Duality of plasmin effect on cytosolic free calcium in human platelets

Effects of Plasmin on von Willebrand Factor and Platelets: A Narrative Review

Plasmin is the major fibrinolytic protease responsible for dissolving thrombi by cleavage of its primary substrate fibrin. In addition, emerging evidence points to other roles of plasmin: (1) as a back-up for ADAMTS13 in proteolysis of ultra-large von Willebrand factor (VWF) multimers and (2) as an activator of platelets. Although the molecular mechanisms of fibrinolysis are well defined, insights on the effects of plasmin on VWF and platelets are relatively scarce and sometimes conflicting. Hence, this review provides an overview of the literature on the effects of plasmin on VWF multimeric structures, on VWF binding to platelets, and on platelet activation. This information is placed in the context of possible applications of thrombolytic therapy for the condition thrombotic thrombocytopenic purpura.

Polymerization-Incompetent Uromodulin in the Pregnant Stroke-Prone Spontaneously Hypertensive Rat

The kidney is centrally involved in blood pressure regulation and undergoes extensive changes during pregnancy. Hypertension during pregnancy may result in an altered urinary peptidome that could be used to indicate new targets of therapeutic or diagnostic interest. The stroke-prone spontaneously hypertensive rat (SHRSP) is a model of maternal chronic hypertension. Capillary electrophoresis-mass spectrometry was conducted to interrogate the urinary peptidome in SHRSP and the control Wistar-Kyoto strain at three time points: prepregnancy and gestational days 12 and 18. The comparison within and between the Wistar-Kyoto and SHRSP peptidome at all time points detected 123 differentially expressed peptides (fold change >1.5; P<0.05). Sequencing of these peptides identified fragments of collagen α-chains, albumin, prothrombin, actin, serpin A3K, proepidermal growth factor, and uromodulin. Uromodulin peptides showed a pregnancy-specific alteration in SHRSP with a 7.8-fold (P<0.01) and 8.8-fold (P<0.05) increase at gestational days 12 and 18, respectively, relative to the Wistar-Kyoto. Further investigation revealed that these peptides belonged to the polymerization-inhibitory region of uromodulin. Two forms of uromodulin (polymerization competent and polymerization incompetent) were found in urine from both Wistar-Kyoto and SHRSP, where the polymerization-incompetent form was increased in a pregnancy-specific manner in SHRSP. Nifedipine-treated pregnant SHRSP showed only polymerization-competent uromodulin, indicating that calcium may be mechanistically involved in uromodulin polymerization. This study highlights, for the first time, a potential role of uromodulin and its polymerization in hypertensive pregnancy.

Pathogenetic Mechanisms of Neurogenic Pulmonary Edema

Neurogenic pulmonary edema (NPE) is a life-threatening complication of central nervous system (CNS) injuries. This review summarizes current knowledge about NPE etiology and pathophysiology with an emphasis on its experimental models, including our spinal cord compression model. NPE may develop as a result of activation of specific CNS trigger zones located in the brainstem, leading to a rapid sympathetic discharge, rise in systemic blood pressure, baroreflex-induced bradycardia, and enhanced venous return resulting in pulmonary vascular congestion characterized by interstitial edema, intra-alveolar accumulation of transudate, and intra-alveolar hemorrhages. The potential etiological role of neurotransmitter changes in NPE trigger zones leading to enhanced sympathetic nerve activity is discussed. Degree of anesthesia is a crucial determinant for the extent of NPE development in experimental models because of its influence on sympathetic nervous system activity. Sympathetic hyperactivity is based on the major activation of either ascending spinal pathways by spinal cord injury or NPE trigger zones by increased intracranial pressure. Attenuation of sympathetic nerve activity or abolition of reflex bradycardia completely prevent NPE development in our experimental model. Suggestions for future research into NPE pathogenesis as well as therapeutic potential of particular drugs and interventions are discussed.

Modulation of inducible nitric oxide synthase (iNOS) expression and cardiovascular responses during static exercise following iNOS antagonism within the ventrolateral medulla

Previous reports indicate that inducible nitric oxide synthase (iNOS) blockade within the rostral ventrolateral medulla (RVLM) and caudal ventrolateral medulla (CVLM) differentially modulated cardiovascular responses, medullary glutamate, and GABA concentrations during static skeletal muscle contraction. In the current study, we determined the role of iNOS antagonism within the RVLM and CVLM on cardiovascular responses and iNOS protein expression during the exercise pressor reflex in anesthetized rats. Following 120 min of bilateral microdialysis of a selective iNOS antagonist, aminoguanidine (AGN; 10 µM), into the RVLM, the pressor responses were attenuated by 72 % and changes in heart rate were reduced by 38 % during a static muscle contraction. Furthermore, western blot analysis of iNOS protein abundance within the RVLM revealed a significant attenuation when compared to control animals. In contrast, bilateral administration of AGN (10 µM) into the CVLM augmented the increases in mean arterial pressure by 60 % and potentiated changes in heart rate by 61 % during muscle contractions, but did not alter expression of the iNOS protein within the CVLM. These results demonstrate that iNOS protein expression within the ventrolateral medulla is differentially regulated by iNOS blockade that may, in part, contribute to the modulation of cardiovascular responses during static exercise.

Blood pressure regulation VIII: resistance vessel tone and implications for a pro-atherogenic conduit artery endothelial cell phenotype

Dysfunction of the endothelium is proposed as the primary initiator of atherosclerotic peripheral artery disease, which occurs mainly in medium- to large-sized conduit arteries of the lower extremities (e.g., iliac, femoral, popliteal arteries). In this review article, we propose the novel concept that conduit artery endothelial cell phenotype is determined, in part, by microvascular tone in skeletal muscle resistance arteries through both changes in arterial blood pressure as well as upstream conduit artery shear stress patterns. First, we summarize the literature supporting the involvement of sympathetic nerve activity (SNA) and nitric oxide (NO) in the modulation of microvascular tone and arterial blood pressure. We then focus on the role of elevated blood pressure and shear stress profiles in modulating conduit artery endothelial cell phenotype. Last, we discuss findings from classic and emerging studies indicating that increased vascular resistance, as it occurs in the context of increased SNA and/or reduced NO bioavailability, is associated with greater oscillatory shear stress (e.g., increased retrograde shear) in upstream conduit arteries. The ideas put forth in this review set the stage for a new paradigm concerning the mechanistic link between increased microvascular tone and development of conduit artery endothelial dysfunction and thus increased risk for peripheral artery disease. Indeed, a vast amount of evidence supports the notion that excessive blood pressure and oscillatory shear stress are potent pro-atherogenic signals to the endothelium.

Central TNF inhibition results in attenuated neurohumoral excitation in heart failure: a role for superoxide and nitric oxide

This study examined the effect of central tumor necrosis factor-alpha (TNF) blockade on the imbalance between nitric oxide and superoxide production in the paraventricular nucleus (PVN) and ventrolateral medulla (VLM), key autonomic regulators, and their contribution to enhanced sympathetic drive in mice with congestive heart failure (CHF). We also used a TNF gene knockout (KO) mouse model to study the involvement of TNF in body fluid homeostasis and sympathoexcitation in CHF. After implantation of intracerebroventricular (ICV) cannulae, myocardial infarction (MI) was induced in wild-type (WT) and KO mice by coronary artery ligation. Osmotic mini-pumps were implanted into one set of WT + MI/Sham mice for continuous ICV infusion of Etanercept (ETN), a TNF receptor fusion protein, or vehicle (VEH). Gene expressions of neuronal nitric oxide synthase (NOS) and angiotensin receptor-type 2 were reduced, while those of inducible NOS, Nox2 homologs, superoxide, peroxynitrite and angiotensin receptor-type 1 were elevated in the brainstem and hypothalamus of MI + VEH. Plasma norepinephrine levels and the number of Fos-positive neurons were also increased in the PVN and VLM in MI + VEH. MI + ETN and KO + MI mice exhibited reduced oxidative stress, reduced sympathoexcitation and an improved cardiac function. These changes in WT + MI were associated with increased sodium and fluid retention. These results indicate that elevated TNF in these autonomic regulatory regions of the brain alter the production of superoxide and nitric oxide, contributing to fluid imbalance and sympathoexcitation in CHF.

