BIOCHEMICAL STEPS IN VISUAL TRANSDUCTION: ROLES FOR NUCLEOTIDES AND CALCIUM IONS*

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Abstract
Abstract— Recent studies from this laboratory permit the suggestion of a scheme for describing molecular mechanisms which may regulate excitation and adaptation in amphibian rod photoreceptor cells. The experiments have studied several chemical changes which occur upon illumination of isolated rod outer segments: (1) activation of cyclic GMP phosphodiesterase which is sensitive to calcium concentration, (2) a resulting rapid drop in cyclic GMP levels which has stoichiometry and time course appropriate for the internal transmitter presumed to mediate between photon absorption in the disc membrane system and the permeability decrease in the plasma membrane, (3) a dephosphorylation of two small proteins whose phosphorylation is controlled by cyclic GMP levels, and (4) a slower hydrolysis of GTP which may drive efflux of calcium from the outer segment.It is suggested that the rapid decrease in sodium conductance which follows illumination is caused by the dephosphorylation of the two small proteins, with their dephosphorylation being controlled by the cyclic GMP decrease. In a slower reaction light activates a GTP‐dependent extrusion of calcium from the cytoplasmic space. This lowering of internal calcium causes desensitization of the light‐sensitive phosphodiesterase enzyme responsible for the cyclic GMP decrease, so that its intensity‐response function now resembles that of the light‐adapted rod photoreceptor. Thus, changes in plasma membrane conductance are regulated by cyclic GMP, and the sensitivity of the system is controlled by slower calcium movements which set the light‐sensitivity of the phosphodiesterase enzyme. Finally, the light‐initiated phosphorylation of rhodopsin also appears to play a role, with phosphorylated rhodopsin causing desensitization of the phosphodiesterase enzyme.