Ligand Sensitivity is Modulated by Ca2+ in Intact Mammalian Cone Photoreceptors

Abstract

Abstract: : Purpose: To determine whether ligand sensitivity of cGMP-gated ion channels in mammalian cones is modulated in a Ca₂₊-dependent manner. Methods: Single cones were isolated by enzymatic dissociation of dark-adapted retina of california ground squirrels (S. beecheyi). Under voltage clamp with tight-seal electrodes, cGMP-gated channels were activated with 8Br-cGMP added to the electrode filling solution. This solution also contained Zaprinast and caged BAPTA (Diazo-2) with free [Ca₂₊] at about 600 nM. Free [Ca₂₊] was suddenly and rapidly reduced by uncaging Diazo-2 with a bright light flash. Results: Uncaging Diazo-2 caused a rapid increase in the whole cell current at a -35 mV holding voltage. Control experiments demonstrate the current enhancement specifically reflects activation of the cGMP-gated channels in the cone and it fails to occur if [Ca₂₊] concentration does not change. Conclusion: In the presence of an unvarying concentration of intracellular ligand, lowering free [Ca₂₊] activates cGMP-gated channels in mammalian cones. The extent of activation is large and comparable to that observed in cones of non-mammalian vertebrates. Although direct evidence is lacking, the ligand-sensitivity of the same channels in mammalian rods, just as rods of non-mammals, is probably not modulated by [Ca₂₊]. This difference is likely to play an important role in explaining the large difference in the light-adaptation range between the two receptor types in mammals.

Keywords: 517 photoreceptors • 445 ion channels • 334 calcium