Regulation of plasmin-induced protease-activated receptor 4 activation in platelets

Plasmin, a major extracellular protease, activates platelets through PAR4 receptors. Plasmin-induced full aggregation is achieved at lower concentrations (0.1 U/mL) in murine platelets as compared to human platelets (1 U/mL). In COS7 cells expressing the murine PAR4 (mPAR4) receptor, 1 U/mL plasmin caused a higher intracellular calcium mobilization than in cells expressing the human PAR4 (hPAR4) receptor. This difference was reversed when the tethered ligand sequences of mPAR4 and hPAR4 were interchanged through site-directed mutagenesis. We further investigated whether PAR3 expressed in murine platelets serves as a co-receptor for PAR4 activation by plasmin. In COS7 cells, co-expressing mPAR3 and mPAR4, plamsin produced a smaller intracellular calcium mobilization compared to cells expressing mPAR4 alone, suggesting that PAR3 might inhibit plasmin-induced PAR4 stimulation. Consistent with these results, PAR3 null murine platelets also showed a greater plasmin-induced calcium mobilization and aggregation compared to wild-type murine platelets. In conclusion, murine platelets are more sensitive to activation by plasmin than human platelets due to differences in the primary sequence of PAR4. In contrast to thrombin-dependent activation of platelets, wherein PAR3 acts as a co-receptor, mPAR3 inhibits plasmin-induced PAR4 activation.

Mitochondrial respiratory enzyme complexes in rostral ventrolateral medulla as cellular targets of nitric oxide and superoxide interaction in the antagonism of antihypertensive action of eNOS transgene

Overproduction of nitric oxide (NO) by gene transduction of endothelial NO synthase (eNOS) in rostral ventrolateral medulla (RVLM), which is responsible for maintenance of vasomotor tone, reduces arterial pressure in spontaneously hypertensive rats (SHR). This NO-induced vasodepression, however, is not sustained and is followed by rebound hypertension. Because superoxide anion (O(2)(*-)) level is increased and synthesis or activity of mitochondrial manganese superoxide dismutase (SOD2) is reduced in RVLM during hypertension, we hypothesized that an interaction between NO and O(2)(*-) in RVLM, using mitochondrial respiratory enzyme complexes (MRC) as the cellular target, contributes to those cardiovascular outcomes after eNOS gene transduction in SHR. The present study assessed this hypothesis using adenoviral vectors to overexpress eNOS (AdeNOS) and/or SOD2 (AdSOD2) in RVLM of SHR or normotensive Wistar-Kyoto (WKY) rats. Microinjection of AdeNOS bilaterally into RVLM elicited 35% depression of MRC-I enzyme activity and evoked 60% and 50% increase in O(2)(*-) and peroxynitrite level in RVLM of SHR, but not WKY rats, which was reversed by cotransduced AdSOD2 or treatment with peroxynitrite decomposition catalyst. Cotransduction of AdeNOS and AdSOD2 in RVLM of SHR elicited significantly greater decreases in arterial pressure and heart rate than those promoted by the individual transgene and prevented the AdeNOS-induced rebound hypertension. We conclude that an interactive action between NO and O(2)(*-) on MRC-I in RVLM via formation of peroxynitrite contributes to the unsustained hypotensive effects of NO after overexpression of eNOS in SHR. The mitochondria-derived O(2)(*-) also mediates the rebound hypertension induced by eNOS transgene in RVLM of SHR.

Simvastatin inhibits central sympathetic outflow in heart failure by a nitric-oxide synthase mechanism

Our previous study demonstrated that oral treatment with simvastatin (SIM) suppressed renal sympathetic nerve activity (RSNA) in the rabbits with chronic heart failure (CHF). The purpose of this experiment was to determine the effects of direct application of SIM to the central nervous system on RSNA and its relevant mechanisms. Experiments were carried out on 21 male New Zealand White rabbits with pacing-induced CHF. The CHF rabbits received infusion of vehicle, SIM, or SIM + N(omega)-nitro-L-arginine methyl ester into the lateral cerebral ventricle via osmotic minipump for 7 days. We found that 1) in CHF rabbits, intracerebroventricular infusion of SIM significantly suppressed basal RSNA (1st day 69.5 +/- 8.9% maximum; 7th day 26.0 +/- 6.0% maximum; P < 0.05, n = 7) and enhanced arterial baroreflex function starting from the 2nd day and lasting through the following 5 days; 2) statin treatment significantly up-regulated neuronal nitric-oxide synthase (nNOS) protein expression in the rostral ventrolateral medulla (RVLM) (control, n = 6, 0.12 +/- 0.04; SIM-treated, n = 7, 0.31 +/- 0.05. P < 0.05); 3) in CATH.a neurons, incubation with SIM significantly up-regulated the nNOS mRNA expression, which was blocked by coincubation with mevalonate, farnesyl-pyrophosphate, or geranylgeranyl-pyrophosphate; and 4) incubation with Y-27632 [(R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide] significantly up-regulated nNOS mRNA expression in these neurons. These results suggest that central treatment with SIM decreased sympathetic outflow in CHF rabbits via up-regulation of nNOS expression in RVLM, which may be due to the inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase and a decrease in Rho kinase by SIM.

PULMONARY VASCULAR REACTIVITY OF SPONTANEOUSLY HYPERTENSIVE RATS IS EXACERBATED IN RESPONSE TO THE CENTRAL ADMINISTRATION OF EXOGENOUS NITRIC OXIDE

1. Centrally, nitric oxide (NO) is a sympathoinhibitory substance. Spontaneously hypertensive rats (SHR) have an impaired central nitroxidergic system and, consequently, NO-mediated decrease in sympathetic activity is exacerbated in SHR compared with Wistar-Kyoto (WKY) rats. We have demonstrated previously that acute hypoxic pulmonary vasoconstriction (HPV) is enhanced by central NO administration. Therefore, in the present study, we hypothesized that accentuation of the HPV by NO would be exacerbated in SHR compared with WKY rats. 2. Mean pulmonary arterial pressure, systemic mean arterial blood pressure, cardiac output and heart rate were measured in pentobarbitone-anaesthetized, artificially ventilated, male SHR and WKY rats. The brief, transient response to a bolus intracerebroventricular (i.c.v.) dose of N(G)-nitro-L-arginine methyl ester (L-NAME; 150 microg in 10 microL) was recorded in all rats. Upon recovery, rats were exposed to acute hypoxia (10% O(2) for 4 min) before and after the i.c.v. administration of the NO donor 3-[4-morpholinyl]-sydnonimine-hydrochloride (SIN-1; 100 microg in 10 microL). 3. In WKY rats, central inhibition of NO synthesis by L-NAME caused a mild increase in tonic pulmonary vascular tone and induced a large systemic pressor response. These responses were not observed in SHR. In contrast, SIN-1 failed to alter tonic pulmonary vascular tone, although it enhanced the HPV in WKY rats and, significantly more so, in SHR. 4. These results confirm that accentuation of the HPV by NO is exacerbated in SHR compared with WKY rats. The mechanism(s) by which the HPV is accentuated by central NO remains to be fully elucidated, but is likely to be associated with the sympathoinhibitory effects of NO and, if so, supports the idea that the nitroxidergic system of the SHR is impaired. Further electrophysiological studies are essential to confirm these assumptions.

Exogenous nitric oxide centrally enhances pulmonary reactivity in the normal and hypertensive rat

1. Chronic hypoxia causes sustained pulmonary hypertension and, although impairment of the pulmonary endothelial nitric oxide (NO) pathway has been implicated, no study has described the central role of NO in modulating pulmonary vascular tone and reactivity. Centrally, NO inhibits sympathetic outflow, so we hypothesised that central NO would modulate pulmonary vascular tone and its reactivity to acute hypoxia, especially in the hypertensive state. 2. Male adult Sprague-Dawley rats were exposed to normoxia (N) or chronic hypoxia (CH; 12% O2) for 14 days. Mean pulmonary arterial pressure (MPAP), systemic mean arterial blood pressure (MABP), cardiac output and heart rate were then measured in pentobarbitone-anaesthetized, artificially ventilated rats. The N and CH rats were exposed to acute hypoxia (10% O2 for 4 min) after the intracerebroventricular (i.c.v.) administration of artificial cerebrospinal fluid (control) and then again after either i.c.v. NG-nitro-L-arginine methyl ester (L-NAME; 150 microg in 10 microL) or 3-morpholino-sydnonimine hydrochloride (SIN-1; 100 microg in 10 microL). 3. Chronic hypoxia caused pulmonary hypertension (MPAP 20+/-1 vs 30+/-1 mmHg in N and CH rats, respectively) and attenuated acute hypoxic pulmonary vasoconstriction (HPV). Central inhibition of NO synthesis (by l-NAME) did not alter baseline MPAP or the acute HPV in either N or CH rats, but it did elevate MABP. The NO donor SIN-1 did not alter baseline MPAP, but it did enhance (N rats) or restore (CH rats) the HPV and decreased MABP. 4. The results of the present study indicate that central NO has a limited role in the tonic modulation of MPAP during normoxia and after chronic hypoxia. However, the acute HPV seems to be enhanced by exogenous NO.

