Ischemic inactivation of G protein-coupled receptor kinase and altered desensitization of canine cardiac β-adrenergic receptors

Background-G protein-coupled receptor kinases (GRKs) modulate myocardial beta -adrenergic receptor (PAR) signaling. We examined whether GRK activity was altered, 24, and 96 hours after left anterior descending coronary artery ligation (LAD GAL) in the dog. Methods and Results-GRK activity was measured in arrhythmogenic subepicardial border zone (EBZ) tissue overlying the infarct and from nonischemic remote-site (RS) subepicardial tissue from the same animal. GRK activity in the ischemic EBZ was 15% of RS (P=0.03, n=6) 24 hours after CAL and appeared to start as early as 6 hours through 96 hours. GRK activity and immunoblot data demonstrated a marked decrease of GRK2 but not GRK5 at 24 hours. EBZ tissue exhibited high-affinity binding for (-)-isoproterenol (K-i of 0.076+/-0.026 nmol/L [SEM]) at 24 hours, which was not significantly different from control tissue from nonoperated animals (1.2+/-0.8 nmol/L, P>0.05, n=6), A significantly lower K-i of 13.8+/-2.8 nmol/L (P<0.001, n=6) was observed for RS taken from the ischemic animals. This was reflected by a 4-fold increase in the EC50 of isoproterenol-stimulated adenylyl cyclase activity from 18 nmol/L in EBZ tissue to 73 nmol/L in RS (P<0.05, n=4). Conclusions-There is a selective decrease in GRK2 activity and a loss of the ability of the arrhythmia-prone EBZ tissue to desensitize to beta -adrenergic stimulation 24 hours after GAL. This correlates temporally with a second (late) peak in sudden cardiac death previously observed between 6 and 24 hours in dog and rat models of myocardial infarction.