Coinfusion Of L-NMMA Research Articles

Nitric oxide contribution to endothelium-mediated vasodilation is attenuated in postmenopausal compared to premenopausal females

Background: Hypertension prevalence increases in females following menopause. Poor endothelial microvascular function increases risk of hypertension. Endothelial-mediated vasodilation is dependent on nitric oxide (NO). Estrogen enhances NO production; therefore, reduced estrogen levels in postmenopausal females may result in a reduced contribution of NO to endothelial-mediated vasodilation, although studies are limited. Herein, we aimed to investigate the contribution of NO to endothelial-mediated vasodilation in premenopausal and postmenopausal females. We hypothesized that postmenopausal females would have attenuated endothelium-mediated vasodilation and reduced contribution of NO compared to premenopausal females. Methods: Ten premenopausal (27±3 yrs) and eleven postmenopausal (59±4 yrs) normotensive females were studied. Resting brachial blood pressures were measured using a standard arm cuff. Brachial arterial catheters were placed for continuous blood pressure measurement and pharmacologic infusions. Forearm blood flow (FBF) was measured using venous occlusion plethysmography at baseline (saline) and during infusion of an endothelium-dependent vasodilator (acetylcholine; 1.0, 2.0, 4.0, and 8.0 μg/100mL tissue/min) with and without a NO synthase inhibitor (L-NG-monomethylarginine [L-NMMA]; 1.0 mg/min) co-infusion. Forearm vascular conductance (FVC) was calculated as FBF/mean arterial pressure✕100. Data were analyzed using a two-way repeated-measures ANOVA. Results: Resting systolic blood pressure (SBP) but not diastolic blood pressure (DBP) was greater in postmenopausal compared to premenopausal females (SBP: 121±15 vs. 103±9 mmHg, respectively; P<0.01; DBP: 72±11 vs. 65±5 mmHg, respectively; P=0.07). FBF responses to acetylcholine did not differ between premenopausal and postmenopausal females (Group Main Effect: P=0.11). When correcting for blood pressure, premenopausal females exhibited greater FVC responses to acetylcholine compared to postmenopausal females (Group Main Effect: P=0.04). In premenopausal females, FBF and FVC responses to acetylcholine were reduced with co-infusion of L-NMMA compared to acetylcholine infusion only (L-NMMA Main Effect: P=0.02 and P=0.01, respectively). In postmenopausal females, the FBF and FVC responses to acetylcholine were unaffected by L-NMMA (L-NMMA Main Effect: P=0.79 and P=0.76, respectively). Conclusion: Compared to premenopausal females, postmenopausal females have impaired endothelial microvascular function. Attenuated NO contribution during endothelium-mediated vasodilation in postmenopausal women may partially contribute to the increased hypertension risk in this population. AHA 14PRE18040000, AHA 898649, NIH HL083947, NIH HL148144, NCATS UL1 TR000135 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Ascorbic acid improves brachial artery vasodilation during progressive handgrip exercise in the elderly through a nitric oxide-mediated mechanism

The proposed mechanistic link between the age-related attenuation in vascular function and free radicals is an attractive hypothesis; however, direct evidence of free radical attenuation and a concomitant improvement in vascular function in the elderly is lacking. Therefore, this study sought to test the hypothesis that ascorbic acid (AA), administered intra-arterially during progressive handgrip exercise, improves brachial artery (BA) vasodilation in a nitric oxide (NO)-dependent manner, by mitigating free radical production. BA vasodilation (Doppler ultrasound) and free radical outflow [electron paramagnetic resonance (EPR) spectroscopy] were measured in seven healthy older adults (69 ± 2 yr) during handgrip exercise at 3, 6, 9, and 12 kg (∼13-52% of maximal voluntary contraction) during the control condition and nitric oxide synthase (NOS) inhibition via N(G)-monomethyl-L-arginine (L-NMMA), AA, and coinfusion of l-NMMA + AA. Baseline BA diameter was not altered by any of the treatments, while L-NMMA and L-NMMA + AA diminished baseline BA blood flow and shear rate. AA improved BA dilation compared with control at 9 kg (control: 6.5 ± 2.2%, AA: 10.9 ± 2.5%, P = 0.01) and 12 kg (control: 9.5 ± 2.7%, AA: 15.9 ± 3.7%, P < 0.01). NOS inhibition blunted BA vasodilation compared with control and when combined with AA eliminated the AA-induced improvement in BA vasodilation. Free radical outflow increased with exercise intensity but, interestingly, was not attenuated by AA. Collectively, these results indicate that AA improves BA vasodilation in the elderly during handgrip exercise through an NO-dependent mechanism; however, this improvement appears not to be the direct consequence of attenuated free radical outflow from the forearm.