Selective Sensitization by Nitric Oxide of Sympathetic Baroreflex in Rostral Ventrolateral Medulla of Conscious Rabbits

Nitric oxide (NO) deficiency in the rostral ventrolateral medulla (RVLM) has been implicated in impaired baroreflex control in hypertensive and heart failure animals. However, the role of local NO in normal baroreflex regulation remains unclear. This study aimed to examine the role of NO in tonic and baroreflex control of blood pressure (BP) in the RVLM of conscious rabbits. Microinjections of NO donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside (5 to 20 nmol), or NO itself (20 to 200 pmol) into the RVLM dose-dependently increased BP. Bilateral microinjections of an NO synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 10 nmol), its inactive enantiomer D-NAME, or soluble guanylate cyclase (sGC) inhibitors, 1-H-[1,2,4]oxadiaolo[4,3-a]quinoxalin-1-one (ODQ, 250 pmol) and methylene blue (10 nmol), into the RVLM did not affect resting BP, heart rate, or renal sympathetic nerve activity (RSNA). However, L-NAME, methylene blue, and ODQ decreased RSNA baroreflex gain by 42% to 55%, whereas D-NAME did not affect this reflex. Co-microinjections of L-NAME and superoxide scavenger tempol (20 nmol) decreased RSNA baroreflex gain by 37+/-8%. Microinjections of a neuronal NOS (nNOS) inhibitor, 7-nitroindazole (500 pmol), into the RVLM decreased RSNA baroreflex gain by 42+/-12%, without altering resting BP, heart rate, or RSNA. Local administration of inducible NOS (iNOS) inhibitors, S-methylisothiourea (0.25 nmol) and aminoguanidine (0.25 and 2.5 nmol), affected neither resting nor baroreflex parameters. These results suggest that nNOS-derived NO facilitates sympathetic baroreflex transmission in the RVLM at least in part via a sGC-dependent, superoxide-independent mechanism. However, local nNOS and iNOS play little role in the tonic support of BP in conscious rabbits.

Overexpression of inducible nitric oxide synthase in rostral ventrolateral medulla causes hypertension and sympathoexcitation via an increase in oxidative stress

The present study examined the role of inducible nitric oxide synthase (iNOS) in the rostral ventrolateral medulla (RVLM) of the brain stem, where the vasomotor center is located, in the control of blood pressure and sympathetic nerve activity. Adenovirus vectors encoding iNOS (AdiNOS) or beta-galactosidase (Adbetagal) were transfected into the RVLM in Wistar-Kyoto (WKY) rats. Blood pressure and heart rate were monitored using a radiotelemetry system. iNOS expression in the RVLM was confirmed by immunohistochemical staining or Western blot analysis. Mean arterial pressure significantly increased from day 6 to day 11 after AdiNOS transfection, but did not change after Adbetagal transfection. Urinary norepinephrine excretion was significantly higher in AdiNOS-transfected rats than in Adbetagal-transfected rats. Microinjection of aminoguanidine or S-methylisothiourea, iNOS inhibitors, or tempol, an antioxidant, significantly attenuated the pressor response evoked by iNOS gene transfer. The levels of thiobarbituric acid-reactive substances, a marker of oxidative stress, were significantly greater in AdiNOS-transfected rats than in Adbetagal-transfected rats. Dihydroethidium fluorescence in the RVLM was increased in AdiNOS-transfected rats. In addition, nitrotyrosine-positive cells were observed in the RVLM only in AdiNOS-transfected rats. Intracisternal infusion of tempol significantly attenuated the pressor response and the increase in the levels of thiobarbituric acid-reactive substances induced by AdiNOS transfection. These results suggest that overexpression of iNOS in the RVLM increases blood pressure via activation of the sympathetic nervous system, which is mediated by an increase in oxidative stress.

Pressure-independent cardiac effects of angiotensin II in pigs

BACKGROUND

Angiotensin II (Ang II) is a potent vasoconstrictor with an important role in the development of cardiovascular disease. Earlier results have shown a positive acute inotropic effect of Ang II in anaesthetized pigs together with significant vasoconstriction. This investigation was designed to study cardiac effects of Ang II, when blood pressure was maintained constant by experimental means.

METHODS

Ang II (200 microg h(-1)) was infused in anaesthetized pigs (n = 10) at two different arterial blood pressures, the first determined by the effects of Ang II alone, and the second maintained at baseline blood pressure with nitroprusside. Cardiac systolic and diastolic function was evaluated by analysis of left ventricular pressure-volume relationships.

RESULTS

Heart rate, end-systolic elastance (Ees) and pre-load adjusted maximal power (PWRmax EDV(-2)) increased at both blood pressure levels, although less when blood pressure was kept constant with nitroprusside. The time constant for isovolumetric relaxation (tau(1/2)) was prolonged with Ang II alone and shortened with Ang II infused together with nitroprusside.

CONCLUSION

Ang II infusion in the pig has inotropic and chronotropic properties independent of arterial blood pressure levels, although the effects seem to be blunted by pharmacological actions of the nitric oxide donor nitroprusside.

3-Morpholinylsydnonimine Inhibits Glutamatergic Transmission in Rat Rostral Ventrolateral Medulla via Peroxynitrite Formation and Adenosine Release

We have previously reported that, depending on the dose, nitric oxide (NO)-generating agents exert a dual facilitatory and inhibitory action on glutamatergic transmission on the rostral ventrolateral medulla (RVLM) neurons. The molecular mechanisms underlying the NO-mediated synaptic inhibition have not yet been defined. Here we show that the amplitude of excitatory postsynaptic currents (EPSCs) was reversibly reduced by the NO donors 3-morpholinylsydnoneimine (SIN-1) (1 mM) and spermine NONOate (1 mM). This effect was antagonized by an active peroxynitrite decomposition catalyst 5,10,15,20-tetrakis(4-sulfonatophenyl)prophyrinato iron (III) chloride, G(i/o)-coupled receptor blockers, N-ethylmaleimide and pertussis toxin, A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine, or adenosine deaminase. However, NO-sensitive guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, GABA(B) receptor antagonist (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH50911), or cannabinoid receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A) had no effect on the inhibitory action of SIN-1 on EPSCs. Perfusion of adenosine mimicked and subsequently occluded the action of SIN-1. Inhibition of EPSC amplitude by SIN-1 was associated with an increase in the paired-pulse ratio of EPSCs. Furthermore, SIN reduced the frequency of spontaneous EPSCs without altering their amplitude of distribution. Pretreatment with N-type Ca(2+)-channel blocker omega-conotoxin GVIA selectively blocked SIN-1-induced inhibition of EPSCs. These results suggest that a higher dose of SIN-1 acts presynaptically to elicit a synaptic depression on the RVLM neurons through an inhibition of presynaptic N-type Ca(2+)-channel activity, leading to reduced glutamate release. The presynaptic action of SIN-1 is mediated by the formation of peroxynitrite, which subsequently acts to release adenosine to activate A(1) adenosine receptors.

Plasmin-mediated Activation of Platelets Occurs by Cleavage of Protease-activated Receptor 4

The activation of plasmin from its circulating precursor plasminogen is the mechanism of several clot-busting drugs used to clinically treat patients who have suffered a stroke; however, plasmin thus generated has been shown to activate platelets directly. There has been speculation as to whether plasmin interacts with the protease-activated receptors (PARs) because of its similarity in amino acid specificity with the classic platelet activator thrombin. We have investigated whether plasmin activates platelets via PAR activation through multiple complementary approaches. At concentrations sufficient to induce human platelet aggregation, plasmin released very little calcium compared with that induced by thrombin, the PAR-1 agonist peptide SFLLRN, or the PAR-4 agonist peptide AYPGKF. Stimulation of platelets with plasmin initially failed to desensitize additional stimulation with SFLLRN or AYPGKF, but a prolonged incubation with plasmin desensitized platelets to further stimulation by thrombin. The desensitization of PAR-1 had no effect on plasmin-induced platelet aggregation and yielded an aggregation profile that was similar to plasmin in response to a low dose of thrombin. However, PAR-4 desensitization completely eliminated aggregation in response to plasmin. Inclusion of the PAR-1-specific antagonist BMS-200261 inhibited platelet aggregation induced by a low dose of thrombin but not by plasmin. Additionally, mouse platelets naturally devoid of PAR-1 showed a full aggregation response to plasmin in comparison to thrombin. Furthermore, human and mouse platelets treated with a PAR-4 antagonist, as well as platelets isolated from PAR-4 homozygous null mice, failed to aggregate in response to plasmin. Finally, a protease-resistant recombinant PAR-4 was refractory to activation by plasmin. We conclude that plasmin induces platelet aggregation primarily through slow cleavage of PAR-4.

