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(Circulation. 1997;95:1783-1790.)
© 1997 American Heart Association, Inc.

Articles

Vascular Effects of Acute Hyperglycemia in Humans Are Reversed by L-Arginine

Evidence for Reduced Availability of Nitric Oxide During Hyperglycemia

Dario Giugliano, MD; Raffaele Marfella, MD; Ludovico Coppola, MD; Giovanni Verrazzo, MD; Rita Acampora, MD; Riccardo Giunta, MD; Francesco Nappo, MD; Carmela Lucarelli, MD; Felice D'Onofrio, MD

From the Department of Geriatrics and Metabolic Diseases (D.G., R.M., L.C., G.V., R.A., R.G., F.N., F.D.) and Institute of Pharmacology and Toxicology (C.L.), Second University of Naples, Italy.

Correspondence to Prof D. Giugliano, MD, Via Emilia 1, 80021 Afragola (NA), Italy.

	Abstract

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Background Acute hyperglycemia may increase vascular tone in normal humans via a glutathione-sensitive, presumably free radical–mediated pathway. The objective of this study was to investigate whether or not the vascular effects of hyperglycemia are related to reduced availability of nitric oxide.

Methods and Results Acute hyperglycemia (15 mmol/L, 270 mg/dL) was induced in 12 healthy subjects with an artificial pancreas. Systolic and diastolic blood pressures, heart rate, and plasma catecholamines showed significant increases (P<.05) starting after 30 minutes of hyperglycemia; leg blood flow decreased significantly (15%; P<.05) at 60 and 90 minutes. Platelet aggregation to ADP and blood viscosity also showed significant increments (P<.05). The infusion of L-arginine (n=7, 1 g/min) but not D-arginine (n=5, 1 g/min) or L-lysine (n=5, 1 g/min) in the last 30 minutes of the hyperglycemic clamp completely reversed all hemodynamic and rheological changes brought about by hyperglycemia. Infusion of N^G-monomethyl-L-arginine (L-NMMA; 2 mg/min) to inhibit endogenous nitric oxide synthesis in 8 normal subjects produced vascular effects qualitatively similar to those of hyperglycemia but quantitatively higher (P<.05); however, heart rate and plasma catecholamine levels decreased during L-NMMA infusion, presumably as a consequence of baroreflex activation. Infusion of L-NMMA during hyperglycemia produced changes not different from those obtained during infusion of L-NMMA alone.

Conclusions The results show that acute hyperglycemia in normal subjects causes significant hemodynamic and rheological changes that are reversed by L-arginine. Moreover, the effects of hyperglycemia are mimicked to a large extent, but not entirely, by infusion of L-NMMA. This suggests that hyperglycemia may reduce nitric oxide availability in humans.

Key Words: hyperglycemia • hemodynamics • rheology • amino acids

	Introduction

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Cardiovascular disease is the leading cause of mortality in patients with diabetes mellitus.¹ Recent prospective studies indicate that glycemic control of diabetes is an important predictor not only of microvascular disease² but also of macrovascular complications, including coronary heart disease,³ peripheral arterial disease,⁴ and lower-extremity amputation.⁵ Moreover, hyperglycemia predicted development of hypertension in women in both the Framingham Study⁶ and the San Antonio Heart Study.⁷

There are several rather compelling reasons why hyperglycemia should be regarded as a risk factor for heart disease.⁸ Development of atherosclerosis based on metabolic disorders is preceded by a relative impairment of endothelium-dependent relaxation, believed to be mediated by NO.⁹ This endothelial dysfunction develops quickly (within minutes or hours) in conditions of hyperglycemia,^{10 11} which makes it unlikely that the mediation of advanced glycosylation end products may quench NO,¹² at least in short-term experiments. On the basis of the finding that a number of free radical scavengers may prevent endothelial dysfunction brought about by hyperglycemia, ¹³ it has been suggested that an increased generation of free radicals from endothelial cells during hyperglycemia is more likely to come into play. Further support for this interpretation comes from recent studies¹⁴ in normal humans in whom acute hyperglycemia produced relevant hemodynamic changes via a glutathione-sensitive, presumably free radical–mediated pathway.

