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Fig. 3. Current–voltage relationship of the glutamate-induced current in
muscle fibres from honeybee. (A) Protocol of voltage ramps running from
–130 mV to +30 mV applied before (ramp 1), during (ramps 2 and 3) and
after (ramp 4) pressure application of L-glutamate (1 mmol
l–1 in Tyrode's solution) for 8 s in one muscle fibre
otherwise voltage-clamped at –80 mV. Voltage ramps induced activation of
voltage-dependent currents and only those with weak voltage-dependent inward
calcium current were used for analysis. Application of glutamate induced a
current with a sustained inward component (with slow inactivation) at
–80 mV. (B) Current–voltage relationships of the whole-cell
current obtained in response to the voltage ramps shown in A in the absence (1
and 4, black) or in the presence (2 and 3, grey) of glutamate, show that
glutamate elicits an inward component at negative potentials. (C) The mean
current–voltage relationship of the glutamate-induced current
(N=4) appears linear below approx. –20 mV and presents an
inward rectification for more positive potential. On the voltage scale
considered, the glutamate-induced steady component was null for membrane
potentials above approx. 0 mV. The grey shading indicates s.e.m. at each
potential explored. Em, voltage command.
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