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(Hypertension. 1996;27:619-625.)
© 1996 American Heart Association, Inc.

Articles

Acute Sympathoinhibitory Actions of Metformin in Spontaneously Hypertensive Rats

Jørgen S. Petersen; Gerald F. DiBona

From the Department of Pharmacology, The Panum Institute, University of Copenhagen (Denmark) (J.S.P.), and Department of Internal Medicine, University of Iowa College of Medicine and Veterans Administration Medical Center, Iowa City (G.F.D.).

	Abstract

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Abstract Chronic treatment with the antihyperglycemic agent metformin prevents hypertension in spontaneously hypertensive rats. This effect has been ascribed to normalization of plasma insulin levels. However, whether metformin affects arterial pressure via changes in sympathetic nerve activity is unknown. Therefore, the objective of this study was to examine whether acute administration of metformin produces changes in mean arterial pressure, heart rate, or efferent renal sympathetic nerve activity in spontaneously hypertensive rats. Rats were anesthetized with alphaxalone-alphadolone (Saffan), paralyzed with pancuronium, and artificially ventilated. Intravenous administration of metformin (0, 1, 10, 100 mg/kg) produced dose-dependent reversible decreases in mean arterial pressure, heart rate, and efferent renal sympathetic nerve activity that were not affected by arterial or cardiopulmonary baroreceptor denervation, nitric oxide synthase inhibition by N-nitro-L-arginine methyl ester, or cyclooxygenase inhibition by indomethacin. Metformin given into the lateral cerebral ventricle (250, 500, 1000 µg) produced dose-dependent decreases in mean arterial pressure, heart rate, and efferent renal sympathetic nerve activity in doses that caused no changes when given intravenously. The sympathoinhibitory response to intracerebroventricular administration of metformin was not affected by ₂-adrenoceptor blockade by intracerebroventricular yohimbine. We conclude that metformin has acute sympathoinhibitory effects (decreased arterial pressure, heart rate, and efferent renal sympathetic nerve activity) that are produced by a direct central nervous system site of action.

Key Words: metformin • indomethacin • L-NAME • blood pressure • baroreflex • vagotomy • central nervous system

	Introduction

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Metformin is an antihyperglycemic agent recently approved by the US Food and Drug Administration for treatment of type II diabetes mellitus. Clinical studies have shown that metformin is an effective antihyperglycemic agent, and in contrast to drugs of the sulfonylurea group, metformin treatment is associated with decreased plasma insulin levels and lack of weight gain.¹ These actions make metformin the recommended oral antihyperglycemic drug in obese patients with type II diabetes mellitus.¹ Furthermore, metformin treatment in such patients is associated with decreased plasma levels of total cholesterol, triglycerides, and low-density lipoprotein cholesterol and an increased ratio of high-density to low-density lipoprotein cholesterol.^{2 3 4 5 6 7 8 9 10 11} In addition, studies have shown that metformin increases fibrinolytic activity.^{3 10 12} Thus, metformin exerts favorable actions on several metabolic risk factors associated with cardiovascular disease.

In an uncontrolled open study in insulin-resistant hypertensive men, Landin et al¹² reported that 6 weeks of metformin treatment increased insulin sensitivity and significantly decreased both systolic (-40±19 mm Hg) and diastolic (-24±5 mm Hg) arterial pressures. SHR and fructose-fed Sprague-Dawley rats are hypertensive rat models with insulin resistance. In these strains, metformin treatment lowers plasma insulin levels and prevents the development of hypertension.^{13 14 15} Hyperinsulinemia increases sympathetic nerve activity in both rats and humans.^{16 17 18} Thus, the correlation between decreased arterial pressure and decreased plasma insulin levels during chronic metformin treatment has been advanced to suggest a causal link between hyperinsulinemia and hypertension.^{14 15 16} However, as suggested by Anderson and Mark,¹⁶ antihyperglycemic agents may lower arterial pressure by other mechanisms that could confound the use of these agents to test the hypothesis of a pathogenetic role of insulin in hypertension.

