Testing the role of calmodulin in the excitation of Limulus photoreceptors

The phototransduction cascade in Limulus ventral photoreceptors involves multiple second messengers, including Ca2+ and cGMP. Light-induced Ca2+ release from intracellular stores is an intermediate step, but the subsequent Ca2+-activated reaction remains to be determined. The possibility that Ca2+/calmodulin (Ca2+/CaM) might be involved is suggested by the high calmodulin content of the transducing lobe. To test whether CaM can excite the transduction cascade we injected a 25 μM Ca2+/CaM solution. This produced a rapid, brief depolarization similar to that produced by light, suggesting a role for CaM in the cascade. However, an important caveat is that Ca2+ dissociating from the Ca2+/CaM complex might excite this process. Several control experiments argue against, but do not entirely eliminate this possibility. To test whether endogenous CaM has a function in excitation, trifluoperazine was pressure injected into the rhabdomeric region. The response to brief flashes was not affected, but the response to steady illumination was transiently attenuated by each injection. We conclude that calmodulin should be considered a candidate to couple intermediate and late stages of the transduction cascade.