Fig. 1. The phototransduction cascade. Light causes photoisomerization of rhodopsin, activating the heterotrimeric G-protein transducin. The GTP-bound -subunit activates phosphodiesterase (PDE), which degrades cGMP to GMP. The decrease in cGMP concentration leads to closure of cyclic-nucleotide-gated (CNG) channels, resulting in two effects, a decrease in Ca²⁺ influx and hyperpolarization of the membrane potential. The resulting decrease in intracellular Ca²⁺ concentration is important for adaptation. Lowered intracellular Ca²⁺ concentration disinhibits guanylate-cyclase-activating protein (GCAP), leading to activation of guanylate cyclase (GC) and resynthesis of cGMP.

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