1. Photoreceptors ofMusca were superfused with solutions containing either fluoride (F−), vanadate (VO 3 − ), or one of the GTP analogs GPPNHP and GTPγS. These agents have been reported to generate discrete voltage fluctuations (bumps) when injected into Limulus photoreceptors. All four agents produced noisy depolarizations in Musca photoreceptors when added to the perfusate.
2. The effects of all four agents were reversible. GPPNHP induced dark noise only after exposure to light, whereas VO 3 − and F− did not require light for their effect.
3. Although individual bumps were too small to be resolved, the power spectra of voltage noise induced by these agents resembled closely those of light-induced noise.
4. F−-induced noise was reduced by adaptation following strong illumination. As F−-induced depolarization increased beyond 6 mV the voltage noise diminished, probably due in part to adaptation. With increasing F−-induced depolarization the power spectrum of F− noise changed shape slightly. Part of this change was probably due to adaptation, and it was similar to but smaller than corresponding changes in the spectrum of light-induced noise.
5. Analysis of F−-induced noise indicated that it was composed of bumps which, though similar in time course, were not identical to light-induced bumps. At small depolarizations, the F−-induced bumps were calculated to be 3–5 times smaller than light bumps and were less strongly affected by adaptation. Although the calculated duration of light-induced bumps diminished as intensity increased, the duration of F−-induced bumps showed no such dependence on depolarization, and was equal to that of light bumps in moderately dim light. Moreover light- and F-induced noise did not summate in the manner expected if they were indistinguishable to the photoreceptor.
6. In addition to noise, we saw other effects as well. The earliest effect of VO 3 − was a smooth depolarization which may have been due to this ion’s role as a Na/K pump inhibitor. In experiments with some of these agents strong light caused a wave of afterdepolarization with no increase in noise. This effect occurred under a variety of conditions, however, and was not specific to the agents used.
7. F−, VO 3 − GPPNHP and GTPγS appear to generate noise inMusca photoreceptors through a common effect on the pathway of phototransduction. In the case of F− in particular, chemically induced bumps are smaller and less affected by adaptation than those produced by light. We discuss possible mechanisms of action consistent with these findings.