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  • Title: Guanosine 3',5'-cyclic monophosphate and the in vitro physiology of frog photoreceptor membranes.
    Author: Woodruff ML, Bownds D, Green SH, Morrisey JL, Shedlovsky A.
    Journal: J Gen Physiol; 1977 May; 69(5):667-79. PubMed ID: 194013.
    Abstract:
    Frog rod outer segments freshly detached from dark-adapted retinas contain approximately 1-2 molecules of guanosine 3',5'-cyclic monophosphate (cyclic GMP) for every 100 molecules of visual pigment present. This cyclic GMP decays to 5'-GMP, and the conversion is accelerated upon illumination of the outer segments. Bleaching one rhodopsin molecule can lead to the hydrolysis of 1,000-2,000 molecules of cyclic GMP within 100-300 ms. The decline in cyclic GMP concentration becomes larger as illumination increases, and varies with the logarithm of light intensity at levels which bleach between 5 X 10(2) and 5 X 10(5) rhodopsin molecules per outer segment-second. Light suppression of plasma membrane permeability, assayed in vitro as light suppression of outer segment swelling in a modified Ringer's solution, occurs over this same range of light intensity. The correlation between cyclic GMP and permeability or swelling is maintained in the presence of two pharmacological perturbations: papaverine, a phosphodiesterase inhibitor, increases both cyclic GMP levels and the dark permeability of the plasma membrane; and beta,gamma-methylene ATP increases the effectiveness of light in suppressing both permeability and cyclic GMP levels.
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