text: Cyclic AMP-dependent protein kinases (protein kinase A) are activated by the signal chain coming from the G protein (that was activated by the receptor) via adenylate cyclase and cyclic AMP (cAMP). In a feedback mechanism, these activated kinases phosphorylate the receptor. The longer the receptor remains active the more kinases are activated and the more receptors are phosphorylated. In β-adrenoceptors, this phosphorylation results in the switching of the coupling from the G class of G-protein to the G class. cAMP-dependent PKA mediated phosphorylation can cause heterologous desensitisation in receptors other than those activated.