Assessing the negative impact of chlorantraniliprole, isoxaflutole, and simazine pesticides on phospholipid membrane models and tilapia gill tissues.

The indiscriminate and, very often, incorrect use of pesticides in Brazil, as well as in other countries, results in severe levels of environmental pollution and intoxication of human life. Herein, we studied plasma membrane models (monolayer and bilayer) of the phospholipid Dioleoyl-sn-glycerol-3-phosphocholine (DOPC) using Langmuir films, and large (LUVs) and giant (GUVs) unilamellar vesicles, to determine the effect of the pesticides chlorantraniliprole (CLTP), isoxaflutole (ISF), and simazine (SMZ), used in sugarcane. CLTP affects the lipid organization of the bioinspired models of DOPC π-A isotherms, while ISF and SMZ pesticides significantly affect the LUVs and GUVs. Furthermore, the in vivo study of the gill tissue in fish in the presence of pesticides (2.0 × 10-10 mol/L for CLTP, 8.3 × 10-9 mol/L for ISF, and SMZ at 9.9 × 10-9 mol/L) was performed using optical and fluorescence images. This investigation was motivated by the gill lipid membranes, which are vital for regulating transporter activity through transmembrane proteins, crucial for maintaining ionic balance in fish gills. In this way, the presence of phospholipids in gills offers a model for understanding their effects on fish health. Histological results show that exposure to CLTP, ISF, and SMZ may interfere with vital gill functions, leading to respiratory disorders and osmoregulation dysfunction. The results indicate that exposure to pesticides caused severe morphological alterations in fish, which could be correlated with their impact on the bioinspired membrane models. Moreover, the effect does not depend on the exposure period (24h and 96h), showing that animals exposed to pesticides for a short period suffer irreparable damage to gill tissue. In summary, we can conclude that the harm caused by pesticides, both in membrane models and in fish gills, occurs due to contamination of the aquatic system with pesticides. Therefore, water quality is vital for the preservation of ecosystems.

Biological responses to imazapic and methyl parathion pesticides in bioinspired lipid membranes and Tilapia fish.

Pesticide misuse has well-documented detrimental effects on ecosystems, with Nile tilapia (Oreochromis niloticus) being particularly vulnerable. The current study focuses on the impact of widely used sugarcane crop pesticides, Imazapic (IMZ) and Methyl Parathion (MP), on tilapia gill tissues and their lipid membranes. This investigation was motivated by the specific role of the lipid membrane in transport regulation. Bioinspired cell membrane models, including Langmuir monolayers and liposomes (LUVs and GUVs), were utilized to explore the interaction of IMZ and MP. The results revealed electrostatic interactions between IMZ and MP and the polar head groups of lipids, inducing morphological alterations in the lipid bilayer. Tilapia gill tissue exposed to the pesticides exhibited hypertrophic increases in primary and secondary lamellae, total lamellar fusion, vasodilation, and lifting of the secondary lamellar epithelium. These alterations can lead to compromised oxygen absorption by fish and subsequent mortality. This study not only highlights the harmful effects of the pesticides IMZ and MP, but also emphasizes the crucial role of water quality in ecosystem well-being, even at minimal pesticide concentrations. Understanding these impacts can better inform management practices to safeguard aquatic organisms and preserve ecosystem health in pesticide-affected environments.

Interaction of cardiopulmonary and carotid baroreflex control of vascular resistance in humans.

Previous studies in experimental animals indicate an important inhibitory interaction between cardiopulmonary and arterial baroreflexes. In the dog, for example, cardiopulmonary vagal afferents modulate carotid baroreflex control of vascular resistance. On the other hand, previous studies in human subjects have not produced convincing evidence of a specific interaction between these baroreceptor reflexes. The purpose of this study was to determine whether unloading of cardiopulmonary baroreceptors in humans with nonhypotensive lower body negative pressure selectively augments the reflex vasoconstrictor responses to simulated carotid hypotension produced by neck pressure. In nine healthy subjects, we measured forearm vascular responses with plethysmography during lower body negative pressure alone (cardiopulmonary baroreflex), during neck pressure alone (carotid baroreflex), and during concomitant lower body negative pressure and neck pressure (baroreflex interaction). Lower body negative pressure produced a greater than twofold augmentation of the forearm vasoconstrictor response to neck pressure. This increase in resistance was significantly greater (P less than 0.05) than the algebraic sum of the increase in resistance from lower body negative pressure alone plus that from neck pressure alone. In contrast, lower body negative pressure did not potentiate the forearm vasoconstrictor responses either to intra-arterial norepinephrine or to the cold pressor test. Thus, the potentiation of the vasoconstrictor response to neck pressure by lower body negative pressure cannot be explained by augmented reactivity to the neurotransmitter or to a nonspecific augmentation of responses to all reflex vasoconstrictor stimuli. In conclusion, nonhypotensive lower body negative pressure selectively augments carotid baroreflex control of forearm vascular resistance. These experiments demonstrate a specific inhibitory cardiopulmonary-carotid baroreflex interaction in humans.

