Models for quick activation of transducin and spontaneous deactivation of activated rhodopsin in rod outer segments.

Serial-parallel activation (S-P) and parallel-parallel activation (P-P) models had been proposed to account for the quick activation of transmitter molecules (transducin) in the visual transduction process in rod outer segments. In these models, unbleached rhodopsin molecules are activated by bleached rhodopsin molecules, and both bleached and activated rhodopsin molecules activate transducin. This activation mechanism enables bleached rhodopsin molecules to transfer the information of photon detection quickly to transducin. In the previous models, however, the rhodopsin photoisomerization process and the deactivation process of activated rhodopsin molecules were excluded. In the present study, these two processes are incorporated into the models. The deactivation process of activated rhodopsin molecules is assumed to proceed spontaneously. It is presented that, in spite of the incorporation of these two processes, the present S-P and P-P models can account for the quick activation of transducin in rod outer segments.