The aim of the present study was to assess whether the increased vascular tone observed during acute hyperglycemia in humans might be related to reduced NO availability. The answers to this question were sought in normal subjects by (1) increasing the availability of L-arginine, the natural precursor of NO formation,¹⁵ and (2) comparing the vascular effects of hyperglycemia with those of L-NMMA, an inhibitor of NO synthesis.¹⁶

	Methods

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Patients
Twenty nonobese subjects (age, 28±1.1 years; 10 men and 10 women; body mass index, 24±0.3 kg/m²; waist-to-hip circumference, 0.82±0.015 [mean±SE]) recruited from the medical and paramedical staff of the Department of Geriatrics and Metabolic Diseases, Second University of Naples, Italy, gave informed consent to participate in this study after they were given a clear explanation of its experimental nature. All had a negative family history of diabetes and hypertension; they were screened by clinical history, physical examination, ECG, chest radiography, and routine chemical analyses and had no evidence of present or past hypertension, hyperlipidemia, cardiovascular disease, diabetes, or any systemic conditions. The subjects were on a weight-maintaining diet with 250 g of carbohydrates per day and had no recent change in body weight or intercurrent illness. None was engaged in physical activity of >3 h/wk. The protocol of the study was approved by the ethics committee of our institution.

Study Protocol
After a 12-hour overnight fast, subjects were placed in a supine comfortable position with a room temperature between 20°C and 24°C. All subjects were instructed to refrain from smoking and from drinking alcoholic beverages or coffee from the night before the study. Intravenous lines were inserted into a large antecubital vein of one arm for infusions and into a dorsal vein of the contralateral arm for blood sampling. Patency was preserved by a slow saline infusion (0.9% NaCl). The subjects were then instrumented for automatic measurements of blood pressure and heart rate.

The study began after the subjects had rested for 30 minutes and after three consecutive measurements of blood pressure and heart rate differing by <5% were recorded.

Study 1
Twelve subjects underwent the following tests in random order and separated by at least a 3-day interval:

(1) Hyperglycemic glucose clamp in which plasma glucose concentrations were acutely raised with a bolus injection of 0.33 g/kg glucose followed by a variable 30% glucose infusion to achieve steady-state plasma glucose levels of 15 mmol/L for 90 minutes. To prevent hypokalemia, 0.26 mmol/L KCl was added to glucose. The test was performed with the aid of an artificial pancreas (Biostator, Life Science, Miles).

(2) The hyperglycemic clamp was repeated on another occasion with the exception that arginine or L-lysine (30 g infused at the rate of 1 g/min) was infused for 60 to 90 minutes. Seven subjects received L-arginine, five subjects received D-arginine, and five received L-lysine. Thus, five subjects were studied three times. Both the subject and the investigator were blinded to the particular amino acid given. L-Arginine monohydrochloride was purchased from Damor Pharmaceuticals as a ready-to-use 30% solution dissolved in saline (0.9% NaCl); D-arginine monohydrochloride was purchased from Clinalfa and L-lysine monohydrochloride from Calbiochem.

Study 2
Eight subjects were studied three times, on separate days and in random order. Once, they were submitted to a hyperglycemic clamp as described above; on another day, they received a 90-minute infusion of L-NMMA (acetate salt, Clinalfa; 2 mg/min), an inhibitor of NO synthesis. Control saline studies were performed in four subjects (two men and two women), while the other four received the infusion of L-NMMA during the hyperglycemic clamp. D-Arginine and L-NMMA solutions were freshly prepared on the morning of the test day with saline (0.9% NaCl) used as vehicle.

Hemodynamics
Heart rate and finger arterial pressure were measured by use of a noninvasive technique (Finapres, Ohmeda 2300) that uses the unloaded principle that has been shown to be as accurate as intra-arterial blood pressure measurements.¹⁷ Data were elaborated by a software program that allowed systolic, diastolic, and mean blood pressures and heart rate to be expressed in graphs. Blood flow in the femoral artery was determined by image-directed duplex ultrasonography combining B-mode imaging and pulsed Doppler beams (Apogeé CX 200, Interspec ATL). Blood flow volumes were automatically calculated as the vessel cross-sectional area multiplied by the time-averaged volume from five repeated measurements for each volume flow estimation. Blood flow did not differ between the two legs, and accordingly, pooled data are presented.