Characterization of Local Vascular Effects of the Nitric Oxide Inhibitor NG‐Monomethyl‐L‐Arginine on Dorsal Hand Veins

Infusion of NG-monomethyl-L-arginine (L-NMMA; 6.4 µmol/min) into hand veins can cause a 20% increase in vein size in specific subjects. This study explored potential underlying mechanisms in healthy male participants. Ten healthy male participants received in phenylephrine (PE)-preconstricted veins a dose-response curve (DRC) to L-NMMA (0.2-6.4 µmol/min) without and with coinfusion of the endothelium-dependent dilator histamine, a DRC to L-arginine with and without coinfusion of L-NMMA, a DRC to NG-monomethyl-D-arginine (D-NMMA), and a DRC to L-NMMA in prostaglandin F(2α)-(PGF(2α))-preconstricted veins. Participants were classified as L-NMMA responders (R) and nonresponders (NR). Infusion of L-NMMA resulted in a maximum venodilation of 38% ± 11% (R) versus 10% ± 5% (NR; P = .005). In PGF(2α)-preconstricted veins, L-NMMA caused venodilation to 26% ± 34% (NS) in responders. Results suggest that endothelial nitric oxide synthase-mediated formation of nitric oxide (NO) from L-NMMA in doses >3.2 µmol/min and continuous PE-induced α-adrenergic stimulation resulting in release of very small amounts of NO from L-NMMA contribute to the observed L-NMMA-induced increase in vein size. Venous reactivity to L-NMMA resulting in a phenotype as R or NR is most likely genetically predetermined, which requires further study.

The Fcγ receptor IIA R131H gene polymorphism is associated with endothelial function in patients with hypercholesterolaemia

ObjectiveA gene polymorphism substituting arginine (R) for histidine (H) at position 131 has been described within the Fcγ receptor IIa (FcγRIIa). The R allele is associated with increased binding of CRP and enhanced activation of monocytes. FcγRIIa is also expressed on endothelial cells, and we hypothesized this polymorphism would be associated with alterations of endothelial function. MethodsGenomic DNA was extracted and allele-specific PCR reactions were used to determine the FcγRIIa H131R polymorphism in 78 hypercholesterolaemic subjects. Using strain gauge plethysmography, forearm blood flow (FBF) responses were determined to intra-arterial infusion of acetylcholine (ACH), for endothelium-dependent vasodilatation (EDV), to nitroprusside (NP), for endothelium-independent vasodilatation (EIV), to NG-monomethyl-l-arginine (l-NMMA), for basal NO activity, and to ACH in the presence of l-NMMA, to assess the contribution of NO release to EDV. ResultsHomozygous carriers of the H allele (n=30) had significantly better EDV than homozygous carriers of the R allele (n=15), while heterozygotes showed an intermediate phenotype (n=33) (e.g. % increase of FBF to ACH 48μg/min: 527±359% in H/H versus 452±262% in H/R versus 332±413% in R/R, p=0.0012 by 2-way ANOVA). EIV and basal NO activity were not affected by genotype, and co-infusion of l-NMMA abolished the differences in EDV. ConclusionsThe R allele of the FcγRIIa polymorphism is associated with impaired EDV and reduced NO activity during endothelial cell stimulation. These data suggest that the functional effects of the FcγRIIa H131R gene polymorphism previously observed in vitro translate into clinically relevant alterations of endothelial function in vivo.

