Discrete voltage fluctuations that occur spontaneously or in response to dim lights can be recorded from the ventral photoreceptors of Limulus. The injection of vanadate or the hydrolysis-resistant analog of guanosine triphosphate, GTP-gamma-S, into ventral photoreceptors induces the production of discrete waves in the dark. The chemically induced discrete waves are similar to those induced by light. Ventral photoreceptors may contain a guanyl nucleotide binding protein whose activation by vanadate or GTP-gamma-S induces the discrete waves.
