Effect of inhibition of nitric oxide synthesis on the cardiovascular and endocrine responses to hemorrhage in conscious rabbits

Roles of nitric oxide and angiotensin II in the impaired baroreflex gain of pregnancy

The present study tested the hypothesis that nitric oxide (NO) contributes to impaired baroreflex gain of pregnancy and that this action is enhanced by angiotensin II. To test these hypotheses, we quantified baroreflex control of heart rate in nonpregnant and pregnant conscious rabbits before and after: 1) blockade of NO synthase (NOS) with Nomega-nitro-L-arginine (20 mg/kg iv); 2) blockade of the angiotensin II AT1 receptor with L-158,809 (5 microg x kg(-1) x min(-1) iv); 3) infusion of angiotensin II (1 ng x kg(-1) x min(-1) nonpregnant, 1.6-4 ng x kg(-1) x min(-1) pregnant iv); 4) combined blockade of angiotensin II AT(1) receptors and NOS; and 5) combined infusion of angiotensin II and blockade of NOS. To determine the potential role of brain neuronal NOS (nNOS), mRNA and protein levels were measured in the paraventricular nucleus, nucleus of the solitary tract, caudal ventrolateral medulla, and rostral ventrolateral medulla in pregnant and nonpregnant rabbits. The decrease in baroreflex gain observed in pregnant rabbits (from 23.3 +/- 3.6 to 7.1 +/- 0.9 beats x min(-1) x mmHg(-1), P < 0.05) was not reversed by NOS blockade (to 8.3 +/- 2.5 beats x min(-1) x mmHg(-1)), angiotensin II blockade (to 5.0 +/- 1.1 beats x min(-1) x mmHg(-1)), or combined blockade (to 12.3 +/- 4.8 beats x min(-1) x mmHg(-1)). Angiotensin II infusion with (to 5.7 +/- 1.0 beats x min(-1) x mmHg(-1)) or without (to 8.4 +/- 2.4 beats x min(-1) x mmHg(-1)) NOS blockade also failed to improve baroreflex gain in pregnant or nonpregnant rabbits. In addition, nNOS mRNA and protein levels in cardiovascular brain regions were not different between nonpregnant and pregnant rabbits. Therefore, we conclude that NO, either alone or via an interaction with angiotensin II, is not responsible for decrease in baroreflex gain during pregnancy.

Effects of nitric oxide on expressions of nitrosocysteine and calcium-activated potassium channels in the supraoptic nuclei and neural lobe of dehydrated rats

Nitric oxide (NO) is an important gas mediator in the signal transduction cascade regulating osmotic function in the hypothalamo-neurohypophysial system. We previously found that increased nitric oxide synthase (NOS) activity in the supraoptic nuclei (SON) and neural lobe following osmotic stimulation and NO could regulate the expression of Ca(2+)-activated K(+) channel (BK channels) protein in the magnocellular system during dehydration. The aim of the current study is to examine the role of NO in the regulation of nitrosocysteine and BK channel protein in the magnocellular system in dehydrated animals. Using Western blot analysis and quantitative immunofluorescent staining study, we found that water deprivation in rats significantly enhanced the expression of nitrosocysteine protein in SON and neural lobes. Immunohistochemistry study indicated that dehydration significantly increased the profiles of SON neurons co-expressing nitrosocysteine with BK-channel protein. Intracerebroventricular administration of L-NAME (an inhibitor of NO synthase) significantly reduced the neuronal profiles of nitrosocysteine, as well as their co-expression with BK-channel in SON of dehydrated rats. However, treatment of sodium nitroprusside (a donor of NO) increased this co-expression. Our results indicate that NO signaling cascade may control the expression of BK channels through the regulation of nitrosocysteine in SON and neural lobe of rats during osmotic regulation.