Prehypertension Is Associated With Impaired Nitric Oxide-Mediated Endothelium-Dependent Vasodilation in Sedentary Adults

Endothelial vasodilator dysfunction contributes to the development of hypertension (blood pressure (BP) ≥ 140/90 mm Hg) and cardiovascular disease (CVD). Prehypertension (BP 120-139/80-89 mm Hg) has recently been identified as an independent risk factor for hypertension and CVD. It is currently unclear whether BP in the prehypertensive range is associated with endothelial vasodilator dysfunction. We tested the hypothesis that BP in the prehypertensive range, independent of other cardiovascular risk factors, is associated with impaired nitric oxide (NO)-mediated endothelium-dependent vasodilation. Forearm blood flow (FBF) responses to intra-arterial acetylcholine (ACh; 8.0-32.0 µg/100 ml tissue/min) and sodium nitroprusside (SNP; 1.0-4.0 µg/100 ml tissue/min) were measured in 20 normotensive (age: 56 ± 1 years; BP: 110/70 ± 1/2 mm Hg) and 20 prehypertensive (56 ± 2 years; 128/79 ± 2/2 mm Hg) adults. In addition, FBF responses to ACh were determined in the absence and presence of the endothelial NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA) (5 mg/min). FBF responses to ACh were significantly lower (~30%) in prehypertensive (from 4.2 ± 0.3 to 11.4 ± 0.7 ml/100 ml tissue/min) compared with normotensive (from 4.6 ± 0.2 to 14.5 ± 0.7 ml/100 ml tissue/min) adults. There were no group differences in FBF responses to SNP. Co-infusion of L-NMMA significantly reduced the FBF response to ACh in the normotensive (~30%; P <0.05) but not the prehypertensive adults. Prehypertension is associated with impaired NO-mediated endothelium-dependent vasodilation. The endothelial vasodilator dysfunction that characterizes hypertension is present at BP levels in the prehypertensive range and may contribute to the increased risk of hypertension and CVD in this population.

Vasodilator prostanoids compensate for attenuated nitric oxide mediated vasodilation in type 1 diabetes

Background Previous research examining endothelial function and the biochemical pathways mediating vasodilation in type 1 diabetes has been conflicting. Both impaired and preserved nitric oxide (NO) mediated vasodilation have been reported whilst some authors have suggested enhanced vasodilator prostanoid (P) activity. The aim of this study was to determine the relative contributions of NO and P to endothelial function in a homogenous group of type 1 diabetic patients free of other confounding factors that may influence vascular behaviour.^f ^Methods and results Endothelial function was assessed using forearm venous plethysmography in 16 patients with uncomplicated type 1 (duration of diabetes 16.8±2.5 years (mean±SEM), HbA1C7.53±0.21% ) and 15 non-diabetic age and sex matched healthy control subjects. Forearm responses to the endothelium-dependent vasodilator, acetylcholine (ACh) (7.5, 15 and 30 µg/min), were recorded at baseline and after intra-arterial infusion of L-NMMA (a NO synthase inhibitor). Responses to ACh were re-examined following co-infusion of L-NMMA and indomethacin (a cyclo-oxygenase inhibitor). Responses to ACh were calculated as areas under the curve (AUC). At baseline vasodilator responses to ACh were similar (p=0.3) in diabetic and non-diabetic subjects. However, L-NMMA reduced ACh mediated responses to a lesser extent in diabetic subjects than control subjects (3±6% versus 18±3%; p&lt;0.03 respectively). Co-infusion with indomethacin further reduced blood flow, but the relative decrease in AUC was greater in the diabetic group (28±3% vs. 14±3%; p&lt;0.001). The contribution of biochemical pathways other than NO and P were similar in both diabetic and control groups (69±7% vs. 68±4%; p=0.45).Conclusions Vasodilator responses to ACh were unchanged in type 1 diabetes but this was reliant up on enhanced P mediated activity compensating for attenuated NO activity. Furthermore, vasoactive substances in addition to NO and P contribute significantly to vascular tone in both diabetic and non-diabetic subjects.