Therefore, the aim of this study was to examine the acute effects of metformin administration on MAP, HR, and ERSNA. We report that metformin has marked acute sympathoinhibitory and antihypertensive actions in SHR. The chemical structure of metformin (dimethylbiguanide) is related to the structure of the serotonin type 3 receptor agonist biphenylbiguanide, which elicits vagal-mediated sympathoinhibitory reflex responses.¹⁹ Thus, to test whether the sympathoinhibitory response to metformin is mediated through stimulation of the vagal nerves, we administered metformin to a group of rats that had undergone VGX. To examine whether the sympathoinhibitory and antihypertensive responses to metformin were prostaglandin dependent²⁰ or mediated through release of nitric oxide,^{21 22 23} we administered metformin to groups of rats pretreated with either the cyclooxygenase inhibitor indomethacin or the nitric oxide synthase inhibitor L-NAME. The actions of metformin on central nervous system control of arterial pressure and renal sympathetic nerve activity were evaluated by intracerebroventricular administration of metformin. To eliminate confounding effects derived from arterial and cardiopulmonary baroreflexes, we performed experiments with pharmacological blockade in rats with combined SAD and VGX.

	Methods

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Animal Preparation.
Male SHR (282±2 g) were anesthetized with 20 mg/kg methohexital IP and placed on a heated micropuncture table maintaining rectal temperature at 37°C to 38°C. Medical grade Tygon catheters were inserted into a femoral artery and vein for measurement of arterial pressure and intravenous infusion of solutions, respectively, and into the right jugular vein for intravenous administration of drugs. After insertion of the femoral vein catheter, a 30-µL IV bolus injection of the steroid anesthetic Saffan (0.9% wt/vol alphaxalone acetate, 0.3% wt/vol alphadolone acetate) was administered; anesthesia was maintained with a continuous infusion of Saffan (5 to 10 µL/min) throughout the experiment. Arterial pressure was measured by a Statham P23 XL pressure transducer and displayed continuously on a Grass model 7E polygraph. HR was recorded by a linear cardiotachometer (Grass model 7P4) triggered by the arterial pressure waveform. An endotracheal tube was inserted, and the rat was paralyzed with pancuronium (1.0 mg/kg IV bolus, followed by 1.0 mg/kg per hour) and artificially ventilated with atmospheric air. The tidal volume and respiratory rate were adjusted to maintain arterial pH between 7.35 and 7.45 throughout the experiment.

The left kidney was exposed via a retroperitoneal approach through a left flank incision. With a dissection microscope (x25), a branch of the renal nerve from the aorticorenal ganglion was carefully isolated and placed on a bipolar platinum electrode. The renal nerve activity was led through a Grass model HIP511 high-impedance probe and amplified (x3000 to x20 000) and filtered (30 to 3000 Hz) with a Grass model P511 band pass amplifier. The amplified and filtered signal was led to an oscilloscope (Textronix 5113) for visual representation and to an audio amplifier/loudspeaker (Grass model AM8 audio monitor) for audial representation as well as a rectifying voltage integrator (Grass model 7P10). After an optimal recording was established, the recording electrode was fixed to the renal nerve branch with silicone adhesive (Wacker Sil-Gel 604, Wacker-Chemie). To eliminate afferent renal nerve activity from the recorded signal, we crushed the renal nerve bundle peripherally.

In seven of eight groups of rats, the sinoaortic baroreceptors were denervated by bilateral cutting of the aortic depressor nerve, the superior laryngeal nerve, the pharyngeal nerve, the superior cervical ganglion, and the carotid sinus nerves.²⁴ The effectiveness of SAD was confirmed by abolishment of bradycardia and sympathoinhibitory response to phenylephrine (1 µg IV). In addition, in six rat groups, the cardiopulmonary baroreceptors were denervated by cervical VGX, with its effectiveness confirmed by abolition of the sympathoinhibitory response (hypotension, bradycardia, decreased ERSNA) to 2-methyl-5-hydroxytryptamine (50 µg/kg IV) compared with the response before VGX.²⁵

One week before the experiment, three rat groups were instrumented with stainless steel cannulas in their left lateral cerebral ventricles. Coordinates for intracerebroventricular cannulas were 0.3 mm posterior to bregma, 1.4 mm lateral to the midline, and 4.5 to 5.0 mm below the skull surface. Brain cannulas were held in place with stainless steel jeweler's screws and cranioplastic cement. At the end of the experiment, 2 µL isotonic saline with methylene blue was injected intracerebroventricularly, and accurate placement of the intracerebroventricular cannula was verified by postmortem examination.