ATP-sensitive potassium channels mediate contraction-induced attenuation of sympathetic vasoconstriction in rat skeletal muscle.

Sympathetic vasoconstriction is sensitive to inhibition by metabolic events in contracting rat and human skeletal muscle, but the underlying cellular mechanisms are unknown. In rats, this inhibition involves mainly alpha2-adrenergic vasoconstriction, which relies heavily on Ca2+ influx through voltage-dependent Ca2+ channels. We therefore hypothesized that contraction-induced inhibition of sympathetic vasoconstriction is mediated by ATP-sensitive potassium (KATP) channels, a hyperpolarizing vasodilator mechanism that could be activated by some metabolic product(s) of skeletal muscle contraction. We tested this hypothesis in anesthetized rats by measuring femoral artery blood flow responses to lumbar sympathetic nerve stimulation or intraarterial hindlimb infusion of the specific alpha2-adrenergic agonist UK 14,304 during KATP channel activation with diazoxide in resting hindlimb and during KATP channel block with glibenclamide in contracting hindlimb. The major new findings are twofold. First, like muscle contraction, pharmacologic activation of KATP channels with diazoxide in resting hindlimb dose dependently attenuated the vasoconstrictor responses to either sympathetic nerve stimulation or intraarterial UK 14,304. Second, the large contraction-induced attenuation in sympathetic vasoconstriction elicited by nerve stimulation or UK 14,304 was partially reversed when the physiologic activation of KATP channels produced by muscle contraction was prevented with glibenclamide. We conclude that contraction-induced activation of KATP channels is a major mechanism underlying metabolic inhibition of sympathetic vasoconstriction in exercising skeletal muscle.

Differential control of heart rate and sympathetic nerve activity during dynamic exercise. Insight from intraneural recordings in humans.

We used microelectrode recordings of muscle sympathetic nerve activity (MSNA) from the peroneal nerve in the leg during arm exercise in conscious humans to test the concept that central command and muscle afferent reflexes produce mass sympathetic discharge at the onset of exercise. Nonischemic rhythmic handgrip and mild arm cycling produced graded increases in heart rate and arterial pressure but did not increase MSNA, whereas ischemic handgrip and moderate arm cycling dramatically increased MSNA. There was a slow onset and offset of the MSNA responses, which suggested metaboreceptor mediation. When forearm ischemia was continued after ischemic handgrip, MSNA remained elevated (muscle chemoreflex stimulation) but heart rate returned to control (elimination of central command). The major new conclusions are that: the onset of dynamic exercise does not produce mass, uniform sympathetic discharge in humans, and muscle chemoreflexes and central command appear to produce differential effects on sympathetic and parasympathetic responses.

Sympathetic nerve discharge is coupled to muscle cell pH during exercise in humans.

We used phosphorus nuclear magnetic resonance spectroscopy (31P-NMR) to probe the cellular events in contracting muscle that initiate the reflex stimulation of sympathetic outflow during exercise. In conscious humans, we performed 31P-NMR on exercising forearm muscle and simultaneously recorded muscle sympathetic nerve activity (MSNA) with microelectrodes in the peroneal nerve to determine if the activation of MSNA is coupled to muscle pH, an index of glycolysis, or to the concentrations (II) of inorganic phosphate (Pi) and adenosine diphosphate (ADP) which are modulators of mitochondrial respiration. During both static and rhythmic handgrip, the onset of sympathetic activation in resting muscle coincided with the development of cellular acidification in active muscle. Furthermore, increases in MSNA were correlated closely with decreases in intracellular pH but dissociated from changes in phosphocreatine [( PCr]), [Pi], and [ADP]. The principal new conclusion is that activation of muscle sympathetic outflow during exercise in humans is coupled to the cellular accumulation of protons in contracting muscle.