Brain nitric oxide synthase activity in spontaneously hypertensive rats during the development of hypertension

OBJECTIVES

Blockade of neuronal nitric oxide synthase (nNOS) in the brain induced an increase in mean arterial pressure of spontaneously hypertensive rats (SHR). We hypothesize that increased nitric oxide (NO) synthesis in the brain compensates for hypertension. Therefore, we measured NOS activity in different brain regions in SHR at prehypertensive, onset and established hypertension, and compared with age-matched Wistar-Kyoto (WKY) rats.

METHOD

NOS activity was measured by the ability of tissue homogenate to convert [3H]l-arginine to [3H]l-citrulline in a Ca2+- and NADPH-dependent manner.

RESULTS

NOS activity was impaired in the cerebral cortex and brainstem of prehypertensive SHR. At established hypertension, SHR showed an augmentation in NOS activity in hypothalamus and brainstem. Chronic treatment of SHR with the angiotensin-1 converting enzyme (ACE)-inhibitor, enalapril, and the AT(1) receptor antagonist, losartan, normalized NOS activity in the hypothalamus but not in the brainstem. Treatment with a peripheral vasodilator, hydralazine, did not affect NOS activity.

CONCLUSION

Attenuated NOS activity in the cortex and brainstem of prehypertensive SHR may play a role in the pathogenesis of hypertension. The upregulated NOS activity in the hypothalamus and brainstem of SHR possibly serves to compensate for hypertension. Hypothalamic, but not brainstem, NO is involved in antihypertensive effects of ACE inhibition and AT(1) receptor blockade. Since a blood pressure decrease per se had no effect on NOS activity, it appears that central sympathetic activity influenced by endogenous angiotensin II, rather than blood pressure, represents the stimulus for the increased NOS activity in the hypothalamus of SHR.

cGMP/protein kinase G-dependent potentiation of glutamatergic transmission induced by nitric oxide in immature rat rostral ventrolateral medulla neurons in vitro

Although both nitric oxide (NO) and glutamate within the rostral ventrolateral medulla (RVLM) are important mediators of the central cardiovascular regulation, little is known about the functional interactions between these two mediators. Herein, we investigated the possible role of NO on the glutamatergic transmission of RVLM neurons. Whole-cell patch-clamp recordings were performed on visualized RVLM neurons in the brainstem slice preparation of rats. We found that bath application of l-arginine, the substrate for NO production, significantly increased the amplitude of excitatory postsynaptic currents (EPSCs). This enhancement was completely abolished by coadministration of the NO synthase inhibitor 7-nitroindazole and mimicked by the NO donors 3-morpholinylsydnoneimine and spermine NONOate. Bath application of a NO-sensitive guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, or a protein kinase G (PKG) inhibitor, Rp-8-bromo-guanosine 3',5'-cyclic monophosphorothioate, fully prevented the l-arginine-, 3-morpholinylsydnoneimine-, and N-[4-[1-(3-aminopropyl)-2-hydroxy-2-nitrosohydrazino]-butyl]-1,3-propanediamin (spermine NONOate)-induced synaptic potentiation. Direct activation of PKG with 8-(4-chlorophenylthio)-cGMP mimicked the action of NO donors. Furthermore, the augmentation by spermine NONOate of EPSC was accompanied by a reduction of the paired-pulse facilitation and synaptic failure rate of EPSCs. Spermine NONOate also significantly increased the frequency of both spontaneous and miniature EPSCs without altering their amplitude distribution. Pretreatment with the N-type Ca2+ channel blocker omega-conotoxin GVIA selectively blocked the spermine NONOate-induced synaptic potentiation. These results suggest that NO acts presynaptically to elicit a synaptic potentiation on the RVLM neurons through an enhancement of presynaptic N-type Ca2+ channel activity leading to facilitating glutamate release. The presynaptic action of NO is mediated by a cGMP/PKG-coupled signaling pathway.

Differential distribution of nitric oxide synthase isoforms in the rostral ventrolateral medulla of the rat

We evaluated the distribution of nitric oxide synthase (NOS) isoforms in the rostral ventrolateral medulla (RVLM), the medullary origin of sympathetic neurogenic vasomotor tone, and the contribution of NOS III to the cardiovascular actions of endogenous NO in the RVLM. Adult Sprague-Dawley rats were used. Reverse transcription-polymerase chain reaction or Western blot analysis revealed that NOS I, II or III was expressed in the ventrolateral medulla at the mRNA or protein level under basal conditions. However, laser scanning confocal microscopic analysis of double-immunofluorescence images showed that whereas NOS I or II immunoreactivity colocalized with cells within the confines of the RVLM that stained positively with the neuronal marker, NeuN, NOS III immunoreactivity was associated primarily with blood vessels. Furthermore, bilateral microinjection into the RVLM of the selective NOS III inhibitor, N(5)-(1-iminoethyl)-L-ornithine, elicited minimal alterations in baseline systemic arterial pressure, heart rate or sympathetic vasomotor outflow in rats anesthetized with propofol. We conclude that whereas NOS I and II are present in neurons within the confines of the RVLM, NOS III is associated primarily with blood vessels. Our results further indicate that NOS III does not appear to contribute to the maintenance of basal sympathetic vasomotor outflows and arterial pressure by the endogenous NO at the RVLM.

Overexpression of eNOS in RVLM improves impaired baroreflex control of heart rate in SHRSP. Rostral ventrolateral medulla. Stroke-prone spontaneously hypertensive rats

We previously demonstrated that the overexpression of endothelial nitric oxide synthase (eNOS) in the rostral ventrolateral medulla (RVLM) decreases blood pressure, heart rate (HR), and sympathetic nerve activity and that these effects are enhanced in stroke-prone spontaneously hypertensive rats (SHRSP). The aim of this study was to determine if an increase in NO production in the RVLM caused by the overexpression of eNOS improves the impaired baroreflex control of HR in SHRSP. We transfected adenovirus vectors encoding eNOS (AdeNOS) into the RVLM of SHRSP or Wistar-Kyoto rats (WKY). Mean arterial pressure and HR were measured by a radio-telemetry system in the conscious state. Reflex changes in HR were elicited by intravenous infusion of either phenylephrine, sodium nitroprusside, or hydralazine at day 7 after the gene transfer. The maximum gain of the baroreflex control of HR was significantly decreased in SHRSP compared with WKY. Overexpression of eNOS in the RVLM of SHRSP improved the impaired maximum gain of the baroreflex control of HR. After treatment with atropine, the maximum gain was still significantly greater in SHRSP in the AdeNOS-transfected group than in the nontransfected group, although it was decreased in both groups. In contrast, after treatment with metoprolol, the maximum gain did not differ between the two groups. These results indicate that an increase in NO production in the RVLM improves the impaired baroreflex control of HR in SHRSP and that these effects may have resulted from a cardiac sympathoinhibitory effect of NO in the RVLM of SHRSP.

Sympathetic neural mechanisms in normal and hypertensive pregnancy in humans

BACKGROUND

Direct recordings from peripheral sympathetic nerves have shown an increased sympathetic drive in pregnancy-induced hypertension (PIH) and preeclampsia (PE). It is unknown whether sympathetic drive is altered in normal pregnancy, when arterial blood pressure can be normal or relatively low. The aim of this study was to measure and compare peripheral sympathetic discharge, its vasoconstrictor effect and its baroreceptor control, during pregnancy and postpartum in women with normal pregnancy (NP) and PIH and in normotensive nonpregnant (NN) women.

METHODS AND RESULTS

Twenty-one women with NP, 18 women with PIH, and 21 NN women had muscle sympathetic nerve activity assessed from multiunit discharges (MSNA) and from single units with defined vasoconstrictor properties (s-MSNA). The s-MSNA in NP (38+/-6.6 impulses/100 beats) was greater (P<0.05) than in NN women (19+/-1.8 impulses/100 beats) despite similar age and body weight but less than in PIH women (P<0.001) (146+/-23.5 impulses/100 beats). MSNA followed a similar trend. Cardiac baroreceptor reflex sensitivity (BRS) was impaired in NP and PIH women relative to NN. After delivery, sympathetic activity decreased to values similar to those obtained in NN, and there was an increase in BRS. In women with NP, the decrease in sympathetic output occurred despite an insignificant change in blood pressure.

CONCLUSIONS

Central sympathetic output was increased in women with normal pregnancy and was even greater in the hypertensive pregnant group. The findings suggest that the moderate sympathetic hyperactivity during the latter months of normal pregnancy may help to return the arterial pressure to nonpregnant levels, although when the increase in activity is excessive, hypertension may ensue.