Experimental Protocol
After surgery, rats were allowed to stabilize for 30 to 60 minutes before the start of the experiment. After this equilibration period, data were collected during a 10-minute control period before the first injection.

In the first series of experiments, metformin was administered intravenously in cumulative doses (0 [isotonic saline], 1, 10, and 100 mg/kg IV). Injection volume was 1 mL/kg at all doses. The rat was allowed 15 minutes of recovery between each injection. The response to intravenous metformin was examined in the following groups: Group 1: intact (n=6); rats with intact baroreflexes. Group 2: SAD (n=6); rats with SAD. Group 3: SAD+VGX (n=6); rats with combined SAD and VGX. Group 4: SAD+VGX+indomethacin (n=6); rats with SAD+VGX pretreated with 10 mg/kg indomethacin IV administered 10 minutes before the first intravenous metformin injection. Group 5: SAD+VGX+L-NAME (n=5); rats with SAD+VGX pretreated with 50 µmol/kg L-NAME IV. Efficacy of nitric oxide synthase inhibition was evaluated by comparing MAP and ERSNA responses to 500 µmol/kg L-arginine IV before and 5 minutes after L-NAME administration and at the end of the experiment. To maintain maximal nitric oxide synthase inhibition throughout the experiment, administration of 50 µmol/kg L-NAME IV was repeated after 30 minutes.

In a second series of experiments, vehicle or metformin was administered intracerebroventricularly. Rats were allowed 60 minutes of equilibration between each dose. Experiments with intracerebroventricular administration were performed in the following groups: Group 6: Vehicle (n=8); cumulative intracerebroventricular injection of vehicle (1.25, 2.5, and 5 µL). At the end of the experiment a test dose of 25 µg guanabenz ICV was administered. Group 7: Metformin (n=12); cumulative intracerebroventricular injection of metformin (250, 500, and 1000 µg). Group 8: Metformin+yohimbine (n=6); cumulative intracerebroventricular injection of metformin (250, 500, and 1000 µg) in rats pretreated with 30 µg yohimbine ICV. Rats were allowed to equilibrate for 60 minutes after yohimbine administration. Efficacy of yohimbine was examined by MAP and ERSNA responses to 25 µg guanabenz ICV at the end of the experiment.

At the end of experiments, rats were killed by an overdose of pentobarbital (25 mg IV), and ERSNA was continuously recorded for a further 30 minutes as a measure of the background signal.

All experimental procedures were in accordance with the University of Iowa and National Institutes of Health guidelines for animal research.

Drugs
Saffan (0.9% wt/vol alphaxalone, 0.3% wt/vol alphadolone acetate) was supplied in 10-mL ampoules (Pitman-Moore Ltd). Pancuronium (pancuronium bromide injection, 1 mg/mL; Astra Pharmaceutical Products, Inc) was dissolved in isotonic saline and stored at room temperature. Phenylephrine HCl (10 mg/mL; Elkins-Sinn, Inc) was diluted in isotonic saline (2.5 mg/mL) and stored at 5°C. 2-Methyl-5-hydroxytryptamine maleate (Research Biochemicals, Inc) was dissolved in isotonic saline (50 mg/L) and stored at -20°C. Indomethacin sodium trihydrate (Merck, Sharp & Dohme) was diluted in isotonic saline (10 mg/mL) with added sodium carbonate (5 mg/mL). L-Arginine (500 µmol/mL) and L-NAME (50 µmol/mL) (Sigma Chemical Co) were dissolved in isotonic saline. Guanabenz (1-[2, 6-dichlorobenzylideneamino]guanidine; Sigma) was dissolved in ethyl alcohol (5 mg/mL) and stored at 5°C. Yohimbine hydrochloride (Sigma) was dissolved in distilled water (6 mg/mL). Metformin (1,1-dimethylbiguanide; Sigma) used for intravenous injection (1, 10, or 100 mg/mL) was dissolved in isotonic saline, and metformin used for intracerebroventricular injection (200 mg/mL) was dissolved in distilled water. Osmolality of intracerebroventricular injection fluid used in intracerebroventricular vehicle experiments was adjusted to the same osmolality as in the metformin solution used for intracerebroventricular injection (1.2 mol/L) by addition of NaCl to distilled water. Solutions with metformin, indomethacin, L-arginine, L-NAME, and yohimbine were prepared fresh on each experimental day.

