Neural mechanisms in nitric-oxide-deficient hypertension

The effect of the long-term inhibition of hydrogen sulfide production on the reactivity of the cardiovascular system in Wistar rats

In this study, we investigated the blood pressure responses of the peripheral bed in vivo after chronic hydrogen sulphide (H2S) inhibition combined with acute nitric oxide (NO) deficiency. We also evaluated the role of endogenously produced H2S in the vasoactive responses of large- and medium-sized arteries in vitro. Changes in integrated blood pressure responses were measured after chronic inhibition of cystathionine-γ-lyase, an enzyme involved in H2S synthesis, with DL-propargylglycine (PPG), and acute inhibition of NO-synthase with nonspecific L-NG-nitro arginine methyl ester (L-NAME), and vasoactive responses of the thoracic aorta (TA) and mesenteric artery (MA) were investigated after acute incubation with PPG. We confirmed that chronic H2S deficiency had no effect on blood pressure, heart trophycity, noradrenaline and H2S donor vasoactive responses but induced renal hypertrophy and decrease in acetylcholine-induced hypotensive and L-NAME-induced hypertensive responses. Acute H2S deficiency led to an increase in basal tone (MA) or active tone (TA), whereas endothelium-dependent vasorelaxation remained unaffected. Long-term administration of PPG revealed a role of endogenous H2S in the bioavailability of endothelial NO in peripheral arteries. When both H2S and NO were lacking, the activation of H2S-independent compensatory mechanisms plays an important role in maintaining the vasodilator responses of the cardiovascular system.

Pattern of sympathetic vasomotor activity in a model of hypertension induced by nitric oxide synthase blockade

We aimed to investigate the effects of nitric oxide (NO) synthesis inhibition by NO synthase inhibitor N‐nitro‐L‐arginine‐methyl ester (L‐NAME) treatment on the sympathetic vasomotor nerve activity (SNA) on two sympathetic vasomotor nerves, the renal and splanchnic. NO plasma level and systemic oxidative stress were assessed. Hypertension was induced by L‐NAME (20 mg/kg per day, by gavage, for seven consecutive days) in male Wistar rats. At the end of the treatment, blood pressure, heart rate, arterial baroreflex sensitivity, renal SNA (rSNA), and splanchnic SNA (sSNA) were assessed in urethane anesthetized rats. L‐NAME‐treated rats presented increased blood pressure (152 ± 2 mmHg, n = 17) compared to the control group (101 ± 2 mmHg, n = 15). Both rSNA (147 ± 10, n = 15 vs. 114 ± 5 Spikes/s, n = 9) and sSNA (137 ± 13, n = 14 vs. 74 ± 13 spikes/s, n = 9) were significantly increased in the L‐NAME‐treated compared to the control group. A differential response on baroreflex sensitivity was found, with a significant reduction for rSNA but not for sSNA arterial baroreceptor sensitivity in L‐NAME‐treated rats. The adjusted regression model revealed that the reduction of systemic NO levels partially explains the variation in sSNA and blood pressure, but not rSNA. Taken together, our data show that hypertension induced by NO synthase blockade is characterized by increased SNA to the rSNA and sSNA. In addition, we found that the rats that had the greatest reduction in NO levels in plasma by L‐NAME were those that developed higher blood pressure levels. The reduction in the NO level partially explains the variations in sSNA but not in rSNA.

