Two light-activated conductances in the eye of the green alga Volvox carteri

Photoreceptor currents of the multicellular green alga Volvox carteri were analyzed using a dissolver mutant. The photocurrents are restricted to the eyespot region of somatic cells. Photocurrents are detectable from intact c ells and excised eyes. The rhodopsin action spectrum suggests that the curr ents are induced by Volvox rhodopsin. Flash-induced photocurrents are a com position of a fast Ca2+-carried current (P-F) and a slower current (P-S), w hich is carried by H+. P-F is a high-intensity response that appears with a delay of less than 50 mu s after flash. The stimulus-response curve of its initial rise is fit by a single exponential and parallels the rhodopsin bl eaching. These two observations suggest that the responsible channel is clo sely connected to the rhodopsin, both forming a tight complex. At low flash energies P-S is dominating. The current delay increases up to 10 ms, and t he P-S amplitude saturates when only a few percent of the rhodopsin is blea ched. The data are in favor of a second signaling system, which includes a signal transducer mediating between rhodopsin and the channel. We present a model of how different modes of signal transduction are accomplished in th is alga under different light conditions.