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α1A-adrenergic receptors activate phospholipase C, but suppress Ins(1,4,5)P3 generation during ischemia-reperfusion in mouse heart

F. Amirahmadi,1 R.M. Graham2 and E.A. Woodcock,1 1Baker Heart Research Institute, Melbourne, Vic, Australia and 2Victor Chang Cardiac Research Institute, Sydney, NSW, Australia.

Reperfusion of ischemic rat or mouse hearts causes noradrenaline (Nor) release, stimulation of α1-adrenergic receptors (α1-AR), phospholipase C (PLC) activation, Ins(1,4,5)P3 generation and consequent arrhythmias. We examined the effect of heightened activity of α1A-AR on these responses. [3H]Inositol-labeled mouse hearts overexpressing α1A-AR (α1A-TG) showed 10 fold higher PLC responses to Nor than wild type controls (α1A-WT). Isolated, perfused α1A-TG and α1A-WT hearts were subjected to 20 or 30 min zero-flow ischemia followed by 2 min reperfusion and [3H]-labeled inositol phosphates were extracted and quantified. Reperfusion of α1A-WT hearts after 30 min ischemia caused substantial [3H]Ins(1,4,5)P3 generation, from 1011 ± 99 to 2274 ± 297 cpm/heart, mean ± SEM, n=6 p<0.01. There was no detectable increase in [3H]Ins(1,4,5)P3 when α1A-WT hearts were reperfused after 20 min ischemia. However, reperfusion after 20 min ischemia with 100 μM Nor added to the perfusate caused generation of [3H]Ins(1,4,5)P3 in α1A-WT, from 662 ± 121 to 2554 ± 773, p < 0.01. In marked contrast to α1A -WT, [3H]Ins(1,4,5)P3 was not generated in α1A-TG hearts after either 20 or 30 min ischemia, even when Nor was added. Despite this, the overall reperfusion-induced PLC response (measured by increases in total [3H]InsPs) was 3-5 fold higher in α1A-TG than in α1A-WT after either 20 or 30 min ischemia. Both α1A-WT and α1A-TG showed substantially heightened responses to added Nor during 2 min reperfusion compared with normoxic conditions, 10 fold for α1A-WT and 2-3 fold for α1A-TG. These findings show that heightened activity of α1A-AR protect the myocardium from large increases in Ins(1,4,5)P3 by a mechanism that does not involve inhibition of PLC.