Data Analysis
Data were sampled on-line via a pulse code modulation recording adapter (model 4000, AR Vetter Co) and stored on videotape with a standard videocassette recorder (Fisher model FVH-6200). Data analysis (MAP, HR, and integrated ERSNA) was performed off-line with an analog-to-digital convertor (model DT2801, Data Translation Inc), appropriate software (Labtech Notebook version 4.2, Laboratory Technologies Corp), and an IBM PS/2 computer. Integrated ERSNA was analyzed as mean integrated voltage (µV·s) per second. The postmortem background signal was subtracted from all nerve activity measurements. In experiments with intravenous injection of metformin, the peak response was defined as the average response during the 5-second period with maximal deflection from baseline. When no well-defined response was observed (after vehicle and 1 mg/kg metformin IV), the reading was performed 40 to 45 seconds after the intravenous injection. In experiments with intracerebroventricular injection of metformin or vehicle, readings were performed 45 to 60 minutes after intracerebroventricular injection. Overall statistical analysis of one-way classified data (treatment group) was performed with one-way ANOVA. Overall statistical analysis of two-way classified data (treatment group and dose) was performed by repeated measures ANOVA. Student's paired or unpaired t test with Bonferroni correction for multiple comparisons was used for comparisons of one-way classified data within or between groups.²⁶ Differences were considered significant at a value of P<.05. All presented values are mean±SE.

	Results

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Baseline values in all groups are shown in the Table. L-NAME administration produced a significant increase in MAP (+61±3 mm Hg) in the absence of changes in ERSNA (0±7%). Thus, MAP was significantly higher in L-NAME–treated rats compared with groups 1 through 4, which had similar MAP values. There were no significant differences in HR among rats given intravenous metformin. Because of the difficulty of comparing absolute values of ERSNA between groups of rats related to differences in the number of active fibers and nerve-electrode contact, statistical comparison was not performed. There were no significant differences in baseline values for MAP or HR among rats given vehicle or intracerebroventricular metformin (Table).

View this table: [in this window] [in a new window]   Table 1. Baseline Values of MAP, HR, and ERSNA in Study Groups

Fig 1 shows a representative tracing after intravenous injection of 100 mg/kg metformin. This demonstrates that intravenous metformin produced a rapid, reversible decrease in MAP, HR, and ERSNA.

View larger version (18K): [in this window] [in a new window]   Figure 1. Representative tracings of MAP, pulsatile arterial pressure (AP), HR, integrated ERSNA, and mean ERSNA during administration of 100 mg/kg metformin IV.

Metformin produced dose-dependent decreases in MAP, HR, and ERSNA that were not affected by SAD, SAD+VGX, cyclooxygenase inhibition by indomethacin, or nitric oxide synthase inhibition by L-NAME (Fig 2).

View larger version (16K): [in this window] [in a new window]   Figure 2. Dose-related changes in MAP (left), HR (middle), and ERSNA (right) after cumulative intravenous administration of metformin in rats with intact baroreflexes, SAD, SAD+VGX, SAD+VGX pretreated with indomethacin, and SAD+VGX pretreated with L-NAME. Mean±SEM.

L-Arginine produced significant hypotensive and sympathoinhibitory responses in rats with SAD+VGX (MAP=-24±3 mm Hg; ERSNA=-56±8%) that were inhibited by L-NAME (at the end of experiment: MAP=0±6 mm Hg; ERSNA=-10±7%; P<.05 versus before L-NAME for both MAP and ERSNA).

Fig 3 shows representative tracings before and after injection of 1000 µg metformin ICV. Intracerebroventricular administration of metformin produced decreases in MAP, HR, and ERSNA of gradual onset and long duration (hours).

View larger version (24K): [in this window] [in a new window]   Figure 3. Representative tracings of MAP, pulsatile arterial pressure (AP), HR, integrated ERSNA, and mean ERSNA before and after administration of 1000 µg metformin ICV.

Intracerebroventricular administration of metformin produced dose-dependent decreases in MAP, HR, and ERSNA that were not affected by intracerebroventricular yohimbine pretreatment. Compared with responses in vehicle-treated rats, there was a significant effect of metformin treatment and a significant dose-treatment interaction on all parameters in both metformin and metformin+yohimbine groups (Fig 4).

