Norepinephrine is released from a neuron and approaches the surface of a smooth muscle cell. Once it arrives it engages with a Beta-2 adrenergic receptor. This interaction causes a conformational change in the receptor, which travels through the phospho-bilipid membrane of the cell. Docked inside the cell, and associated with the receptor, is a G-protein complex consisting of an Alpha, and Beta-Gamma subunit with an associated GDP.

The conformational change of the receptor causes the Alpha Subunit to change configuration, and dissasociate the Beta-Gamma subunit and replace the GDP with the more reactive GTP.