Nitric oxide up-regulates the expression of calcium-dependent potassium channels in the supraoptic nuclei and neural lobe of rats following dehydration

Nitric oxide (NO) is a gas molecule to signal neurotransmission in the hypothalamo-neurohypophysial system during osmotic regulation. We previously reported that osmotic stimulation increased nitric oxide synthase (NOS) activity in the supraoptic nuclei (SON) and neural lobe. The aim of this study is to define the role of NO in the regulation of Ca(2+)-activated K(+) channels (BK channels) expression in the magnocellular system following dehydration. We used Western blot analysis and quantitative immunocytochemistry to conduct the experiment in rats. In the immunoblot study, we found that water deprivation significantly increased the expression of BK channels in the SON and neural lobes. Dehydration also enhanced the profiles of neurons expressing vasopressin and oxytocin significantly. In about 70% of these neurons, BK channels were co-localized in the same neuron, and their expression increased significantly during dehydration. We further examined the effects of intracerebroventricular administration of sodium nitroprusside (a donor of NO) and L-NAME (an inhibitor of NO synthase) on expression of BK channels in the SON. We found that compared to animals treated with the donor of NO, there were significant decreases in the expression of BK proteins in animals receiving L-NAME. These results suggest that NO may enhance the expression of BK channels in the supraoptic nuclei and neural lobe of rats following dehydration.

Selective inducible nitric oxide synthase (iNOS) inhibition attenuates remote acute lung injury in a model of ruptured abdominal aortic aneurysm

OBJECTIVE

Abdominal aortic aneurysm rupture is associated with a systemic inflammatory response syndrome and acute lung injury. Using a selective inducible nitric oxide synthase (iNOS) inhibitor, N(6)-(iminoethyl)-lysine (L-NIL), we explored the role of iNOS in the early pro-inflammatory signaling and acute lung injury in experimental abdominal aortic aneurysm rupture.

MATERIALS AND METHODS

Anesthetized rats were randomized to sham control or shock and clamp (s + c) groups, which underwent one hour of hemorrhagic shock, followed by 45 minutes of supramesenteric aortic clamping, and then two hours resuscitated reperfusion. Animals in s + c were randomized to receive intravenous L-NIL at 50 microg/kg/h or saline at the start of reperfusion. Pulmonary permeability to (125)I-labeled albumin, myeloperoxidase (MPO) activity, cytokine levels, and semi-quantitative RT-PCR for mRNA were indicators of microvascular permeability, leuco-sequestration, and pro-inflammatory signaling, respectively.

RESULTS

Lung permeability index were significantly increased in s + c compared to sham (4.43 +/- 0.96 versus 1.30 +/- 0.17, P < 0.01), and attenuated by L-NIL treatment (2.14 +/- 0.70, P < 0.05). Lung tissue MPO activity was significantly increased in s + c compared to sham (2.80 +/- 0.32 versus 1.03 +/- 0.29, P < 0.002), and attenuated by L-NIL treatment (1.50 +/- 0.20, P < 0.007). Lung tissue iNOS activity was significantly increased in s + c compared to sham animals (P < 0.05), and attenuated by L-NIL treatment (P < 0.05). Lung tissue iNOS mRNA was upregulated 8-fold in s + c compared to sham (P < 0.05). Data represents mean +/- standard error mean, comparisons with ANOVA.

CONCLUSIONS

These data suggest that in our model of ruptured abdominal aortic aneurysm iNOS plays a crucial role in reperfusion lung injury. Selective inhibition of iNOS during early reperfusion prevents neutrophil mediated acute lung injury.

