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

Abstract

Background - G protein-coupled receptor kinases (GRKs) modulate myocardial β-adrenergic receptor (βAR) signaling. We examined whether GRK activity was altered 6, 24, and 96 hours after left anterior descending coronary artery ligation (LAD CAL) 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 [SEMI]) 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). Canclusions - There is a selective decrease in GRK2 activity and a loss of the ability of the arrhythmia-prone EBZ tissue to desensitize to β-adrenergic stimulation 24 hours after CAL. 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.