Meta-Biol

• Pathways

The cGMP-gated (CNG) channels are heterotetramers of three alpha subunits (CNGA1) and one beta subunit (CNGB1). Although the subunits bear structural similarities to voltage-gated potassium channels, the rod channel is only weakly voltage sensitive (Chen et al. 1993). Opening occurs with cGMP bound and rarely occurs in the absence of cGMP (hence the closed state is designated as CNG channel and the open status is designated as cGMP:CNG channel). In darkness, cGMP concentration is relatively high and cGMP-gated cation (CNG) channels are open to allow the influx of cations into the rod outer segment. The inward current is composed mainly of Na+ ions, with lesser contributions from Ca2+ ions and Mg2+ ions (Dhallan et al. 1992). The channel is outwardly rectifying so that over physiological membrane potentials, the inward current is proportional to the number of channels open and is nearly independent of voltage. Mutations in the CNG channel subunits can cause a recessive form of retinitis pigmentosa (https://sph.uth.edu/retnet/).