Protons block the dark current of isolated retinal rods

Area of Science:

• Cellular Physiology
• Phototransduction
• Ion Channel Regulation

Background:

• Frog rods are key photoreceptor cells responsible for vision in dim light.
• Membrane currents, particularly sodium conductance, are crucial for rod function.
• The role of intracellular protons (H+) in modulating these currents is not fully understood.

Purpose of the Study:

• To investigate the effect of intracellular protons (H+) on membrane currents in isolated frog rods.
• To elucidate the mechanism by which H+ influences sodium (Na+) conductance.
• To determine the relationship between proton sensitivity, calcium (Ca2+) levels, and light sensitivity.

Main Methods:

• Recording of membrane currents in isolated frog rods using the suction pipette technique.
• Perfusion techniques to control and test the sensitivity of rods to varying H+ concentrations.
• Analysis of the relationship between H+ concentration, Ca2+ concentration, and Na+ conductance.

Main Results:

• Increased intracellular H+ rapidly and reversibly suppresses the Na+ conductance of the outer segment.
• The concentration of H+ required for a 50% decrease in Na+ conductance is inversely proportional to Ca2+ concentration.
• H+ sensitivity and light sensitivity exhibit the same dependence on Ca2+ concentration, suggesting a shared pathway.

Conclusions:

• Protons (H+) act internally within frog rods to suppress membrane Na+ conductance, similar to light.
• The effect of H+ appears to be mediated through Ca2+, potentially involving Ca2+ liberation from disks.
• A proposed mechanism involves light-activated hydrolysis of cyclic GMP (cGMP) producing protons that release Ca2+ via ion exchange.