Arterial baroreflex buffering of sympathetic activation during exercise-induced elevations in arterial pressure.

Static muscle contraction activates metabolically sensitive muscle afferents that reflexively increase sympathetic nerve activity and arterial pressure. To determine if this contraction-induced reflex is modulated by the sinoaortic baroreflex, we performed microelectrode recordings of sympathetic nerve activity to resting leg muscle during static handgrip in humans while attempting to clamp the level of baroreflex stimulation by controlling the exercise-induced rise in blood pressure with pharmacologic agents. The principal new finding is that partial pharmacologic suppression of the rise in blood pressure during static handgrip (nitroprusside infusion) augmented the exercise-induced increases in heart rate and sympathetic activity by greater than 300%. Pharmacologic accentuation of the exercise-induced rise in blood pressure (phenylephrine infusion) attenuated these reflex increases by greater than 50%. In contrast, these pharmacologic manipulations in arterial pressure had little or no effect on: (a) forearm muscle cell pH, an index of the metabolic stimulus to skeletal muscle afferents; or (b) central venous pressure, an index of the mechanical stimulus to cardiopulmonary afferents. We conclude that in humans the sinoaortic baroreflex is much more effective than previously thought in buffering the reflex sympathetic activation caused by static muscle contraction.

Differential sympathetic neural control of oxygenation in resting and exercising human skeletal muscle.

Metabolic products of skeletal muscle contraction activate metaboreceptor muscle afferents that reflexively increase sympathetic nerve activity (SNA) targeted to both resting and exercising skeletal muscle. To determine effects of the increased sympathetic vasoconstrictor drive on muscle oxygenation, we measured changes in tissue oxygen stores and mitochondrial cytochrome a,a3 redox state in rhythmically contracting human forearm muscles with near infrared spectroscopy while simultaneously measuring muscle SNA with microelectrodes. The major new finding is that the ability of reflex-sympathetic activation to decrease muscle oxygenation is abolished when the muscle is exercised at an intensity > 10% of maximal voluntary contraction (MVC). During high intensity handgrip, (45% MVC), contraction-induced decreases in muscle oxygenation remained stable despite progressive metaboreceptor-mediated reflex increases in SNA. During mild to moderate handgrips (20-33% MVC) that do not evoke reflex-sympathetic activation, experimentally induced increases in muscle SNA had no effect on oxygenation in exercising muscles but produced robust decreases in oxygenation in resting muscles. The latter decreases were evident even during maximal metabolic vasodilation accompanying reactive hyperemia. We conclude that in humans sympathetic neural control of skeletal muscle oxygenation is sensitive to modulation by metabolic events in the contracting muscles. These events are different from those involved in either metaboreceptor muscle afferent activation or reactive hyperemia.

Impairment of sympathetic activation during static exercise in patients with muscle phosphorylase deficiency (McArdle's disease).

Static exercise in normal humans causes reflex increases in muscle sympathetic nerve activity (MSNA) that are closely coupled to the contraction-induced decrease in muscle cell pH, an index of glycogen degradation and glycolytic flux. To determine if sympathetic activation is attenuated when muscle glycogenolysis is blocked due to myophosphorylase deficiency (McArdle's disease), an inborn enzymatic defect localized to skeletal muscle, we now have performed microelectrode recordings of MSNA in four patients with McArdle's disease during static handgrip contraction. A level of static handgrip that more than doubled MSNA in normal humans had no effect on MSNA and caused an attenuated rise in blood pressure in the patients with myophosphorylase deficiency. In contrast, two nonexercise sympathetic stimuli, Valsalva's maneuver and cold pressor stimulation, evoked comparably large increases in MSNA in patients and normals. The principal new conclusion is that defective glycogen degradation in human skeletal muscle is associated with a specific reflex impairment in sympathetic activation during static exercise.

Epinephrine facilitates neurogenic vasoconstriction in humans.

