IK channel
# IK channel

The $I_K$ channel $(K_{Ca}3.1),$ which has a conductance of 20–80 pS, is expressed mainly in peripheral tissues such as those of the haematopoietic system, colon, placenta, lung and pancreas. The $K_{Ca}3.1$ channel in red blood cells was the first $Ca^{2+}–sensitive$ $K^+$ channel to be identified and it has been implicated in a wide range of cell functions, including vasodilation of the microvasculature, $K^+$ flux across endothelial cells of brain capillaries and the phagocytic activity of neutrophils. $K_{Ca}3.1$ is of primary importance in the relationship between $K^+$ channels and cell proliferation.

In the latter case, a human hIKCa1 gene encodes the channel found in T cells, which is responsible for the hyperpolarization that is required to keep $Ca^{2+}$ flowing into the cell through the $I_{CRAC}$ channels.

In comparison with the large-conductance (BK) channels, $K_{Ca}3.1$ is much more sensitive to $Ca^{2+}$ and can thus respond to the global level of $Ca^{2+}.$ This high affinity for $Ca^{2+}$ depends upon four resident calmodulin molecules tightly bound to the cytoplasmic tails of the four pore-forming α-subunits. Before the channel can open, $Ca^{2+}$ must bind to each of the calmodulins to induce the co-operative conformational change that opens the gate, which explains why this process has a Hill coefficient of 4. This $Ca^{2+}–induced$ gating process resembles that which has been described for the small-conductance (SK) channels. The fact that calmodulin is prebound to its effector enables the channels to respond to $Ca^{2+}$ very quickly.

The PtdIns3P signaling cassette may play a role in regulating the activity of $K_{Ca}3.1.$ If this signaling lipid is hydrolysed by MTMR6, which is one of the myotubularins, there is a decrease in the activity of the $Ca^{2+}–activated$ channel.

