Abstract
BackgroundAdrenomedullin (ADM) exerts its biological functions through the receptor-mediated enzymatic mechanisms that involve protein kinase A (PKA), or neuronal nitric oxide synthase (nNOS). We previously demonstrated that the receptor-mediated cAMP/PKA pathway involves in ADM-enhanced baroreceptor reflex (BRR) response. It remains unclear whether ADM may enhance BRR response via activation of nNOS-dependent mechanism in the nucleus tractus solitarii (NTS).MethodsIntravenous injection of phenylephrine was administered to evoke the BRR before and at 10, 30, and 60 min after microinjection of the test agents into NTS of Sprague-Dawley rats. Western blotting analysis was used to measure the level and phosphorylation of proteins that involved in BRR-enhancing effects of ADM (0.2 pmol) in NTS. The colocalization of PKA and nNOS was examined by immunohistochemical staining and observed with a laser confocal microscope.ResultsWe found that ADM-induced enhancement of BRR response was blunted by microinjection of NPLA or Rp-8-Br-cGMP, a selective inhibitor of nNOS or protein kinase G (PKG) respectively, into NTS. Western blot analysis further revealed that ADM induced an increase in the protein level of PKG-I which could be attenuated by co-microinjection with the ADM receptor antagonist ADM22-52 or NPLA. Moreover, we observed an increase in phosphorylation at Ser1416 of nNOS at 10, 30, and 60 min after intra-NTS administration of ADM. As such, nNOS/PKG signaling may also account for the enhancing effect of ADM on BRR response. Interestingly, biochemical evidence further showed that ADM-induced increase of nNOS phosphorylation was prevented by co-microinjection with Rp-8-Br-cAMP, a PKA inhibitor. The possibility of PKA-dependent nNOS activation was substantiated by immunohistochemical demonstration of co-localization of PKA and nNOS in putative NTS neurons.ConclusionsThe novel finding of this study is that the signal transduction cascade that underlies the enhancement of BRR response by ADM in NTS is composed sequentially of cAMP/PKA and nNOS/PKG pathways.
Highlights
Adrenomedullin (ADM) exerts its biological functions through the receptor-mediated enzymatic mechanisms that involve protein kinase A (PKA), or neuronal nitric oxide synthase
Involvement of neuronal nitric oxide synthase (nNOS) in baroreceptor reflex (BRR)-enhancing effect of ADM in nucleus tractus solitarii (NTS) In our previous study [12], we have demonstrated that microinjection of ADM (0.2 pmol) into NTS significantly augmented BRR response with a maximal enhancement at 60 min after administration
The ADM-induced BRR enhancement was substantially unaffected by co-microinjections with the relatively selective inducible NOS inhibitor SMT (250 pmol) (Figure 1C), or L-NIO (100 pmol), a preferential endothelial NOS inhibitor (Figure 1D)
Summary
Adrenomedullin (ADM) exerts its biological functions through the receptor-mediated enzymatic mechanisms that involve protein kinase A (PKA), or neuronal nitric oxide synthase (nNOS). In addition to distribution in the cardiovascular system, ADM and ADMRs are expressed in the central nervous system (CNS) and are localized to the autonomic nuclei, including nucleus tractus solitarii (NTS), lateral parabrachial nucleus (LPBN), and rostral ventrolateral medulla (RVLM) [4,5,6]. These findings suggested a possible role for ADM in central regulation of cardiovascular functions. In our recent study [12], we demonstrated that microinjection of ADM into NTS, the termination site of primary baroreceptor afferents in the brain stem [13], significantly increases BRR response and sensitivity in a time- and dose-dependent manner, without producing discernible changes in basal arterial pressure and heart rate
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