View larger version (15K): [in this window] [in a new window]   Figure 4. Dose-related changes in MAP (left), HR (middle), and ERSNA (right) after cumulative intracerebroventricular administration of vehicle (saline), metformin, or metformin in intracerebroventricularly yohimbine-pretreated rats. All rats had SAD and VGX. Mean±SEM. *P<.05.

Within 5 to 10 minutes after intracerebroventricular yohimbine administration, significant decreases in MAP, HR, and ERSNA were observed (MAP=-39±8 mm Hg; HR=-28±3 beats per minute; ERSNA=-17±2%). However, all parameters returned to preyohimbine levels within 60 minutes. At the end of the experiment, the hypotensive and sympathoinhibitory responses to 25 µg guanabenz ICV were inhibited in yohimbine-treated rats (MAP=-21±7 mm Hg; ERSNA=-41±8%) compared with vehicle-treated rats (MAP=-63±7 mm Hg; ERSNA=-67±11%; P<.05 for both MAP and ERSNA versus yohimbine-treated rats).

	Discussion

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The main finding of this study was that acute intravenous and intracerebroventricular administration of metformin produced a marked sympathoinhibitory response associated with decreased arterial pressure, bradycardia, and decreased ERSNA. The sympathoinhibitory response could be elicited by intracerebroventricular injection of metformin in doses that had no effect on MAP, HR, or ERSNA when administered intravenously. These data suggest that metformin inhibits peripheral sympathetic nerve activity by a central nervous system site of action.

Metformin consistently has been reported to improve peripheral insulin sensitivity, decrease plasma lipid concentrations, and increase fibrinolytic activity, but the reported effects on arterial pressure are conflicting. In normotensive subjects, metformin does not affect arterial pressure in either normal subjects¹⁰ or patients with insulin resistance and hyperinsulinemia.^{3 7 27} However, antihypertensive effects of metformin have been reported in hypertensive patients. In an uncontrolled, open study, Landin et al¹² found that 6 weeks of metformin treatment in nonobese men with insulin resistance and mild hypertension produced a significant reduction in arterial pressure that was reversed after cessation of treatment. However, two controlled studies have failed to demonstrate an antihypertensive effect of metformin in mild hypertension.^{28 29} Moreover, Gudbjörnsdottir et al²⁹ found no change in renal norepinephrine spillover or muscle sympathetic nerve activity during basal conditions or insulin-induced sympathoexcitation after 6 weeks of metformin treatment in obese, insulin-resistant, normoglycemic, mildly hypertensive men. In a double-blind, randomized study with a crossover design, Giugliano et al⁸ found that 12 weeks of metformin treatment reduced arterial pressure significantly in obese, hypertensive women, and the reduction in blood pressure was associated with decreased plasma norepinephrine concentration (-19%) and decreased left ventricular mass index. The same authors examined the effect of metformin on metabolic control and blood pressure in obese, type II diabetic patients who were poorly controlled by insulin alone.⁹ In the latter prospective, randomized, placebo-controlled, double-blind trial, 6 months of metformin treatment caused a significant reduction in arterial pressure that was closely related to the antihyperglycemic effect.⁹ These studies suggest that the antihypertensive effect of metformin is only evident in obese, type II diabetic patients with hypertension. The decrease in plasma norepinephrine concentration during chronic metformin treatment in obese, hypertensive women⁸ is compatible with a significant sympathoinhibitory contribution to the antihypertensive effect of metformin. Moreover, since moderate to severe obesity, essential hypertension, and hyperinsulinemia are associated with elevated peripheral sympathetic nerve activity,³⁰ it is probable that the antihypertensive effect of metformin in these subjects may be related to a higher level of sympathetic nerve activity in this subgroup of hypertensive patients.

