myo-inositol polyphosphate may be a messenger for visual excitation in Limulus photoreceptors

Abstract

Photoreceptor excitation begins with the absorption of a photon by rhodopsin and proceeds through an unknown sequence of steps that leads to changes in specific ionic conductances. These conductance changes produce the receptor potential. It has been proposed^1–3 that hydrolysis of phosphoinositides is involved in the control of a variety of physiological processes. Recent studies have implicated inositol 1,4,5-trisphosphate as an intracellular messenger in the cascade mediating hormone-stimulated secretion^4–7. We propose that one of the steps in the excitatory cascade^8,9 in Limulus ventral photoreceptors may be an increase in intracellular concentration of myo-inositol polyphosphates, derived from hydrolysis of the membrane component phosphatidyl-inositol bisphosphate by a phospholipase. Here we present biochemical and electrophysiological evidence that an inositol polyphosphate may be an intracellular messenger in the cascade mediating excitation, based on the following criteria: (1) the cells possess the synthetic and degradative metabolism for the messenger; (2) the natural stimulus leads to a change in the concentration of the messenger within the cells; and (3) intracellular injection of exogenous messenger mimics naturally occurring electrophysiological events.