Rapid Nongenomic Effects of Aldosterone on the Renal Vasculature in Humans

There is increasing evidence for the importance of rapid nongenomic effects of aldosterone on the human vasculature. In vitro animal experiments in renal arterioles also suggest the presence of such effects on the renal vasculature. We conducted a clinical study to explore these effects in vivo in humans. Thirteen healthy male volunteers were examined. Aldosterone (500 microg) or placebo was injected intravenously with or without coinfusion of N(G) monomethyl-L-arginine (L-NMMA) in a randomized, double-blinded 4-fold crossover design. Renal plasma flow and glomerular filtration rate were measured by constant infusion clearance technique using inulin and para-aminohippuric acid. Injection of aldosterone without concomitant infusion of L-NMMA changed the renal plasma flow and glomerular filtration rate not statistically significant compared with placebo. Coinfusion of L-NMMA unmasked the effect of aldosterone: aldosterone with L-NMMA decreased the glomerular filtration rate slightly (-1.4+/-6.2 mL/min), whereas infusion of L-NMMA alone increased the glomerular filtration rate (8.3+/-9.8 mL/min; P=0.004). L-NMMA alone decreased renal plasma flow by 58.2+/-97.5 mL/min, and aldosterone with L-NMMA decreased renal plasma flow by 190.0+/-213.7 mL/min (P=0.074). Accordingly, Aldosterone with L-NMMA increased renal vascular resistance much more than L-NMMA alone (1588+/-237 versus 614+/-240 dynxsxcm(-5); P=0.014). These data indicate that aldosterone acts via rapid nongenomic effects in vivo in humans at the renal vasculature. Antagonizing the endothelial NO synthase unmasks these effects. Therefore, rapid nongenomic aldosterone effects increase renal vascular resistance and thereby mediate arterial hypertension if endothelial dysfunction is present.

Lipid-independent effects of statins on endothelial function and bioavailability of nitric oxide in hypercholesterolemic patients

Experimental evidence suggests a lipid-independent effect of statins on endothelial function and nitric oxide (NO) availability in humans. We investigated whether improvement in NO availability in hypercholesterolemia can be achieved rapidly with statins before lipid-lowering therapy is complete. We studied 41 patients (52 +/- 11 years) with low-density lipoprotein (LDL) cholesterol > or = 130 mg/dL (179 +/- 45 mg/dL) randomly assigned to treatment either with atorvastatin (20 mg/day) or cerivastatin (0.4 mg/day). Endothelium-dependent vasodilation of the forearm vasculature was measured by plethysmography and intra-arterial infusion of acetylcholine (ACh) after 3 days (n = 18) and 14 days (n = 39) of treatment. NO availability and oxidative stress were assessed by coinfusion of l-NMMA and vitamin C. After 3 days of treatment, LDL-cholesterol decreased by 11.9% with a further decrease to 29.6% after 14 days ( P < .001). Endothelium-dependent vasodilation improved by +46.7% after 3 days of statin therapy compared with before therapy (ACh 48 microg/min: +15.7 +/- 10.6 vs +10.7 +/- 10.8 mL/min per 100 mL, P < .05). No further improvement in endothelium-dependent vasodilation (+42.7% compared with before therapy) could be demonstrated after 14 days of treatment (ACh 48 microg/min: +17.7 +/- 10.3 vs +12.4 +/- 9.3 mL/min per 100 mL before therapy, P < .001). Coinfusion of ACh plus vitamin C was able to improve endothelium-dependent vasodilation before but not after 3 or 14 days of statin therapy either. The improvement in endothelium-dependent vasodilation after therapy was no longer observed when the NO-synthase inhibitor l-NMMA was coinfused together with ACh. Short-term lipid-lowering therapy with statins is able to improve endothelial function and NO availability almost completely after 3 days in hypercholesterolemic patients probably by decreasing oxidative stress. This improvement seems to be more rapid than the accompanying decline in LDL-cholesterol and not related to these lipid changes. This finding can support the concept of lipid-independent effects of statins in humans.