In SHR and fructose-fed Sprague-Dawley rats, chronic metformin treatment has been shown to decrease plasma insulin levels and prevent the development of hypertension. Furthermore, the antihypertensive effect of metformin could be reversed when plasma insulin levels were increased to the levels seen in untreated rats.^{13 14 15} These findings have been used to support the hypothesis of an important pathogenetic role of insulin in the development of hypertension.¹⁶ However, chronic metformin treatment does not affect arterial pressure in Dahl salt-sensitive rats or one-kidney, one clip hypertensive Sprague-Dawley rats.³¹ Moreover, treatment of SHR with troglitazone or Dahl salt-sensitive rats with pioglitazone improves insulin sensitivity in both hypertensive strains without any effect on arterial pressure.^{31 32} These results suggest that metformin has antihypertensive effects in SHR and fructose-fed Sprague-Dawley rats but not in Dahl salt-sensitive rats. Since both metformin and troglitazone treatments improve insulin sensitivity and decrease the plasma insulin concentration in SHR, whereas only metformin treatment produces a decrease in arterial pressure,^{14 32} the antihypertensive effect of metformin in SHR may be related to factors other than changes in plasma insulin level. In support for an important sympathoinhibitory contribution to the antihypertensive action of metformin in SHR, Morgan and Mark³³ have shown that the adrenal sympathoexcitatory response to acute hyperinsulinemia in SHR is abolished after 1 week of treatment with metformin. In the present study, we found that metformin decreases MAP, HR, and ERSNA after intracerebroventricular administration of doses that were without effects when given intravenously. These findings suggest that metformin has antihypertensive effects in SHR that are independent of changes in peripheral insulin sensitivity. However, the results are still compatible with the notion that hyperinsulinemia can produce sympathoexcitation that, along with impaired insulin-induced inhibition of norepinephrine-induced vasoconstriction in SHR, may contribute to the development of hypertension in SHR.^{16 18 34} Thus, in agreement with our results and the clinical trials in obese hypertensive patients,^{8 9} the observations discussed above suggest that metformin may be a potentially useful antihypertensive agent during states in which both sympathetic nerve activity and plasma insulin concentrations are increased (ie, the obese, hypertensive, type II diabetic). In humans, the maximal dose of metformin is 3000 mg/d or about 40 mg/kg per day in a 70-kg person. Thus, the metformin doses that elicit marked acute sympathoinhibitory responses in rats are substantially higher than the doses used for treatment of patients with type II diabetes mellitus. Whether metformin has acute sympathoinhibitory actions in doses used clinically is unknown.

The antihypertensive and sympathoinhibitory responses to intravenous metformin were similar in intact, SAD, and SAD+VGX rats. This suggests that the acute central nervous system sympathoinhibitory action of metformin overrides the sympathoexcitatory reflex response that would be expected during the unloading of arterial and cardiopulmonary baroreceptors (ie, hypotension). Further studies are warranted to examine whether chronic metformin treatment affects arterial and cardiopulmonary baroreflex functions.

Recent studies suggest that the increase in arterial pressure during systemic nitric oxide synthase inhibition is largely mediated by disinhibition of sympathetic nerve activity due to blockade of nitric oxide synthesis in the medulla oblongata.^{21 22 23} In this study, L-NAME produced no significant changes in ERSNA in baroreceptor-denervated rats. However, intravenous injection of L-arginine elicited significant depressor and sympathoinhibitory responses that were abolished by L-NAME. The lack of a sympathoexcitatory response to L-NAME but preserved sympathoinhibitory response to L-arginine suggests that basal ERSNA was rather elevated during these experimental conditions in SHR (ie, anesthesia, surgery, SAD).

In summary, we have shown that acute intravenous administration of metformin in SHR elicited marked sympathoinhibitory, bradycardic, and hypotensive responses that were not affected by arterial and cardiopulmonary baroreflex denervation, cyclooxygenase, or nitric oxide synthase inhibition. Metformin given into the lateral cerebral ventricle decreased MAP, HR, and ERSNA in doses that produced no changes when administered intravenously. The central nervous system actions of metformin were not modified by central nervous system ₂-adrenoceptor blockade with yohimbine. We conclude that metformin has acute sympathoinhibitory effects produced by a direct central nervous system site of action.

	Selected Abbreviations and Acronyms

 

ERSNA = efferent renal sympathetic nerve activity HR = heart rate L-NAME = N-nitro-L-arginine methyl ester MAP = mean arterial pressure SAD = sinoaortic denervation SHR = spontaneously hypertensive rat(s) VGX = bilateral vagotomy

	Acknowledgments

 
This work was performed during the tenure of J.S.P. as Visiting Assistant Professor, Department of Internal Medicine, University of Iowa College of Medicine. This study was supported by grants DK-15843 and HL-44546 from the National Institutes of Health and grants from the Department of Veterans Affairs to G.F.D. J.S.P. was supported by grants from the Novo Nordisk Foundation and from the Foundation of King Christian the Xth.