Blockade of Rostral Ventrolateral Medulla Apelin Receptors Does Not Attenuate Arterial Pressure in SHR and L-NAME-Induced Hypertensive Rats

Dysfunction of the apelinergic system, comprised of the neuropeptide apelin mediating its effects via the G protein-coupled apelin receptor (APJ), may underlie the onset of cardiovascular disease such as hypertension. Apelin expression is increased in the rostral ventrolateral medulla (RVLM) in spontaneously hypertensive rats (SHRs) compared to Wistar-Kyoto (WKY) normotensive rats, however, evidence that the apelinergic system chronically influences mean arterial blood pressure (MABP) under pathophysiological conditions remains to be established. In this study we investigated, in conscious unrestrained rats, whether APJ contributes to MABP and sympathetic vasomotor tone in the progression of two models of hypertension - SHR and _L-NAME-treated rats - and whether APJ contributes to the development of hypertension in pre-hypertensive SHR. In SHR we showed that APJ gene (aplnr) expression was elevated in the RVLM, and there was a greater MABP increase following microinjection of [Pyr¹]apelin-13 to the RVLM of SHR compared to WKY rats. Bilateral microinjection of a lentiviral APJ-specific-shRNA construct into the RVLM of WKY, SHR, and _L-NAME-treated rats, chronically implanted with radiotelemeters to measure MABP, decreased aplnr expression in the RVLM and abolished acute [Pyr¹]apelin-13-induced increases in MABP. However, chronic knockdown of aplnr in the RVLM did not affect MABP in either SHR or _L-NAME-treated rats. Moreover, knockdown of aplnr in the RVLM of prehypertensive SHR did not protect against the development of hypertension. These results show that endogenous apelin, acting via APJ, is not involved in the genesis or maintenance of hypertension in either animal model used in this study.

Blood pressure regulation in stress: focus on nitric oxide-dependent mechanisms

Stress is considered a risk factor associated with the development of various civilization diseases including cardiovascular diseases, malignant tumors and mental disorders. Research investigating mechanisms involved in stress-induced hypertension have attracted much attention of physicians and researchers, however, there are still ambiguous results concerning a causal relationship between stress and long-term elevation of blood pressure (BP). Several studies have observed that mechanisms involved in the development of stress-induced hypertension include increased activity of sympathetic nervous system (SNS), glucocorticoid (GC) overload and altered endothelial function including decreased nitric oxide (NO) bioavailability. Nitric oxide is well known neurotransmitter, neuromodulator and vasodilator involved in regulation of neuroendocrine mechanisms and cardiovascular responses to stressors. Thus NO plays a crucial role in the regulation of the stress systems and thereby in the BP regulation in stress. Elevated NO synthesis, especially in the initial phase of stress, may be considered a stress-limiting mechanism, facilitating the recovery from stress to the resting levels via attenuation of both GC release and SNS activity as well as by increased NO-dependent vasorelaxation. On the other hand, reduced levels of NO were observed in the later phases of stress and in subjects with genetic predisposition to hypertension, irrespectively, in which reduced NO bioavailability may account for disruption of NO-mediated BP regulatory mechanisms and accentuated SNS and GC effects. This review summarizes current knowledge on the role of stress in development of hypertension with a special focus on the interactions among NO and other biological systems affecting blood pressure and vascular function.

Tetrahydrobiopterin ameliorates the exaggerated exercise pressor response in patients with chronic kidney disease: a randomized controlled trial

Chronic kidney disease (CKD) patients have an exaggerated increase in blood pressure (BP) during rhythmic handgrip exercise (RHG 20%) and static handgrip exercise (SHG 30%). Nitric oxide levels increase during exercise and help prevent excessive hypertension by both increasing vasodilation and reducing sympathetic nerve activity (SNA). Therefore, we hypothesized that tetrahydrobiopterin (BH4), an essential cofactor for nitric oxide synthase, would ameliorate the exaggerated exercise pressor response in CKD patients. In a randomized, double-blinded, placebo-controlled trial, we tested the effects of 12 wk of sapropterin dihydrochloride (6R-BH4; n = 18) versus placebo (n = 14) treatement on BP and muscle SNA (MSNA) responses during RHG 20% and SHG 30% in CKD patients. The 6R-BH4-treated group had a significantly lower systolic BP (+6 ± 1 vs. +13 ± 2 mmHg, P = 0.002) and mean arterial pressure response (+5 ± 1 vs. +10 ± 2 mmHg, P = 0.020) during RHG 20% and a significantly lower systolic BP response (+19 ± 3 vs. +28 ± 3 mmHg, P = 0.043) during SHG 30%. Under baseline conditions, there was no significant difference in MSNA responses between the groups; however, when the BP response during exercise was equalized between the groups using nitroprusside, the 6R-BH4-treated group had a significantly lower MSNA response during RHG 20% (6R-BH4 vs. placebo, +12 ± 1 vs. +21 ± 2 bursts/min, P = 0.004) but not during SHG 30%. These findings suggest that 6R-BH4 ameliorates the augmented BP response during RHG 20% and SHG 30% in CKD patients. A reduction in reflex activation of SNA may contribute to the decreased exercise pressor response during RHG 20% but not during SHG 30% in CKD patients.