Nitric oxide modulation of the hypothalamo-neurohypophyseal system

Nitric oxide (NO), a free radical gas produced endogenously from the amino acid L-arginine by NO synthase (NOS), has important functions in modulating vasopressin and oxytocin secretion from the hypothalamo-neurohypophyseal system. NO production is stimulated during increased functional activity of magnocellular neurons, in parallel with plastic changes of the supraoptic nucleus (SON) and paraventricular nucleus. Electrophysiological data recorded from the SON of hypothalamic slices indicate that NO inhibits firing of phasic and non-phasic neurons, while L-NAME, an NOS inhibitor, increases their activity. Results from measurement of neurohypophyseal hormones are more variable. Overall, however, it appears that NO, tonically produced in the forebrain, inhibits vasopressin and oxytocin secretion during normovolemic, isosmotic conditions. During osmotic stimulation, dehydration, hypovolemia and hemorrhage, as well as high plasma levels of angiotensin II, NO inhibition of vasopressin neurons is removed, while that of oxytocin neurons is enhanced. This produces a preferential release of vasopressin over oxytocin important for correction of fluid imbalance. During late pregnancy and throughout lactation, fluid homeostasis is altered and expression of NOS in the SON is down- and up-regulated, respectively, in parallel with plastic changes of the magnocellular system. NO inhibition of magnocellular neurons involves GABA and prostaglandin synthesis and the signal-transduction mechanism is independent of the cGMP-pathway. Plasma hormone levels are unaffected by i.c.v. 1H-[1, 2, 4]oxadiazolo-[4,3-a]quinoxalin-1-one (a soluble guanylyl cyclase inhibitor) or 8-Br-cGMP administered to conscious rats. Moreover, cGMP does not increase in homogenates of the neural lobe and in microdialysates of the SON when NO synthesis is enhanced during osmotic stimulation. Among alternative signal-transduction pathways, nitrosylation of target proteins affecting activity of ion channels is considered.

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.

Effect of N(G)-nitro-L-arginine methyl ester on autonomic modulation of heart rate variability during hypovolemic shock

OBJECTIVE

To study the changes in neuroautonomic regulation of heart rate and the effects of N(G)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of nitric oxide synthase, on efferent sympathetic cardiac activity and blood pressure during hypovolemic shock. Hypotension during hypovolemic shock may be attributable, in part, to the failure of neuroautonomic regulation of heart rate and blood pressure. In addition, the release of nitric oxide may contribute to hypotension through vasodilation and inhibition of efferent sympathetic activity.

DESIGN

Prospective, controlled trial.

SETTING

Experimental laboratory in a university hospital.

SUBJECTS

Seventeen anesthetized adult male New Zealand White rabbits.

INTERVENTIONS

The rabbits were divided into four groups: control (n = 3), control plus L-NAME (n = 5), hypovolemic (n = 4), and hypovolemic plus L-NAME (n = 5). Hypovolemic rabbits were bled of 10% of their circulating blood volume (85 mL/kg) every 10 mins until 30% cumulative hypovolemia was reached. Rabbits received either three doses of saline 1 mL/kg every 10 mins or L-NAME 10 mg/kg in 1 mL/kg of saline solution administered after each hemorrhage for a total of three doses. Changes in heart rate, respiratory rate, mean arterial pressure, plasma catecholamine levels, and heart rate power spectra were recorded every 10 mins during serial hypovolemia and during a 30-min recovery period.

MEASUREMENTS AND MAIN RESULTS

During hypovolemic shock there was a decrease in log low-frequency heart rate power (p = .001) and in systolic (p = .003), diastolic (p < .001), and mean (p < .001) blood pressures compared with control rabbits. Treatment with L-NAME during hypovolemia resulted in increased log low-frequency heart rate power (p = .03) and systolic (p = .01), diastolic (p = .007), and mean (p = .009) blood pressures compared with hypovolemic rabbits who received saline placebo.

CONCLUSIONS

We found that treatment with L-NAME increased efferent sympathetic cardiac activity and mean arterial pressure during hypovolemic shock compared with control rabbits. We conclude that L-NAME may blunt hypotension during hypovolemic shock by inhibiting nitric oxide synthase and may act to restore neuroautonomic cardiovascular reactivity. Spectral analysis of heart rate variability may allow for insights into the pathophysiology of shock and provide a means of monitoring the neuroautonomic cardiovascular response to therapy.