	Footnotes

 
Reprint requests to Jørgen Søberg Petersen, MD, PhD, Department of Pharmacology, The Panum Institute, Building 18.6.44, 3 Blegdamsvej, DK-2200 Copenhagen N, Denmark.

	References

Top Abstract Introduction Methods Results Discussion References

 
1. United Kingdom Prospective Diabetes Study Group. United Kingdom prospective diabetes study (UKPDS) 13: relative efficacy of randomly allowed diet, sulfonylurea, insulin, or metformin in patients with newly diagnosed non-insulin dependent diabetes followed for three years. Br Med J. 1995;310:83-88. [Abstract/Free Full Text]

2. Montaguti U, Celin D, Ceredi C, Descovich GC. Efficacy of the long-term administration of metformin in hyperlipidaemic patients. Res Clin Forums. 1979;1:95-103.

3. Campbell IW, Duncan C, Patton NW, Broadhead T, Tucker GT, Woods HF. The effect of metformin on glycemic control, intermediary metabolism and blood pressure in non-insulin-dependent diabetes mellitus. Diabet Med. 1987;4:337-341. [Medline] [Order article via Infotrieve]

4. Vague P, Juhan-Vague I, Alessi MC, Badier C, Valadier J. Metformin decreases the high plasminogen activator inhibition capacity, plasma insulin and triglyceride levels in non-diabetic obese subjects. Thromb Haemost. 1987;57:326-328. [Medline] [Order article via Infotrieve]

5. De Fronzo RA, Barzilai N, Simonson DC. Mechanism of metformin action in obese and lean noninsulin-dependent diabetic subjects. J Clin Endocrinol Metab. 1991;73:1294-1301. [Abstract/Free Full Text]

6. Haupt E, Knick B, Koschinsky T, Liebermeister H, Schneider J, Hirche H. Oral antidiabetic combination therapy with sulphonylureas and metformin. Diabete Metab. 1991;17:224-231. [Medline] [Order article via Infotrieve]

7. Chan JCN, Tomlinson B, Critchley JAJH, Cockram CS, Walden RJ. Metabolic and hemodynamic effects of metformin and glibenclamide in normotensive NIDDM patients. Diabetes Care. 1993;16:1035-1038. [Abstract]

8. Giugliano D, De Rosa N, Di Maro G, Marfella R, Acampora R, Buininconti R, D'Onofrio F. Metformin improves glucose, lipid metabolism, and reduces blood pressure in hypertensive, obese women. Diabetes Care. 1993;16:1387-1390. [Abstract]

9. Giugliano D, Quatraro A, Consoli G, Minei A, Ceriello A, De Rosa N, D'Onofrio F. Metformin for obese, insulin-treated diabetic patients: improvement in glycaemic control and reduction of metabolic risk factors. Eur J Clin Pharmacol. 1993;44:107-112. [Medline] [Order article via Infotrieve]

10. Landin K, Tengborn L, Smith U. Metformin and metoprolol CR treatment in non-obese men. J Intern Med. 1994;235:335-341. [Medline] [Order article via Infotrieve]

11. DeFronzo RA, Goodman AM, and the Multicenter Metformin Study Group. Efficacy of metformin in patients with non-insulin-dependent diabetes mellitus. N Engl J Med. 1995;333:541-549. [Abstract/Free Full Text]

12. Landin K, Tengborn L, Smith U. Treating insulin resistance in hypertension with metformin reduces both blood pressure and metabolic risk factors. J Intern Med. 1991;229:181-187. [Medline] [Order article via Infotrieve]

13. Morgan DA, Ray CA, Ballon TW, Mark AL. Metformin increases insulin sensitivity and lowers arterial pressure in spontaneously hypertensive rats. Hypertension. 1992;20:421. Abstract.

