Control of light-sensitive current in salamander rods.

Area of Science:

• Phototransduction in retinal rods
• Cellular signaling pathways in vision

Background:

• The light-sensitive current in retinal rods is critical for vision.
• Cyclic guanosine monophosphate (cGMP) is a key second messenger in phototransduction.
• Phosphodiesterase (PDE) hydrolyzes cGMP, regulating its levels.

Purpose of the Study:

• To investigate the role of phosphodiesterase (PDE) activity in the decline of light-sensitive current.
• To elucidate the relationship between PDE activity, cyclic GMP hydrolysis, and the flash response in retinal rods.
• To explore the involvement of calcium ions and guanylate cyclase in phototransduction regulation.

Main Methods:

• Utilized the Na+/Li+ switch method in toad and salamander rods to measure the rate constant (b) of current decline.
• Employed suction electrodes to record light-sensitive currents.
• Administered 3-isobutyl-1-methylxanthine (IBMX), a PDE inhibitor, and varied light intensities.
• Applied light flashes of controlled strengths (e.g., 140 Rh, 5000-10,000 Rh).

Main Results:

• The rate constant (b) of current decline, indicative of PDE activity, increased significantly with light exposure and IBMX treatment.
• Moderate flashes (140 Rh) increased 'b' approximately 7-fold within 0.5 seconds, with subsequent decline.
• Strong flashes (5000-10,000 Rh) elevated 'b' substantially, suggesting PDE activity changes are sufficient to explain the flash response's rising phase.
• Light-sensitive current recovery was faster than the decline in 'b', suggesting a delayed increase in guanylate cyclase activity.

Conclusions:

• The activity of phosphodiesterase (PDE) is a primary determinant of the exponential decline in light-sensitive current following light stimulation.
• Changes in PDE activity, influenced by light and calcium ions, are sufficient to account for the rising phase of the visual flash response.
• A delayed activation of guanylate cyclase, possibly due to reduced calcium inhibition, explains the rapid current recovery post-flash.