Heightened alpha1A-adrenergic receptor activity suppresses ischaemia/reperfusion-induced Ins(1,4,5)P3 generation in the mouse heart: a comparison with ischaemic preconditioning

Abstract

Reperfusion of ischaemic rat or mouse hearts causes NE [noradrenaline ('norepinephrine')] release, stimulation of alpha(1)-ARs (alpha(1)-adrenergic receptors), PLC (phospholipase C) activation, Ins(1,4,5)P(3) generation and the development of arrhythmias. In the present study, we examined the effect of increased alpha(1A)-AR drive on these responses. In hearts from non-transgenic mice (alpha(1A)-WT), Ins(1,4,5)P(3) generation was observed after 2 min of reperfusion following 30 min of zero-flow ischaemia. No Ins(1,4,5)P(3) response was observed in hearts from transgenic mice with 66-fold overexpression of alpha(1A)-AR (alpha(1A)-TG). This was despite the fact that alpha(1A)-TG hearts had 8-10-fold higher PLC responses to NE than alpha(1A)-WT under normoxic conditions. The immediate phospholipid precursor of Ins(1,4,5)P(3), PtdIns(4,5)P(2), responded to ischaemia and reperfusion similarly in alpha(1A)-WT and alpha(1A)-TG mice. Thus the lack of Ins(1,4,5)P(3) generation in alpha(1A)-TG mice is not caused by limited availability of PtdIns(4,5)P(2). Overall, alpha(1)-AR-mediated PLC activity was markedly enhanced in alpha(1A)-WT mice under reperfusion conditions, but responses in alpha(1A)-TG mice were not significantly different in normoxia and post-ischaemic reperfusion. Ischaemic preconditioning prevented Ins(1,4,5)P(3) generation after 30 min of ischaemic insult in alpha(1A)-WT mice. However, the precursor lipid PtdIns(4,5)P(2) was also reduced by preconditioning, whereas heightened alpha(1A)-AR activity did not influence PtdIns(4,5)P(2) responses in reperfusion. Thus preconditioning and alpha(1A)-AR overexpression have different effects on early signalling responses, even though both prevented Ins(1,4,5)P(3) generation. These studies demonstrate a selective inhibitory action of heightened alpha(1A)-AR activity on immediate post-receptor signalling responses in early post-ischaemic reperfusion.