A quantitative model of phototransduction and light adaptation in amphibian rod photoreceptors.

The physical events underlying phototransduction are now so well understood that phototransduction has become a problem nearing its solution. Both the kinetics and many details of the underlying biochemistry have been determined, so that it is now possible to construct a mathematical model of the transduction process. The proposed model, which consists of 7 coupled differential equations, aims at identifying the essential mechanisms underlying phototransduction. Six of the equations are first-order kinetic equations for the intracellular concentrations of the main molecules involved in phototransduction. The seventh is a kinetic equation for cGMP concentration that incorporates the known nonlinear dependence on calcium concentration. The model is able to reproduce quantitatively most known electrophysiological results on phototransduction. Photocurrent depends directly and nonlinearly on cGMP concentration. A key aspect of the model is the nonlinear interaction between calcium and cGMP through which calcium tends to decrease cGMP concentration and cGMP tends to increase calcium concentration. By giving insight into these interactions, the model provides a better understanding of the regulation of second messengers during transduction, a problem that has so far defied experimental analysis.