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Modulation of rod, but not cone, cGMP-gated photoreceptor channels by calcium-calmodulin

Published online by Cambridge University Press:  02 June 2009

Lawrence W. Haynes  and
Stephanie C. Stotz
Lawrence W. Haynes
Affiliation:
Department of Physiology and Biophysics and Neuroscience Research Group, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada
Stephanie C. Stotz
Affiliation:
Department of Physiology and Biophysics and Neuroscience Research Group, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada
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Abstract

Inside-out patches containing cGMP-gated channels were excised from catfish rod or cone outer segments and held under voltage clamp. The net cGMP-dependent currents elicited by saturating and subsaturating concentrations of cGMP at ±30 mV were measured and the dependence of current upon cGMP concentration was determined. The apparent affinity of the channel for its ligand was estimated by fitting these data with the Hill equation. The concentration of cGMP required to give half the maximum current (K1/2) in rod and cone channels at +30 mV was ~28 μM and ~37 μM, respectively, and was weakly voltage dependent. Thus, cone channels have an intrinsically higher K1/2 than rod channels. For both types of channel, the Hill coefficient was ~2.3. In the presence of calcium-calmodulin, the apparent affinity of the rod channel for cGMP decreased by about twofold, but the apparent affinity of the cone channels was unaffected. These results indicate that the open probability of the cone channel for its ligand cannot be modulated by calmodulin. This represents the first significant departure between rod and cone photoreceptors in mechanisms used by phototransduction and suggests that the β subunit of the cone channel must be different from that of the rod channel.

Keywords

PhototransductionRetinal rodRetinal coneIon channel
Type
Research Articles
Information
Visual Neuroscience , Volume 14 , Issue 2 , March 1997 , pp. 233 - 239
Copyright
Copyright © Cambridge University Press 1997

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