The excitation cascade of Limulus ventral photoreceptors: guanylate cyclase as the link between InsP(3)-mediated Ca(2+ )release and the opening of cGMP-gated channels

BACKGROUND: Early stages in the excitation cascade of Limulus photoreceptors are mediated by activation of G(q )by rhodopsin, generation of inositol-1,4,5-trisphosphate by phospholipase-C and the release of Ca(2+). At the end of the cascade, cGMP-gated channels open and generate the depolarizing receptor potential. A major unresolved issue is the intermediate process by which Ca(2+ )elevation leads to channel opening. RESULTS: To explore the role of guanylate cyclase (GC) as a potential intermediate, we used the GC inhibitor guanosine 5'-tetraphosphate (GtetP). Its specificity in vivo was supported by its ability to reduce the depolarization produced by the phosphodiesterase inhibitor IBMX. To determine if GC acts subsequent to InsP(3 )production in the cascade, we examined the effect of intracellular injection of GtetP on the excitation caused by InsP(3 )injection. This form of excitation and the response to light were both greatly reduced by GtetP, and they recovered in parallel. Similarly, GtetP reduced the excitation caused by intracellular injection of Ca(2+). In contrast, this GC inhibitor did not affect the excitation produced by injection of a cGMP analog. CONCLUSION: We conclude that GC is downstream of InsP3-induced Ca2+ release and is the final enzymatic step of the excitation cascade. This is the first invertebrate rhabdomeric photoreceptor for which transduction can be traced from rhodopsin photoisomerization to ion channel opening.