On the relation between rapid light-induced Ca2+ release and proton uptake in rod outer segment disk membranes

Area of Science:

• Phototransduction in photoreceptor cells
• Membrane biophysics and ion transport
• Cellular signaling mechanisms

Background:

• Rod outer segment (ROS) disk membranes are crucial for visual signal transduction.
• Light-induced calcium (Ca2+) release and proton uptake are key events in ROS signaling.
• Understanding the precise location and regulation of these ion fluxes is essential.

Purpose of the Study:

• To pinpoint the intracellular site of light-induced Ca2+ release in ROS disk membranes.
• To investigate the role of electrolytes in modulating Ca2+ release.
• To elucidate the relationship between Ca2+ release, proton uptake, and metarhodopsin II formation kinetics.

Main Methods:

• Flash-spectrophotometry utilizing arsenazo III and bromcresol purple indicator dyes.
• Experiments on intact ROS, leaky ROS, and sonicated ROS vesicles.
• Use of ionophore A23187 to facilitate ion communication across membranes.
• Electrolysis of suspension media to study electrolyte effects.

Main Results:

• Ca2+ release occurs at the luminal side of the disk membrane and is not transported across it within 20s.
• Light-induced Ca2+ release is inhibited by electrolytes, with effectiveness increasing with cation charge (monovalent < divalent < trivalent).
• Electrolytes screen the negative surface charge of the disk membrane, affecting surface potential and bound Ca2+.
• Kinetics of Ca2+ release and proton uptake are structurally dependent, showing distinct Arrhenius plots for intact vs. sonicated ROS.

Conclusions:

• Ca2+ release is localized to the luminal side of the ROS disk membrane.
• Electrolytes regulate Ca2+ release by modulating membrane surface potential.
• A model is proposed where metarhodopsin II formation triggers proton uptake, reducing surface potential and causing Ca2+ release.