Numerous studies have suggested that epinephrine may facilitate neural release of NE. There have been no studies in humans that demonstrate the functional significance of this action. To determine whether epinephrine facilitates neurogenic vasoconstriction in humans, we contrasted forearm vasoconstrictor responses to a reflex stimulus (lower body negative pressure [LBNP]) and to intraarterial NE before, during, and 30 min after infusion of epinephrine (50 ng/min) or isoproterenol (10 or 25 ng/min) into a brachial artery. These doses had no systemic effects. We reasoned that if prejunctional stimulation of beta receptors by epinephrine and isoproterenol had functional significance, the vasoconstrictor response to LBNP would be potentiated in comparison to the response to NE (postjunctional mechanism). Studies were done on 23 normal male volunteers. Forearm blood flow was measured with a strain gauge plethysmograph and intraarterial pressure was recorded. The ratio of vasoconstrictor responses to LBNP/NE was used as an index of neural release of the neurotransmitter NE. This ratio increased during infusions of both epinephrine and isoproterenol. 30 min after epinephrine the vasoconstrictor response to LBNP (n = 15) was augmented from +9.9 +/- 2.2 (SE) resistance units (RU) before epinephrine to +16.4 +/- 3.2 RU (P less than 0.05); whereas the response to NE (n = 8) tended to decrease from +8.8 +/- 3.1 RU before to +4.2 +/- 1.2 RU after epinephrine (P greater than 0.05). In contrast, 30 min after isoproterenol the vasoconstrictor responses to LBNP and NE were the same as before isoproterenol. The augmented ratio of responses to LBNP/NE after epinephrine and not after isoproterenol supports the concept that epinephrine, but not isoproterenol, is taken up by the adrenergic terminal, is released subsequently during reflex stimulation, and augments the release of the neurotransmitter NE. These experiments provide the first hemodynamic evidence in humans that epinephrine and isoproterenol facilitate neurogenic vasoconstriction. The sustained effect of epinephrine in contrast to isoproterenol suggests that the late facilitation by epinephrine is related to its neural uptake and subsequent release.

Paradoxical withdrawal of reflex vasoconstriction as a cause of hemodialysis-induced hypotension.

Acute hypotension is an important complication of hemodialysis, but the underlying mechanisms remain poorly understood. Because hemorrhage-induced hypovolemia can trigger a sudden decrease in sympathetic activity resulting in bradycardia and vasodilation, we hypothesized that hemodialysis-induced hypovolemia also can trigger the same type of vasodepressor reaction, which would exacerbate the volume-dependent fall in blood pressure. We therefore measured blood pressure, vascular resistance, and sympathetic nerve activity (intraneural microelectrodes) during sessions of maintenance hemodialysis in 7 patients with and 16 patients without a history of hemodialysis-induced hypotension. During hemodialysis, blood pressure at first remained unchanged as calf resistance increased in both hypotension-resistant (from 37 +/- 4 to 49 +/- 5 U, P < 0.05) and hypotension-prone (from 42 +/- 6 to 66 +/- 12 U, P < 0.05) patients; sympathetic activity increased comparably in the subset of patients in whom it could be measured. With continued hemodialysis, calf resistance and sympathetic activity increased further in the hypotension-resistant patients, but in the hypotension-prone patients the precipitous decrease in blood pressure was accompanied by decreases in sympathetic activity, vascular resistance, and heart rate as well as symptoms of vasodepressor syncope. On an interdialysis day, both groups of patients increased vascular resistance normally during unloading of cardiopulmonary baroreceptors with lower body negative pressure and increased heart rate normally during unloading of arterial baroreceptors with infusion of nitroprusside. These findings indicate that in a group of hemodialysis patients without diabetes or other conditions known to impair autonomic reflexes, hemodialysis-induced hypotension is not caused by chronic uremic impairment in arterial or cardiopulmonary baroreflexes but rather by acute, paradoxical withdrawal of sympathetic vasoconstrictor drive producing vasodepressor syncope.

Effects of intranasal cocaine on sympathetic nerve discharge in humans.

