NO-dependent and -independent elevation of plasma levels of insulin and glucose in rats by L-arginine

Pharmacotherapy in patients with vasomotor disorders

Background

Anginal symptoms in patients with non-obstructive coronary artery disease are frequently related to vasomotor disorders of the coronary circulation. Although frequently overlooked, a distinct diagnosis of different vasomotor disorders can be made by intracoronary function testing. Early detection and treatment seems beneficial, but little evidence is available for the medical treatment of these disorders. Nevertheless, there are several pharmacotherapeutic options available to treat these patients and improve quality of life.

Methods & findings

We performed an extensive yet non-systematic literature search to explore available pharmacotherapeutic strategies for addressing vasomotor disorders in individuals experiencing angina and non-obstructive coronary artery disease. This article presents a comprehensive overview of therapeutic possibilities for patients exhibiting abnormal vasoconstriction (such as spasm) and abnormal vasodilation (like coronary microvascular dysfunction).

Conclusion

Treatment of vasomotor disorders can be very challenging, but a general treatment algorithm based on the existing evidence and the best available current practice is feasible.

Arginine protects muscle cells from wasting in vitro in an mTORC1-dependent and NO-independent manner

Amino acids are potent regulators of muscle protein synthesis and breakdown and have received considerable attention for the treatment of muscle wasting conditions. Arginine is critically involved in numerous physiological functions including providing substrate for the production of creatine, urea and nitric oxide (NO) and in the synthesis of new proteins. However, little is known about the direct effects of arginine on skeletal muscle protein synthesis during catabolic conditions. The aims of this study were to determine whether exogenous arginine could protect skeletal muscle cells from wasting directly and whether this effect was dependent on production of NO and/or activation of the rapamycin-sensitive mechanistic target of rapamycin complex 1 (mTORC1) signalling pathway. To explore these aims, we deprived mature C2C12 myotubes from nutrients and growth factors by incubating them in HEPES buffered saline with arginine or equimolar concentrations of alanine (control). Our results show that arginine: increased the ratio of phosphorylated to total mTOR (146 %), S6 (40 %) and 4EBP1 (69 %); increased protein synthesis (69 %) during the first hour of treatment; and increased myotube diameter by ~15 %. Experiments using the NO synthase inhibitor L-NG-Nitroarginine Methyl Ester showed a NO-independent protection from muscle wasting. On the other hand, the mTORC1 inhibitor rapamycin prevented increases in phosphorylated S6, protein synthesis and myotube diameter. The activation of mTORC1 and protein synthesis by arginine was not associated with changes in the phosphorylation status of Akt, but rather increased the expression of the amino acid-sensitive type III PI3-kinase Vps34 signalling protein. These data support a direct role for arginine in the regulation of mTORC1 in skeletal muscle.

Acute intravenous L-arginine infusion decreases endothelin-1 levels and improves endothelial function in patients with angina pectoris and normal coronary arteriograms: correlation with asymmetric dimethylarginine levels

BACKGROUND

We tested the hypothesis that asymmetric dimethylarginine (ADMA) levels could be elevated and influence endothelin-1 and nitric oxide release and action in patients with cardiac syndrome X (CSX). In addition, we evaluated whether an intravenous infusion of L-arginine would improve endothelial function in these subjects.

METHODS AND RESULTS

Nine patients with CSX and 14 control subjects underwent a continuous infusion of L-arginine (0.125 g/min) or saline for 120 minutes. Sixty minutes after L-arginine or saline infusions, an intravenous insulin bolus (0.1 U/kg) combined with a euglycemic clamp was performed. Basal ADMA and endothelin-1 levels were higher in patients with CSX than in controls. At the end of the first hour of infusion, compared with saline, L-arginine infusion increased basal forearm blood flow, nitrite and nitrate (NOx), and forearm cGMP release and decreased endothelin-1. After insulin bolus, during saline, insulin-induced NOx, endothelin-1, and forearm cGMP release was almost abolished. Conversely, L-arginine restored a physiological profile of all endothelial variables compared with control subjects. In control subjects, compared with saline infusion, L-arginine infusion did not modify any parameter. ADMA levels were positively correlated with basal endothelin-1 levels and negatively correlated with insulin-induced incremental levels of NOx and forearm cGMP release.

CONCLUSIONS

Plasma ADMA levels are increased in patients with CSX, and they are correlated with increases in endothelin-1 and reductions in insulin-induced increments in plasma NOx and cGMP, effects that are reversed by intravenous L-arginine. These data suggest that increased ADMA levels play a role in the abnormal vascular reactivity that is observed in patients with CSX.

