2nd messengers
As an example, cyclic AMP is a signal transducer in smooth muscle in response to the action of adrenaline at the beta-adrenoreceptor. The beta-receptor-linked Gs protein enhances AC activity, resulting in an increases in cAMP. cAMP binds to the regulatory subunits on PKA, releasing the catalytic subunits which are then free to phosphorylate substrate proteins. One substrate, glycogen synthase, catylzes conversion of glucose into glycogen.