Ca2+ is an obligatory intermediate in the excitation cascade of limulus photoreceptors.
We have investigated the role of Ca2+ in the excitation of Limulus photoreceptors by intracellular injection of the Ca2+ buffer, 5,5'-dibromo-BAPTA. Buffer with free Ca2+ of 0.5 or 5 microM slowed the rising edge of the light response over 100-fold and greatly reduced both the transient and plateau phases of the light response, as expected if Ca2+ elevation is necessary for all phases of excitation. Injection of buffers with free Ca2+ of 5 or 45 microM, levels normally reached during light, evoked sustained inward current as expected if Ca2+ is sufficient for excitation. The transduction cascade appears due to a single pathway that sequentially involves 1,4,5-trisphosphate inositol, Ca2+, and cyclic GMP.
Shin
J
J
Department of Biology, Brandeis University, Waltham, Massachusetts 02254-9110.
Richard
E A
EA
Lisman
J E
JE
D000818:Animals; D002021:Buffers; D002118:Calcium / Q000502:physiology*; D003198:Computer Simulation; D006737:Horseshoe Crabs / Q000502:physiology*; D007267:Injections; D008297:Male; D008959:Models, Neurological; D017956:Photoreceptor Cells, Invertebrate / Q000502:physiology*
D002021:Buffers; D002118:Calcium