Insulin resistance, a new target for nitric oxide-delivery drugs

In the Western hemisphere, the incidence of insulin resistance and its complications has been growing rapidly and is reaching epidemic proportions. Over the past decade, evidence has accumulated, indicating that nitric oxide (NO) plays a key role in the regulation of metabolic and cardiovascular homeostasis. Defective endothelial nitric oxide synthase (eNOS) driven NO synthesis causes insulin resistance, arterial hypertension and dyslipidemia in mice, and characterizes insulin-resistant humans. On the other hand, stimulation of inducible nitric oxide synthase (iNOS) and NO overproduction in mice, may also cause metabolic insulin resistance, suggesting a Yin-Yang effect of NO in the regulation of glucose homeostasis. Here, we will review the evidence for this novel concept, and thereby provide the conceptual framework for the use of NO-delivery drugs and pharmacological agents that modulate the bioavailability of endogenously produced NO for the treatment of insulin resistance.

Overexpression of arginase alters circulating and tissue amino acids and guanidino compounds and affects neuromotor behavior in mice

Arginine is an intermediate of the ornithine cycle and serves as a precursor for the synthesis of nitric oxide, creatine, agmatine and proteins. It is considered to be a conditionally essential amino acid because endogenous synthesis only barely meets daily requirements. In rapidly growing suckling neonates, endogenous arginine biosynthesis is crucial to compensate for the insufficient supply of arginine via the milk. Evidence is accumulating that the intestine rather than the kidney plays a major role in arginine synthesis in this period. Accordingly, ectopic expression of hepatic arginase in murine enterocytes by genetic modification induces a selective arginine deficiency. The ensuing phenotype, whose severity correlates with the level of transgene expression in the enterocytes, could be reversed with arginine supplementation. We analyzed the effect of arginine deficiency on guanidine metabolism and neuromotor behavior. Arginine-deficient transgenic mice continued to suffer from an arginine deficiency after the arginine biosynthetic enzymes had disappeared from the enterocytes. Postweaning catch-up growth in arginine-deficient mice was characterized by increased levels of all measured amino acids except arginine. Furthermore, plasma total amino acid concentration, including arginine, was significantly lower in adult male than in adult female transgenic mice. Decreases in the concentration of plasma and tissue arginine led to significant decreases in most metabolites of arginine. However, the accumulation of the toxic guanidino compounds, guanidinosuccinic acid and methylguanidine, corresponded inversely with circulating arginine concentration, possibly reflecting a higher oxidative stress under hypoargininemic conditions. In addition, hypoargininemia was associated with disturbed neuromotor behavior, although brain levels of toxic guanidino compounds and ammonia were normal.

Inhibition of nitric oxide synthesis accentuates blood pressure elevation in hyperinsulinemic rats

OBJECTIVES

To examine the role of endogenous nitric oxide (NO) in the pathogenesis of hypertension and insulin resistance in chronic hyperinsulinemic rats.

METHODS

Sustained hyperinsulinemia was achieved by insulin infusion (21.5 pmol/kg per min) via subcutaneous osmotic minipump for 6 weeks. NO synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME, 5 mg/kg per day) was given orally after 4 weeks of vehicle or insulin infusion. The systolic blood pressure (SBP) was measured under conscious state by an electrosphygmomanometer before and after drug treatments.

RESULTS

Insulin infusion alone significantly increased SBP from 134 +/- 3 to 156 +/- 2 mmHg by week 4 and further to 158 +/- 3 mmHg by week 6 of insulin infusion. The insulin-infused rats had markedly decreased insulin sensitivity, as reflected by an elevated steady-state plasma glucose level estimated by the insulin suppression test. There were no significant differences in plasma glucose and triglyceride levels between rats with and without insulin infusion. When hypertension had been established in rats receiving insulin infusion for 4 weeks, superimposed L-NAME on insulin infusion for additional 2 weeks further increased SBP by 18 +/- 2 mmHg (from 157 +/- 2 to 175 +/- 2 mmHg). Plasma levels of NO metabolites (NOx) significantly decreased from 13.7 +/- 1.1 micromol/l during the control period to 6.1 +/- 0.6 micromol/l after 4 weeks of insulin infusion and further reduced to 4.1 +/- 0.5 micromol/l after combined infusion of L-NAME and insulin. L-NAME treatment alone for 2 weeks in control rats significantly increased SBP by 33 +/- 2 mmHg (from 133 +/- 2 to 166 +/- 2 mmHg) and plasma insulin levels, as a consequence of insulin resistance. Despite marked increases in blood pressure due to infusion of insulin alone or in combination with L-NAME, the sodium balance, urinary sodium and water excretions, water intake and body weight gain of insulin/L-NAME-treated rats were not significantly different from rats without insulin infusion.

