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Calcium is an essential signaling molecule required for various cellular functions. Calcium pumps and ion channels on cell and organellar membranes, such as those on the endoplasmic reticulum (ER), regulate calcium concentrations inside the cell. They remain closed, keeping the cytosolic calcium levels low at a resting state.
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Mechanically-gated ion channels are proteins found in eukaryotic and prokaryotic cell membranes that open in response to mechanical stress. Tension, compression, swelling, and shear stress can alter the conformation of the protein, opening a transmembrane channel that allows the passage of ions for signal transmission. In eukaryotes, mechanically-gated channels are distributed in several regions like the neurons, lungs, skin, bladder, and heart, where they play critical roles in numerous...
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Single-Cell Calcium Imaging for Studying the Activation of Calcium Ion Channels
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Tunable calcium current through TRPV1 receptor channels.

Damien S K Samways1, Baljit S Khakh, Terrance M Egan

  • 1Department of Pharmacological and Physiological Science, Saint Louis University School of Medicine, St. Louis, Missouri 63104, USA. samwaysds@slu.edu

The Journal of Biological Chemistry
|September 9, 2008
PubMed
Summary

The calcium (Ca2+) current through TRPV1 receptors changes based on how they are activated. Proton activation results in a smaller Ca2+ current compared to capsaicin activation due to specific amino acid interactions.

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Area of Science:

  • Ion channel physiology
  • Molecular biology
  • Neuroscience

Background:

  • Transient Receptor Potential Vanilloid 1 (TRPV1) channels are crucial for sensing heat, pain, and chemicals like capsaicin.
  • Calcium ions (Ca2+) play a vital role in TRPV1 signaling pathways.
  • Understanding the precise contribution of Ca2+ to TRPV1 responses is essential for elucidating pain mechanisms.

Purpose of the Study:

  • To quantify the exact contribution of Ca2+ to TRPV1 channel activity under different activation modes.
  • To investigate how the mode of TRPV1 receptor activation influences the fraction of current carried by Ca2+.
  • To identify the molecular mechanisms underlying variations in Ca2+ permeability during TRPV1 activation.

Main Methods:

  • Patch clamp photometry was employed to measure ion currents and Ca2+ flux through TRPV1.
  • Site-directed mutagenesis was used to alter specific amino acid residues within the TRPV1 pore.
  • TRPV1 receptors were activated using capsaicin and changes in pH (protons) to compare Ca2+ currents.

Main Results:

  • The fraction of current carried by Ca2+ (Pf%) was significantly lower when TRPV1 was activated by protons compared to capsaicin.
  • Protonation of three acidic amino acids (Asp646, Glu648, Glu651) in the pore mouth was identified as the cause for the reduced Ca2+ current under acidic conditions.
  • This demonstrates a dynamic and activation-dependent Ca2+ permeability in TRPV1 receptors.

Conclusions:

  • TRPV1 receptor's Ca2+ current is not static but dynamically regulated by the specific activating stimulus.
  • Acidic conditions (low pH) reduce Ca2+ influx through TRPV1 by altering interactions within the pore, impacting physiological signaling.
  • These findings reveal a novel layer of regulation for TRPV1 channel function, relevant to pain and sensory perception.