Ionic aspects of excitation in rod outer segments.

The current status of the problem of ionic mechanisms underlying excitation of vertebrate photoreceptors is reviewed. Evidence is presented that the ionic dark current of retinal rods is sustained by the action of a ouabain-sensitive Na-K exchange pump driven by oxidative metabolism. The photoreceptors are depleted of K when the pump is stopped by ouabain. Considerations of cell membrane topology, kinetics of the light response, and signal amplification indicate that the light-induced suppression of the ionic dark current is mediated by a diffusible internal chemical transmitter substance. The desensitizing effect of low CA2+ Ringer's on the light responses of vertebrate rods, along with other indirect evidence, suggests that the transmitter substance may be calcium ions released from the internal membranous disks of the outer segments.