Cytosolic phospholipase A2α is critical for angiotensin II-induced hypertension and associated cardiovascular pathophysiology

Angiotensin II activates cytosolic phospholipase A(2)α (cPLA2α) and releases arachidonic acid from tissue phospholipids, which mediate or modulate ≥1 cardiovascular effects of angiotensin II and has been implicated in hypertension. Because arachidonic acid release is the rate limiting step in eicosanoid production, cPLA2α might play a central role in the development of angiotensin II-induced hypertension. To test this hypothesis, we investigated the effect of angiotensin II infusion for 13 days by micro-osmotic pumps on systolic blood pressure and associated pathogenesis in wild type (cPLA2α(+/+)) and cPLA2α(-/-) mice. Angiotensin II-induced increase in systolic blood pressure in cPLA2α(+/+) mice was abolished in cPLA2α(-/-) mice; increased systolic blood pressure was also abolished by the arachidonic acid metabolism inhibitor, 5,8,11,14-eicosatetraynoic acid in cPLA2α(+/+) mice. Angiotensin II in cPLA2α(+/+) mice increased cardiac cPLA2 activity and urinary eicosanoid excretion, decreased cardiac output, caused cardiovascular remodeling with endothelial dysfunction, and increased vascular reactivity in cPLA2α(+/+) mice; these changes were diminished in cPLA2α(-/-) mice. Angiotensin II also increased cardiac infiltration of F4/80(+) macrophages and CD3(+) T lymphocytes, cardiovascular oxidative stress, expression of endoplasmic reticulum stress markers p58(IPK), and CHOP in cPLA2α(+/+) but not cPLA2α(-/-) mice. Angiotensin II increased cardiac activity of ERK1/2 and cSrc in cPLA2α(+/+) but not cPLA2α(-/-) mice. These data suggest that angiotensin II-induced hypertension and associated cardiovascular pathophysiological changes are mediated by cPLA2α activation, most likely through the release of arachidonic acid and generation of eicosanoids with predominant prohypertensive effects and activation of ≥1 signaling molecules, including ERK1/2 and cSrc.

Cardiovascular risk in chronic kidney disease: role of the sympathetic nervous system

Patients with chronic kidney disease are at significantly increased risk for cardiovascular disease and sudden cardiac death. One mechanism underlying increased cardiovascular risk in patients with renal failure includes overactivation of the sympathetic nervous system (SNS). Multiple human and animal studies have shown that central sympathetic outflow is chronically elevated in patients with both end-stage renal disease (ESRD) and chronic kidney disease (CKD). SNS overactivation, in turn, increases the risk of cardiovascular disease and sudden death by increasing arterial blood pressure, arrythmogenicity, left ventricular hypertrophy, and coronary vasoconstriction and contributes to the progression renal disease. This paper will examine the evidence for SNS overactivation in renal failure from both human and experimental studies and discuss mechanisms of SNS overactivity in CKD and therapeutic implications.