Cocaine-induced cardiovascular emergencies are mediated by excessive adrenergic stimulation. Animal studies suggest that cocaine not only blocks norepinephrine reuptake peripherally but also inhibits the baroreceptors, thereby reflexively increasing sympathetic nerve discharge. However, the effect of cocaine on sympathetic nerve discharge in humans is unknown. In 12 healthy volunteers, we recorded blood pressure and sympathetic nerve discharge to the skeletal muscle vasculature using intraneural microelectrodes (peroneal nerve) during intranasal cocaine (2 mg/kg, n = 8) or lidocaine (2%, n = 4), an internal local anesthetic control, or intravenous phenylephrine (0.5-2.0 microg/kg, n = 4), an internal sympathomimetic control. Experiments were repeated while minimizing the cocaine-induced rise in blood pressure with intravenous nitroprusside to negate sinoaortic baroreceptor stimulation. After lidocaine, blood pressure and sympathetic nerve discharge were unchanged. After cocaine, blood pressure increased abruptly and remained elevated for 60 min while sympathetic nerve discharge initially was unchanged and then decreased progressively over 60 min to a nadir that was only 2+/-1% of baseline (P < 0.05); however, plasma venous norepinephrine concentrations (n = 5) were unchanged up to 60 min after cocaine. Sympathetic nerve discharge fell more rapidly but to the same nadir when blood pressure was increased similarly with phenylephrine. When the cocaine-induced increase in blood pressure was minimized (nitroprusside), sympathetic nerve discharge did not decrease but rather increased by 2.9 times over baseline (P < 0.05). Baroreflex gain was comparable before and after cocaine. We conclude that in conscious humans the primary effect of intranasal cocaine is to increase sympathetic nerve discharge to the skeletal muscle bed. Furthermore, sinoaortic baroreflexes play a pivotal role in modulating the cocaine-induced sympathetic excitation. The interplay between these excitatory and inhibitory neural influences determines the net effect of cocaine on sympathetic discharge targeted to the human skeletal muscle circulation.

Impaired modulation of sympathetic vasoconstriction in contracting skeletal muscle of rats with chronic myocardial infarctions: role of oxidative stress.

Skeletal muscle perfusion during exercise is impaired in heart failure, but the underlying mechanisms are poorly understood. One possibility is that sympathetic vasoconstriction is enhanced in exercising muscle in heart failure as a result of impaired counterregulatory mechanisms that normally act to attenuate vasoconstrictor responses. In healthy animals, sympathetic vasoconstriction in contracting skeletal muscle is attenuated by endogenously produced nitric oxide (NO). Because the NO pathway may be dysfunctional in heart failure, we hypothesized that reduced NO in contracting muscle would result in enhanced sympathetic vasoconstriction. In sham rats and rats with chronic myocardial infarctions (MIs) produced by coronary artery ligation, we measured arterial pressure and femoral artery blood flow responses to sympathetic nerve stimulation (1, 2.5, and 5 Hz) in resting and contracting hindlimb. In resting hindlimb, sympathetic stimulation decreased femoral vascular conductance similarly in sham and MI rats. In contracting hindlimb, these vasoconstrictor responses were attenuated to a greater extent in sham than in MI rats. NO synthase inhibition enhanced sympathetic vasoconstriction in contracting hindlimb of sham, but not MI, rats. Conversely, infusion of L-arginine or a superoxide scavenger, tempol or tiron, attenuated sympathetic vasoconstriction in contracting hindlimb of MI rats. NO synthase expression was similar, but malondialdehyde (a marker of free radical damage) was greater in skeletal muscle from MI than from sham rats. These data suggest that impaired metabolic modulation of sympathetic vasoconstriction in contracting skeletal muscle of MI rats is a consequence of superoxide-mediated disruption of the NO pathway.

Cocaine stimulates the human cardiovascular system via a central mechanism of action.

BACKGROUND: Cocaine is thought to stimulate the cardiovascular system by blocking peripheral norepinephrine reuptake. This study was designed to test the novel hypotheses that cocaine also stimulates the human cardiovascular system by (1) increasing central sympathetic outflow, or (2) decreasing parasympathetic control of heart rate. METHODS AND RESULTS: In 14 healthy cocaine-naive humans, we measured blood pressure, heart rate, and skin sympathetic nerve activity (SNA) with intraneural microelectrodes before, during, and for 90 minutes after intranasal cocaine (2 mg/kg, n=7) or lidocaine (2 mg/kg, n=7). Intranasal cocaine caused an initial but transient 3. 3-fold increase in skin SNA during the period of intranasal administration followed by a sustained 2.4-fold increase lasting for up to 90 minutes after cocaine. Unlike cocaine, intranasal lidocaine caused only a small transient increase in skin SNA due to local nasal irritation. The cocaine-induced increase in SNA was accompanied by decreased skin blood flow, increased skin vascular resistance, and increased heart rate. In 11 additional subjects, we showed that the cocaine-induced increase in heart rate was eliminated by beta-adrenergic receptor blockade (propranolol) but unaffected by muscarinic receptor blockade (atropine), indicating sympathetic mediation. CONCLUSIONS: These studies provide direct microneurographic evidence in humans that intranasal cocaine stimulates central sympathetic outflow. This central sympathetic activation appears to be targeted not only to the cutaneous circulation promoting peripheral vasoconstriction but also to the heart promoting tachycardia.