Effect of blockade of nitric oxide synthesis on renin secretion in human subjects

Nitric oxide (NO) has been implicated in the control of renin secretion in experimental animals but little information is available concerning its role in humans. The aim of the present study was to investigate the effects of inhibition of NO synthesis on resting renin secretion and on the renin secretory responses to activation of the macula densa and sympathetic neural mechanisms controlling renin secretion. In eight healthy subjects, injection of furosemide increased plasma renin activity (PRA) with little or no change in blood pressure or heart rate. Injection of the NO synthase inhibitor L-NMMA increased blood pressure and decreased heart rate and PRA, but failed to alter the PRA response to furosemide. In another ten subjects, standing increased PRA. L-NMMA again decreased PRA but failed to alter the PRA response to standing. These results suggest that NO participates in the regulation of resting renin secretion in humans, and provide preliminary evidence that NO does not contribute significantly to the renin responses to activation of the macula densa or sympathetic mechanisms controlling renin secretion.

Role of nitric oxide in regulation of renal sympathetic nerve activity during hemorrhage in conscious rats

The effect of inhibition of nitric oxide (NO) synthesis on the responses of blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity (RSNA) during hemorrhaging was examined with the use of an NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), in conscious rats. In the 0.9% saline group, hemorrhage (10 ml/kg body wt) did not alter BP but significantly increased HR and RSNA by 88 +/- 12 beats/min and 67 +/- 12%, respectively. Intravenous infusion of L-NAME (50 microg. kg(-1). min(-1)) significantly attenuated these tachycardic and sympathoexcitatory responses to hemorrhage (14 +/- 7 beats/min and 26 +/- 12%, respectively). Pretreatment of L-arginine (87 mg/kg) recovered the attenuation of HR and RSNA responses induced by L-NAME (92 +/- 6 beats/min and 64 +/- 10%, respectively). L-NAME by itself did not alter the baroreceptor reflex control of HR and RSNA. Hemorrhage increased the plasma vasopressin concentration, and its increment in the L-NAME-treated group was significantly higher than that in the 0.9% saline group. Pretreatment with the vascular arginine vasopressin V(1)-receptor antagonist OPC-21268 (5 mg/kg) recovered the attenuation of RSNA response induced by L-NAME (54 +/- 7%). These results indicate that NO modulated HR and RSNA responses to hemorrhage but did not directly affect the baroreceptor reflex arch. It can be assumed that NO modulated the baroreflex function by altering the secretion of vasopressin induced by hemorrhage.

Role of nitric oxide in the control of renin secretion

Because of the significant constitutive expression of NO synthases in the juxtaglomerular apparatus, nitric oxide (NO) is considered as a likely modulator of renin secretion. In most instances, NO appears as a tonic enhancer of renin secretion, acting via inhibition of cAMP degradation through the action of cGMP. Depending on as yet unknown factors, the stimulatory effect of NO on renin secretion may also switch to an inhibitory one that is compatible with the inhibition of renin secretion by cGMP-dependent protein kinase activity. Whether NO plays a direct regulatory role or a more permissive role in the control of renin secretion remains to be answered.

Effects of L-NAME on cerebral metabolic, vasopressin, oxytocin, and blood pressure responses in hemorrhaged rats

NG-nitro-L-arginine methyl ester (L-NAME; 250 micrograms/5 microliters), an inhibitor of NO synthase, or the vehicle artificial cerebrospinal fluid (aCSF; 5 microliters) was administered intracerebroventricularly to conscious rats hemorrhaged (0.7 ml/min) to a 20% volume depletion. Hypotension was maximal 5 min after hemorrhage ended, with compensatory recovery to basal levels 20 min later, regardless of drug treatment. L-NAME, however, elevated (P < 0.05) blood pressure (vs. aCSF controls) 40-45 min after intracerebroventricular administration. In normovolemic rats, L-NAME produced a significant pressor response and increased plasma levels of vasopressin (VP) and oxytocin (OT). After hemorrhage, both hormone levels increased, but only OT was further enhanced by L-NAME. Thus centrally produced NO tonically inhibits OT and VP secretion under basal normovolemic conditions and selectively inhibits OT release during hypovolemia. Hemorrhage increased the rates of glucose utilization in the neural lobe, indicative of enhanced efferent neural functional activity. L-NAME further enhanced the metabolic activity in the entire hypothalamoneurohypophysial system of hemorrhaged animals. Several other brain structures involved in the regulation of blood pressure and the stress response were also metabolically affected by the hemorrhage and L-NAME.