Altered neural and vascular mechanisms in hypertension

Essential hypertension is a multifactorial disorder which belongs to the main risk factors responsible for renal and cardiovascular complications. This review is focused on the experimental research of neural and vascular mechanisms involved in the high blood pressure control. The attention is paid to the abnormalities in the regulation of sympathetic nervous system activity and adrenoceptor alterations as well as the changes of membrane and intracellular processes in the vascular smooth muscle cells of spontaneously hypertensive rats. These abnormalities lead to increased vascular tone arising from altered regulation of calcium influx through L-VDCC channels, which has a crucial role for excitation-contraction coupling, as well as for so-called "calcium sensitization" mediated by the RhoA/Rho-kinase pathway. Regulation of both pathways is dependent on the complex interplay of various vasodilator and vasoconstrictor stimuli. Two major antagonistic players in the regulation of blood pressure, i.e. sympathetic nervous system (by stimulation of adrenoceptors coupled to stimulatory and inhibitory G proteins) and nitric oxide (by cGMP signaling pathway), elicit their actions via the control of calcium influx through L-VDCC. However, L-type calcium current can also be regulated by the changes in membrane potential elicited by the activation of potassium channels, the impaired function of which was detected in hypertensive animals. The dominant role of enhanced calcium influx in the pathogenesis of high blood pressure of genetically hypertensive animals is confirmed not only by therapeutic efficacy of calcium antagonists but especially by the absence of hypertension in animals in which L-type calcium current was diminished by pertussis toxin-induced inactivation of inhibitory G proteins. Although there is considerable information on the complex neural and vascular alterations in rats with established hypertension, the detailed description of their appearance during the induction of hypertension is still missing.

Inhibition of nitric oxide synthase evokes central sympatho-excitation in healthy humans

Animal studies have indicated that nitric oxide is a key signalling molecule involved in the tonic restraint of central sympathetic outflow from the brainstem. Extension of these findings to humans has been difficult because systemic infusion of nitric oxide synthase (NOS) inhibitors increases blood pressure due to inhibition of endothelial NOS, resulting in activation of the arterial baroreflex and subsequent inhibition of central sympathetic outflow. To overcome this confounding inhibitory influence of the baroreflex, in the current study we directly measured skin sympathetic nerve activity (SNA), which is not under baroreceptor control. Healthy, normotensive humans were studied before, during a 60 min intravenous infusion of the NOS inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME; 4 mg kg(1)), and for 120 min following the infusion (i.e. 180 min total). Skin SNA and arterial blood pressure (BP) were continuously measured. BP was increased from baseline at the end of the l-NAME infusion (14 +/- 2 mmHg; P < 0.05) and remained significantly elevated for the remainder of the experiment (18 +/- 3 mmHg; P < 0.05). Similarly, systemic NOS inhibition produced time-dependent increases in skin SNA, such that skin SNA was elevated at the end of the l-NAME infusion (total activity, 200 +/- 22% baseline; P = 0.08) and was further increased at the end of the study protocol (total activity, 350 +/- 41% baseline; P < 0.05). Importantly, skin SNA remained unchanged during time and hypertensive (phenylephrine) control experiments. These findings indicate that pharmacological inhibition of NOS causes sympathetic activation and support a role of nitric oxide in central sympathetic control in humans.

The endocrine system in chronic nitric oxide deficiency

The experimental model of chronic inhibition of nitric oxide (NO) production has proven to be a useful tool to study cardiovascular and renal lesions produced by this type of hypertension, which are similar to those found in human hypertension. It also offers a unique opportunity to study the interaction of NO with the humoral systems, known to have a role in the normal physiology of vascular tone and renal function. This review provides a thorough and updated analysis of the interactions of NO with the endocrine system. There is special focus on the main vasoactive factors, including the renin-angiotensin-aldosterone system, catecholamines, vasopressin, and endothelin among others. Recent discoveries of crosstalk between the endocrine system and NO are also reported. Study of these humoral interactions indicates that NO is a molecule with ubiquitous function and that its inhibition alters virtually to all other known regulatory systems. Thus, hypothyroidism attenuates the pressor effect of NO inhibitor N-nitro-L-arginine methyl ester, whereas hyperthyroidism aggravates the effects of NO synthesis inhibition; the sex hormone environment determines the blood pressure response to NO blockade; NO may play a homeostatic role against the prohypertensive effects of mineralocorticoids, thyroid hormones and insulin; and finally, NO deficiency affects not only blood pressure but also glucose and lipid homeostasis, mimicking the human metabolic syndrome X, suggesting that NO deficiency may be a link between metabolic and cardiovascular disease.

