Divalent cation effects on acetylcholine-activated channels at the frog neuromuscular junction
- 32 Downloads
The effects of the divalent cations Ca and Mg on the properties of ACh-activated channels at the frog neuromuscular junction were studied using a two-microelectrode voltage clamp.
The divalent cation concentration was varied from 2 to 40 mM in solutions containing 50% normal Na. The reversal potential was determined by interpolation of the acetylcholine (ACh)-induced current versus voltage relationship. The single-channel conductance and the mean channel lifetime were calculated from fluctuation analysis of the ACh-induced end-plate current.
Extracellular Na and/or divalent cations affected the reversal potential of endplate channels in a way that cannot be described by the Goldman-Hodgkin-Katz equation or by a simple two-barrier, one-binding site model of the channel if the assumption was made that permeability ratios were constant and not a function of ion concentrations.
Increasing the divalent cation concentration decreased the single-channel conductance to approximately 10 pS in solutions with 50% Na and 40 mM divalent cation concentrations.
The effect of the divalent cations Ca and Mg on the mean channel lifetime was complex and dependent on whether the divalent cation was Ca or Mg. The mean channel lifetime was not significantly changed in most solutions with increased Ca concentration, while it was slightly prolonged by increased Mg concentration.
Key wordsdivalent cations acetylcholine receptors ion channels neuromuscular junction Rana pipiens
Unable to display preview. Download preview PDF.
- Adams, D. J., Dwyer, T. M., and Hille, B. (1980). The permeability of endplate channels to monovalent and divalent metal cations.J. Gen. Physiol. 75493–510.Google Scholar
- Anderson, C. R., and Stevens, C. F. (1973). Voltage clamp analysis of acetylcholine produced end-plate current fluctuations at frog neuromuscular junction.J. Physiol Lond. 235655–691.Google Scholar
- Auerbach, A., and Sachs, F. (1984). Patch clamp studies of single ionic channels.Annu. Rev. Biophys. Bioeng. 13269–302.Google Scholar
- Begenisich, T. B., and Cahalan, M. D. (1980). Sodium channel permeation in squid axons. I. Reversal potential experiments.J. Physiol. Lond. 307217–242.Google Scholar
- Bregestovski, P. D., Miledi, R., and Parker, I. (1979). Calcium conductance of acetylcholine-induced end-plate channels.Nature Lond. 279638–639.Google Scholar
- Butler, J. N. (1968). The thermodynamic activity of calcium ion in sodium chloride-calcium chloride electrolytes.Biophys. J. 81426–1433.Google Scholar
- Cohen, I., and Van der Kloot, W. (1978). Effects of (Ca++) and (Mg++) on the decay of miniature end-plate currents.Nature Lond. 27177–79.Google Scholar
- Cohen, I., and Van der Kloot, W. (1982). The interaction of extracellular H+, Na+, Ca2+, and Sr2+ on the decay of miniature end-plate currents.Brain Res. 241285–290.Google Scholar
- Dani, J. A., and Eisenman, G. (1984). Acetylcholine-activated channel current-voltage relations in symmetrical Na solutions.Biophys. J. 4510–12.Google Scholar
- Dwyer, T. M., and Farley, J. M. (1984). Permeability properties of chick myotube acetylcholine-activated channels.Biophys. J. 45529–539.Google Scholar
- Eisenman, G., and Horn, R. (1983). Ionic selectivity revisited: The role of kinetic and equilibrium processes in ion permeation through channels.J. Membrane Biol. 76197–225.Google Scholar
- Goldman, D. E. (1943). Potential, impedance and rectification in membranes.J. Gen. Physiol. 2737–60.Google Scholar
- Hille, B., and Schwarz, T. (1978). Potassium channels as multi-ion single-file pores.J. Gen. Physiol. 72409–442.Google Scholar
- Hodgkin, A. L., and Katz, B. (1949). The effect of sodium ions on the electrical activity of the giant axon of the squid.J. Physiol. Lond. 10837–77.Google Scholar
- Horn, R., and Brodwick, M. S. (1980). Acetylcholine-induced current in perfused rat myoballs.J. Gen. Physiol. 75297–321.Google Scholar
- Horn, R., and Stevens, C. F. (1980). Relation between structure and function of ion channels.Comm. Mol. Cell. Biophys. 157–68.Google Scholar
- Krasne, S. (1978). Ion selectivity in membrane permeation. InMembrane Physiology (Andreoli, T. E., Hoffman, J. F., and Fanestil, D. D., Eds.), Plenum, New York, pp. 217–241.Google Scholar
- Lauger, P., Stephan, W., and Frehland, E. (1980). Fluctuations of barrier structure in ionic channels.Biochim. Biophys. Acta. 602167–180.Google Scholar
- Lewis, C. A. (1979). Ion-concentration dependence of the reversal potential and the single channel conductance of ion channels at the frog neuromuscular junction.J. Physiol. Lond. 286417–445.Google Scholar
- Lewis, C. A. (1984). Seasonal changes in the properties of frog end-plate channels.Biophys. J. 46273–276.Google Scholar
- Lewis, C. A., and Stevens, C. F. (1979). Mechanism of ion permeation through channels in a postsynaptic membrane. InMembrane Transport Processes, Vol. 3 (Stevens, C. F., and Tsien, R. W., Eds.), Raven Press, New York, pp. 133–151.Google Scholar
- Lewis, C. A., and Stevens, C. F. (1983). Acetylcholine receptor channel ionic selectivity: Ions experience an aqueous environment.Proc. Natl. Acad. Sci. 806110–6113.Google Scholar
- Magleby, K. L., and Weinstock, M. M. (1980). Nickel and calcium ions modify the characteristics of the acetylcholine receptor-channel complex at the frog neuromuscular junction.J. Physiol. Lond. 299203–218.Google Scholar
- McLarnon, J. G., and Quastel, D. M. J. (1983). Postsynaptic effects of magnesium and calcium at the mouse neuromuscular junction.J. Neurosci. 31626–1633.Google Scholar
- Miledi, R., and Parker, I. (1980). Effects of strontium ions on end-plate channel properties.J. Physiol. Lond. 306567–577.Google Scholar
- Robinson, R. A., and Stokes, R. H. (1959).Electrolyte Solutions, Butterworths, London.Google Scholar
- Shatkay, A. (1968). Individual activity of calcium ions in pure solutions of CaCl2 and in mixtures.Biophys. J. 8912–919.Google Scholar
- Stevens, C. F. (1972). Inferences about membrane properties from electrical noise measurements.Biophys. J. 121028–1047.Google Scholar
- Takeda, K., Gage, P. W., and Barry, P. H. (1982). Effects of divalent cations on toad end-plate channels.J. Membrane Biol. 6455–66.Google Scholar
- Takeuchi, N. (1963). Effects of calcium on the conductance change of the end-plate membrane during the action of transmitter.J. Physiol. Lond. 167141–155.Google Scholar