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<p>The phototransduction cascade in vertebrate retinal photoreceptors uses cyclic guanosine monophosphate (cGMP) as second messenger. cGMP levels control the conductance of plasma membrane channels, so the rate at which cGMP levels change is critical for photoreceptor performance. There is an apparent contradiction between the high total cGMP concentration in the outer segments and the experimentally measured high rate of cGMP turnover. This contradiction can be resolved by presuming that most cGMP is bound, such that the dynamic fraction represents a small proportion of the total cGMP but is the fraction undergoing changes in response to light and controlling channel conductance. This review examines evidence for the partitioning of the total cGMP pool into bound and free fractions, probable cGMP binding sites, and the possible functional significance of the existence of the two fractions.</p>

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Free and Bound cGMP Fractions in the Outer Segment of Vertebrate Photoreceptors

  • 期刊论文

  • 发表日期：

    2026年2月24日

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  • M. L. Firsov

摘要

The phototransduction cascade in vertebrate retinal photoreceptors uses cyclic guanosine monophosphate (cGMP) as second messenger. cGMP levels control the conductance of plasma membrane channels, so the rate at which cGMP levels change is critical for photoreceptor performance. There is an apparent contradiction between the high total cGMP concentration in the outer segments and the experimentally measured high rate of cGMP turnover. This contradiction can be resolved by presuming that most cGMP is bound, such that the dynamic fraction represents a small proportion of the total cGMP but is the fraction undergoing changes in response to light and controlling channel conductance. This review examines evidence for the partitioning of the total cGMP pool into bound and free fractions, probable cGMP binding sites, and the possible functional significance of the existence of the two fractions.