Vanadyl sulfate prevents fructose-induced hyperinsulinemia and hypertension in rats

Mechanisms of insulin resistance in rat models of hypertension and their relationships with salt sensitivity

Several lines of evidence suggest that insulin resistance and the resultant hyperinsulinaemia are causally related to hypertension. Insulin actions are initiated by binding to a high-affinity transmembrane protein receptor which is present in all mammalian cells. These effects are predominant in skeletal muscle, liver, and fat and involve a number of tissue-specific and biochemically diverse events. Less well known are effects of insulin occurring in tissues not usually considered as insulin targets, which are hypothetical contributors to the pro-hypertensive action of the hormone. These effects include activation of renal sodium reabsorption, stimulation of the sympathetic nervous system, growth-promoting activity on vascular smooth muscle cells, and modulation of transmembrane cation transport. Epidemiological investigations have implicated sodium intake in the pathogenesis of hypertension. Because of the sodium-retaining effects of insulin, it has been postulated that insulin resistance with associated hyperinsulinaemia may be critical for the pathogenesis of salt-sensitivity in essential hypertensive subjects. Insulin resistance is present also in strains of rats with genetic hypertension that can be utilized as models to study the molecular mechanisms of this abnormality. In the present article, we summarize the current knowledge of the mechanisms of insulin resistance in rat models of arterial hypertension in which decreased sensitivity to insulin occurs and propose a rationale hypothesis that links insulin resistance with salt-sensitivity and hypertension.

In vivo regulation of protein-serine kinases by insulin in skeletal muscle of fructose-hypertensive rats

The effects of tail-vein insulin injection (2 U/kg) on the regulation of protein-serine kinases in hindlimb skeletal muscle were investigated in hyperinsulinemic hypertensive fructose-fed (FF) animals that had been fasted overnight. Basal protein kinase B (PKB) activity was elevated about twofold in FF rats and was not further stimulated by insulin. Phosphatidylinositol 3-kinase (PI3K), which lies upstream of PKB, was increased approximately 3.5-fold within 2-5 min by insulin in control rats. Basal and insulin-activated PI3K activities were further enhanced up to 2-fold and 1.3-fold, respectively, in FF rats. The 70-kDa S6 kinase (S6K) was stimulated about twofold by insulin in control rats. Both basal and insulin-stimulated S6K activity was further enhanced up to 1.5-fold and 3.5-fold, respectively, in FF rats. In control rats, insulin caused a 40-50% reduction of the phosphotransferase activity of the beta-isoform of glycogen synthase kinase 3 (GSK-3beta), which is a PKB target in vitro. Basal GSK-3beta activity was decreased by approximately 40% in FF rats and remained unchanged after insulin treatment. In summary, 1) the PI3K --> PKB --> S6K pathway was upregulated under basal conditions, and 2) insulin stimulation of PI3K and S6K activities was enhanced, but both PKB and GSK-3 were refractory to the effects of insulin in FF rats.

Insulin-induced endothelin release and vasoreactivity in hypertriglyceridemic and hypertensive rats

Insulin-elicited endothelin release in hypertriglyceridemic, hypertensive, hyperinsulinemic (HTG) rats was shown. Weanling male Wistar rats were given 30% sucrose in their drinking water for 20-24 wk. In vitro contractions of aorta and femoral arteries were elicited with 40 mM KCl. Endothelin release induced with KCl plus 50 microU/ml insulin resulted in increases in contractile responses: 41 +/- 5.9 and 57 +/- 6% for control and 65.5 +/- 6 and 95 +/- 9% for HTG aortas and femoral arteries, respectively. The endothelin ET(B)-receptor blocker BQ-788 decreased responses to KCl + insulin by 39 +/- 8 and 53 +/- 5% in control and 48 +/- 13 and 79 +/- 3.5% in HTG aortas and femoral arteries, respectively. The ET(A)-receptor antagonist PD-151242 inhibited these responses by 12 +/- 10 and 1 +/- 9% in control and by 51.5 +/- 9 and 58.5 +/- 1% in HTG aortas and femoral arteries, respectively. These results suggest that endothelin may contribute to the hypertension in this model.

Exogenous hyperinsulinemia causes insulin resistance, hyperendothelinemia, and subsequent hypertension in rats

In many clinical and animal studies, hypertension and insulin resistance coexist, but their mechanistic relationship is unclear. We explored the causal link between these two parameters in a rat model with chronic hyperinsulinemia induced with human insulin (1 U/d) released from subcutaneously implanted minipumps. Rats with saline minipumps served as a control. During the first experiment, plasma levels of insulin and glucose and the systolic blood pressure of the two groups were continuously monitored for 17 days. In the subsequent four experiments, rats were killed on days 10 and 13 to measure plasma endothelin-1 (ET-1) levels and the glucose transport into and insulin and ET-1 binding of isolated adipocytes. In one experiment, rats were tested for oral glucose tolerance on days 10 and 13. In another experiment, ET-1 binding to the aortic plasma membrane was also determined. The results showed that rats became hyperinsulinemic throughout the experimental period by the instillation of exogenous insulin. Hyperinsulinemic rats were consistently hypoglycemic during the first day, but they became euglycemic thereafter, indicating an insulin-resistant state. Glucose intolerance was obvious by day 10, but significant hypertension was not detected until the 11th day on insulin infusion. Compared with the saline controls, insulin-infused rats had an increase of plasma ET-1 levels but a decrease of both basal and insulin-stimulated glucose transport into adipocytes. ET-1 binding to adipocytes of the insulin-infused group was elevated significantly from day 10 through day 13. ET-1 binding to the aortic membranes, supposedly downregulated by the increased plasma ET-1 and hypertension, was similar to that found in the controls on day 13. These results imply that hyperinsulinemia in rats could lead to hypertension via the elevation of plasma ET-1 levels together with an unaltered vascular binding of ET-1, which was probably unrelated to the insulin resistance.