Reduced functional expression and molecular synthesis of inducible nitric oxide synthase in rostral ventrolateral medulla of spontaneously hypertensive rats

BACKGROUND

We demonstrated recently that the prevalence of neuronal (nNOS) over inducible (iNOS) nitric oxide synthase activity at the rostral ventrolateral medulla (RVLM), the medullary origin of sympathetic neurogenic vasomotor tone, and the associated dominance of sympathoexcitation over sympathoinhibition underlie the maintenance of sympathetic vasomotor outflow by the endogenous NO. Here, we evaluated the hypothesis that a significant downregulation of iNOS at the RVLM may play a crucial role in the genesis of augmented sympathetic vasomotor tone during hypertension.

METHODS AND RESULTS

Spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats anesthetized with propofol were used. Compared with SHR, the hypotension, bradycardia, or depression in sympathetic vasomotor tone induced by bilateral microinjection of lipopolysaccharide (5 or 10 ng) into the RVLM of WKY rats exhibited significantly shorter-onset latency, appreciably steeper slope, and a greater incidence of mortality. All these effects of lipopolysaccharide (10 ng) were significantly blunted by coadministration of the selective iNOS inhibitor S-methylisothiourea (250 pmol). Reverse transcription-polymerase chain reaction and Western blot analyses further revealed significantly lower iNOS mRNA and protein levels at the ventrolateral medulla in SHR under basal conditions or on activation by lipopolysaccharide (10 ng). Conversely, nNOS mRNA and protein levels remained constant in the RVLM and were comparable in both strains of rats.

CONCLUSIONS

We conclude that a significant downregulation in both functional expression and molecular synthesis of iNOS at the RVLM may underlie the augmented sympathetic vasomotor tone during hypertension.

Role of nitric oxide in central sympathetic outflow

The gaseous molecule nitric oxide (NO) plays an important role in cardiovascular homeostasis. It plays this role by its action on both the central and peripheral autonomic nervous systems. In this review, the central role of NO in the regulation of sympathetic outflow and subsequent cardiovascular control is examined. After a brief introduction concerning the location of NO synthase (NOS) containing neurons in the central nervous system (CNS), studies that demonstrate the central effect of NO by systemic administration of NO modulators will be presented. The central effects of NO as assessed by intracerebroventricular, intracisternal, or direct injection within the specific central areas is also discussed. Our studies demonstrating specific medullary and hypothalamic sites involved in sympathetic outflow are summarized. The review will be concluded with a discussion of the role of central NO mechanisms in the altered sympathetic outflow in disease states such as hypertension and heart failure.

Overexpression of eNOS in the RVLM Causes Hypotension and Bradycardia Via GABA Release

In this study, we examine the role of NO located in the rostral ventrolateral medulla (RVLM) in the control of blood pressure and the activity of the sympathetic nervous system. To determine the effect of an increase in NO production in the RVLM on blood pressure in conscious rats, adenovirus vectors encoding either endothelial NO synthase (AdeNOS) or β-galactosidase (Adβgal) were transfected into the bilateral RVLM. The local expression of endothelial NO synthase (eNOS) protein in the RVLM was confirmed by immunohistochemical staining for the eNOS protein and by Western blot analysis. Mean arterial blood pressure (MAP) and heart rate, which were monitored using a radio-telemetry system, were significantly decreased in the AdeNOS-treated group from day 5 to day 10 after the gene transfer. Urinary norepinephrine excretion was decreased on day 7 after the gene transfer in the AdeNOS-treated group. Microinjection of either N G -monomethyl- l -arginine (L-NMMA) or bicuculine, a γ-amino butyric acid (GABA) receptor antagonist, into the RVLM at day 7 after the gene transfer increased MAP to significantly greater levels in the AdeNOS-treated group. However, microinjection of kynurenic acid into the RVLM on day 7 after the gene transfer did not alter MAP levels in either group. GABA and glutamate levels in the RVLM, when measured by in vivo microdialysis, were significantly increased in the AdeNOS-treated group. These results suggest that the increase in NO production caused by the overexpression of eNOS in the bilateral RVLM decreases blood pressure, heart rate, and sympathetic nerve activity in conscious rats. Furthermore, these responses may be mediated by an increased release of GABA in the RVLM.

Role of central nervous system nitric oxide in the development of neurogenic pulmonary edema in rats

OBJECTIVE

The present study was undertaken to evaluate roles of nitric oxide in the central nervous system in the development of neurogenic pulmonary edema. Nitric oxide donor compounds have been reported to be effective for controlling some kinds of pulmonary edema.

DESIGN

Randomized trial.

SETTING

Experimental university pharmacology laboratory.

SUBJECTS

Wistar rats anesthetized with pentobarbital.

INTERVENTIONS

Neurogenic pulmonary edema was induced by injections of fibrinogen and thrombin into the cisterna magna. Physiologic roles of nitric oxide were evaluated by using NG-nitro-l-arginine methyl ester (a nitric oxide synthase inhibitor) or l-arginine (a nitric oxide donor compound). Vagus nerves were either left intact or bilaterally severed 20 mins before the injections of fibrinogen and thrombin.

MEASUREMENTS AND MAIN RESULTS

Because enhanced sympathetic nerve activity mediates neurogenic pulmonary edema, the concentration of neuropeptide Y, a neurotransmitter, in edema fluid was measured by using enzyme-linked immunosorbent assay. To evaluate the severity of pulmonary edema and pulmonary vascular permeability, lung water content and protein concentration in edema fluid were analyzed. In rats with intact vagus nerves, injection of NG-nitro-l-arginine methyl ester into the cisterna magna worsened the pulmonary edema, whereas l-arginine had no effect. In contrast, in vagotomized rats, l-arginine abrogated pulmonary edema, whereas NG-nitro-l-arginine methyl ester exerted no influence. Likewise, the ratio of edema fluid protein to serum protein and the neuropeptide Y concentration were increased by NG-nitro-l-arginine methyl ester in rats with the vagus nerves intact and were diminished by l-arginine in vagotomized rats.

CONCLUSIONS

Neurogenic pulmonary edema is characterized by elevated pulmonary vascular permeability and may be inhibited by nitric oxide production in the medulla oblongata.

Role of nitric oxide on pressor mechanisms within the dorsomedial and rostral ventrolateral medulla in anaesthetized cats

1. The role of nitric oxide (NO) in central cardiovascular regulation and the correlation between NO and glutamate-induced mechanisms is not clear. Microinjection of glutamate (3 nmol/30 nL) into dorsomedial medulla (DM) and rostral ventrolateral medulla (RVLM) increased arterial blood pressure (BP) and sympathetic vertebral nerve activity (VNA). Thus, in the present study, we examined the modulation by NO of glutamate-induced pressor responses in the DM and RVLM of cats. 2. Histochemical methods using nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) as a marker to stain neurons containing NO synthase (NOS), showed positive findings of NOS in both the DM and RVLM. 3. Microinjection of N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, into the DM or RVLM did not alter resting BP and VNA, but it did cause a dose-dependent attenuation of glutamate-induced pressor responses. Interestingly, the increase in NO levels that resulted from pretreatment with L-arginine (L-Arg) or sodium nitroprusside (SNP) did not alter resting BP and VNA, but still inhibited glutamate-induced pressor responses in the DM and RVLM in a dose-dependent manner. 4. We also examined whether NO modulated the pressor responses induced by activation of different excitatory amino acid receptors. N-Methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) were used. Consistent with the results from the initial glutamate studies, we observed that not only L-NAME, but also L-Arg and SNP attenuated pressor responses induced by NMDA and AMPA. No difference was found between the effects of NO on NMDA- and AMPA-induced pressor responses. 5. To investigate the possibility of a loss of agonist selectivity, the effects of D-2-amino-5-phosphonovalerate (D-AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on AMPA and NMDA responses in the DM were examined. The results showed that CNQX did not alter NMDA-induced pressor responses, while D-AP5 failed to alter AMPA-induced responses. 6. Our results suggest that activation of the glutamate-induced pressor mechanism is regulated by changes in NO levels in the DM and RVLM. This implies that NO may play a permissive role to allow operation of the glutamate-activation mechanism.

Neuronal nitric oxide strongly suppresses sympathetic outflow in high-salt Dahl rats

OBJECTIVE

To investigate the effects of a selective inhibitor of neuronal nitric oxide synthase (nNOS), 7-nitroindazole, on peripheral sympathetic outflow in Dahl rats.

