MoBio Signaling via Ion Channels Chapter 6

Ion channels are membrane proteins that contain a pore for ions to pass through. In terms of ion selectivity, they are classified as calcium channels, sodium channels, potassium channels, etc. In terms of gating (how channels are opened), they may be classified as voltage-gated channels, ligand-gated channels, etc.

Image

Figure 6-C-1. Two types of Ca2+ channels. (1) The voltage-activated Ca2+ channels located in the plasma membrane. (2) The IP3-sensitive Ca2+ channels located in the membrane of endoplasmic reticulum.

The major role of sodium and potassium ions is to regulate membrane potential. However, the calcium ion also plays important roles in other cellular functions, since many enzymes are calcium-dependent. The above figure shows two pathways that Ca2+ may enter the cytosol.

  1. Through voltage-activated Ca2+ channels whose opening probability depends on the membrane potential.
  2. Through IP3-sensitive Ca2+ channels whose opening probability is regulated by IP3 (inositol trisphosphate).

The entry of Ca2+ ions stored in the endoplasmic reticulum into the cytosol may result in calcium waves.

Calmodulin is an important molecule that can bind to Ca2+ to modulate its activity.