The phosphoinositide (PI) intracellular signaling
pathway, which triggers Ca2+ release from intracellular
stores, appears to be a central feature of phototransduction
in most invertebrate species studied. Procedures designed
to inhibit PI-pathway reactions cause suppression of excitation
to dim lights. However, in Limulus photoreceptors,
responses to bright stimuli are in fact enhanced by some
of these procedures, suggesting that PI metabolism is not
obligatory for light-induced excitation. Other studies,
however, suggest that Ca2+ release is obligatory
for excitation. We studied this issue by examining the
effects of PI-pathway inhibitor, Li+, on electrophysiological
responses to light in Limulus photoreceptors.
Li+ is reported to cause depletion of intracellular
PI-pathway intermediate, inositol; and it offers the pharmacological
advantage that its block can be bypassed by introducing
exogenous inositol. Introduction of Li+ caused
a very slowly developing but complete suppression of responses
to dim stimuli. In contrast, Li+ caused a rapidly
developing but partial suppression of responses to bright
stimuli. Li+-induced suppression was reversed
by exogenous introduction of inositol. In addition, inositol
prevented Li+-induced suppression of excitation.
Li+ enhanced light adaptation (light-induced
desensitization) but slowed response deactivation, indicating
a difference in the processes underlying these phenomena.
Li+ slowed dark adaptation, the recovery of
sensitivity following light adaptation. All of these effects
were prevented or rescued by extracellularly applied inositol,
suggesting the presence of a transmembrane inositol transport
system. The overall results suggest that PI-dependent signaling
is central and obligatory for excitation in Limulus,
at least for responses to dim to moderate illumination.
The failure of Li+ to suppress bright light-induced
excitation completely may be due to a failure of Li+
to block PI metabolism completely, as in other systems;
however, it may point to a parallel, PI-independent excitation
pathway possessing very low light sensitivity when PI metabolism
is inhibited.
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