Abstract
The role for Ca 2+ in the excitation process by which light opens membrane channels in Limulus photoreceptors is discussed. Light initiates a phospholipase C/IP 3 pathway that results in a rapid elevation of intracellular Ca 2+, but whether this elevation is causal in triggering the light response or merely synergistic to some other second messenger pathway has been unclear. We have developed a procedure using progressive injection of Ca 2+ buffers that distinguishes between mediation and synergy models [Shin J-H. Richard EA. Lisman JE. (1992) Ca 2+ is an obligatory intermediate in the excitation cascade of Limulus photoreceptors. Neuron, 11, 845–855]. Our conclusion is that Ca 2+ mediates all phases of the light-response. Models of this kind had previously been rejected because intracellular injection of Ca 2+ buffer can lead to an increase of the late component (> 200 ms) of the response to bright, sustained light. We have used computer simulations of IP 3 mediated Ca 2+ release to show that the positive and negative regulation of this process by Ca 2+ itself together with other feedback loops can explain counterintuitive effects of Ca 2+ buffers.
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