03.02.1.1 G-Protein Coupled Receptors

Adrenaline is a hormone released from adrenal medulla that has widespread effects including stimulating bronchial GPC b₂-adrenoceptors to induce bronchodilation via cell-signaling involving adenylate cyclise (Figure 3.16).

Figure 3.16 GPCR – β₂-adrenoceptor (Copyright QUT University, Sheila Doggrell)

The β₂-adrenoceptor spans the membrane. This allows the adrenaline, which does not enter the cell, to talk to the inside of the cell. The G proteins are so named as they require GTP (guanosine triphosphate) for action. There are lots of different G proteins. The one linked to the β₂-adrenoceptor is stimulatory G-protein (G_s), and amplifies the message, leading to stimulation of a membrane bound enzyme, adenylate cyclase, which catalyses the conversion of ATP (adenosine triphosphate) to cAMP, which eventually leads to relaxation of the bronchial muscle, bronchodilation.

Salbutamol is a selective agonist at β₂-adrenoceptor. Salbutamol does not mimic all the effects of adrenaline, just those mediated by β₂-adrenoceptor. This leads to the relaxation of bronchial smooth muscle, and salbutamol is used as a bronchodilator in asthma.