In Intact Mammalian Photoreceptors, Ca2+-dependent Modulation of cGMP-gated Ion Channels Is Detectable in Cones but Not in Rods

Dec 31, 2003The Journal of general physiology

Calcium-dependent control of light-sensitive channels occurs in cone but not rod photoreceptors

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Abstract

At 600 nM free cytoplasmic Ca2+, cone CNG channels show a sensitivity midpoint to 8Br-cGMP of approximately 2.3 microM.

Cone photoreceptors can signal small luminance differences despite large changes in background intensity.
Rod photoreceptors exhibit minimal adaptation only at intensities that nearly saturate their photoresponse.
The sensitivity of cone CNG channels to cyclic GMP is greater than that of rod channels.
Rapid decreases in cytoplasmic Ca2+ activate a significant inward current in cones, but not in rods.
The activation of current in cones is linked to an increase in the affinity of CNG channels for cyclic nucleotides.
The time course of current activation in cones varies with temperature, ranging from approximately 480 ms at 20-21 degrees C to 138 ms at 32 degrees C.

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In the mammalian retina, cone photoreceptors efficiently adapt to changing background light intensity and, therefore, are able to signal small differences in luminance between objects and backgrounds, even when the absolute intensity of the background changes over five to six orders of magnitude. Mammalian rod photoreceptors, in contrast, adapt very little and only at intensities that nearly saturate the amplitude of their photoresponse. In search of a molecular explanation for this observation we assessed Ca2+-dependent modulation of ligand sensitivity in cyclic GMP-gated (CNG) ion channels of intact mammalian rods and cones. Solitary photoreceptors were isolated by gentle proteolysis of ground squirrel retina. Rods and cones were distinguished by whether or not their outer segments bind PNA lectin. We measured membrane currents under voltage-clamp in photoreceptors loaded with Diazo-2, a caged Ca2+ chelator, and fixed concentrations of 8Br-cGMP. At 600 nM free cytoplasmic Ca2+ the midpoint of the cone CNG channels sensitivity to 8BrcGMP, 8BrcGMPK1/2, is approximately 2.3 microM. The ligand sensitivity is less in rod than in cone channels. Instantly decreasing cytoplasmic Ca2+ to <30 nM activates a large inward membrane current in cones, but not in rods. Current activation arises from a Ca2+ -dependent modulation of cone CNG channels, presumably because of an increase in their affinity to the cyclic nucleotide. The time course of current activation is temperature dependent; it is well described by a single exponential process of approximately 480 ms time constant at 20-21 degrees C and 138 ms at 32 degrees C. The absence of detectable Ca2+-dependent CNG current modulation in intact rods, in view of the known channel modulation by calmodulin in-vitro, affirms the modulation in intact rods may only occur at low Ca2+ concentrations, those expected at intensities that nearly saturate the rod photoresponse. The correspondence between Ca2+ dependence of CNG modulation and the ability to light adapt suggest these events are correlated in photoreceptors.

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In Intact Mammalian Photoreceptors, Ca2+-dependent Modulation of cGMP-gated Ion Channels Is Detectable in Cones but Not in Rods

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