(full paper is archived in the Miller Library)
Title: Cyclic GMP increases channel openings in squid photoreceptors
Student Author(s): Chang, Bryna T.
Faculty Advisor(s): Thompson, Stuart
Location: Final Papers Biology 175H
Date: June 1996
Abstract: Cyclic guanosine monophosphate (cGMP) gated channels serve as the downstream target in vertebrate visual transduction. However, it is not known whether cGMP-gated channels play a similar role in photoreceptors from invertebrates. Inside-out patch clamping of solitary photorecptor cells isolated from the retina of the squid(Loligo opalescens) revealed that the probability of channel opening increases from a baseline of Po=0.20 in the absence of cGMP to near saturation (Po=0.91) at 100 uM cGMP, suggesting the existence of cGMP-gated channels. The probability of channel openings was independent of membrane potential over the range -60 to +60 mV. Data from these macroscopic patches revealed multiple channel types with unitary conductances ranging from 10 to 125 pS in symmetrical Na+ solutions. Conductance of a 100 pS channel was blocked by the addition of 2 mM Ca2+ to the inner face of the patch. However, channel activity persists in the presence of 60 mM Mg2+, a divalent cation concentration sufficient to block cGMP-gated channels in vertebrate photoreceptors. This suggests a significant difference in vertebrate and invertebrate cGMP-gated channel structure. The results of this study indicate that cGMP-gated channels are the effectors in invertebrate phototransduction.