In retinal cones, membrane depolarization in darkness activates the cGMP-dependent conductance. A model of Ca homeostasis and the regulation of guanylate cyclase.

Jun 1, 1993The Journal of general physiology

How Dark-Induced Membrane Changes in Retinal Cones Control Calcium Levels and Enzyme Activity

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Abstract

Outer segment currents in single cone photoreceptors of striped bass exhibit a dark, voltage-activated current that increases in amplitude over 0.75-1.5 seconds.

In darkness, when membrane voltage exceeds 10 mV, an outward current is activated in the outer segment of cone photoreceptors.
This dark, voltage-activated current (DVAC) reaches a steady-state value after a sigmoidal time course.
DVAC is completely suppressed by light, indicating a direct relationship between light exposure and current activity.
The characteristics of DVAC are similar to light-sensitive currents, suggesting a shared underlying mechanism.
A decrease in cytoplasmic calcium concentration is linked to the activation of guanylate cyclase, resulting in the synthesis of cGMP.
A quantitative model correlates cytoplasmic calcium dynamics with DVAC characteristics, proposing that guanylate cyclase activity in cones mirrors that in rods.

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We measured outer segment currents under voltage clamp in solitary, single cone photoreceptors isolated from the retina of striped bass. In darkness, changes in membrane voltage to values more positive than 10 mV activate a time- and voltage-dependent outward current in the outer segment. This dark, voltage-activated current (DVAC) increases in amplitude with a sigmoidal time course up to a steady-state value, reached in 0.75-1.5 s. DVAC is entirely suppressed by light, and its current-voltage characteristics and reversal potential are the same as those of the light-sensitive currents. DVAC, therefore, arises from the activation by voltage in the dark of the light-sensitive, cGMP-gated channels of the cone outer segment. Since these channels are not directly gated by voltage, we explain DVAC as arising from a voltage-dependent decrease in cytoplasmic Ca concentration that, in turn, activates only guanylate cyclase and results in net synthesis of cGMP. This explanation is supported by the finding that the Ca buffer BAPTA, loaded into the cytoplasm of the cone outer segment, blocks DVAC. To link a decrease in cytoplasmic Ca concentration to the synthesis of cGMP and the characteristics of DVAC, we develop a quantitative model that assumes cytoplasmic Ca concentration can be continuously calculated from the balance between passive Ca influx via the cGMP-gated channel and its active efflux via a Na/Ca,K exchanger, and that further assumes that guanylate cyclase is activated by decreasing cytoplasmic Ca concentration with characteristics identical to those described for the enzyme in rods. The model successfully simulates experimental data by adjusting the Ca conductance of the cGMP-gated channels as a function of voltage and the Ca buffering power of the cytoplasm. This success suggests that the activity of guanylate cyclase in cone outer segments is indistinguishable from that in rods.

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In retinal cones, membrane depolarization in darkness activates the cGMP-dependent conductance. A model of Ca homeostasis and the regulation of guanylate cyclase.

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