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