Antihypertensive Mechanisms of Chronic Captopril or N-Acetylcysteine Treatment in L-NAME Hypertensive Rats

Hypertension due to chronic inhibition of NO synthase (NOS) by Nomega-nitro-L-arginine methyl ester (L-NAME) administration is characterized by both impaired NO-dependent vasodilation and enhanced sympathetic vasoconstriction. The aim of our study was to evaluate changes in the participation of major vasoactive systems in L-NAME-treated rats which were subjected to simultaneous antihypertensive (captopril) or antioxidant (N-acetylcysteine, NAC) treatment. Three-month-old Wistar males treated with L-NAME (60 mg/kg/day) for 5 weeks were compared to rats in which L-NAME treatment was combined with simultaneous chronic administration of captopril or NAC. Basal blood pressure (BP) and its acute responses to consecutive i.v. injections of captopril (10 mg/kg), pentolinium (5 mg/kg), L-NAME (30 mg/kg), tetraethylammonium (TEA, 16 mg/kg) and nitroprusside (NP, 20 microg/kg) were determined in conscious rats at the end of the study. The development of L-NAME hypertension was prevented by captopril treatment, whereas NAC treatment caused only a moderate BP reduction. Captopril treatment normalized the sympathetic BP component and significantly reduced residual BP (measured at full NP-induced vasodilation). In contrast, chronic NAC treatment did not modify the sympathetic BP component or residual BP, but significantly enhanced NO-dependent vasodilation. Neither captopril nor NAC treatment influenced the compensatory increase of TEA-sensitive vasodilation mediated by endothelium-derived hyperpolarizing factor in L-NAME-treated rats. Chronic captopril treatment prevented L-NAME hypertension by lowering of sympathetic tone, whereas chronic NAC treatment attenuated L-NAME hypertension by reduction in the vasodilator deficit due to enhanced NO-dependent vasodilation.

L-NAME- and ADMA-induced sympathetic neural activation in conscious rats

Although studies in anesthetized, sino-aortic denervated animals indicate that inhibition of central nitric oxide (NO) causes an excitatory influence on efferent sympathetic nerve activity (SNA) that is normally offset by baroreflex activation, studies in conscious animals have not provided clear-cut evidence for a sympathoexcitatory effect of N(omega)-nitro-l-arginine methyl ester (L-NAME) or the endogenous circulating NO synthase (NOS) inhibitor asymmetric dimethylarginine (ADMA). Thus our goals were to 1) use surgical sino-aortic denervation to test for a sympathoexcititatory effect of intravenous l-NAME in conscious rats, and 2) to determine whether SNA responses to intravenous L-NAME can be extrapolated directly to intravenous ADMA. We recorded mean arterial blood pressure and renal SNA in both intact and sino-aortic-denervated conscious rats during 3 h of continuous intravenous infusion with either L-NAME or ADMA. When we eliminated the confounding influence of the sino-aortic baroreceptors, L-NAME produced a progressive increase in SNA with the peak response exceeding the baseline level of nerve firing by 150%. The same type of frank sympathetic activation was observed with intravenous ADMA. Taken together, these data offer straightforward evidence for l-NAME, as well as ADMA-induced sympathetic activation with direct recordings of SNA in conscious animals. These data confirm and extend the concept that circulating endogenous NOS inhibitors can constitute an excitatory signal to SNA.

