Reduced reperfusion-induced Ins(1,4,5)P3 generation and arrhythmias in hearts expressing constitutively active α1B-adrenergic receptors

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