Heightened α_1A-adrenergic receptor activity suppresses ischaemia/reperfusion-induced Ins(1,4,5)P₃ generation in the mouse heart: a comparison with ischaemic preconditioning

Reperfusion of ischaemic rat or mouse hearts causes NE [noradrenaline (‘norepinephrine’)] release, stimulation of α₁-ARs (α₁-adrenergic receptors), PLC (phospholipase C) activation, Ins(1,4,5)P₃ generation and the development of arrhythmias. In the present study, we examined the effect of increased α_1A-AR drive on these responses. In hearts from non-transgenic mice (α_1A-WT), Ins(1,4,5)P₃ generation was observed after 2 min of reperfusion following 30 min of zero-flow ischaemia. No Ins(1,4,5)P₃ response was observed in hearts from transgenic mice with 66-fold overexpression of α_1A-AR (α_1A-TG). This was despite the fact that α_1A-TG hearts had 8–10-fold higher PLC responses to NE than α_1A-WT under normoxic conditions. The immediate phospholipid precursor of Ins(1,4,5)P₃, PtdIns(4,5)P₂, responded to ischaemia and reperfusion similarly in α_1A-WT and α_1A-TG mice. Thus the lack of Ins(1,4,5)P₃ generation in α_1A-TG mice is not caused by limited availability of PtdIns(4,5)P₂. Overall, α₁-AR-mediated PLC activity was markedly enhanced in α_1A-WT mice under reperfusion conditions, but responses in α_1A-TG mice were not significantly different in normoxia and post-ischaemic reperfusion. Ischaemic preconditioning prevented Ins(1,4,5)P₃ generation after 30 min of ischaemic insult in α_1A-WT mice. However, the precursor lipid PtdIns(4,5)P₂ was also reduced by preconditioning, whereas heightened α_1A-AR activity did not influence PtdIns(4,5)P₂ responses in reperfusion. Thus preconditioning and α_1A-AR overexpression have different effects on early signalling responses, even though both prevented Ins(1,4,5)P₃ generation. These studies demonstrate a selective inhibitory action of heightened α_1A-AR activity on immediate post-receptor signalling responses in early post-ischaemic reperfusion.