Central neuropeptide Y signaling ameliorates N(omega)-nitro-L-arginine methyl ester hypertension in the rat through a Y1 receptor mechanism

Neuropeptide Y is a potent inhibitory neurotransmitter expressed in the central neurons that control blood pressure. NO also serves as an inhibitory neurotransmitter, and its deficit causes sympathetic overactivity, which then contributes to hypertension. This study tested the hypothesis that neuropeptide Y functions as a central neurotransmitter to lower blood pressure, therefore its increased signaling ameliorates hypertension induced by NO deficiency. Conscious neuropeptide Y transgenic male rats, overexpressing the peptide under its natural promoter, and nontransgenic littermates (controls) were used in this study. Neuropeptide Y, Y1 receptor antagonist BIBP3226, or vehicle (saline) were administered continuously for 14 days into the cerebral lateral ventricle in unrestrained animals using osmotic pumps. Blood pressure was measured by radiotelemetry. Compared with control animals, transgenic overexpression of neuropeptide Y significantly ameliorated (by 9.7+/-1.5 mm Hg) NO deficiency hypertension (induced by administration of N(omega)-nitro-L-arginine methyl ester in the drinking water). This hypotensive effect of neuropeptide Y upregulation was associated with reduced proteinuria and cardiac hypertrophy and fibrosis. Central administration of neuropeptide Y in nontransgenic rats also reduced (by 10.2+/-1.6 mm Hg) the NO deficiency hypertension, whereas a neuropeptide Y1 receptor antagonist centrally administered in the transgenic subjects during NO deficiency hypertension completely attenuated the depressor effect of neuropeptide Y upregulation. Thus, acting at the level of the central nervous system distinctively via a Y1 receptor-mediated mechanism, endogenous neuropeptide Y exerted a potent antihypertensive function, and its enhanced signaling ameliorated NO deficiency hypertension.

The vascular response to the K+ channel inhibitor 4-aminopyridine in hypertensive rats

The K+ channel inhibitor 4-aminopyridine induced an immediate increase in blood pressure and tension in spontaneously hypertensive rats (SHR). Further analysis strongly suggested this to be due to closure of vascular smooth muscle K+ channels, as previously concluded for normotensive rats (WKY). The tension response was greater in SHR than WKY, suggesting an increased channel activity in order to compensate for the high total peripheral vascular resistance in SHR. The response was enhanced after nitric oxide (NO) synthase inhibitor in both strains, probably reflecting increased channel activity after elimination of the NO-cGMP pathway. The response in SHR but not WKY was increased after alpha(1)-adrenoceptor inhibition and adrenalectomy but not sympathetic nerve transmitter depletion. It increased also after angiotensin AT(1) and endothelin ET(A) receptor antagonists and protein kinase C inhibitor. These results indicated an increased adrenal catecholamine, angiotensin AT(1) and endothelin ET(A) activation of the phospholipase C-protein kinase C pathway in SHR, inhibiting the 4-aminopyridine-sensitive K+ channels.

Nitric oxide opposes glucose-induced hypertension by suppressing sympathetic activity

We have reported that glucose infusion in L-NAME-treated rats increased arterial pressure more than the additive responses to glucose and L-NAME alone. This suggested that nitric oxide synthesis inhibition potentiated the hypertensive response to chronic glucose infusion, and the heart rate data suggested an important role for the sympathetic nervous system. This study tested the role of the sympathetic nervous system by infusing glucose for 7 days in 4 groups of rats: L-NAME (L), L-NAME plus alpha- and beta-adrenergic receptor blockade (LB), vehicle, or vehicle plus adrenergic receptor blockade (blockers). Mean arterial pressure (MAP, 24 hours per day) increased significantly in both the vehicle and blockers groups, confirming our previous reports. Likewise, MAP increased significantly more during glucose infusion in the L rats, from 120+/-3 mm Hg to 158+/-4 mm Hg by day 7, which was >3 times the increase in the vehicle rats. Heart rate also increased significantly in the L rats, from 391+/-4 to 426+/-8 bpm, and that increase was prevented completely in the LB rats. However, although the increase in MAP in the LB rats was significantly less than in the L rats, the hypertension was not prevented completely. The explanation for that partial inhibition is not clear, but the overall effectiveness of adrenergic receptor blockade to attenuate the potentiated hypertensive and tachycardic responses to glucose infusion in the L-NAME-treated rats versus the normal rats suggests that nitric oxide may help protect against hypertension during glucose infusion through suppression of sympathetic activity.