DESIGN AND METHODS

Dahl salt-sensitive and salt-resistant rats were fed either a regular-salt (0.4% NaCl) or a high-salt (8% NaCl) diet for 4 weeks. In chronically instrumented conscious rats, renal sympathetic nerve activity (RSNA) was measured in both baroreceptor-loaded and baroreceptor-unloaded states. The baroreceptor unload was performed by decreasing arterial pressure with occlusion of the inferior vena cava.

RESULTS

7-Nitroindazole (307 micromol/kg intraperitoneally) increased resting RSNA from 24 +/- 3% to 38 +/- 6% with an increase in mean arterial pressure of 15 +/- 3 mmHg, and increased baroreceptor-unloaded RSNA from 100% to 278 +/- 16% in salt-sensitive Dahl rats receiving a high-salt diet However, 7-nitroindazole did not increase resting RSNA, but did increase baroreceptor-unloaded RSNA from 100% to 179 +/- 15%, 177 +/- 15%, and 133 +/- 4% in salt-sensitive Dahl rats receiving a regular-salt diet, salt-resistant Dahl rats receiving a high-salt diet, and salt-resistant Dahl rats receiving a regular-salt diet, respectively. The high-salt diet significantly increased the baroreceptor-unloaded RSNA more than the regular-salt diet did, in both salt-sensitive and salt-resistant rats. After administration of the vehicle for 7-nitroindazole (peanut oil), L-arginine (100 micromol/kg per min for 10 min) decreased both resting and baroreceptor-unloaded RSNA, whereas after pretreatment with 7-nitroindazole, the L-arginine-induced suppression was reversed, in Dahl salt-sensitive rats receiving a high-salt diet.

CONCLUSIONS

Neuronal nitric oxide may suppress the sympathetic discharge generated before baroreflex-mediated inhibition in all rats. This neuronal nitric oxide-mediated suppression was enhanced by the salt load in both salt-resistant and salt-sensitive Dahl rats. Finally, the neuronal nitric oxide-mediated suppression in tonic peripheral sympathetic outflow may be greatly enhanced in salt-sensitive hypertension.

Nitric oxide limits pressor responses to sympathetic activation in rat spinal cord

N-methyl D-aspartate (NMDA) receptor stimulation is known to activate nitric oxide (NO) synthase, an enzyme present in a high proportion of sympathetic preganglionic neurons. In this study, we have examined the possibility that NO modulates the pressor responses elicited by NMDA receptor stimulation in the spinal cord. In experiments on anesthetized rats, we determined whether intrathecal administration of either 3-morpholinylsydnoneimine chloride (SIN-1), an NO donor, or N:(G)-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, affected the response to stimulation of spinal NMDA receptors by NMDA (1 pmol to 1 micromol in 10-microL intrathecal administration). Intrathecal NMDA resulted in dose-dependent increases in blood pressure. SIN-1 (100 nmol) attenuated the pressor responses to NMDA (F(1,70)=12, P=0.001). Conversely, L-NAME (1 nmol to 1 micromol) augmented the pressor response to NMDA in a dose-dependent manner (F(3,161)=28.3, P<0.001). The effect of L-NAME to amplify the pressor response to NMDA was reversed by L-arginine but not by D-arginine. These results indicate that endogenous synthesis of NO in the spinal cord limits the pressor response to stimulation of spinal NMDA receptors.

Overexpression of eNOS in NTS causes hypotension and bradycardia in vivo

The role of nitric oxide (NO) in the brain in the control of blood pressure and the sympathetic nervous system is debated. This study examined the effect of overexpression of endothelial NO synthase (eNOS) in the nucleus tractus solitarii (NTS) on blood pressure in conscious rats. Adenovirus vectors encoding either eNOS (AdeNOS) or ss-galactosidase were transfected into the NTS in vivo. In the AdeNOS-treated rats, the local expression of eNOS in the NTS was confirmed by immunohistochemical staining and Western blot analysis for the eNOS protein and by increased production of nitrite/nitrate in the NTS measured by in vivo microdialysis. Blood pressure and heart rate, monitored by the use of a radiotelemetry system in a conscious state, were significantly decreased in the AdeNOS-treated group at day 5 to day 10 after the gene transfer. Urinary norepinephrine excretion also was decreased at day 7 after the gene transfer in the AdeNOS-treated group. Our results indicate that overexpression of eNOS in the NTS decreases blood pressure, heart rate, and sympathetic nerve activity in conscious rats.

Proteolysis of the exodomain of recombinant protease-activated receptors: prediction of receptor activation or inactivation by MALDI mass spectrometry

Protease-activated receptors (PARs) mediate cell activation after proteolytic cleavage of their extracellular amino terminus. Thrombin selectively cleaves PAR1, PAR3, and PAR4 to induce activation of platelets and vascular cells, while PAR2 is preferentially cleaved by trypsin. In pathological situations, other proteolytic enzymes may be generated in the circulation and could modify the responses of PARs by cleaving their extracellular domains. To assess the ability of such proteases to activate or inactivate PARs, we designed a strategy for locating cleavage sites on the exofacial NH(2)-terminal fragments of the receptors. The first extracellular segments of PAR1 (PAR1E) and PAR2 (PAR2E) expressed as recombinant proteins in Escherichia coli were incubated with a series of proteases likely to be encountered in the circulation during thrombosis or inflammation. Kinetic and dose-response studies were performed, and the cleavage products were analyzed by MALDI-TOF mass spectrometry. Thrombin cleaved PAR1E at the Arg41-Ser42 activation site at concentrations known to induce cellular activation, supporting a native conformation of the recombinant polypeptide. Plasmin, calpain and leukocyte elastase, cathepsin G, and proteinase 3 cleaved at multiple sites and would be expected to disable PAR1 by cleaving COOH-terminal to the activation site. Cleavage specificities were further confirmed using activation site defective PAR1E S42P mutant polypeptides. Surface plasmon resonance studies on immobilized PAR1E or PAR1E S42P were consistent with cleavage results obtained in solution and allowed us to determine affinities of PAR1E-thrombin binding. FACS analyses of intact platelets confirmed the cleavage of PAR1 downstream of the Arg41-Ser42 site. Mass spectrometry studies of PAR2E predicted activation of PAR2 by trypsin through cleavage at the Arg36-Ser37 site, no effect of thrombin, and inactivation of the receptor by plasmin, calpain and leukocyte elastase, cathepsin G, and proteinase 3. The inhibitory effect of elastase was confirmed on native PAR1 and PAR2 on the basis of Ca(2+) signaling studies in endothelial cells. It was concluded that none of the main proteases generated during fibrinolysis or inflammation appears to be able to signal through PAR1 or PAR2. This strategy provides results which can be extended to the native receptor to predict its activation or inactivation, and it could likewise be used to study other PARs or protease-dependent processes.

On the mechanism of plasmin-induced platelet aggregation. Implications of the dual role of granule ADP

Plasmin-induced platelet aggregation has been considered to be a cause of reocclusion after thrombolytic treatment with plasminogen activators. However, little is known regarding the mechanism and regulation of plasmin-induced platelet aggregation. In this study, we demonstrated that plasmin causes the degranulation of platelets, and that ADP released from granules plays a crucial role in the induction of platelet aggregation. This conclusion is supported by results showing that both ADP antagonists and ADPase can inhibit the effect of plasmin on platelets. We also demonstrated that pretreatment of platelets with ADP makes the platelets more sensitive to plasmin, and plasmin-induced platelet aggregation is, therefore, observed at lower concentrations where no aggregation occurs in quiescent platelets. In other words, it is thought that ADP potentiates the plasmin-induced aggregation. The effect of ADP was inhibited by N(6)-[2-(methylthio)-ethyl]-2-(3,3, 3-trifluoropropyl)thio-5'-adenylic acid, monoanhydride with dichloromethylenebisphosphonic acid (AR-C69931), a selective antagonist for the P2T(AC) subtype of P2 receptor, but not by the P2Y1 receptor-selective antagonist adenosine 3'-phosphate 5'-phosphosulfate (A3P5PS). The P2X1 receptor agonist alpha, beta-methylene adenosine 5'-triphosphate (alpha,beta-MeATP) did not mimic the action of ADP. These data indicate that ADP potentiates plasmin-induced platelet aggregation via the P2T(AC) receptor. In addition, epinephrine, a typical G(i) agonist against platelets, could potentiate the plasmin-induced platelet aggregation, suggesting that the signal via the G(i) protein is involved in potentiating the plasmin-induced platelet aggregation, ADP is secreted from platelet granules, and concomitantly works in conjunction with plasmin in a P2T(AC) receptor-mediated manner.

