Reduced reperfusion-induced Ins(1,4,5)P3 generation and arrhythmias in hearts expressing constitutively active alpha1B-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 alpha1-adrenergic receptors (ARs), but the subtype of receptor involved has not been identified. Under normoxic conditions, hearts from transgenic animals expressing constitutively active alpha1B-ARs in heart (alpha1B-constitutively active mutant [CAM]) showed higher [3H] inositol phosphate responses to norepinephrine (2.3-fold) than hearts from nontransgenic animals (alpha1B-WT) (1.6-fold). alpha1B-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 alpha1B-CAM animals showed no Ins(1,4,5)P3 response in early reperfusion. In parallel studies, alpha1B-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 alpha1B-CAM hearts (95% to 62% for VPB and 47% to 12% for ventricular tachycardia; both P<0.05). The resistance of the alpha1B-CAM hearts was not due to alpha1B-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 alpha1A-receptor activity in the alpha1B-CAM hearts, expression of the mRNA for alpha1A- and alpha1B-receptors was measured. alpha1B-WT hearts contained mRNA for both receptor subtypes, but the levels of alpha1B-receptor mRNA were 5-fold higher than alpha1A-receptor mRNA. alpha1B-CAM hearts contained very high levels of alpha1B-receptor mRNA (26-fold increase), but the expression of mRNA for the alpha1A-receptors (0.141+/-0.035 amol/ microg RNA; mean+/-SEM; n=6) was reduced by 50% relative to alpha1B-WT controls (0.276+/-0.046 amol/ microg RNA; n=6; P<0.01). The reduction in arrhythmogenic and Ins(1,4,5)P3 responses in alpha1B-CAM hearts provides evidence that these response are not mediated by alpha1B-receptors.
Harrison
S N
SN
Cellular Biochemistry, Molecular Physiology and Experimental Cardiology Laboratories, Baker Medical Research Institute, Melbourne, Victoria, Australia.
Autelitano
D J
DJ
Wang
B H
BH
Milano
C
C
Du
X J
XJ
Woodcock
E A
EA
D000818:Animals; D001145:Arrhythmias, Cardiac / Q000378:metabolism* / Q000503:physiopathology*; D009320:Atrial Natriuretic Factor / Q000235:genetics; D005260:Female; D006321:Heart / Q000033:anatomy & histology; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000096:biosynthesis*; D007295:Inositol Phosphates / Q000096:biosynthesis; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D009203:Myocardial Infarction / Q000473:pathology; D015428:Myocardial Reperfusion Injury / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism*; D009929:Organ Size; D012333:RNA, Messenger / Q000096:biosynthesis; D018340:Receptors, Adrenergic, alpha-1 / Q000096:biosynthesis*; D018179:Receptors, Thrombin / Q000378:metabolism; D014158:Transcription, Genetic
C547714:Adra1a protein, mouse; C547715:Adra1a protein, rat; C547717:Adra1b protein, mouse; C547718:Adra1b protein, rat; D007295:Inositol Phosphates; D012333:RNA, Messenger; D018340:Receptors, Adrenergic, alpha-1; D018179:Receptors, Thrombin; D015544:Inositol 1,4,5-Trisphosphate; D009320:Atrial Natriuretic Factor