CONCLUSIONS

Sustained hyperinsulinemia causes partial impairment of NO production that may contribute to the development of insulin resistance and hypertension. Additional inhibition of NO synthesis by L-NAME accentuates the blood pressure elevation and insulin resistance in hyperinsulinemic rats. Furthermore, a rightward shift of the renal arterial pressure-natriuretic function relationship occurred in this hypertensive model.

Long-term oral L-arginine administration improves peripheral and hepatic insulin sensitivity in type 2 diabetic patients

The aim of this study was to evaluate whether long-term administration of arginine acting through a normalization of NO/cyclic-guanosine-3' 5'-cyclic monophosphate (cGMP) pathway was able to ameliorate peripheral and hepatic insulin sensitivity in 12 lean type 2 diabetic patients.

RESEARCH DESIGN AND METHODS

A double-blind study was performed for 3 months. In the first month, patients were treated with their usual diet. Then they were randomly allocated into to groups. In group 1, patients were treated with diet plus placebo (orally three times per day) for 2 months. In group 2 patients were treated for 1 month with diet plus placebo orally, three times per day) and then for 1 month with diet plus L-arginine (3 g three times per day). At the end of the first and the second month of therapy, patients underwent a euglycemic-hyperinsulinemic clamp combined with [6,6-2H2] glucose infusion. A total of 10 normal subjects underwent the same test as control subjects.

RESULTS

In group 1, no changes in basal cGMP levels, systolic blood pressure, forearm blood flow, glucose disposal, and endogenous glucose production were observed throughout. In group 2, L-arginine normalized basal cGMP levels and significantly increased forearm blood flow by 36% and glucose disposal during the clamp by 34% whereas it decreased systolic blood pressure and endogenous glucose production by 14 and 29%, respectively. However, compared with normal subjects, L-arginine treatment was not able to completely overcome the defect in glucose disposal.

CONCLUSIONS

L-Arginine treatment significantly improves but does not completely normalizc peripheral and hepatic insulin sensitivity in type 2 diabetic patients.

The effect of nitric oxide inhibition on blood pressure depends on rat strain

BACKGROUND

Nitric oxide is a continuously released endothelium-derived vasodilator and plays an important role in the maintenance of blood pressure (BP). Rat strains appear to differ in their resting BP and their response to the intravenous administration of N(omega)-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase inhibitor. The presence of diabetes and hypertension also leads to differences in BP responses to l-NAME. We postulated that the contribution of NO to resting BP varies between rat strains and certain strains may be more sensitive to the effects of NO blockade.

METHODS

Blood pressure was continuously measured using a carotid arterial catheter and the responses to l-NAME were compared in anesthetized Lewis and Sprague-Dawley rats during a 2-h control period and a 2-h experimental period. l-NAME was given by a 50 mg/kg bolus followed by a 10 mg/kg/h infusion via a mesenteric vein.

RESULTS

During the control period, the Lewis animals had lower systolic and diastolic BPs of 103 +/- 1 and 80 +/- 1 mm Hg compared with 127 +/- 1 and 105 +/- 1 mm Hg measured in Sprague-Dawley rats (P < 0.01). Although l-NAME infusion increased systolic BP in both strains compared with control values (P < 0.00005), the magnitude was significantly greater in Sprague-Dawley than Lewis animals (P = 0.0142); additionally, the BP was unstable in the Lewis animals. Furthermore, pulse pressure decreased during l-NAME in Lewis animals but increased in Sprague-Dawley animals (P < 0.00005). There were no significant changes in serum concentrations of aspartate transaminase nor of nitrite plus nitrate after l-NAME in either group.

CONCLUSION

These results indicate that the effect of l-NAME on systemic BP differs markedly in Sprague-Dawley and Lewis rats, suggesting that the role of nitric oxide in regulation of resting vascular resistance may differ significantly between these rat strains. Rat strain is an important consideration for valid comparisons between studies.