Centrally produced nitric oxide and the regulation of body fluid and blood pressure homeostases

1. Nitric oxide (NO) tonically inhibits the basal release of vasopressin and oxytocin into plasma. 2. Nitric oxide inhibition on vasopressin secretion is removed, while that on oxytocin is enhanced, during water deprivation, hypovolaemia, moderate osmotic stimulation and angiotensin (Ang)II. This results in a preferential release of vasopressin over oxytocin that promotes conservation of water. 3. Nitric oxide facilitates drinking behaviour stimulated by water deprivation, osmotic stimulation, haemorrhage and AngII. Together with the hormonal response, NO produces a positive water balance during reductions in intracellular and intravascular volumes. 4. Nitric oxide produced within the central nervous system maintains resting arterial blood pressure partially by attenuating the pressor actions of AngII and prostaglandins. 5. Central production of NO is enhanced during osmotic stimulation to counterbalance the salt-induced pressor response. 6. Paradoxically, central production of NO is also enhanced during haemorrhage, presumably to maintain peripheral vasodilation and blood flow to vital organs.

Role of nitric oxide in the control of the heart rate within the nucleus ambiguus of rats

The aim of this study was to determine whether NO plays a role in the control of heart rate (HR) within the nucleus ambiguus (NA). Experiments were performed in 29 male Wistar rats anaesthetized with urethane. Microinjections of the NO-donor sodium nitroprusside (SNP; 5 mmol) as well as of L-arginine (L-arg; 50 mmol) into functionally identified cardioinhibitory sites within the NA significantly decreased HR (-57.7 +/- 8.4 and -53.8 +/- 3.2 bpm, respectively), whereas the NO-synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) significantly increased HR (+40 +/- 2.7 bpm). Bilateral vagotomy and i.v. injection of atropine (0.5mg/kg) always abolished the HR decrease induced by SNP and L-arg, whereas propranolol did not affect the HR responses. These results demonstrated that NO mechanisms within the NA play a role in the parasympathetic control of the HR.

Nitric oxide-dependent guanylyl cyclase participates in the glutamatergic neurotransmission within the rostral ventrolateral medulla of awake rats

A well-known action of nitric oxide (NO) is to stimulate the soluble form of guanylyl cyclase, evoking an accumulation of cyclic GMP in target cells. The aim of the present study was to examine the effects of inhibition of guanylyl cyclase dependent on NO during cardiovascular responses induced by L-glutamate and S-nitrosoglutathione (SNOG) microinjected into the rostral ventrolateral medulla (RVLM) of awake rats. Three days before the experiments, adult male Wistar rats (280 to 320 g) were anesthetized for implantation of guide cannulas to the desired stereotaxic position (AP=-2.5 mm, L=1.8 mm) in relation to lambda. The cannulas were fixed to the skull with acrylic cement. Twenty-four hours before the experiments, a femoral artery and vein were cannulated for recording arterial pressure (AP) and heart rate (HR) and injection of anesthetic. Unilateral microinjections (100 nL) of L-glutamate (5 nmol/L) and SNOG (2.5 nmol/L) were made into the histologically confirmed RVLM. The cardiovascular responses to these drugs were evaluated before and after microinjection (3 nmol/L, 200 nL) of either methylene blue or oxodiazoloquinoxaline (ODQ). The hypertensive effect of L-glutamate was attenuated by 74% after methylene blue (DeltaAP=49+/-8 to 13+/-4 mm Hg) and by 80.5% after ODQ (DeltaAP=30+/-2 to 6+/-2 mm Hg). The increase in AP produced by SNOG was fully blocked by ODQ (DeltaAP=39+/-8 to 1+/-2 mm Hg). These data indicate that cyclic GMP mechanisms have a key role in glutamatergic neurotransmission in the RVLM of awake rats, and it is most probable that NO participates in this response.

Sympathoinhibitory effects of clonidine are transmitted by the caudal ventrolateral medulla in cats

We examined mechanisms of the central sympathoinhibitory actions of systemically administered clonidine in anesthetized cats. To avoid influences of sympathetic chemo- and baroreflexes, the animals were deafferentated by cutting the carotid sinus and vagal nerves bilaterally. Intravenous (i.v.) injections of clonidine (25-250 nmol/kg) caused significant (50-90%) decreases in preganglionic sympathetic nerve activity (SNA) recorded from the white ramus of the third thoracic segment. Microinjections (500 nl) into the rostral ventrolateral medulla (RVLM) of clonidine at doses (50-500 pmol in 500 nl), which probably produced higher local concentrations than produced by systemic administration, caused only slight reductions of SNA and small decreases in arterial blood pressure (BP). Furthermore, sympathoinhibition and hypotension caused by intravenous clonidine was almost unaffected by prior microinjection of alpha2-receptor antagonist rauwolscine (500 pmol) into the RVLM. Microinjections of clonidine into the caudal ventrolateral medulla (CVLM), which provides important inhibitory input to the RVLM, had no significant effects. However, chemical lesions of the CVLM with kainate (5.0 nmol), effectively blocked the sympathoinhibitory effects of subsequently administered intravenous clonidine. The results suggest that the central sympathoinhibitory effects of therapeutically relevant doses of systemically administered clonidine may be primarily mediated by pathways that activate the CVLM rather than by direct actions within the RVLM.

Neural mechanisms in nitric-oxide-deficient hypertension

Nitric oxide is hypothesized to be an inhibitory modulator of central sympathetic nervous outflow, and deficient neuronal nitric oxide production to cause sympathetic overactivity, which then contributes to nitric-oxide-deficient hypertension. The biochemical and neuroanatomical basis for this concept revolves around nitric oxide modulation of glutamatergic neurotransmission within brainstem vasomotor centers. The functional consequence of neuronal nitric oxide in blood pressure regulation is, however, marked by an apparent conflict in the literature. On one hand, conscious animal studies using sympathetic blockade suggest a significant role for neuronal nitric oxide deficiency in the development of nitric-oxide-deficient hypertension, and on the other hand, there is evidence against such a role derived from 'knock-out' mice lacking nitric-oxide synthase 1, the major source of neuronal nitric oxide.

Role of nNOS in blood pressure regulation in eNOS null mutant mice

The role of neural nitric oxide synthase (nNOS) in regulating blood pressure (BP) remains uncertain. Recently it was reported that in mice lacking functional endothelial NOS (eNOS) genes (-/-), acute administration of a nonselective NOS inhibitor, Nw-nitro-L-arginine, decreased mean BP, suggesting that NO released by non-eNOS isoforms increases BP. Because the inducible NOS isoform is not constitutively expressed and when induced causes hypotension, we hypothesize that it is NO produced by nNOS that increases BP in the absence of eNOS activity. To test this hypothesis, we studied the acute effect of selective and nonselective nNOS inhibitors on BP and cerebellar NOS activity in eNOS (-/-), wild-type (+/+), and heterozygous (+/-) mice as well as in +/+ mice with renovascular hypertension. Because it is not known whether the decrease in BP caused by acute NOS inhibition in -/- mice can occur chronically, we also studied the effect of chronic NOS inhibition on both BP and cerebellar NOS activity. eNOS (-/-) mice had higher BP than +/+ or +/-mice, and acute administration of the selective nNOS inhibitor 7-nitroindazole (7-NI) decreased their mean BP from 137+/-13 to 124+/-12 mm Hg (P<0.01). In +/+, +/-, or renovascular hypertensive +/+ mice, 7-NI caused a small but insignificant rise from 105+/-5 to 110+/-6 mm Hg, from 115+/-9 to 119+/-13 mm Hg, and from 146+/-6 to 150+/-6 mm Hg, respectively. Fifteen minutes after administration of 7-NI, cerebellar NOS activity decreased by 70%; however, this inhibitory effect was brief, since 2 hours after 7-NI administration NOS returned toward control values. Chronic oral or intraperitoneal administration of 7-NI did not inhibit cerebellar NOS activity, whereas the nonselective NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME) decreased this activity by 50%. Therefore, we studied the effect of chronic L-NAME administration (4 weeks) on BP. In -/- mice, chronic L-NAME administration decreased BP from 135+/-4 to 120+/-3 mm Hg (P<0.05), whereas in +/+ and +/-mice, as expected, it increased BP from 109+/-2 to 125+/-3 mm Hg (P<0.001) and from 107+/-6 to 119+/-5 mm Hg (P<0.02), respectively. After L-NAME administration was stopped, BP returned to baseline. These results suggest that in eNOS -/- mice, NO derived from nNOS increases BP both acutely and chronically.

