The Amplifying Transduction Cascade Triggered by Rhodopsin in Visual Receptor Cells Biochemical and Biophysical Approaches
After a long rivalry with Hagins’ “calcium hypothesis” cyclic GMP is now solidly established as the cytoplasmic transmitter of the visual excitation process in the vertebrate photoreceptor1,2: in these cells cGMP directly controls the conductance of the Na+ channels in the plasma membrane; in this respect, the visual transduction process differs from the usual pathway of cyclic nucleoside dependent kinase activation, found in the transduction process of many hormonal or neuronal signals. In many other respects the light sensitive and the hormone sensitive systems present striking similarities. The Rhodopsin-Transducin-cGMP phosphodiesterase cascade parallels exactly that of hormone receptor-G protein-AMPcyclase. It became clear a few years ago that transducin, the GTP binding protein of the visual system, is a member of the growing family of G proteins responsible for the coupling of hormonal or neuronal membrane receptors to their various intracellular effectors: AMP cyclase, phosphodiesterase or phospholipase specific for phosphatidylinositol hydrolysis. This analogy between the transducin cascade and the other G protein mediated processes had led a few years ago to the suggestion that rhodopsin might be considered as a special type of hormone receptor3.
KeywordsLight Flash Release Signal Inhibitory Subunit Forward Detector cGMP Phosphodiesterase
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