Involvement of nitric oxide in nickel-induced hyperglycemia in rats

Nitric oxide is an important bioactive signaling molecule that mediates a variety of normal physiological functions which, if altered, could contribute to the genesis of many pathological conditions, including diabetes. In the present study we have shown the involvement of NO in nickel-induced hyperglycemia in male albino rats. Administration of nickel chloride (25 to 100 micromol/kg; ip) to overnight-fasted rats resulted in significant dose and time-dependent increase in plasma glucose, attaining maximum level at 1 h posttreatment and thereafter decreasing to normal levels by 4 h. The involvement of NO in nickel-induced hyperglycemia was evident by the observation that pretreatment of rats with NG-monomethyl-l-arginine (10 to 50 micromol/kg; ip), an inhibitor of nitric oxide synthase (NOS), significantly attenuated the nickel-mediated increase in the plasma glucose levels in a dose-dependent fashion. The activity of Ca(2+)-dependent NOS (constitutive form, c-NOS) was found to be significantly elevated in adrenals (5.5-fold) and brain (1.4-fold) at 1 and 2 h posttreatment, attaining normal levels by 4 h. In contrast, the activity of c-NOS in pancreas was significantly decreased (2.8-fold) with a concomitant increase (11.6-fold) in inducible NOS (i-NOS) at the same time interval. As observed by immunoblot analysis, a significant increase in i-NOS protein expression in the pancreas was observed at 1 and 2 h posttreatment. This was associated with a significant elevation in cGMP levels in adrenals, brain, and pancreas, possibly via the stimulation of cytosolic guanylate cyclase. This elevation in cGMP was abolished by low concentration of hemoglobin. These effects were associated with the accumulation of nickel in the target tissues. Taken together, our data suggest that nickel causes a significant increase in the levels of (i) cGMP and c-NOS in adrenals and brain and (ii) i-NOS in pancreas. These events may be responsible for modulating the release of insulin from pancreas finally leading to hyperglycemic condition in rats.

The insulin secretory response to intravenous glucose in the rat is independent of NO formation

In isolated pancreative beta cells from rats the insulin secretory response to glucose is amplified by L-arginine. Since this effect is inhibited by NO synthesis inhibitors, and since L-arginine is precursor of NO, the observation indicates a role for NO in insulin secretion from beta cells. We recently reported that i.v. L-arginine elicited insulin secretion in anaesthetized rats by a mechanism that was partly NO dependent. The aim of the present study was to assess if the insulin secretory response to an intravenous infusion of glucose also requires an intact NO formation. Anaesthetized rats were given D-glucose (100 mg kg-1 min-1 i.v. for 30 min). Plasma insulin (PI), blood glucose (BG) levels and mean arterial blood pressure (MAP) were assessed from before and until 15 min after the end of the infusion. One group of rats were untreated and served as controls. The two other groups were pretreated with either of the NO synthase inhibitors NW-nitro-L-arginine methyl ester (L-NAME, 50 mg kg-1 i.v.), or NG-monomethyl-L-arginine (L-NMMA, 100 mg kg-1 i.v.). In controls infusion of glucose elevated PI by up to 25 +/- 3 U L-1, and BG by up to 27 +/- 1 mmol L-1. Pretreatment with L-NAME elevated MAP from 74 +/- 6 to 132 +/- 4 mmHg, indicating that NO synthase was inhibited.(ABSTRACT TRUNCATED AT 250 WORDS)

L-arginine-induced hypotension in the rat: evidence that NO synthesis is not involved

L-Arginine is the biological precursor for nitric oxide (NO). NO is formed continuously in endothelial cells and maintains a certain degree of vasodilator tone under physiological conditions. Although the formation of NO is not primarily controlled by precursor availability, the extent to which extra supplementation with L-arginine may affect endothelial NO formation, and hence, vasodilator tone and systemic blood pressure, is not entirely clear. To address this issue, we infused L-arginine i.v. in anaesthetized normotensive rats pretreated with NG-nitro-L-arginine methyl ester (L-NAME, 50 or 200 mg-1) and in untreated controls, under continued recording of mean arterial pressure (MAP). In control animals L-arginine (25 or 100 mg kg-1 min-1) had no effect on systemic MAP (111 +/- 3 mm Hg), while L-arginine (200 mg kg-1 min-1) lowered MAP (to 70 +/- 6 mm Hg). D-Arginine (200 mg kg-1 min-1) also induced hypotension; during infusion of D-arginine MAP fell from 106 +/- 4 to 64 +/- 4 mm Hg. Pretreatment with L-NAME (50 and 200 mg kg-1) elevated MAP to 140 +/- 2 and 147 +/- 3 mm Hg, respectively, but failed to affect the hypotensive response to L-arginine; during infusion of L-arginine (200 mg kg-1 min-1) in rats pretreated with L-NAME (50 and 200 mg kg-1) MAP fell to 86 +/- 9 and 104 +/- 6 mm Hg, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)