Reduced reperfusion-induced Ins(1,4,5)P-3 generation and arrhythmias in hearts expressing constitutively active alpha(1B)-adrenergic receptors

Reperfusion of globally ischemic rat hearts causes the generation of inosit ol(1,4,5)trisphosphate [Ins(1,4,5)P-3] and the initiation of arrhythmias. T hese responses are mediated by alpha(1)-adrenergic receptors (ARs), but the subtype of receptor involved has not been identified. Under normoxic condi tions, hearts from transgenic animals expressing constitutively active alph a(1B)-ARs in heart (alpha(1B)-constitutively active mutant [CAM]) showed hi gher [H-3] inositol phosphate responses to norepinephrine (2.3-fold) than h earts from nontransgenic animals (alpha(1B)-WT) (1.6-fold). alpha(1B)-WT he arts responded to 2 minutes of reperfusion after 20 minutes of global ische mia by generation of Ins(1,4,5)P-3 (5301+/-1310 to 11 413+/-1597 CPM/g tiss ue; 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 nor moxia, hearts from alpha(1B)-CAM animals showed no Ins(1,4,5)P-3 response i n early reperfusion. In parallel studies, alpha(1B)-WT hearts developed ven tricular tachycardia and ventricular premature beats (VPB) during 5 minutes of reperfusion after 20 minutes of ischemia The incidence of these arrhyth mias 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 th e 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 alp ha(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-f old increase), but the expression of mRNA for the alpha(1A)-receptors (0.14 1+/-0.035 amol/mu g RNA; mean+/-SEM; n=6) was reduced by 50% relative to al pha(1B)-WT controls (0.276+/-0.046 amol/mu g RNA; n=6; P<0.01). The reducti on in arrhythmogenic and Ins(1,4,5)P-3 responses in alpha(1B)-CAM hearts pr ovides evidence that these response are not mediated by alpha(1B)-receptors .