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Transduction ion channels directly gated by sugars on the insect taste cell.

Murakami M, Kijima H - J. Gen. Physiol. (2000)

Bottom Line: An inhibitor of G-protein activation, GDP-beta-S, did not significantly decrease the sucrose response.These results strongly suggested that the channel is an ionotropic receptor (a receptor/channel complex), activated directly by sucrose without mediation by second messengers or G protein.We also report transduction ion channels of the receptor/channel complex type directly gated by fructose and those gated by L-valine located on the sensory process.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan. midori@bio.phys.nagoya-u.ac.jp

ABSTRACT
Insects detect sugars and amino acids by a specialized taste cell, the sugar receptor cell, in the taste hairs located on their labela and tarsi. We patch-clamped sensory processes of taste cells regenerated from the cut end of the taste hairs on the labelum of the flashfly isolated from the pupa approximately 20 h before emergence. We recorded both single channel and ensemble currents of novel ion channels located on the distal membrane of the sensory process of the sugar receptor cell. In the stable outside-out patch membrane excised from the sensory processes, we could repeatedly record sucrose-induced currents for tens of minutes without appreciable decrease. An inhibitor of G-protein activation, GDP-beta-S, did not significantly decrease the sucrose response. These results strongly suggested that the channel is an ionotropic receptor (a receptor/channel complex), activated directly by sucrose without mediation by second messengers or G protein. The channel was shown to be a nonselective cation channel. Analyses of single channel currents showed that the sucrose-gated channel has a single channel conductance of approximately 30 pS and has a very short mean open time of approximately 0.23 ms. It is inhibited by external Ca(2+) and the dose-current amplitude relation could be described by a Michaelis-Menten curve with an apparent dissociation constant of approximately 270 mM. We also report transduction ion channels of the receptor/channel complex type directly gated by fructose and those gated by L-valine located on the sensory process.

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Ionic properties of sucrose-activated channels in the outside-out patches. (A) A sucrose-induced current record of an outside-out patch voltage clamped at −60 mV, in which two voltage ramps from −60 to +60 mV were inserted in the period of steady sucrose-induced current (right) and in the control period without sucrose application (left). (B) I-V relationships of the ensemble current induced by 250 mM sucrose obtained from the current traces responding to voltage ramps from −60 to +60 mV in the external Ca2+-free Ringer (NaCl) and in solutions in which Na+ was replaced by K+ (KCl, gray trace) and choline+ (choline chloride). The trace of NaCl was obtained from the record shown in A. (C) I-V relationships in external solutions in which NaCl was half (50% TEA+, gray trace) or fully (100% TEA+) replaced by TEA chloride. TEA+ blocked sucrose-activated channels from the extracellular side both inwardly and outwardly. (D) Inhibition of sucrose-induced currents by Ca2+ in the stimulant solution. Current traces induced by 250 mM sucrose on an outside-out patch in various concentrations of Ca2+ (given in millimolar) are shown. The period of stimulation is shown by a shadowed bar. (E) A plot of responses against Ca2+ concentration. Relative magnitude of conductances activated by 250 mM sucrose with voltage ramps from −60 to +60 mV (n = 3, open bars) and the variances of current fluctuations at −60 mV(♦) in various Ca2+ concentrations. SEMs are shown by vertical lines above (conductances) or below (variances) the data points.
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Figure 7: Ionic properties of sucrose-activated channels in the outside-out patches. (A) A sucrose-induced current record of an outside-out patch voltage clamped at −60 mV, in which two voltage ramps from −60 to +60 mV were inserted in the period of steady sucrose-induced current (right) and in the control period without sucrose application (left). (B) I-V relationships of the ensemble current induced by 250 mM sucrose obtained from the current traces responding to voltage ramps from −60 to +60 mV in the external Ca2+-free Ringer (NaCl) and in solutions in which Na+ was replaced by K+ (KCl, gray trace) and choline+ (choline chloride). The trace of NaCl was obtained from the record shown in A. (C) I-V relationships in external solutions in which NaCl was half (50% TEA+, gray trace) or fully (100% TEA+) replaced by TEA chloride. TEA+ blocked sucrose-activated channels from the extracellular side both inwardly and outwardly. (D) Inhibition of sucrose-induced currents by Ca2+ in the stimulant solution. Current traces induced by 250 mM sucrose on an outside-out patch in various concentrations of Ca2+ (given in millimolar) are shown. The period of stimulation is shown by a shadowed bar. (E) A plot of responses against Ca2+ concentration. Relative magnitude of conductances activated by 250 mM sucrose with voltage ramps from −60 to +60 mV (n = 3, open bars) and the variances of current fluctuations at −60 mV(♦) in various Ca2+ concentrations. SEMs are shown by vertical lines above (conductances) or below (variances) the data points.

Mentions: The reversal potentials were also estimated from the sucrose-induced ensemble currents. The I-V relations of the ensemble currents were obtained by applying a ramp voltage from −60 to +60 mV (see the traces of NaCl in Fig. 7B and Fig. C). It was 5.3 ± 0.9 mV (n = 9), in good agreement with that of the single-channel current, and suggested that the channel may permeate cations nonselectively.


Transduction ion channels directly gated by sugars on the insect taste cell.

