Heme oxygenase-1 plays a pro-life role in experimental brain stem death via nitric oxide synthase I/protein kinase G signaling at rostral ventrolateral medulla

Background: despite its clinical importance,a dearth of information exists on the cellular and molecular mechanisms that underpin brain stem death. a suitable neural substrate for mechanistic delineation on brain stem death resides in the rostral ventrolateral medulla (rvlm) because it is the origin of a life-and-death signal that sequentially increases (pro-life) and decreases (pro-death) to reflect the advancing central cardiovascular regulatory dysfunction during the progression towards brain stem death in critically ill patients. the present study evaluated the hypothesis that heme oxygnase-1 (ho-1) may play a pro-life role as an interposing signal between hypoxiainducible factor-1 (hif-1) and nitric oxide synthase i (nos i)/protein kinase g (pkg) cascade in rvlm,which sustains central cardiovascular regulatory functions during brain stem death. methods: we performed cardiovascular,pharmacological,biochemical and confocal microscopy experiments in conjunction with an experimental model of brain stem death that employed microinjection of the organophosphate insecticide mevinphos (mev; 10 nmol) bilaterally into rvlm of adult male sprague-dawley rats. results: western blot analysis coupled with laser scanning confocal microscopy revealed that augmented ho-1 expression that was confined to the cytoplasm of rvlm neurons occurred preferentially during the pro-life phase of experimental brain stem death and was antagonized by immunoneutralization of hif-1a or hif-1 b in rvlm. on the other hand,the cytoplasmic presence of ho-2 in rvlm neurons manifested insignificant changes during both phases. furthermore,immunoneutralization of ho-1 or knockdown of ho-1 gene in rvlm blunted the augmented life-and-death signals exhibited during the pro-life phase. those pretreatments also blocked the upregulated prolife nos i/pkg signaling without affecting the pro-death nos ii/peroxynitrite cascade in rvlm. conclusions: we conclude that transcriptional upregulation of ho-1 on activation by hif-1 in rvlm plays a preferential pro-life role by sustaining central cardiovascular regulatory functions during brain stem death via upregulation of nos i/pkg signaling pathway. our results further showed that the pro-dead nos ii/peroxynitrite cascade in rvlm is not included in this repertoire of cellular events. © 2010 dai et al.