Gi-coupled protein receptor

The Gi-(inhibitory) proteins serves two main functions. First the Gi α-subunit inhibits the adenylate cyclase and thus prevents the increase in cAMP levels. The second function of the Gi-protein is the activation of potassium-channels, resulting in a potassium-ion influx. Both actions of the Gi-protein result in the production/activation of certain enzymes, ion channels and transport proteins.
The following receptors are examples of GPCRs coupling to the Gi subunit. In the TRC, the Gi coupled receptor is always represented as red GPCR receptor:
Acetylcholine M2 receptor
Adrenergic α2 receptor