G-Protein Gated Ion Channels

When acetylcholine binds a GPCR in heart muscle, it activates inhibitory G protein. The Gβγ subunit binds and opens potassium channels, allowing K+ ions to exit the cytosol and hyperpolarize the membrane. Simultaneously, Gαi inhibits adenylyl cyclase, reducing cAMP and closing calcium channels. This dual mechanism delays membrane depolarization and slows heart rate.

In olfactory neurons, odorant binding activates Golf, which stimulates adenylyl cyclase to produce cAMP as a second messenger. Increased cAMP levels open cAMP-gated cation channels, allowing Na+ influx and rapid membrane depolarization. This electrical signal transmits nerve impulses to the brain, enabling smell perception.

Light causes rhodopsin to undergo conformational change and activate transducin G protein. GTP-bound Gtα stimulates cyclic GMP phosphodiesterase, which hydrolyzes cyclic GMP to 5'-GMP, decreasing intracellular levels. Lower cyclic GMP closes cation channels, restricting Na+ and Ca2+ entry and inducing membrane hyperpolarization.

Direct regulation occurs when Gβγ subunits physically bind and open ion channels, as seen with potassium channels in cardiac tissue. Indirect regulation uses second messengers like cAMP or cyclic GMP to modulate channel activity. Both mechanisms alter membrane potential but operate through distinct molecular pathways.

Taste receptor cells express specialized GPCRs including 30 bitter-taste receptors (T2Rs) and the sweet-taste receptor (T1R2-T1R3 heterodimer). These receptors bind taste molecules and activate G proteins to modulate ion channel opening. The resulting changes in membrane potential transmit flavor information to the brain.

Hyperpolarization and reduced cAMP-mediated calcium entry both slow the return to depolarized states. A hyperpolarized membrane takes longer to reach the threshold needed for muscle contraction. This delay decreases the frequency of heart muscle contractions, lowering heart rate and allowing the body to rest.

Rod photoreceptors contain an outer segment with stacked membrane-bound discs packed with rhodopsins. A plasma membrane encloses this segment and contains cyclic GMP-gated cation channels. In darkness, high cyclic GMP keeps channels open, maintaining depolarization. Light-induced cyclic GMP reduction closes channels and triggers hyperpolarization.