Rheology
Platelet aggregation was determined according to Born.¹⁸ Briefly, PRP was obtained by centrifuging each blood sample anticoagulated with sodium citrate at 200g for 10 minutes, and PPP was prepared by centrifuging the remaining volume of blood at 2000g for 10 minutes. The aggregometer was adjusted before each test so that the light transmission was 0% for PRP and 100% for PPP. Aggregation was induced with a final concentration of 1.25 µmol/L ADP. Aliquots of whole blood anticoagulated with 0.77 mol/L EDTA (ratio of blood to EDTA was 1:20) were used to assess blood viscosity at different rates of shear (225 and 45 s^-1) using a Brookfield Digital Viscosimeter (Brookfield Engineering Laboratories). Hematocrit values were determined by centrifuging blood samples in glass capillary tubes for 5 minutes at 5000g. All determinations were made in duplicate by a person who was blinded to subjects and treatments. Coefficients of variation were 4% for blood viscosity and 5% for platelet aggregation.

In separate experiments, we tested the effects of L-arginine, D-arginine, L-NMMA, and glucose on ADP-induced platelet aggregation in PRP from euglycemic subjects. The final concentrations of the drugs were 4.75 mmol/L for both L-arginine and D-arginine, 15 mmol/L for glucose, and 2.00 mmol/L for L-NMMA.

Analyses
Samples for analysis of plasma glucose were collected in tubes containing a trace of sodium fluoride and those for insulin in tubes containing a mixture (0.1 mL/mL of blood) of EDTA-Trasylol solution (500 U/mL Trasylol, Bayer; 1.2 g/L disodium EDTA). Blood samples for plasma catecholamines were collected in ice-cold heparinized tubes, and the plasma was separated within 120 minutes and stored at -70°C until assayed. Plasma glucose was determined by the glucose oxidase method using an autoanalyzer (Beckman). Plasma insulin levels were determined by radioimmunoassay.¹⁹ Plasma catecholamines were measured with high-pressure liquid chromatography after extraction with alumina²⁰ ; in our laboratory, the assay has a detection limit of 20 ng/mL, with intra-assay and interassay coefficients of variation of 8.1% and 8.8%, respectively.

Statistical Analysis
Results are given as mean±SE. One-way ANOVA was used to compare baseline data followed by Scheffé's test for multiple comparisons to allow pairwise testing for significant differences between groups. To detect differences within and between the groups in response to the various agents used, a two-way ANOVA for repeated measures was performed followed by a pairwise multiple-comparison procedure (Student-Newman-Keuls test) to locate the significant difference indicated by ANOVA. A value of P<.05 was considered significant.

	Results

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Hyperglycemic Clamp
In 12 control subjects, fasting glucose and insulin levels were 4.9±0.2 mmol/L (89.0±3.6 mg/dL) and 45±10 pmol/L (6.4±1.5 µU/mL), respectively. During the clamp, plasma glucose stabilized at 15 mmol/L (270 mg/dL) (Table 1). A biphasic pattern of insulin release was observed with an early rise at 10 minutes (315±56 pmol/L; 45±8 µU/mL) followed by a gradual and sustained increase. Systolic and diastolic blood pressures and heart rate increased during the clamp and remained significantly elevated above baseline until the end of the infusion starting from 40 minutes of infusion (Fig 1). Leg blood flow was 0.275±0.025 L/min in the basal state and decreased significantly at 60 and 90 minutes (0.247±0.025 and 0.238±0.024 L/min; P<.05).

View this table: [in this window] [in a new window]   Table 1. Effects of Acute Hyperglycemia and L-Arginine or D-Arginine Superimposed During the Last 30 Minutes of a Hyperglycemic Clamp on ADP (1.25 µmol/L)-Stimulated Platelet Aggregation, Blood Viscosity, Plasma Glucose, Insulin, and Catecholamine Levels in Healthy Subjects

View larger version (22K): [in this window] [in a new window]   Figure 1. Effects of acute hyperglycemia (plasma glucose clamped at 15 mmol/L, 270 mg/dL) on systolic and diastolic blood pressures, heart rate, and leg blood flow in 12 healthy subjects (control study). In the same subjects on another occasion, an infusion of L-arginine (n=7, 1 g/min) was superimposed during the last 30 minutes of the clamp. Values are given as mean±SE. *Significant differences between the control study and the L-arginine study (P<.05, two-way ANOVA for repeated measures).

Plasma norepinephrine rose from a basal value of 146±12 to 170±13 (P<.05) and 175±14 ng/L (P<.05) after 60 and 90 minutes of the clamp, respectively. Plasma epinephrine rose from a basal value of 51±5 to 65±6 and 72±6 ng/L, respectively (P<.05 for both) (Table 1).