Sympathetic modulation of nitrergic neurogenic vasodilation in cerebral arteries

The presence of close apposition between the adrenergic and the non-adrenergic or nitrergic nerve terminals in large cerebral arteries in several species is well documented. The axo-axonal distance between these different types of nerve terminals is substantially closer than the synaptic distance between the adventitial nerve terminals and the outermost layer of smooth muscle in the media. This feature suggests that a functional axo-axonal interaction between nerve terminals is more likely to occur than that between the nerve and muscle. Thus, transmitters released from one nerve terminal may modulate release of transmitters from the neighboring nerve terminals, resulting in a neurogenic response. We have reported that nicotine-induced nitric oxide (NO)-mediated neurogenic vasodilation is dependent on intact sympathetic innervation in porcine and cat cerebral arteries. Evidence also has been presented to indicate that nicotine acts on alpha7-nicotinic receptors located on sympathetic nerve terminals, resulting in release of norepinephrine which then diffuses to act on beta2-adrenoceptos located on the neighboring nitrergic nerve terminals to release NO and therefore vasodilation. The predominant facilitatory effect of beta2-adrenoceptors in releasing NO is compromised by presynaptic alpha2-adrenoceptors located on the same nerves. Activation of cerebral sympathetic nerves may cause NO-mediated dilation in large cerebral arteries at the base of the brain.

Inhibition of nitric oxide synthesis increases erythrocyte membrane fluidity and unsaturated fatty acid content

Changes in the lipid composition of the membrane affect its fluidity and function. These variables are altered in various forms of hypertension. Our hypothesis was that the rapid increase in blood pressure (BP) caused by inhibition of nitric oxide production would lead to alterations in membrane fluidity similar to those observed in genetic hypertension. We used Nomega-nitro L-arginine methyl ester (L-NAME) and vehicle-treated (3 weeks) Wistar-Kyoto rats to study the effects of nitric oxide synthase (NOS) inhibition on membrane fluidity and lipid composition. Erythrocyte membrane fluidity was measured by fluorescence anisotropy. Membrane lipids were separated using Sep-Pak and thin-layer chromatography. Fatty acid methyl esters were produced and analyzed by gas chromatography-mass spectrometry. Nomega-nitro L-arginine methyl ester treatment increased BP and erythrocyte membrane fluidity. The phospholipid and unsaturated fatty acid levels in the membranes from the L-NAME-treated rats were consistent with the increase in fluidity (ie, more unsaturated fatty acid, in particular, arachidonic and docosahexaenoic acid) and a reduction in membrane sphingomyelin content. Fatty acid analysis of individual lipid groups suggested the changes in membrane fatty acid composition may be asymmetric, with the majority of the changes occurring in the outer leaflet. Inhibition of NOS results in changes in membrane composition that may explain the concurrent changes in fluidity. The increased membrane fluidity observed here contrasts with the reduced fluidity observed in genetic hypertension or unchanged fluidity in secondary hypertension. The effects could be related to NOS inhibition or may be a direct effect of L-NAME.

