How does phosphodiesterase change cGMP levels in a photoreceptor?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

details of the mammalian eye — Photoreception The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.

In vertebrate photoreceptors, the enzyme phosphodiesterase plays a critical role in the phototransduction cascade by regulating the concentration of the second messenger cGMP (3′,5′-cyclic guanosine monophosphate).

When a photon of light strikes the rhodopsin molecule, it triggers the activation of a G-protein known as transducin. This activated transducin subsequently activates phosphodiesterase. Once active, phosphodiesterase functions by cleaving cGMP into 5′-GMP. This enzymatic activity directly reduces the total amount of cGMP available within the cell.1

This reduction is essential for the cell's electrical response to light. In the dark, cGMP binds to sodium channels, keeping them open and maintaining the cell in a depolarized state. As phosphodiesterase lowers cGMP levels in the presence of light, these channels close, cutting off the inward flow of sodium ions and causing the photoreceptor to become hyperpolarized.1