The smooth muscle sphincters of the irises of many vertebrate species contract when visible light is shined on them. They do so with an action spectrum like the absorption spectrum of rhodopsin, the retinal rod pigment. This is not mediated by neural elements or even any electrical event at the cell membrane. It is proposed to determine whether the myocytes contain rhodopsin as well as various other proteins suspected of being involved, in phototransduction and regulation of smooth muscle contraction including: transducin, a rod protein, and three proteins commonly found in the contractile apparatus of smooth muscle; calmodulin, myosin light chain kinase and myosin. A mathematical description of the biochemical reactions thought to be involved has been derived and this project proposes to test the predictions of the theory by a variety of cellular physiological techniques. These would include making the cells very permeable to molecules which otherwise would not pass the cell membrane. In this way the concentrations of molecules can be set experimentally. Another technique to be used is to release biologically active compounds from photolabile complexes by strong flashes of light. The direct activation of the frog iris muscle by light involves a combination of biochemical reactions that couple the bleaching of a photopigment to contraction of the smooth muscle by an actin-myosin based system. These biochemical reactions are involved in signal transduction in a variety of cell types. Thus, further study of this unique system may shed new light on signal transduction mechanisms in general.