Subpressor dose of L-NAME unmasks hypertensive effect of chronic hyperinsulinemia

We previously found that chronic exogenous hyperinsulinemia without sugar supplementation does not elevate blood pressure. This may be partially explained by the ability of insulin to release nitric oxide and cause vasodilatation. To test this hypothesis, we studied 4 groups of rats: 9 rats (body weight, 213+/-14 g) treated with a gradual increase of a sustained-release subcutaneous insulin pellet; 9 rats (body weight, 213+/-9 g) treated with N:(G)-nitro-L-arginine methyl ester (L-NAME) in drinking water 50 mg/L; 19 rats (body weight, 217+/-11 g) treated with the combination of L-NAME and insulin; and 9 control rats (body weight, 218+/-11 g). Blood pressure was followed weekly for 6 weeks, and then rats were studied in metabolic cages. Weight gain was not different during the 6 weeks. Renal function did not differ between the 4 groups, but 24-hour urinary nitrite/nitrate excretion was lower (P<0.02) in L-NAME-treated and higher in insulin-treated rats. Plasma insulin doubled (P<0.002) in the insulin-treated rats, but there was no hypoglycemia and, by week 6, fructosamine levels were 2.1+/-0.2, 2.1+/-0.2, 2.3+/-0.1, and 2.3+/-0.2 mmol/L in control rats and rats treated with L-NAME, insulin, and L-NAME plus insulin, respectively. Systolic blood pressure, which did not differ at baseline, at week 3 was 122+/-17, 118+/-17, and 118+/-24 mm Hg in the control, L-NAME, and insulin groups and 136+/-14 mm Hg (P<0.03) in the combination group. At week 6, systolic blood pressure was 128+/-14, 127+/-15, and 118+/-13 mm Hg in the control, L-NAME, and insulin groups, respectively, and 150+/-14 mm Hg (P<0.0005) in the combination group. In a subsequent experiment, L-arginine 2 g/L abrogated the effects of L-NAME and insulin combination. In conclusion, chronic exogenous hyperinsulinemia does not affect blood pressure but may cause hypertension when endothelial function is compromised.

Presynaptic beta(2)-adrenoceptors mediate nicotine-induced NOergic neurogenic dilation in porcine basilar arteries

We previously reported that nicotine-induced nitric oxide (NO)-mediated cerebral neurogenic vasodilation was dependent on intact sympathetic innervation. We hypothesized that nicotine acted on sympathetic nerve terminals to release norepinephrine (NE), which then acted on adrenoceptors located on the neighboring nitric oxidergic (NOergic) nerve terminals to release NO, resulting in vasodilation. The adrenoceptor subtype in mediating nicotine-induced vasodilation in isolated porcine basilar arterial rings denuded of endothelium was therefore examined pharmacologically and immunohistochemically. Results from using an in vitro tissue bath technique indicated that propranolol and preferential beta(2)-adrenoceptor antagonists (ICI-118,551 and butoxamine), in a concentration-dependent manner, blocked the relaxation induced by nicotine (100 microM) without affecting the relaxation elicited by transmural nerve stimulation (TNS, 8 Hz). In contrast, preferential beta(1)-adrenoceptor antagonists (atenolol and CGP-20712A) did not affect either nicotine- or TNS-induced relaxation. Results of double-labeling studies indicated that beta(2)-adrenoceptor immunoreactivities and NADPH diaphorase reactivities were colocalized in the same nerve fibers in basilar and middle cerebral arteries. These findings suggest that NE, which is released from sympathetic nerves upon application of nicotine, acts on presynaptic beta(2)-adrenoceptors located on the NOergic nerve terminals to release NO, resulting in vasodilation. In addition, nicotine-induced relaxation was enhanced by yohimbine, an alpha(2)-adrenoceptor antagonist, which, however, did not affect the relaxation elicited by TNS. Prazosin, an alpha(1)-adrenoceptor antagonist, on the other hand, did not have any effect on relaxation induced by either nicotine or TNS. The predominant facilitatory effect of beta(2)-adrenoceptors in releasing NO may be compromised by presynaptic alpha(2)-adrenoceptors.