Methodological validity and feasibility of the nitric oxide clamp technique for nitric oxide research in human resistant vessels.

N(G)-methyl-L-arginine (L-NMMA) has been widely used for nitric oxide (NO) research, particularly for the assessment of NO-dependent vasodilatation evoked by agonists. However, such experiments may not be straightforward because L-NMMA causes vasoconstriction, which itself must non-specifically affect responses to any vasoactive agents. Therefore, in order to more accurately estimate the roles of NO in human vessels in vivo, we developed an NO clamp technique that uses co-infusion of an NO donor with L-NMMA. To assess the validity and feasibility of this technique, we compared the effects of intra-arterial infusion of L-NMMA on the forearm blood flow responses to vasodilators with and without the NO clamp technique in healthy males. All drugs were intra-arterially infused and changes in forearm blood flow (FBF) were measured by strain-gauge plethysmography. Vasodilatation evoked by atrial natriuretic peptide was significantly attenuated by L-NMMA alone (p = 0.001) but not by the NO clamp technique. L-NMMA significantly attenuated the responses to acetylcholine either with or without the NO clamp technique. However, the ratio of the area under the curve (AUC) of acetylcholine with L-NMMA to that without L-NMMA was significantly higher when the NO clamp technique was not used (AUC ratio: 0.62 +/- 0.13 vs. 0.48 +/- 0.14, respectively; p = 0.031). The contribution of NO to the FBF responses to vasodilators may be more properly assessed by the co-infusion of L-NMMA with the NO clamp technique than by L-NMMA alone. Our NO clamp technique thus appears to be valid and feasible for human NO research.

Relative contribution of vasodilator prostanoids, NO, and KATP channels to human forearm metabolic vasodilation.

Isolated ATP-sensitive K(+) (K(ATP)) channel inhibition with glibenclamide does not alter exercise-induced forearm metabolic vasodilation. Whether forearm metabolic vasodilation would be influenced by K(ATP) channel inhibition in the setting of impaired nitric oxide (NO)- and prostanoid-mediated vasodilation is unknown. Thirty-seven healthy subjects were recruited. Forearm blood flow (FBF) was assessed using venous occlusion plethysmography, and functional hyperemic blood flow (FHBF) was induced by isotonic wrist exercise. Infusion of N(G)-monomethyl-l-arginine (l-NMMA), aspirin, or the combination reduced resting FBF compared with vehicle (P < 0.05). Addition of glibenclamide to l-NMMA, aspirin, or the combination did not further reduce resting FBF. l-NMMA decreased peak FHBF by 26%, and volume was restored after 5 min (P < 0.05). Aspirin reduced peak FHBF by 13%, and volume repaid after 5 min (P < 0.05). Coinfusion of l-NMMA and aspirin reduced peak FHBF by 21% (P < 0.01), and volume was restored after 5 min (P < 0.05). Addition of glibenclamide to l-NMMA and aspirin did not further decrease FHBF. Vascular K(ATP) channel blockade with glibenclamide does not affect resting FBF or FHBF in the setting of NO and vasodilator prostanoid inhibition.

Nitric oxide regulates local arterial distensibility in vivo.

