The Cytosolic Concentration of Free Ca2+ in Vertebrate Rod Outer Segments

  • W. G. Owen
  • G. M. Ratto
Part of the NATO ASI Series book series (NSSA, volume 194)


The role of calcium in the mechanism of phototransduction has yet to be defined. The observation of Yoshikami and Hagins (1971), that an application of calcium to the outside of the rod caused a reduction in the dark-current, and thus mimicked the effect of light, led to the well-known “calcium hypothesis” according to which, calcium ions, released by light from an intracellular store, were internal messengers that diffused to the plasma membrane and caused the closure of “light-sensitive” channels. This hypothesis stimulated an enormous interest in phototransduction during the subsequent decade and a half. During that time, however, several lines of evidence began to point to cyclic GMP as the internal transmitter, the recent demonstration, by Fesenko, Kolesnikov and Lyubarsky (1985, 1986), that the channels in patches of plasma membrane excised from rod outer segments (ROS) are activated by cGMP and are not blocked by calcium ions, being, perhaps, the most direct (see also Yau and Nakatani, 1985b). These channels have properties identical to those of the light-sensitive channels (Matthews, 1986).


Outer Segment Guanylate Cyclase Free Calcium Light Adaptation Dark Level 
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Copyright information

© Plenum Press, New York 1990

Authors and Affiliations

  • W. G. Owen
    • 1
  • G. M. Ratto
    • 1
    • 2
  1. 1.Department of Molecular and Cell BiologyUniversity of CaliforniaBerkeleyUSA
  2. 2.Istituto di NeurofisiologiaCNRPisaItaly

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