Murakami M, Kijima H - J. Gen. Physiol. (2000)

Ionic properties of sucrose-activated channels in the outside-out patches. (A) A sucrose-induced current record of an outside-out patch voltage clamped at −60 mV, in which two voltage ramps from −60 to +60 mV were inserted in the period of steady sucrose-induced current (right) and in the control period without sucrose application (left). (B) I-V relationships of the ensemble current induced by 250 mM sucrose obtained from the current traces responding to voltage ramps from −60 to +60 mV in the external Ca2+-free Ringer (NaCl) and in solutions in which Na+ was replaced by K+ (KCl, gray trace) and choline+ (choline chloride). The trace of NaCl was obtained from the record shown in A. (C) I-V relationships in external solutions in which NaCl was half (50% TEA+, gray trace) or fully (100% TEA+) replaced by TEA chloride. TEA+ blocked sucrose-activated channels from the extracellular side both inwardly and outwardly. (D) Inhibition of sucrose-induced currents by Ca2+ in the stimulant solution. Current traces induced by 250 mM sucrose on an outside-out patch in various concentrations of Ca2+ (given in millimolar) are shown. The period of stimulation is shown by a shadowed bar. (E) A plot of responses against Ca2+ concentration. Relative magnitude of conductances activated by 250 mM sucrose with voltage ramps from −60 to +60 mV (n = 3, open bars) and the variances of current fluctuations at −60 mV(♦) in various Ca2+ concentrations. SEMs are shown by vertical lines above (conductances) or below (variances) the data points.
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Related In: Results  -  Collection

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Figure 7: Ionic properties of sucrose-activated channels in the outside-out patches. (A) A sucrose-induced current record of an outside-out patch voltage clamped at −60 mV, in which two voltage ramps from −60 to +60 mV were inserted in the period of steady sucrose-induced current (right) and in the control period without sucrose application (left). (B) I-V relationships of the ensemble current induced by 250 mM sucrose obtained from the current traces responding to voltage ramps from −60 to +60 mV in the external Ca2+-free Ringer (NaCl) and in solutions in which Na+ was replaced by K+ (KCl, gray trace) and choline+ (choline chloride). The trace of NaCl was obtained from the record shown in A. (C) I-V relationships in external solutions in which NaCl was half (50% TEA+, gray trace) or fully (100% TEA+) replaced by TEA chloride. TEA+ blocked sucrose-activated channels from the extracellular side both inwardly and outwardly. (D) Inhibition of sucrose-induced currents by Ca2+ in the stimulant solution. Current traces induced by 250 mM sucrose on an outside-out patch in various concentrations of Ca2+ (given in millimolar) are shown. The period of stimulation is shown by a shadowed bar. (E) A plot of responses against Ca2+ concentration. Relative magnitude of conductances activated by 250 mM sucrose with voltage ramps from −60 to +60 mV (n = 3, open bars) and the variances of current fluctuations at −60 mV(♦) in various Ca2+ concentrations. SEMs are shown by vertical lines above (conductances) or below (variances) the data points.
Mentions: The reversal potentials were also estimated from the sucrose-induced ensemble currents. The I-V relations of the ensemble currents were obtained by applying a ramp voltage from −60 to +60 mV (see the traces of NaCl in Fig. 7B and Fig. C). It was 5.3 ± 0.9 mV (n = 9), in good agreement with that of the single-channel current, and suggested that the channel may permeate cations nonselectively.

Bottom Line: An inhibitor of G-protein activation, GDP-beta-S, did not significantly decrease the sucrose response.These results strongly suggested that the channel is an ionotropic receptor (a receptor/channel complex), activated directly by sucrose without mediation by second messengers or G protein.We also report transduction ion channels of the receptor/channel complex type directly gated by fructose and those gated by L-valine located on the sensory process.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan. midori@bio.phys.nagoya-u.ac.jp

ABSTRACT
Insects detect sugars and amino acids by a specialized taste cell, the sugar receptor cell, in the taste hairs located on their labela and tarsi. We patch-clamped sensory processes of taste cells regenerated from the cut end of the taste hairs on the labelum of the flashfly isolated from the pupa approximately 20 h before emergence. We recorded both single channel and ensemble currents of novel ion channels located on the distal membrane of the sensory process of the sugar receptor cell. In the stable outside-out patch membrane excised from the sensory processes, we could repeatedly record sucrose-induced currents for tens of minutes without appreciable decrease. An inhibitor of G-protein activation, GDP-beta-S, did not significantly decrease the sucrose response. These results strongly suggested that the channel is an ionotropic receptor (a receptor/channel complex), activated directly by sucrose without mediation by second messengers or G protein. The channel was shown to be a nonselective cation channel. Analyses of single channel currents showed that the sucrose-gated channel has a single channel conductance of approximately 30 pS and has a very short mean open time of approximately 0.23 ms. It is inhibited by external Ca(2+) and the dose-current amplitude relation could be described by a Michaelis-Menten curve with an apparent dissociation constant of approximately 270 mM. We also report transduction ion channels of the receptor/channel complex type directly gated by fructose and those gated by L-valine located on the sensory process.

Show MeSH
Related in: MedlinePlus