The second messenger systems involved in the final stages of
the phototransduction cascade in Limulus photoreceptors
remain unclear. Excised patches of transducing membrane contain
cGMP-gated channels, suggesting the involvement of cGMP in the
excitation process. To further explore this possibility, we tested
the effects of inhibitors and agonists of guanylate cyclase. The
active site cyclase inhibitors guanosine 5′-tetraphosphate
and adenosine 5′-tetraphosphate produced a reversible reduction
of the response to light without affecting resting membrane properties.
The cyclase inhibitor Rp-GTPαS produced
a similar reduction, but the effect was only slightly reversible. The
reduction in the response produced by these inhibitors was robust, often
producing over a 95% decrease in the amplitude of the light
response. Previous work had shown that an end-product cyclase
inhibitor, imidodiphosphate, also inhibited the response. The
consistent results with four different guanylate cyclase inhibitors
strongly support the involvement of this enzyme in the
phototransduction cascade. To determine whether the guanylate
cyclase involved is the NO-dependent soluble form, we applied
inhibitors and activators of the nitric oxide synthase/guanylate
cyclase pathway such as L-N5-(1-iminoethyl) ornithine,
sodium nitroprusside, and carboxy-PTIO. None of these agents had any
substantial effect on phototransduction. Taken together, these results
support a role for a particulate guanylate cyclase in Limulus
photoreceptor excitation.