Testing the role of calmodulin in the excitation of Limulus photoreceptors.
The phototransduction cascade in Limulus ventral photoreceptors involves multiple second messengers, including Ca(2+) and cGMP. Light-induced Ca(2+) release from intracellular stores is an intermediate step, but the subsequent Ca(2+)-activated reaction remains to be determined. The possibility that Ca(2+)/calmodulin (Ca(2+)/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 microM Ca(2+)/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 Ca(2+) dissociating from the Ca(2+)/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.
Garger
Alexander V
AV
Department of Biology, Brandeis University, Waltham, MA 02454-9110, USA.
Richard
Edwin A
EA
Lisman
John E
JE
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D002147:Calmodulin / Q000378:metabolism / Q000494:pharmacology / Q000502:physiology*; D006737:Horseshoe Crabs / Q000166:cytology / Q000378:metabolism*; D007424:Intracellular Fluid / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D009474:Neurons / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D010775:Photic Stimulation; D017956:Photoreceptor Cells, Invertebrate / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D015290:Second Messenger Systems / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D014785:Vision, Ocular / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects
D002147:Calmodulin; D002118:Calcium