated channels

Abstract

The acetylcholine-(ACh-)activated channels of chick myotubes were studied by the patch-clamp method. Single-channel amplitudes were measured over a wide range of potentials in solutions of cesium, arginine, and three small amines. Symmetrical, isotonic cesium solutions gave a linear I-V relationship with the single-channel conductance, gamma, of 42 pS at 11 degrees C. Dilutions of cesium by mannitol shifted the reversal potential 23.9 mV per e-fold change in internal cesium concentration. Selectivity, as defined by reversal potential criteria, depended on the molecular size of the permeant cation. The Q10 of gamma for the symmetrical isotonic cesium solutions as well as internal isotonic methylamine was 1.3-1.4. These properties are qualitatively similar to those seen at the ACh-activated channel of the frog neuromuscular junction. Partially substituting arginine for internal cesium depressed outward currents. 80 mM arginine acted equally well from the inside or the outside, as if arginine transiently blocks the ACh-activated channel in a current dependent way. Diluting internal cesium almost 10-fold, from 320 to 40 mM, increased the permeability of the channel calculated from Goldman-Hodgkin-Katz equations by almost threefold. Thus, cesium itself appears to block with a dissociation constant of 135 mM. Methylamine blocked the channel approximately as well as did cesium. Ammonia and ethylamine blocked the channel somewhat more than cesium. We conclude that (a) the channel is qualitatively similar to that of frog neuromuscular junction, (b) cations bind within the channel, and (c) arginine decreases channel conductance equally whether applied from the inside or the outside.

Single acetylcholine receptor channel currents recorded at high hydrostatic pressures.

“…Open-time histograms obtained in this way were also fitted by single exponentials, as shown in Fig. 4 (19) and from chick myotubes (20). In this temperature range both ro and rc showed a Q1o around 3.0(± 0.3).…”

Ion permeation through single channels activated by acetylcholine in denervated toad sartorius skeletal muscle fibers: Effects of alkali cations

“…Firstly, the I-V curves for K and Cs were found to be essentially linear over the voltage examined, suggesting a symmetrical energy profile for the major conductive element of the channel. Secondly, as has been pointed out earlier, results recently obtained using excised patches bathed in symmetrical alkali cation solutions all indicate that the energy profile of the major conductive element of the ACh channel is symmetrical (Horn & Patlak, 1980;Redmann et al, 1982;Dwyer & Farley, 1984;Dwyer, 1986;Sanchez et al, 1986). In contrast to the 2B1S model, both the electrodiffusion and the 4B3S ratetheory models gave good fits to the experimental I-V curves for Na, Li and Cs.…”

“…The current-voltage curve obtained by Sine and Steinbach (1984a,b) shows that the rectification only begins to become prominent at potentials greater than about +50 mV, outside the range of the present study. There is no evidence for rectification at the same potentials for symmetrical Na + solutions (Horn & Patlak, 1980) or symmetrical Cs + solutions (Dwyer & Farley, 1984).…”

“…Hence under these experimental conditions (potassium and caesium as external cations) any significant energy barrier asymmetry in the channel shoC ' have been revealed by the finding of a significantly nonsymmetrical current-voltage curve. Secondly, recent studies using excised patches from cultured cells have found that the current-voltage curve for ACh channels bathed in symmetrical alkali cation solutions is linear (Horn & Patlak, 1980;Redmann et al, 1982;Dwyer & Farley, 1984;Dwyer, 1986;Sanchez et al, 1986).…”

“…Recordings from excised patches in symmetrical solutions have shown that the current-voltage curve is linear for Na + (Horn & Patlak, 1980;Sanchez et al, 1986, rat myotubes), Cs § (Dwyer & Farley, 1984;Dwyer, 1986, chick myotubes), and Li § (Redmann et al, 1982, cultured BC3H1 cells) but has a slight inward rectification for K + (G.A. Redmann, personal communication; Sine & Steinbach, 1984a,b, both BC3H1 cells).…”