Nitric oxide control of drinking, vasopressin and oxytocin release and blood pressure in dehydrated rats

Intracerebroventricular (i.c.v.) injection of the inhibitor of NO synthase (NOS), N(G)-nitro-L-arginine methyl ester (L-NAME) (250 microg/5 microL) attenuated the drinking response in rats deprived of water for 24 h. Moreover, oxytocin (OT) levels in plasma increased after 2 min, whereas both oxytocin and vasopressin levels were elevated at 120 min after intracerebroventricular injection. The delayed effect of L-NAME on both hormones was not observed in dehydrated animals allowed to drink water. Blood pressure remained stable after injection of artificial cerebrospinal fluid (aCSF) in dehydrated rats not allowed to drink. In rats having access to water, however, there was an immediate but transient pressor response (0-5 min) with a delayed hypotension from 45 to 120 min. L-NAME consistently increased blood pressure in a biphasic mode, whether the animals drank or not, with an early peak at 5 min that decayed after 15-30 min and a second pressor response beginning at 30-45 min and remaining elevated at 120 min when the experiment ended. These pressor responses were independent of the adrenal glands. Thus, centrally produced nitric oxide facilitates drinking, inhibits release of vasopressin and oxytocin from the magnocellular system, and maintains resting arterial blood pressure in normally hydrated and dehydrated rats.

Nitric oxide and shock

Shock can be defined as the failure of the circulatory system to provide necessary cellular nutrients, including oxygen, and to remove metabolic wastes. Although it is now recognized that more than 100 different forms of shock exist, this recognition is more a reflection of the widespread use of the term to describe a variety of disease states. For the purpose of this monograph, we concentrate on various forms of cardiovascular shock, in particular, shock that may be linked to inappropriate vasodilation from overproduction of the endogenous vasodilator, nitric oxide. Some forms of shock have been extensively studied, and convincing evidence exists for the role of nitric oxide. Other disease states have been less well characterized in terms of their association with excess nitric oxide production. Available evidence of a role for nitric oxide is discussed in the hope of stimulating the interest of investigators to explore these areas more thoroughly.

Nitric oxide and the control of renin secretion

Research during recent years has established nitric oxide as a unique signaling molecule that plays important roles in the regulation of the cardiovascular, nervous, renal, immune and other systems. Nitric oxide has also been implicated in the control of the secretion of hormones by the pancreas, hypothalamus, pituitary and other endocrine glands, and evidence is accumulating that it contributes to the regulation of the secretion of renin by the kidneys. The enzyme nitric oxide synthetase is present in vascular and tubular elements of the kidney, particularly in cells of the macula densa, a structure that plays an important role in the control of renin secretion. Guanylyl cyclase, a major target for nitric oxide, is also present in the kidney and is responsive to changes in nitric oxide levels. Drugs that inhibit nitric oxide synthesis generally suppress renin release in vivo and in vitro, suggesting a stimulatory role for the L-arginine-nitric oxide pathway in the control of renin secretion. Under some conditions, however, blockade of nitric oxide synthesis increases renin secretion. Recent studies indicate that nitric oxide not only contributes to the regulation of basal renin secretion, but also participates in the renin secretory responses to activation of the renal baroreceptor, macula densa and beta adrenoceptor mechanisms that regulate renin secretion. Future research should clarify the mechanisms by which nitric oxide regulates the secretion of renin and establish the physiological significance of this regulation.