SELECTIVE-INHIBITION OF THE PHOSPHOLIPASE-C PATHWAY BLOCKS ONE LIGHT-ACTIVATED CURRENT COMPONENT IN LIMULUS PHOTORECEPTOR

Light-activated receptor currents were measured in Limulus ventral ner ve photoreceptors by a two electrode voltage clamp. Short hashes stimu late three different current components (C-1, C-2, C-3). Currents were measured before and after injection of neomycin, heparin and BAPTA. T hese substances inhibit the inositol phosphate pathway at different st ages. Each substance selectively blocked the C-2 component. After the block of C-2 the other two components could be further stimulated, but their amplitudes were usually reduced. High concentrations of BAPTA, however, increased the amplitude (two-fold) and decay time constant (e ight-fold) of C-1, suggesting that calcium is necessary for the deacti vation of the C-1 current. The time to maximum for C-2 is known to be dependent on the light-adaptation state and the stimulus intensity. It is shown here that the time to the maximum of C-2 remains nearly unch anged when C-2 is gradually inhibited by BAPTA, but is prolonged when C-2 is inhibited by neomycin. Since the kinetics of an enzyme reaction depend on the substrate concentration, these results indicate, as exp ected, that neomycin changes the substrate concentration and thus chan ges the rate and gain of the cascade, while BAPTA changes the product concentration, i.e. binds the released calcium. Therefore, these obser vations support the hypothesis that the negative feedback of adaptatio n regulates the phospholipase C cascade at the same early stage, where the neomycin acts. The same consideration suggests that after the cal cium release no further amplification occurs in the transduction casca de since the time to maximum with BAPTA remains unchanged. The results are explained by three parallel transduction pathways. It is suggeste d that the inositol phosphate cycle regulates the gain of the other tw o pathways by the release of calcium ions.