Spontaneous Bumps of the Limulus Photoreceptor Cell are Probably Triggered by the Spontaneous Activation of Single G-Protein Molecules

The elementary responses, the “bumps”, o f the ventral photoreceptor o f Limulus polyphemus were studied under voltage clamp conditions. We compared sponta­ neous bumps with those induced by light and we applied the G-protein activator A1F4. The amplitude o f the spontaneous bumps is on the average 7 8 times smaller than that o f the light-induced bumps. Bumps induced by A1F4_ have identical size and time parameters when compared with spontaneous bumps. Thus at least a large part o f the spontaneous bumps is probably triggered by the activation o f single G-protein molecules. However, if the next step o f the transduction chain is gainless, it is also possible the spontaneous bump originate from this step. The different size o f spontaneous and light-induced bumps can be explained assuming that a light-activated metarhodopsin molecule leads to the activation o f 3 to 12 G-protein molecules.