Arterial stiffness is an important determinant of cardiovascular risk. Several lines of evidence support a role for the endothelium in regulating arterial stiffness by release of vasoactive mediators. We hypothesized that nitric oxide (NO) acting locally regulates arterial stiffness in vivo, and the aim of this experiment was to test this hypothesis in an ovine hind-limb preparation. All studies were conducted in anesthetized sheep. Pulse wave velocity (PWV) was calculated by the foot-to-foot methodology from 2 pressure waveforms recorded simultaneously with a high-fidelity dual pressure-sensing catheter placed in the common iliac artery. Intra-arterial infusion of N(G)-monomethyl-L-arginine (L-NMMA) increased iliac PWV significantly, by 3+/-2% (P<0.01). Infusion of acetylcholine and glyceryl trinitrate reduced PWV significantly, by 6+/-4% (P=0.03) and 5+/-2% (P<0.01), respectively. Only the effect of acetylcholine, however, was significantly inhibited during coinfusion of L-NMMA (P=0.03). There was no change in systemic arterial pressure throughout the studies. Importantly, infusion of L-NMMA or acetylcholine distal to the common iliac artery (via the sheath) did not affect PWV. These results demonstrate, for the first time, that basal NO production influences large-artery distensibility. In addition, exogenous acetylcholine and glyceryl trinitrate both increase arterial distensibility, the former mainly through NO production. This may help explain why conditions that exhibit endothelial dysfunction are also associated with increased arterial stiffness. Therefore, reversal of endothelial dysfunction or drugs that are large-artery vasorelaxants may be effective in reducing large-artery stiffness in humans, and thus cardiovascular risk.

Assessment of endothelial function of the renal vasculature in human subjects

Background l-Arginine, the substrate of nitric oxide (NO) synthase, and N G-monomethyl- l-arginine (L-NMMA), a competitive inhibitor of endothelial NO synthase, are used to analyze endothelial function of the renal vasculature. However, little is known about the appropriate dose of l-arginine to be used and the duration of action of l-arginine and L-NMMA. Methods Twenty-nine healthy male subjects (age, 27 ± 1 years) were examined. In protocol 1 ( N = 17), l-arginine at low (100 mg/kg) and high dose (250 mg/kg), and high-dose l-arginine combined either with L-NMMA (total dose, 4.25 mg/kg; N = 9) or placebo ( N = 8) were given. In protocol 2 ( N = 12), L-NMMA was given before l-arginine infusion (100 mg/kg). Glomerular filtration rate (GFR) and renal plasma flow (RPF) were measured at rest and at the end of each infusion step. Results In protocol 1, l-arginine dose dependently increased RPF and GFR (RPF: 599 ± 19 v 630 ± 18 v 690 ± 24 mL/min, P <.05; GFR: 111 ± 3 v 115 ± 3 v 121 ± 3 mL/min, P <.01; for baseline, l-arginine 100 mg/kg and 250 mg/kg, respectively). However, these changes could not be antagonized by coinfusion of L-NMMA to l-arginine 250 mg/kg: RPF and GFR remained unchanged in both the placebo and the L-NMMA group. In protocol 2, L-NMMA decreased RPF (492 ± 18 v 567 ± 27 mL/min, P <.01) and increased GFR (122 ± 4 v 118 ± 3 mL/min, P <.05). These changes could only be partially reversed by subsequent infusion of l-arginine (RPF: 533 ± 15 mL/min; GFR: 121 ± 4 mL/min; both parameters P = NS v L-NMMA and v baseline). Conclusions l-arginine at a dose of 100 mg/kg is sufficient to analyze endothelial function of the renal vasculature. The prolonged effect of L-NMMA and l-arginine must be taken into account in study protocols using both substances. Thus, stimulation and blockade of NO synthase cannot be examined in the same protocol.

