Structure and function of adenosine receptors and their genes

Four adenosine receptors have been cloned from many mammalian and some non-mammalian species. In each case the translated part of the receptor is encoded by two separate exons. Two separate promoters regulate the A(1) receptor expression, and a similar situation may pertain also for the other receptors. The receptors are expressed in a cell and tissue specific manner, even though A(1) and A(2B) receptors are found in many different cell types. Emerging data indicate that the receptor protein is targeted to specific parts of the cell. A(1) and A(3) receptors activate the G(i) family of G proteins, whereas A(2A) and A(2B) receptors activate the G(s) family. However, other G proteins can also be activated even though the physiological significance of this is unknown. Following the activation of G proteins several cellular effector pathways can be affected. Signaling via adenosine receptors is also known to interact in functionally important ways with signaling initiated via other receptors.