14. Verma S, Bhanot S, McNeil JH. Metformin decreases plasma insulin levels and systolic blood pressure in spontaneously hypertensive rats. Am J Physiol. 1994;267:H1250-H1253. [Abstract/Free Full Text]

15. Verma S, Bhanot S, McNeil JH. Antihypertensive effects of metformin in fructose-fed hyperinsulinemic, hypertensive rats. J Pharmacol Exp Ther. 1994;271:1334-1337. [Abstract/Free Full Text]

16. Anderson EA, Mark AL. The vasodilator action of insulin: implications for the insulin hypothesis in hypertension. Hypertension. 1993;21:136-141. [Free Full Text]

17. Morgan DA, Ballon TW, Ginsberg B, Mark AL. Nonuniform regional sympathetic nerve responses to hyperinsulinemia in rats. Am J Physiol. 1993;264:R423-R427. [Abstract/Free Full Text]

18. Muntzel M, Beltz T, Mark AL, Johnson AK. Anteroventral third ventricle lesions abolish lumbar sympathetic responses to insulin. Hypertension. 1994;23:1059-1062. [Abstract/Free Full Text]

19. Veelken R, Sawin LL, DiBona GF. Epicardial serotonin receptors in circulatory control in conscious Sprague-Dawley rats. Am J Physiol. 1990;258:H466-H472. [Abstract/Free Full Text]

20. Petersen JS, DiBona GF. Furosemide elicits non-uniform reflex responses via cardiac sympathetic afferents. Hypertension. 1994;23:924-930. [Abstract/Free Full Text]

21. Togashi H, Sakuma I, Yoshioka M, Kobayashi T, Yasuda H, Kitabatake A, Saito H, Gross SS, Levi R. A central nervous system action of nitric oxide in blood pressure regulation. J Pharmacol Exp Ther. 1992;262:343-347. [Abstract/Free Full Text]

22. Zanzinger J, Czachurski J, Seller H. Inhibition of sympathetic vasoconstriction is a major principle of vasodilation by nitric oxide in vivo. Circ Res. 1994;75:1073-1077. [Abstract/Free Full Text]

23. Zanzinger J, Czachurski J, Seller H. Inhibition of basal and reflex-mediated sympathetic activity in the RVLM by nitric oxide. Am J Physiol. 1995;268:R958-R962. [Abstract/Free Full Text]

24. Krieger EM. Neurogenic hypertension in the rat. Circ Res. 1964;15:511-521. [Abstract/Free Full Text]

25. Petersen JS, Hinojosa-Laborde C, DiBona GF. Sympathoinhibitory responses to 2-methyl-serotonin during changes in sodium intake. Hypertension. 1993;21:1000-1004. [Abstract/Free Full Text]

26. Wallenstein S, Zucker CL, Fleiss JL. Some statistical methods useful in circulation research. Circ Res. 1980;47:1-9. [Abstract/Free Full Text]

27. Hermann LS, Scherstén B, Bitzén P-O, Kjellström T, Lindgärde F, Melander A. Therapeutic comparison of metformin and sulphonylurea, alone and in various combinations: a double-blind controlled study. Diabetes Care. 1994;17:1100-1109. [Abstract]

28. Semplicini A, Del Prato S, Giusto M, Campagnolo M, Palatini P, Rossi GP, Valle R, Dorella M, Albertini G, Pessina AC. Short-term effects of metformin on insulin sensitivity and sodium homeostasis in essential hypertensives. J Hypertens. 1993;11(suppl 5):S276-S277.

29. Gudbjörnsdottir S, Friberg P, Elam M, Attvall S, Lönnroth P, Wallin BG. The effect of metformin and insulin on sympathetic nerve activity, norepinephrine spillover and blood pressure in obese, insulin resistant, normoglycemic, hypertensive men. Blood Pressure. 1994;3:394-403. [Medline] [Order article via Infotrieve]

30. Tuck ML. Obesity, the sympathetic nervous system, and essential hypertension. Hypertension. 1992;19(suppl I):I-68-I-77.

31. Zhang HY, Reddy SR, Kotchen TA. Antihypertensive effect of pioglitazone is not invariably associated with increased insulin sensitivity. Hypertension. 1994;25:106-110.

32. Katayama S, Abe M, Kashiwabara H, Kosegawa I, Ishii J. Evidence against a role of insulin in hypertension in spontaneously hypertensive rats: CS-045 does not lower blood pressure despite improvement of insulin resistance. Hypertension. 1994;23:1071-1974. [Abstract/Free Full Text]

33. Morgan DA, Mark AL. Contrasting effects of metformin and ciglitazone on sympathetic nerve responses to insulin. FASEB J. 1993;7:A653. Abstract.

34. Lembo G, Iaccarino G, Vecchione C, Rendina V, Trimarco B. Insulin modulation of vascular reactivity is already impaired in prehypertensive spontaneously hypertensive rats. Hypertension. 1995;26:290-293.[Abstract/Free Full Text]

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