When activated by b-adrenergic receptors, the G protein:

When norepinephrine binds to a beta-adrenergic receptor, it activates a G protein (i.e., a guanosine triphosphate (GTP) binding protein), which, in turn, activates adenylate cyclase. Adenylate cyclase catalyzes the formation of cyclic adenosine 3',5'-monophosphate (cAMP), which activates a variety of kinases. One of these kinases (protein kinase A) phosphorylates phospholamban, which reduces the inhibition of the sarcoplasmic reticular Ca2+ pump, increasing sequestration of Ca2+ from the cytoplasm. Ref: Biaggioni I., Robeson D. (2012). Chapter 9. Adrenoceptor Agonists & Sympathomimetic Drugs. In B.G. Katzung, S.B. Masters, A.J. Trevor (Eds),Basic & Clinical Pharmacology, 12e.