Suppose a mutation greatly increases the affinity of a rod’s sodium channels for cgmp, such that once cgmp binds to a channel, it remains bound for much longer than normal. what would be the effect of this mutation on the rod’s sodium channels?

Out of the following given choices;

A.      The sodium channels would open and close randomly, regardless of the presence of light.

B.      The sodium channels would remain closed in the light and the dark, even when cGMP decreases after light stimulation.

C.      The sodium channels would remain open in the dark and close rapidly to the light.

D.      The sodium channels would remain open longer, even when cGMP decreases after light stimulation.

The answer is D. cGMP maintain the sodium channels open when there is no light stimulation. This ensures that there is high intracellular Ca2+ concentration due to the Na+/Ca2+-K+ exchanger. When cGMP is hydrolyzed in light stimulation cascade, the channels close causing hyperpolarization of the rod cells.