Improvement in Endothelial Function by Angiotensin-converting Enzyme Inhibition and Vitamin C in Essential Hypertension

Background and Objectives:The endothelium plays an important role in maintaining vascular tone and function. This essential hypertension may be associated with alterations in endothelial function. The effects of antihypertensive agents on endothelial function have not been fully evaluated in human hypertension and data on the forearm circulation of humans is controversial. The aim of this study was 1 to evaluate endothelial function in hypertensive patients 2 to investigate whether vitamin C administration has a benefit on endothelial function and 3 to determine whether treatment with ACE inhibitor improve improves endothelial dysfunction in hypertensive patients. Materials and Methods:The endothelial function was estimated using venous occlusion plethysmography (VOP in 8 hypertensive patients and 8 healthy volunteers. The patients in the hypertension group were treated with enalapril, then reexamined. The change of forearm blood flow (FBF was measured under conditions of acetylcholine infusion through the brachial artery as well as with intra- arterial vitamin C. The measurement of forearm volume change was repeated 7 times for each stage. Results:The forearm blood flow response to acetylcholine was significantly enhanced with inra-arterial infusion of vitamin C in the hypertensive group before antihypertensive treatment (302±58%→446±43%. Co-infusion of L-NMMA, an inhibitor of nitric oxide synthase, the blunted forearm blood flow response to acetylcholine (Vit C (+; 446±43%→Vit C+L-NMMA;229±23%. After Following antihypertensive treatment with enalapril for 2 months in the hypertensive group, endothelium-dependent vasorelaxation (vasodilatory response to acetylcholine was significantly improved in the treated group as compared to mea- surements taken before enalapril treatment (302±58%→643±78%. Conclusions:Although the mechanisms leading to depressed endothelial function in essential hypertension remains to be elucidated, our study shows

Reversal of glibenclamide-induced coronary vasoconstriction by enhanced perfusion pulsatility: possible role for nitric oxide.

ATP-sensitive potassium channels (K+ATP) prominently contribute to basal coronary tone; however, flow reserve during exercise remains unchanged despite channel blockade with glibenclamide (GLI). We hypothesized that increasing perfusion pulsatility, as accompanies exercise, offsets vasoconstriction from K+ATP-channel blockade, and that this effect is blunted by nitric oxide synthase (NOS) inhibition. In 31 anaesthetized dogs the left anterior descending artery was blood-perfused by computer-controlled servo-pump, with real-time arterial perfusion pulse pressure (PP) varied from 40 and 100 mm Hg at a constant mean pressure and cardiac workload. At control PP (40 mm Hg), GLI (50 micrograms/min/kg, i.c.) lowered mean regional coronary flow from 37 +/- 5 to 25 +/- 4 ml/min (P < 0.001). However, this was not observed at 100 mm Hg PP (41 +/- 2 vs. 45 +/- 4). NOS inhibition by NG-monomethyl-L-arginine (L-NMMA) did not alter basal flow at 40 mm Hg PP, but modestly lowered flow (-5%, P < 0.001) at higher PP (100 mm Hg), reducing PP-flow augmentation by -36%, and acetylcholine (ACh) induced flow elevation by -39%. Co-infusion of L-NMMA with GLI resulted in net vasoconstriction at both PP levels (-60% and -40% at 40 and 100 mm Hg PP, respectively). Unlike GLI, vasoconstriction by vasopressin (-43 +/- 3% flow reduction at 40 mm Hg PP) or quinacrine (-23 +/- 7%) was not offset at higher pulsatility (-44 +/- 4 and -23 +/- 6%, respectively). Neither of the latter agents inhibited ACh- or PP-induced flow responses, nor did they modify the effect of L-NMMA on these responses. Increased coronary flow pulsatility offsets vasoconstriction from K+ATP blockade by likely enhancing NO release. This mechanism may assist exercise-mediated dilation in settings where K+ATP opening is partially compromised.

NG-monomethyl-L-arginine inhibits the blood flow but not the insulin-like response of forearm muscle to IGF- I: possible role of nitric oxide in muscle protein synthesis.

