Channel-linked receptors function in the gating of an ion channel into the open or closed states, in response to ligand-receptor binding. These ligand-gated channels are usually very specific for a particular ion and fall into two categories: fast and slow channels. Fast channels are those where the receptor and channel are directly coupled, whereas with slow channels, the receptor and channel are linked by confusing a G protein. The binding of a ligand to a fast channel receptor will increase the likelihood that an ion channel will open. That channel will only remain open for a short period of time, allowing the flux of a particular ion across the membrane either into or out of the cell, depending upon that ion's electrochemical gradient. The flux of ions across a membrane can alter cellular function in two ways: by changing that cell's membrane potential, and/or the ion can act as a second messenger inside the cell. The movement of sodium into a cell induces a rapid depolarization. Alternatively, if the channel is specific for calcium, then the influx of calcium can trigger a series of responses by binding to the cytosolic protein
calmodulin or by binding to other proteins. The
calcium-calmodulin complex activates a protein kinase which catalyzes phosphorylation of a protein. Calcium is well suited to this role due to its low cytosolic concentration, making the cell very responsive to changes in intracellular calcium. The cytosolic concentration of calcium is kept low by 1) the active transport of calcium out of the cell, 2) sequestration of calcium by cytosolic binding proteins, and 3) the active transport of calcium into the
smooth endoplasmic reticulum and
mitochondria.
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