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Single-channel properties and gating of Na+ and K+ channels in the squid giant axon
Bezanilla, F.; Correa, A.M. (1995). Single-channel properties and gating of Na+ and K+ channels in the squid giant axon, in: Abbott, N.J. et al. (Ed.) Cephalopod neurobiology: neuroscience studies in squid, octopus and cuttlefish. pp. 131-151
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| Auteurs | | Top |
- Bezanilla, F.
- Correa, A.M.
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| Abstract |
This chapter describes patch clamp recordings made from the inner membrane surface of the cut-open squid giant axon, allowing examination of macroscopic ionic currents, gating currents, and single channel fluctuations. Sodium currents could be studied in the absence of divalent cations, and showed a ~ 15 mV shift to more negative potentials of the voltage-dependence of all parameters of activation and inactivation, suggesting that divalent cations neutralize external negative charge near the voltage sensor of the channel. Batrachotoxin (BTX) eliminated fast and slow inactivation of a fraction of the Na channels and reduced single channel conductance from 33 pS to 11 pS. The conductance of the Na channel increased with [Na+], saturating at about 1M. The results fitted a single-ion pore, 3 barrier 2 site model, with fixed charge at the pore vestibules. Modelling of the open-closed states of the Na channels requires several inactivated states to account for charge immobilization, and several closed states to account for the lag of the ionic current and absence of rising phase in the gating current. At positive potentials the currents show a second open state with the same conductance value, and this second state is responsible for the incomplete inactivation observed at positive potentials. Use of BTX permits analysis of the effect of voltage and temperature on Na channel kinetics, providing information about the underlying molecular events, and refinement of the channel model. Several types of K channels can also be detected: a 20 pS (delayed rectifier) channel, and less abundant 10 pS and 40 pS channels. The delayed rectifier is modulated by internal ATP, and phosphorylation increased the probability of opening; the results could be modelled by shifts in the voltage-dependence of activation and inactivation as a result of addition of negative charges. |
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