In human skeletal muscle, insulin-like growth factor-I (IGF-I) exerts both growth hormone-like (increase in protein synthesis) and insulin-like (decrease in protein degradation and increase in glucose uptake) actions and augments forearm blood flow two- to threefold. This study was designed to address whether (a) the increase in blood flow due to IGF-I could be blocked by an inhibitor of nitric oxide synthase; and (b) the metabolic actions of IGF-I were altered by use of a nitric oxide synthase inhibitor. Forearm blood flow, glucose, lactate, oxygen, nitrite, and phenylalanine balances and phenylalanine kinetics were studied in a total of 17 healthy, adult volunteers after an overnight fast in two different protocols. In protocol 1, after basal samples IGF-I was infused alone for 4 h with samples repeated during the last 30 min. After the 4-h sample period, NG-monomethyl-L-arginine (L-NMMA) was infused into the brachial artery for 2 h to bring flow back to baseline and repeat samples were taken (6 h). In response to IGF-I alone, forearm blood flow rose from 3.8 +/- 1.0 (bas) to 7.9 +/- l.9 (4 h) ml/min/100 ml (P < 0.01) and was reduced back to baseline by L-NMMA at 6 h (P < 0.01). In protocol 1, IGF-I alone increased forearm nitrite release at 4 h (P < 0.03), which was reduced back to baseline by L-NMMA at 6 h (P < 0.05). Despite the reduction in flow with L-NMMA, IGF+L-NMMA yielded increases in glucose uptake (P < 0.005), lactate release (P < 0.04), oxygen uptake (P < 0.01), and a positive shift in phenylalanine balance (P < 0.01) due to both an increase in muscle protein synthesis (P < 0.02) and a decrease in protein degradation (P < 0.03). In protocol 2, L-NMMA was coinfused with IGF-I for 6 h, with the dose titrated to keep blood flow +/- 25% of baseline. Coinfusion of L-NMMA restrained blood flow to baseline and also yielded the same, significant metabolic effects, except that no significant increase in muscle protein synthesis was detected. These observations suggest: (a) that IGF-I increases blood flow through a nitric oxide-dependent mechanism; (b) that total blood flow does not affect the insulin-like response of muscle to IGF-I; and (c) that nitric oxide may be required for the protein synthetic (growth hormone-like) response of muscle to IGF-I.

Differential inhibition by NG‐monomethyl‐l‐arginine of vasodilator effects of acetylcholine and methacholine in human forearm vasculature

1. We compared the effects of NG-monomethyl-L-arginine (L-NMMA), an NO synthase inhibitor, on vasodilatation produced by acetylcholine and methacholine in human forearm vasculature. 2. Acetylcholine (83 nmol min-1) infused into the brachial artery of 8 healthy volunteers caused a submaximal increase in forearm blood flow, measured by venous occlusion plethysmography, from 3.3 +/- 0.5 (mean +/- s.e. mean) to 13.3 +/- 1.7 ml min-1 100 ml-1. 3. Co-infusion of L-NMMA (4 mumol min-1) with acetylcholine (83 nmol min-1) over 6 min resulted in a 58% +/- 12% fall in the response to acetylcholine whereas during co-infusion of saline over the same time period in the same subjects (n = 8) on a different day the response to acetylcholine fell by only 9% +/- 17% (P < 0.01). 4. Methacholine (1.5 and 15 nmol min-1) increased forearm blood flow from 2.5 +/- 0.4 to 5.9 +/- 0.9 and from 3.2 +/- 0.4 to 17.0 +/- 1.9 ml min-1 100 ml-1 respectively. 5. Co-infusion of L-NMMA (4 mumol min-1) had no significant effect on the response to methacholine (1.5 or 15 nmol min-1) when compared with saline control (n = 8). Co-infusion of a higher dose of L-NMMA (8 mumol min-1) with methacholine (1.5 nmol min-1) did not significantly inhibit the vasodilator response (n = 7). 6. These results suggest that, in human forearm vasculature, methacholine acts predominantly through mechanisms other than the L-arginine/nitric oxide pathway.