/ arr est in ▁system ▁serves ▁as ▁a ▁complex ▁signal ing ▁switch ▁for ▁G ▁protein - cou ple d ▁re cept ors . ▁ ▁GR K 2 ▁and ▁the ▁closely - related ▁GR K 3 ▁ph osph ory late ▁re cept ors ▁at ▁sites ▁that ▁encou rage ▁arrest in - medi ated ▁re ceptor ▁des ens it ization , ▁internal ization ▁and ▁tra ff ick ing ▁rather ▁than ▁arrest in - medi ated ▁signal ing ▁( in ▁contrast ▁to ▁GR K 5 ▁and ▁GR K 6 , ▁which ▁have ▁the ▁opposite ▁effect ). ▁This ▁difference ▁is ▁one ▁basis ▁for ▁ph arm ac ological ▁bi ased ▁ag on ism ▁( also ▁called ▁functional ▁select ivity ), ▁where ▁a ▁drug ▁binding ▁to ▁a ▁re ceptor ▁may ▁bias ▁that ▁re ceptor ’ s ▁signal ing ▁toward ▁a ▁particular ▁subset ▁of ▁the ▁actions ▁stim ulated ▁by ▁that ▁re ceptor . ▁ ▁GR K 2 ▁is ▁expressed ▁broad ly ▁in ▁t issues , ▁but ▁generally ▁at ▁higher ▁levels ▁than ▁the ▁related ▁GR K 3 . ▁GR K 2 ▁was ▁originally ▁identified ▁as ▁a ▁protein ▁kin ase ▁that ▁ph osph ory l ated ▁the ▁ β 2 - adr ener g ic ▁re ceptor , ▁and ▁has ▁been ▁most ▁extens ively ▁studied ▁as ▁a ▁reg ulator ▁of ▁ad r ener g ic ▁re cept ors ▁( and ▁other ▁G PC R s ) ▁in ▁the ▁heart , ▁where ▁it ▁has ▁been ▁proposed ▁as ▁a ▁drug ▁target ▁to ▁treat ▁heart ▁failure . ▁Str ateg ies ▁to ▁in hib it ▁GR