Platelet aggregation and blood viscosity showed significant increases during the clamp (Table 1).

Hyperglycemic Clamp Plus Arginine or Lysine
Basal metabolic, hemodynamic, and rheological parameters were not significantly different from those obtained in the same subjects during the clamp alone, nor was there any difference in the changes obtained during the first 60 minutes of the clamp (Figs 1 and 2; Table 1). When L-arginine was infused in the last 30 minutes of the clamp in seven normal subjects, there was a rapid fall in blood pressure levels (systolic and diastolic) already evident 10 minutes after L-arginine infusion was begun and persisting throughout the entire infusion (P<.001 versus 60-minute value). Leg blood flow was 0.208±0.021 L/min 60 minutes after glucose infusion and increased to 0.325±0.042 L/min at 90 minutes (P<.01). Heart rate and plasma catecholamine levels recorded during hyperglycemic clamp were not modified by L-arginine infusion (Fig 1 and Table 1). Platelet aggregation and blood viscosity showed a significant decrease after L-arginine infusion was added (Table 1).

View larger version (21K): [in this window] [in a new window]   Figure 2. Effects of acute hyperglycemia (plasma glucose clamped at 15 mmol/L, 270 mg/dL) on systolic and diastolic blood pressures, heart rate, and leg blood flow in healthy subjects. An infusion of D-arginine (n=5, 1 g/min) or L-lysine (n=5, 1 g/min) was superimposed during the last 30 minutes of the clamp. Values are given as mean±SE.

No significant changes in blood pressure, heart rate, or leg blood flow occurred when D-arginine or L-lysine was infused in the last 30 minutes of the clamp (Fig 2). Similarly, platelet aggregation, blood viscosity, and plasma catecholamine levels were not influenced by the infusion of D-arginine (Table 1) or L-lysine (data not shown).

Infusion of L-NMMA
Infusion of L-NMMA in eight normal volunteers increased both systolic and diastolic blood pressure levels, which was significant starting from the 10th minute of the infusion and persisted throughout the entire infusion. Both systolic and diastolic blood pressures increased during the hyperglycemic clamp, and the rise was significant versus baseline values starting from the 40th minute. The rise in blood pressure during L-NMMA was more pronounced and occurred significantly earlier than that occurring during hyperglycemia in the same subject, with a significant difference between treatments (Fig 3). Leg blood flow decreased significantly below baseline with both treatments, with significant differences in favor of L-NMMA at 30 and 60 minutes (Fig 3). There was an opposing effect on heart rate, which increased during hyperglycemia and decreased during L-NMMA infusion. Similarly, plasma catecholamines decreased during L-NMMA infusion (epinephrine: 42±4, 34±4, and 30±4 ng/L at baseline, 60 minutes, and 90 minutes, respectively [P<.05]; norepinephrine: 196±16, 181±14, and 179±14 ng/L at baseline, 60 minutes, and 90 minutes, respectively [P<.05]) (Table 2).

View larger version (21K): [in this window] [in a new window]   Figure 3. Effects of acute hyperglycemia (plasma glucose clamped at 15 mmol/L, 270 mg/dL) or L-NMMA (2 mg/min) on hemodynamic parameters in eight normal subjects. The hyperglycemic clamp and systemic L-NMMA infusion lasted 90 minutes. Values are given as mean±SE. *Significant differences between treatments (P<.05, two-way ANOVA for repeated measures).

View this table: [in this window] [in a new window]   Table 2. Effects of Hyperglycemia, L-NMMA, and Hyperglycemia Plus L-NMMA on ADP (1.25 µmol/L)-Stimulated Platelet Aggregation, Blood Viscosity, Plasma Glucose, Insulin, and Catecholamine Levels in Healthy Subjects

The effects of L-NMMA on platelet aggregation, blood viscosity, plasma glucose, and insulin concentrations are given in Table 2. Significant increases of both platelet aggregation and blood viscosity occurred with both treatments (hyperglycemia and L-NMMA). Infusion of L-NMMA produced no significant effect on glucose and insulin levels.

The infusion of L-NMMA during the hyperglycemic clamp produced hemodynamic and rheological effects that were not significantly different from those occurring during infusion of L-NMMA alone (Table 2). In particular, maximal increases of either systolic or diastolic blood pressure were 9.0±1.5 and 7.1±1.1 mm Hg, respectively, during infusion of L-NMMA alone, and 7.8±1.8 and 5.9±1 mm Hg during the combined infusion of glucose and L-NMMA (P=NS). Heart rate and leg blood flow decreased by a maximal value of 6.2±1.3 bpm and 0.06±0.01 L/min, respectively, during infusion of L-NMMA alone, and by 5.3±1.1 bpm and 0.048±0.01 L/min during infusion of glucose plus L-NMMA (P=NS).