Enhanced depressor response to nitric oxide in the rostral ventrolateral medulla of spontaneously hypertensive rats

Possible impairment of the L-arginine-nitric oxide (NO) pathway in the rostral ventrolateral medulla of adult spontaneously hypertensive rats (SHR) was investigated by microinjecting N(G)-nitro-L-arginine methyl ester (L-NAME), NOC 18 (an NO donor), or L-arginine. Unilateral injection of L-NAME (10 nmol/50 nL) into the rostral ventrolateral medulla significantly increased mean arterial pressure (MAP) in both SHR and Wistar-Kyoto rats (WKY). The increases in MAP did not differ significantly between the two strains (15+/-3 versus 10+/-2 mm Hg, respectively; n=8). In contrast, microinjection of L-arginine elicited significant (P<.05) dose-dependent decreases in MAP in both strains, and these depressor responses were significantly greater in SHR than in WKY (in 10 nmol of L-arginine: -29+/-2 versus -15+/-2 mm Hg, respectively; n=8, P<.01). Similarly, microinjection of NOC 18 (10 nmol/50 nL) reduced MAP in both strains, and the depressor response was also significantly greater in SHR than in WKY (-38+/-7 versus -22+/-3 mm Hg, respectively; n=8, P<.05). These results suggest that the L-arginine-NO pathway in the rostral ventrolateral medulla is impaired in SHR and that this impairment may contribute to the increase in arterial pressure in this animal model of genetic hypertension.

Angiotensin II-nitric oxide interaction on sympathetic outflow in conscious rabbits

Increasing evidence suggests that endogenous NO inhibits sympathetic outflow in anesthetized animals. However, in a recent study from this laboratory, we were unable to find any evidence of increased renal sympathetic nerve activity (RSNA) in response to blockade of NO synthesis in conscious rabbits. Because angiotensin II (Ang II) increases sympathetic outflow, one factor for this discrepancy may be the difference in the resting level of Ang II, which may be lower in well-trained conscious animals. In the present study, the effects of blockade of NO synthesis with Nomega-nitro-L-arginine methyl ester (L-NAME, 30 mg/kg IV) on resting RSNA with and without a background intravenous infusion of Ang II (10 ng.kg(-1).min(-1)) was investigated in conscious rabbits. Intravenous administration of L-NAME (30 mg/kg) caused an increase in mean arterial blood pressure (MAP, from 80.4+/-2.9 to 92.8+/-2.5; P=.0001) and a decrease in RSNA (from 100+/-0% to 53.4+/-8.6%, P=.0016). When the elevated blood pressure was returned to control by infusion of hydralazine (0.01 to 0.06 mg.kg(-1).min(-1)), RSNA returned to the level before L-NAME administration. During a sustained infusion of Ang II (10 ng.kg(-1).min(-1)), L-NAME increased MAP from 89.2+/-2.9 to 109.0+/-4.3 mm Hg (P=.0101) and decreased RSNA from 100.0+/-0% to 53.7+/-7.5% (P=.0013). Under this circumstance, however, when the MAP was returned to the level that existed before the administration of L-NAME, RSNA increased significantly above the level that existed before the administration of L-NAME (164.5+/-17.7% versus 100+/-0%, P=.0151). The enhancement of the sympathetic response by Ang II was completely blocked by the AT1 receptor antagonist, losartan. In contrast, during a background infusion of phenylephrine, which increased MAP to the same level as produced by Ang II, L-NAME had no effect on RSNA when MAP was returned to the control level. Nomega-Nitro-D-arginine methyl ester had no effect on MAP and RSNA. Intravenous infusion of Ang II alone for 75 minutes had no effect on RSNA when MAP was returned to control levels. These data suggest that an elevated level of Ang II is critical for the inhibitory effect of NO on sympathetic outflow in conscious rabbits and imply that these two substances have a major impact on the regulation of sympathetic outflow.

Activation by hypotension of neurons in the hypothalamic paraventricular nucleus that project to the brainstem

To investigate the involvement of neuronal nitric oxide (NO) in the response of the brain to changes in blood pressure, we studied the activation of putative NO-producing neurons in the paraventricular nucleus of the hypothalamus (PVN) in rats whose mean arterial pressures (MAPs) were decreased by 40-50% with hemorrhage (HEM) or infusion of sodium nitroprusside (NP). Activation was assessed on the basis of expression of the immediate early gene, c-fos; putative NO-producing neurons were identified with the histochemical stain for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d); and the proportions of neurons projecting to the nucleus of the tractus solitarius (NTS) and/or caudal ventrolateral medulla (CVLM) were determined with retrograde tracing techniques. No differences were found for results obtained from HEM and NP animals. Three to four percent of activated PVN neurons projected to the NTS or CVLM. Conversely, approximately 33% and 16% of neurons projecting to the NTS and CVLM, respectively, were activated. About 43% of NADPH-d neurons in the PVN were activated. Of PVN neurons projecting to the NTS or CVLM, 38% and 32%, respectively, were NADPH-d positive. About 11% of NADPH-d PVN neurons projected to the NTS or CVLM. An average of 3 NADPH-d neurons per section were activated and projected to either target. Finally, 7 PVN cells per section sent collateral branches to the NTS and CVLM; 2 or 3 of these cells per section were also activated by decreases in arterial pressure. No NADPH-d cells were found that sent collateral branches to the NTS and CVLM. This study shows that decreases in MAP activate PVN neurons that project, singly and through collaterals, to the NTS and CVLM. A relatively high proportion of the singly projecting neurons is NADPH-d positive. These results support the contention that descending projections from the PVN to the brainstem play an important role in the physiological response to decreases in arterial pressure and suggest that NO may participate in this response.

The sympathetic nervous system is involved in the maintenance but not initiation of the hypertension induced by N(omega)-nitro-L-arginine methyl ester

Studies in anesthetized animals have advanced the theory that there is an important neurogenic component to the hypertension caused by pharmacological inhibition of nitric oxide, but studies in conscious animals have produced conflicting evidence for and against this theory. To try to reconcile the seemingly contradictory data, we hypothesized that the neurogenic component of this hypertension is time dependent such that the sympathetic nervous system is involved primarily in the maintenance, rather than the initiation, of the hypertension. We measured intra-arterial pressure in conscious, unrestrained rats with and without guanethidine-induced sympathectomy during varying durations of intravenous N(omega)-nitro-L-arginine methyl ester (L-NAME). The major new finding is that sympathectomy had no effect on the hypertensive response to bolus injections of L-NAME but in the same rats it produced a greater than 50% attenuation in the hypertension seen after 6 days of continuous L-NAME (change in mean arterial pressure, 23+/-4 versus 55+/-4 mm Hg, P<.01, sympathectomy versus control). Using 8-hour infusions of L-NAME, we found that 60 minutes was the minimum time required for detecting a sympathectomy-sensitive component of L-NAME-induced hypertension. Furthermore, we demonstrate that the magnitude of this component increases further between 8 hours to 6 days of continuous L-NAME: it accounted for only 18% of the total hypertensive response at 8 hours but 61% after 6 days. From these experiments, we conclude that the importance of the sympathetic system in the pathogenesis of L-NAME-induced hypertension accrues slowly over hours and days, and thus its importance can be overlooked by focusing on the initial phase of the hypertension.

Neuronal nitric oxide reduces sympathetic excitability by modulation of central glutamate effects in pigs

Mechanisms of the modulation of sympathetic activity by neuronal NO were studied in vagotomized anesthetized pigs. Inhibition of neuronal NO synthase (nNOS) within the brain stem by intracerebroventricular (ICV) administration of 7-nitroindazole (7-NI, 1 mmol/L) or S-methyl-L-thiocitrulline (MeTC, 0.1 mmol/L) caused slight increases in renal sympathetic nerve activity (RSNA) but did not affect arterial blood pressure (BP) or cardiac output (CO). However, the sympathoexcitatory effects of glutamate (0.5 mL, 0.1 mol/L ICV) that were associated with marked increases in BP, CO, and heart rate were potentiated by both nNOS inhibitors. Furthermore, 7-NI and MeTC significantly enhanced the responses of RSNA, BP, and CO to activation of somatosympathetic reflexes via stimulation of the left greater sciatic nerve (nervus ischiadicus, 10 to 20 V, 30 Hz, 1-millisecond pulses). Subsequent systemic inhibition of either the neuronal (by 7-NI) or all isoforms of NOS by NG-nitro-L-arginine-methyl ester (20 mg/kg) had no significant additional effects on these responses. The effects of NOS inhibition were effectively counteracted by the endogenous NOS substrate L-arginine and by S-nitroso-N-acetyl-penicillamine (SNAP), a stable analogue of endogenous S-nitroso factors. Disruption of sympathoinhibitory baroreflex mechanisms by bilateral cutting of the carotid sinus nerves caused increases in RSNA and slightly increased responses to all excitatory stimuli but had no effects on the actions of the NOS inhibitors or SNAP. These results suggest that modulation of glutamate effects by nNOS-derived NO may be an important mechanism by which NO affects sympathetic activity in pigs.