Reduced reperfusion-induced Ins(1,4,5)P3 generation and arrhythmias in hearts expressing constitutively active α(1B)-adrenergic receptors

Abstract

Reperfusion of globally ischemic rat hearts causes the generation of inositol(1,4,5)trisphosphate [Ins(1,4,5)P3] and the initiation of arrhythmias. These responses are mediated by α1-adrenergic receptors (ARs), but the subtype of receptor involved has not been identified. Under normoxic conditions, hearts from transgenic animals expressing constitutively active α(1B)-ARs in heart (α(1B)-constitutively active mutant [CAM]) showed higher [3H] inositol phosphate responses to norepinephrine (2.3-fold) than hearts from nontransgenic animals (α(1B)-WT) (1.6-fold). α(1B)-WT hearts responded to 2 minutes of reperfusion after 20 minutes of global ischemia by generation of Ins(1,4,5)P3 (5301±1310 to 11 413±1597 CPM/g tissue; mean±SEM; n=6; P<0.01 in [3H] labeling studies and 3.8±0.2 to 6.3±0.6 nmol/g by mass analysis, n=6; P<0.05). In contrast to findings in normoxia, hearts from α(1B)-CAM animals showed no Ins(1,4,5)P3 response in early reperfusion. In parallel studies, α(1B)-WT hearts developed ventricular tachycardia and ventricular premature beats (VPB) during 5 minutes of reperfusion after 20 minutes of ischemia. The incidence of these arrhythmias was reduced in the α(1B)-CAM hearts (95% to 62% for VPB and 47% to 12% for ventricular tachycardia; both P<0.05). The resistance of the α(1B)-CAM hearts was not due to α(1B)-AR-mediated preconditioning, as the Ins(1,4,5)P3 response to thrombin receptor activation during reperfusion was not different between the 2 groups. To investigate the possibility of reduced α(1A)-receptor activity in the α(1B)-CAM hearts, expression of the mRNA for α(1A)- and α(1B)- receptors was measured. α(1B)-WT hearts contained mRNA for both receptor subtypes, but the levels of α(1B)-receptor mRNA were 5-fold higher than α(1A)-receptor mRNA. α(1B)-CAM hearts contained very high levels of α(1B)- receptor mRNA (26-fold increase), but the expression of mRNA for the α(1A)- receptors (0.141±0.035 amol/μg RNA; mean±SEM; n=6) was reduced by 50% relative to α(1B)-WT controls (0.276±0.046 amol/μg RNA; n=6; P<0.01). The reduction in arrhythmogenic and Ins(1,4,5)P3 responses in α(1B)-CAM hearts provides evidence that these response are not mediated by α(1B)-receptors.