The phototransduction cascade in rod photoreceptors involves G protein signalling. Which effector enzyme is activated by transducin (Gt), and what is the downstream effect on the dark current?
- A Adenylyl cyclase is activated by transducin; increased cAMP closes K+ channels, causing depolarisation in response to light
- B Phospholipase C is activated by transducin; IP3 releases intracellular Ca2+ which hyperpolarises the rod via Ca2+-activated K+ channels
- C Guanylyl cyclase is directly activated by transducin; increased cGMP opens CNG channels, causing the depolarisation seen after light exposure
- D cGMP phosphodiesterase (PDE6) is activated by transducin α-subunit; PDE6 hydrolyses cGMP, closing cGMP-gated cation channels (CNG channels) and reducing the dark inward current (hyperpolarisation of the rod) ✓
Explanation
In darkness, high intracellular cGMP keeps cGMP-gated (CNG) channels open in the outer segment, allowing Na+/Ca2+ inward current — the 'dark current' — keeping the rod depolarised (~-40 mV). Light → rhodopsin activation → transducin (Gt) α-subunit dissociates and activates PDE6 (cGMP-specific phosphodiesterase) → rapid cGMP hydrolysis → CNG channel closure → dark current decreases → rod hyperpolarises (to ~-70 mV) → reduced glutamate release onto bipolar cells. This sign-inverted signal (hyperpolarisation in response to light) is a unique property of vertebrate photoreceptors. Guanylyl cyclase (GC) actually regenerates cGMP during recovery (dark adaptation), not during light exposure.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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