Ionic permeabilities ofl-glutamate activated, excitatory synaptic channel in crayfish muscle

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Abstract
Excitatory single channel currents triggered byl-glutamate were measured in outside-out excised patches of crayfish muscle membrane. If an ‘intracellular’ solution was present in the pipette and normal extracellular solution with added glutamate (10−3 M) passed the outside of the patch, the single channel currents,i 1, had amplitudes of −8 pA at a patch potential of −70 mV. If in the extracellular solution Na+ was replaced by Li+ or Ca2+, the amplitudes of single channel currents were reduced by about 30%. Only about 20% of the channel current amplitude remained on replacement of Na+ by choline. Replacement of Na+ reduced the variance of channel amplitude distributions to the level of the baseline. Presence of Na+ thus induces an additional variance of open channel current. When the proportions of Na+/choline were varied, the resulting channel currents could be separated in Na+, Ca2+ and choline components. The amplitude of the Na+ component,i 1,Na, could be described by a constant channel permeability πNa = 110 10−15 cm3 s−1 according to the constant field equation. Ba2+ could replace Ca2+ without change in single channel current, while replacement of Ca2+ by Mg2+ reduced the channel currents by 20%. The following permeabilities of the single channel were estimated (in 10−15 cm3 s−1): πNa = 110, πK = 86, πCa = 30, πMg = 24, πBa = 30, πLi = 84 and πcholine = 11. These permeabilities were obtained inserting ionic concentrations. The respective permeabilities are listed also as calculated on the basis of ionic activities. In presence of high Na+ the single channel current amplitudes were not affected appreciably by a reduction of the extracellular Ca2+ from 13.5 to 1 mM, or by application of ‘calcium channel blockers’ like La3+ or Cd2+.