Protein kinase c activators inhibit the visual cascade in Limulus ventral photoreceptors at an early stage

The phosphoinositide cascade mediates visual transduction in invertebrate p hotoreceptors. Phospholipase C (PLC) catalyzes the hydrolysis of phosphatid ylinositol bisphosphate, producing inositol trisphosphate (InsP(3)) and dia cylglycerol (DAG). Protein kinase C (PKC) is a major target of DAG in many cell types. We have used PKC activators to investigate the function of the kinase in the phototransduction cascade in Limulus polyphemus ventral photo receptors. Extracellular application of (-)-indolactam V (0.03-30 mu M) or phorbol-12,13-dibutyrate (10 mu M) reversibly reduced the sensitivity of th e electrical response of the photoreceptors to light by up to 1000-fold. Th e inert stereoisomer (+)-indolactam V and 4 alpha-phorbol had no effect. Th e effect of (-)-indolactam V was antagonized by the PKC inhibitors bisindol ylmaleimide I and Go 6976. Coapplication of bisindolylmaleimide V, used as a negative control compound for PKC inhibition, did not reduce the effectiv eness of (-)-indolactam V. These findings are consistent with (-)-indolacta m V activating PKC and desensitizing the light response. Furthermore, our p harmacological results indicate that PKC activation does not appear to play a role in light adaptation. We localized the position of the target of PKC in the visual cascade. We chemically excited the cascade at various stages to determine the kinase's target. PKC activation by (-)-indolactam V decre ased the light-induced elevation of intracellular calcium but had no effect on the photoreceptor's excitatory response to intracellular injection of I nsP(3). However, the PKC activator greatly reduced the excitation caused by GTP-gamma-S injection. We propose that PKC inhibits the visual transductio n cascade at the G-protein and/or PLC stage.