No side effects were reported by the subjects or observed by the investigators during or after the tests.

In Vitro Studies
The effects of L-arginine, D-arginine, glucose, and L-NMMA on platelet aggregation in vitro in samples of PRP from seven euglycemic subjects are reported in Table 3. A significant decrease in ADP-induced platelet aggregation was observed when L-arginine was added to the PRP sample, but only after 10 minutes of incubation. D-Arginine and high levels of glucose had no significant effects. Incubation of PRP samples with L-NMMA produced significant enhancement of platelet aggregation at all incubation times.

View this table: [in this window] [in a new window]   Table 3. Effects of L-Arginine, D-Arginine, Glucose, and L-NMMA on Platelet Aggregation In Vitro in PRP

	Discussion

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We found that short-term elevation of plasma glucose levels in normal subjects produced significant increases in systolic and diastolic blood pressures, heart rate, and plasma catecholamine levels and decreased leg blood flow, suggesting vasoconstriction. These hemodynamic changes are unlikely to be mediated by the simultaneous increase of plasma insulin, which may stimulate sympathetic noradrenergic activity,²¹ because blockade of insulin secretion with the somatostatin analogue octreotide does not influence the vascular effect of hyperglycemia.¹⁴ Similar findings have been obtained by Williams et al,²² who reported that local forearm hyperglycemia (300 mg/dL for 6 hours) attenuated endothelium-dependent vasodilatation in healthy humans; again, the hyperglycemia-induced decrease in forearm blood flow was not affected by simultaneous octreotide infusion. The possibility that hyperglycemia may increase blood pressure due to its osmotic effect cannot be excluded; however, glutathione, a free radical scavenger that lowers oxidative stress in normal humans,²³ completely prevents the hemodynamic changes brought about by hyperglycemia.¹⁴ This led us to hypothesize that as occurs in animal studies,^{10 11 12 13} acute hyperglycemia may increase the production of free radicals, which may quench NO, reducing its availability for target cells.

Endothelium-derived NO is now recognized as the most potent vasodilating substance²⁴ and is formed from the semiessential amino acid L-arginine by the constitutive form of NOS. We reasoned that increasing the availability of the precursor L-arginine might counterbalance the reduced availability of NO brought about by hyperglycemia. We found that intravenous L-arginine infusion in normal subjects superimposed with ongoing hyperglycemia completely normalized both the increase in blood pressure values and the decrease in leg blood flow. The drop in blood pressure levels that occurred during L-arginine infusion might have triggered sympathetic drive, thus preventing normalization of heart rate and plasma catecholamine levels. The dose of L-arginine used in the present study has been shown to produce significant hemodynamic changes in normal humans under euglycemic conditions.²⁵ Systemic L-arginine administration in normal subjects has been found to increase the urinary excretion of cGMP and NO₃^- (the stable end product of NO metabolism),²⁵ plasma cGMP levels,²⁶ and exhaled NO,^{27 28} suggesting that the amount of L-arginine for NOS may be rate limiting and stereospecific for the formation of endogenous NO. Supporting this interpretation, similar doses of D-arginine, which is not used for endogenous NO synthesis,²⁴ and L-lysine, which uses the same cationic amino acid transport system as L-arginine,²⁹ have no effect on hyperglycemia-induced vascular changes. On the other hand, we cannot exclude the possibility that part of the effect of L-arginine to increase NO availability may be mediated by the simultaneous increase in endogenous insulin that may activate the NO system.²¹

