Rhodopsin inactivation is a modulated process in Limulus photoreceptors

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Abstract
Many G-protein-coupled receptors are only transiently active because an inactivation process stops the receptor from activating G protein molecules. Although this inactivation has been investigated in vitro, the real kinetics of the process can only be obtained from intact cells. Here we describe a method for measuring the inactivation of rhodopsin in intact photoreceptors and the application of this method to the ultraviolet rhodopsin of Limulus median eye. The results show that the inactivation process is very rapid (less than 150 ms) and occurs well before the peak of the receptor potential. We have also investigated whether the inactivation process can itself be modulated. Our results show that light-adaptation accelerates inactivation by about 10-fold, providing evidence that G-protein-mediated transduction can be modulated at this first stage.