Reduced Reperfusion–Induced Ins(1,4,5)P3 Generation and Arrhythmias in Hearts Expressing Constitutively Active a1B-Adrenergic Receptors

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 a1-adrenergic receptors (ARs), but the subtype of receptor involved has not been identified. Under normoxic conditions, hearts from transgenic animals expressing constitutively active a1B-ARs in heart (a1B-constitutively active mutant [CAM]) showed higher [ H] inositol phosphate responses to norepinephrine (2.3-fold) than hearts from nontransgenic animals (a1B-WT) (1.6-fold). a1B-WT hearts responded to 2 minutes of reperfusion after 20 minutes of global ischemia by generation of Ins(1,4,5)P3 (530161310 to 11 41361597 CPM/g tissue; mean6SEM; n56; P,0.01 in [H] labeling studies and 3.860.2 to 6.360.6 nmol/g by mass analysis, n56; P,0.05). In contrast to findings in normoxia, hearts from a1B-CAM animals showed no Ins(1,4,5)P3 response in early reperfusion. In parallel studies, a1B-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 a1B-CAM hearts (95% to 62% for VPB and 47% to 12% for ventricular tachycardia; both P,0.05). The resistance of the a1B-CAM hearts was not due to a1B-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 a1A-receptor activity in the a1B-CAM hearts, expression of the mRNA for a1Aand a1B-receptors was measured. a1B-WT hearts contained mRNA for both receptor subtypes, but the levels of a1B-receptor mRNA were 5-fold higher than a1A-receptor mRNA. a1B-CAM hearts contained very high levels of a1B-receptor mRNA (26-fold increase), but the expression of mRNA for the a1A-receptors (0.14160.035 amol/mg RNA; mean6SEM; n56) was reduced by 50% relative to a1B-WT controls (0.27660.046 amol/mg RNA; n56; P,0.01). The reduction in arrhythmogenic and Ins(1,4,5)P3 responses in a1B-CAM hearts provides evidence that these response are not mediated by a1B-receptors. (Circ Res. 1998;83:1232-1240.)