Transdermal estrogen replacement therapy decreases sympathetic activity in postmenopausal women.

BACKGROUND: Menopause heralds a dramatic increase in incident hypertension, suggesting a protective effect of estrogen on blood pressure (BP). In female rats, estrogen has been shown to decrease sympathetic nerve discharge (SND) and BP. SND, however, has not been recorded during estrogen replacement therapy (ERT) in humans. Methods and Results-In 12 normotensive postmenopausal women, we conducted a randomized crossover placebo-controlled study to test whether chronic ERT caused a sustained decrease in SND and BP. Twenty-four-hour ambulatory BP, SND, and arterial baroreflex sensitivity were measured before and after 8 weeks of transdermal estradiol (200 microgram/d), oral conjugated estrogens (0.625 mg/d), or placebo. To test the acute effects of estrogen on SND, additional studies were performed in the same women receiving intravenous conjugated estrogens or sublingual estradiol. After 8 weeks of transdermal ERT, the basal rate of SND decreased by 30% (from 40+/-4 to 27+/-4 bursts per minute, P=0.0001) and ambulatory diastolic BP fell by 5+/-2 mm Hg (P=0.0003). In contrast, SND and BP were unaffected either by 8 weeks of oral ERT or by acute estrogen administration. Neither transdermal nor oral ERT had any effects on baroreflex sensitivity. CONCLUSIONS: In normotensive postmenopausal women, chronic transdermal ERT decreases SND without augmenting arterial baroreflexes and causes a small but statistically significant decrease in ambulatory BP. Sympathetic inhibition is evident only with chronic rather than acute estrogen administration, implying a genomic mechanism of action. Because the effects of transdermal ERT are larger than those of oral ERT, the route of administration may be an important consideration in optimizing the beneficial effects of ERT on BP and overall cardiovascular health.

Exercise-induced attenuation of alpha-adrenoceptor mediated vasoconstriction in humans: evidence from phase-contrast MRI.

OBJECTIVE: We recently provided evidence for contraction-induced attenuation of reflex sympathetic vasoconstriction in human skeletal muscle microcirculation. We now asked whether contraction-induced modulation of alpha-adrenoceptor mediated vasoconstriction in the human forearm (a) is evident in a large artery supplying the contracting skeletal muscle and (b) implicates a post-junctional site of action. METHODS AND RESULTS: To address these questions in humans, we used phase-contrast magnetic resonance imaging to measure blood flow velocity and cross-sectional area of the brachial artery during brachial-artery infusion of the alpha-adrenoceptor agonist norepinephrine (NE) (1.1 g/min for 5 min) at rest and during mild ipsilateral rhythmic handgrip (20% of maximum). At rest, brachial artery conductance decreased progressively during the entire 5 min period of infusion (baseline to first half to second half of infusion: 0.421 +/- 0.157 to 0.255 +/- 0.187 to 0.012 +/- 0.014 ml/min/mmHg, P < 0.05). When NE was superimposed on handgrip, conductance at first decreased sharply (1.205 +/- 0.127 to 0.330 +/- 0.097 ml/min/mmHg, P < 0.05). However, during the second half of the infusion, conductance did not decrease further but rather returned progressively toward baseline (0.476 +/- 0.199 ml/min/mmHg at the end of the exercise, P < 0.05 vs. NE alone). CONCLUSION: These data provide new evidence in humans that alpha-adrenoceptor mediated vasoconstriction is sensitive to modulation by skeletal muscle contraction. Such modulation is evident at the level of a large conduit artery and it involves a post-junctional mechanism of action.

Is nitric oxide involved in the tonic inhibition of central sympathetic outflow in humans?