Acute hyperglycemia also induced significant increases of platelet aggregation and blood viscosity. Both effects were counteracted by L-arginine but not D-arginine. To the best of our knowledge, this is the first report showing an effect of L-arginine on blood viscosity, which is now regarded as an important cardiovascular risk factor.³⁰ Interestingly, inhibition of platelet aggregation and blood viscosity has recently been shown in normal humans during glyceryl trinitrate administration,³¹ the vascular effects of which are thought to derive from metabolic conversion to NO.³² Moreover, NO is known to inhibit platelet aggregation.³³ All this does not constitute direct proof of stimulation of NO synthesis by L-arginine, but the similarity between the vascular effects of L-arginine and some of those currently attributed to NO is intriguing. Further supporting this similarity are results of in vitro studies showing that L-arginine but not D-arginine can inhibit platelet aggregation directly. NOS has been shown to be present in human platelets,³³ and L-arginine might affect NO synthesis in both endothelial cells and platelets. In agreement with previous findings,³⁴ we found no significant effect of high levels of glucose on platelet aggregation in vitro. This seems to be in apparent contrast with the finding that acute hyperglycemia may directly inhibit NO synthesis in porcine aortic endothelial cells, as suggested by the 40% to 60% reduction in the conversion of ¹⁴C-L-arginine to ¹⁴C-citrulline after 3 hours' incubation with 44 mmol/L (792 mg/dL) glucose.³⁵ Differences in experimental protocols may help explain these divergent findings.

A basal release of endothelium-derived NO has been demonstrated in the past to be involved in the physiological regulation of blood pressure, mainly in studies showing that blockade of NO synthesis increased blood pressure.³⁶ NO synthesis is inhibited by arginine analogues, such as L-NMMA and L-NAME, which compete with endogenous L-arginine; this effect is overcome by L-arginine excess.¹⁶ We found that L-NMMA infused in normal humans at a dose that causes systemic hemodynamic changes produced effects similar to those brought about by hyperglycemia. In particular, L-NMMA increased systolic and diastolic blood pressures and reduced leg blood flow; in contrast to hyperglycemia, this was associated with reduced heart rate and plasma catecholamines, presumably through the activation of the baroreflex.³⁷ NO is thought to be involved in the regulation of catecholamine release from sympathetic nerves³⁸ because inhibition of NO synthesis has been shown to lead to an activation of the sympathetic nervous system. Our findings that L-NMMA infusion in normal humans produces significant decreases in epinephrine and norepinephrine plasma levels raise the possibility that stimulation of sympathetic nerve activity, which is supposed to occur during inhibition of NO release in humans,³⁹ is masked and overcome by the sinoaortic baroreflex. Furthermore, a reduction in cardiac output via the decrease in heart rate mediated by the baroreceptor might have contributed to the reduction in leg blood flow during the systemic infusion of L-NMMA.

The time courses of the effects of both hyperglycemia and L-NMMA were somewhat different from each other: the increase in blood pressure induced by L-NMMA was more rapid than that of hyperglycemia. This may be related to the time required for hyperglycemia to activate those metabolic pathways (glucose auto-oxidation, polyol pathway, and cyclooxygenase pathway) presumed to mediate its effects on vasculature. All these pathways are strictly associated with hyperglycemia and may increase the generation of oxygen-reactive substances, mainly superoxide anions, which may quench endogenous NO.⁸ Moreover, differences in ambient plasma insulin concentrations might have attenuated the vascular effects of hyperglycemia, given the ability of insulin to produce NO-mediated vasodilation.²¹ Both enhanced platelet aggregation and increased blood viscosity during L-NMMA infusion suggest that endogenous NO may be implicated in these processes, and this is compatible with the contribution of NO to the protective role of endothelium.⁴⁰ Because the vascular effects of acute hyperglycemia and systemic L-NMMA infusion were neither synergistic nor additive, it seems appropriate to hypothesize a common mechanism for both, ie, reduced NO availability. We cannot exclude the possibility, however, that the rate of NO production may be unchanged or even increased during hyperglycemia, which, by reducing NO availability, may hide the biological effects of the eventual raised NO production. Further studies are needed to clarify this topic.

In conclusion, we have shown that acute hyperglycemia causes significant hemodynamic and rheological changes in normal subjects that are reversed by L-arginine but not D-arginine or L-lysine. Hyperglycemia may act through a reduction of NO availability, but this is still unproved; however, the similarity to the vascular effects brought about by systemic L-NMMA infusion is intriguing. Although the relevance of these short-term changes to chronic diabetic vascular complications is speculative at present, these results may offer an additional mechanism through which hyperglycemia acts as an independent contributor to the development of cardiovascular complications in diabetes mellitus.^{2 3 4 5 6}

	Selected Abbreviations and Acronyms

 

L-NMMA = NG-monomethyl-L-arginine NO = nitric oxide NOS = nitric oxide synthase PPP = platelet-poor plasma PRP = platelet-rich plasma

Received May 23, 1996; revision received November 9, 1996; accepted November 25, 1996.

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