Beta-blockers are thought to cause “unopposed alpha receptor agonism.” Since the beta receptors would be occupied by the beta-blockers, substances (epinephrine, norepinephrine, etc.) ...

G-protein coupled receptors regulate hormone secretions ... Within islets, beta cells secrete insulin, and alpha cells secrete glucagon. Scientists have identified three additional cell types ...

In unstimulated cells, the state of G alpha (orange circles) is defined by its interaction with GDP, G beta-gamma (purple circles), and a G-protein-coupled receptor (GPCR; light green loops).