Recent studies in experimental animals have advanced the concept that neuronal nitric oxide is an important component of the signal transduction pathways that tonically restrain sympathetic vasoconstrictor outflow from the brain stem. To determine whether or not this concept can be extended to the control of sympathetic outflow in humans, we recorded muscle sympathetic nerve activity (microelectrodes, peroneal nerve) in healthy human subjects during intravenous infusion of the nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) (3.6 to 6.7 mg/kg). The major new finding is that during intravenous L-NMMA mean arterial pressure increased (10 +/- 2 mm Hg, P < .05), whereas heart rate and sympathetic nerve activity decreased (P < .05) by 10 +/- 2 beats per minute and 61 +/- 5%, respectively. These reflex decreases were indistinguishable from those produced when blood pressure was increased comparably with phenylephrine, an internal vasoconstrictor control. When the L-NMMA-induced increase in blood pressure was attenuated experimentally to minimize baroreflex activation, sympathetic nerve activity and heart rate were unchanged. Furthermore, during infusion of L-arginine (323 to 513 mg/kg IV) to increase nitric oxide synthesis, mean arterial pressure decreased (12 +/- 2 mm Hg, P < .05), but heart rate and sympathetic nerve activity increased (P < .05) by 11 +/- 2 beats per minute and 98 +/- 27%, respectively. Thus, our experiments in humans provide no support for the emerging concept that nitric oxide is involved in the tonic restraint of central sympathetic outflow.

Sympathetically mediated hypertension caused by chronic inhibition of nitric oxide.

Pharmacological inhibition of nitric oxide synthase causes sustained hypertension in many animal species. Although this hypertension has been attributed to inhibition of endothelium-dependent vasodilation, short-term studies in anesthetized preparations have advanced the hypothesis that there could be a sympathetic component to this hypertension. To test this hypothesis we measured intra-arterial pressure directly before and after 1 week of treatment with the nitric oxide synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, approximately 80 mg/kg per day in drinking water) in conscious unrestrained rats with or without chronic guanethidine-induced sympathectomy. The major new finding is that the hypertensive response to L-NAME was greatly attenuated by sympathectomy. With L-NAME, mean arterial pressure increased from 101 +/- 3 to 152 +/- 6 mm Hg in rats without sympathectomy (n = 11) but only from 96 +/- 2 to 122 +/- 3 mm Hg in rats with sympathectomy (n = 15, +52 +/- 5 versus +27 +/- 4 mm Hg, P < .01). Sympathectomy did not alter maximal endothelium-dependent vasodilation assessed by femoral vascular responses to intra-arterial acetylcholine or bradykinin, indicating that the differing hypertensive responses to L-NAME in rats with versus without sympathectomy could be related to inhibition of neuronal rather than endothelial nitric oxide synthesis. We also found that L-NAME-induced hypertension, once developed, is completely reversed by acute ganglionic blockade. In conclusion, these findings identify an important sympathetic neural component to the sustained hypertension produced by pharmacological inhibition of nitric oxide in the rat.

A large blood pressure-raising effect of nitric oxide synthase inhibition in humans.

In experimental animals, systemic administration of nitric oxide synthase (NOS) inhibitors causes large increases in blood pressure that are in part sympathetically mediated. The aim of this study was to determine the extent to which these conclusions can be extrapolated to humans. In healthy normotensive humans, we measured blood pressure in response to two NOS inhibitors, NG-monomethyl-L-arginine (L-NMMA) and NG-nitro-L-arginine methyl ester (L-NAME), the latter of which recently became available for use in humans. The major new findings are 3-fold. First, L-NAME produced robust increases in blood pressure that were more than 2 times larger than those previously reported in humans with L-NMMA and approximated those seen in experimental animals. L-NAME (4 mg/kg) raised mean arterial pressure by 24+/-2 mm Hg (n=27, P<0.001), whereas in subjects who received both inhibitors, a 12-fold higher dose of L-NMMA (50 mg/kg) raised mean arterial pressure by 15+/-2 mm Hg (n=4, P<0.05 vs L-NAME). Second, the L-NAME-induced increases in blood pressure were caused specifically by NOS inhibition because they were reversed by L-arginine (200 mg/kg, n=12) but not D-arginine (200 mg/kg, n=6) and because NG-nitro-D-arginine methyl ester (4 mg/kg, n=5) had no effect on blood pressure. Third, in humans, there is an important sympathetic component to the blood pressure-raising effect of NOS inhibition. alpha-Adrenergic blockade with phentolamine (0.2 mg/kg, n=9) attenuated the L-NAME-induced increase in blood pressure by 40% (P<0.05). From these data, we conclude that pharmacological inhibition of NOS causes large increases in blood pressure that are in part sympathetically mediated in humans as well as experimental animals.

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.