Differential contributions of NOS isoforms in the rostral ventrolateral medulla to cardiovascular responses associated with mevinphos intoxication in the rat

The organophosphate poison mevinphos (Mev) elicits cardiovascular responses via nitric oxide (NO) produced on activation of M2 muscarinic receptors (M2R) in the rostral ventrolateral medulla (RVLM), where sympathetic vasomotor tone originates. This study further evaluated the contribution of nitric oxide synthase (NOS) isoforms at the RVLM to this process, using adult Sprague-Dawley rats. Bilateral co-microinjection into the RVLM of the selective NOS I inhibitor (250 pmol), 7-nitroindazole or N(omega)-propyl-L-arginine antagonized the initial sympathoexcitatory cardiovascular responses to Mev (10 nmol). Co-administration of a selective NOS II inhibitor, N6-(1-iminoethyl)-L-lysine (250 or 500 pmol) further enhanced these cardiovascular responses and reversed the secondary sympathoinhibitory actions of Mev. A potent NOS III inhibitor, N5-(1-iminoethyl)-L-ornithine (46 or 92 nmol) was ineffective. We also found that M2R co-localized only with NOS I- or NOS II-immunoreactive RVLM neurons. Furthermore, only NOS I or II in the ventrolateral medulla exhibited an elevation in mRNA or protein levels during the sympathoexcitatory phase, with further up-regulated synthesis of NOS II during the sympathoinhibitory phase of Mev intoxication. We conclude that whereas NOS III is not engaged, NO produced by NOS I and II in the RVLM plays, respectively, a sympathoexcitatory and sympathoinhibitory role in the cardiovascular responses during Mev intoxication.     

Contribution of peroxynitrite to fatal cardiovascular depression induced by overproduction of nitric oxide in rostral ventrolateral medulla of the rat

We evaluated the contribution of peroxynitrite to the fatal cardiovascular depression induced by overproduction of nitric oxide (NO) after activation of inducible NO synthase (iNOS) in the rostral ventrolateral medulla (RVLM), the origin of sympathetic vasomotor tone. In Sprague-Dawley rats maintained under propofol anesthesia, microinjection of E. coli lipopolysaccharide (LPS) bilaterally into the RVLM elicited significant hypotension, bradycardia, reduction in sympathetic vasomotor tone and mortality. There was also a discernible elevation of iNOS expression in the ventrolateral medulla, followed by a massive production of nitrotyrosine, an experimental index for peroxynitrite. Co-administration bilaterally into the RVLM of the selective iNOS inhibitor, S-methylisothiourea (50, 100 or 250 pmol), an active peroxynitrite decomposition catalyst, 5,10,15,20-tetrakis- (N-methyl-4'-pyridyl)-porphyrinato iron (III) (10 or 50 pmol), a peroxynitrite scavenger, L-cysteine (5, 50 or 100 pmol), or a superoxide dismutase mimetic, Mn(III)-tetrakis-(4-benzoic acid) porphyrin (1 or 10 pmol), significantly prevented mortality, reduced nitrotyrosine production and reversed the NO-induced cardiovascular suppression after application of LPS into the RVLM. We conclude that the formation of peroxynitrite by a reaction between superoxide anion and NO is primarily responsible for the fatal cardiovascular depression induced by overproduction of NO after activation of iNOS at the RVLM.     

Contribution of cGMP but not peroxynitrite to negative feedback regulation of penile erection elicited by nitric oxide in the hippocampal formation of the rat

We established previously that nitric oxide (NO) in the hippocampal formation (HF) participates actively in negative feedback regulation of penile erection. This study further evaluated whether this process engaged soluble guanylyl cyclase (sGC)/cGMP cascade or peroxynitrite in the HF. Intracavernous pressure (ICP) recorded from the penis in adult, male Sprague-Dawley rats anesthetized with chloral hydrate was employed as our experimental index for penile erection. Microinjection bilaterally of a NO-independent sGC activator, YC-1 (0.1 or 1 nmol) or a cGMP analog, 8-Bromo-cGMP (0.1 or 1 nmol), into the HF elicited a significant reduction in baseline ICP. Bilateral application into the HF of equimolar doses (0.5 or 1 nmol) of a sGC inhibitor, LY83583 or a NO-sensitive sGC inhibitor, ODQ significantly antagonized the decrease in baseline ICP induced by co-administration of the NO precursor, L-arginine (5 nmol), along with significant enhancement of the magnitude of papaverine-induced elevation in ICP. In contrast, a peroxynitrite scavenger, L-cysteine (50 or 100 pmol), or an active peroxynitrite decomposition catalyst, 5,10,15,20-tetrakis-(N-methyl-4'-pyridyl)-porphyrinato iron (III) (10 or 50 pmol), was ineffective in both events. These results suggest that NO may participate in negative feedback regulation of penile erection by activating the sGC/cGMP cascade in the HF selectively.