Inhibition of phospholipase C by U-73122 blocks one component of the receptor current in Limulus photoreceptor

In the ventral nerve photoreceptor of Limulus a short, intense flash evokes a receptor current consisting of three components. In contrast to other hypotheses, we suggested previously that only the second component of the current is activated by the phospholipase C pathway, which releases calcium from intracellular stores by inositol trisphosphate. The present paper gives further evidence to our suggestion. It is demonstrated that U-73122, a specific inhibitor for the phospholipase C, selectively blocks the second component. The first and third components are moderately affected and could still be activated after the complete block of the second one. Results support the idea that the first and third components areactivated by pathways operating independently of phospholipase C.

Spontaneous Bumps Of The Limulus Photoreceptor Cell Are Probably Triggered By The Spontaneous Activation Of Single G-Protein Molecules

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