Photomechanical coupling in the vertebrate sphincter pupillae.

The photomechanical response of the vertebrate iris sphincter pupillae isolated from irises of many species of vertebrates contract when light is shined on them. It appears that the cell membranes of the constituent smooth muscle cells contain rhodopsin which triggers the photomechanical response (PMR) when bleached. In amphibians and some fish this mechanism of pupillary control is more important than the more well-known retinal reflex. In the mammals the retinal reflex is more important; however, even in the mammals the exact role of the innervation is not understood. The PMR can be inhibited by beta adrenergic agonists but not by alpha adrenergic agonists. The activation sequence of the PM probably involves (1) rhodopsin activated G-protein, (2) phospholipase C, (3) inositol triphosphate, and (4) a calcium-calmodulin-myosin light chain kinase cascade. A simple mathematical version of the phosphorylation theory of smooth muscle contraction accurately predicts the time courses of PMRs to light stimuli of different durations and intensities.