Endothelial dysfunction precedes hypertension in diet-induced insulin resistance

The insulin-resistant (IR) syndrome may be an impetus for the development of hypertension (HTN). Unfortunately, the mechanism by which this could occur is unclear. Our laboratory and others have described impaired endothelium-mediated relaxation in IR, mildly hypertensive rats. The purpose of the current study is to determine if HTN is most likely a cause or result of impaired endothelial function. Sprague-Dawley rats were randomized to receive a fructose-rich diet for 3, 7, 10, 14, 18, or 28 days or were placed in a control group. The control group received rat chow. After diet treatment, animals were instrumented with arterial cannulas, and while awake and unrestrained, their blood pressure (BP) was measured. Subsequently, endothelium-mediated relaxation to acetylcholine was determined (in vitro) by measuring intraluminal diameter of phenylephrine-preconstricted mesenteric arteries ( approximately 250 microM). Serum insulin levels were significantly elevated in all groups receiving fructose feeding compared with control, whereas there were no differences in serum glucose levels between groups. Impairment of endothelium-mediated relaxation starts by day 14 [mean percent maximal relaxation (Emax): 69 +/- 10% of baseline] and becomes significant by day 18 (Emax: 52 +/- 11% of baseline; P < 0.01). However, the mean BP (mmHg) does not become significantly elevated until day 28 [BP: 132 +/- 1 (day 28) vs. 116 +/- 3 (control); P < 0.05]. These findings demonstrate that both IR and endothelial dysfunction occur before HTN in this model and suggest that endothelial dysfunction may be a mechanism linking insulin resistance and essential HTN.

Comparative effects of chromium, vanadium and gymnema sylvestre on sugar-induced blood pressure elevations in SHR

OBJECTIVE

Effects on systolic blood pressure (SBP) of ingesting three agents reported to influence insulin metabolism, i.e., chromium polynicotinate, bis(maltolato)oxovanadium (BMOV), and the herb, Gymnema sylvestre, were assessed simultaneously in spontaneously hypertensive rats (SHR).

METHODS

In the first study, SHR were fed either a starch, sugar, or sugar diet containing chromium polynicotinate, bis(maltolato)oxovanadium (BMOV), or G. sylvestre. Tail SBP was estimated indirectly and various blood chemistries were measured. TBARS formation was determined in hepatic and renal tissue. In a second study, tail SBP was measured in SHR ingesting diets containing different concentrations of BMOV.

RESULTS

Compared to starch, SHR consuming sucrose showed a significant elevation of SBP within days that was maintained for the duration of study. Addition of chromium polynicotinate to the sucrose diet at the beginning of study prevented the sucrose-induced elevation of SBP for 2 weeks, but SBP rose significantly after that. BMOV at high concentrations overcame the sucrose-induced rise in SBP and even decreased SBP below values seen in SHR eating the starch diet, but marked weight loss was noted. A second study examined different concentrations of BMOV. At 0.01% w/w concentration of BMOV, SBP was still significantly decreased, even though SHR did not lose body weight (BW) early on. SHR consuming G. sylvestre showed no change or even elevated SBP. Hepatic thiobarbituric acid reacting substances (TBARS) formation, an estimate of lipid peroxidation, was decreased by chromium polynicotinate and BMOV, and renal TBARS by chromium polynicotinate. Circulating cholesterol concentrations were decreased in the SHR consuming G. sylvestre.

CONCLUSIONS

Chromium decreases the portion of SBP elevated by high sucrose intake as shown previously, but high levels of sucrose ingestion can eventually overcome this. BMOV overcame sucrose-induced elevation of SBP as well as some of the "genetic hypertension." Different from chromium, this decrease was not overcome by high levels of dietary sucrose. The significant lowering of cholesterol with G. sylvestre ingestion indicates some effect on metabolism, but G. sylvestre did not lower and even raised SBP.

Role of aldehydes in fructose induced hypertension

Aldehydes are formed in tissues of humans and animals as intermediates of glucose and fructose metabolism and due to lipid peroxidation. N-acetyl cysteine (NAC), an analogue of the dietary amino acid cysteine, binds aldehydes thus preventing their damaging effect on physiological proteins. We measured systolic blood pressure (SBP), platelet cytosolic free calcium [Ca2+]i and tissue aldehyde conjugates in fructose induced hypertensive Wistar-Kyoto (WKY) rats and examined the effect of NAC in the diet on these parameters. Animals age 7 weeks were divided into three groups of 6 animals each and were treated as follows: WKY-control (chow diet and normal drinking water); WKY-Fructose (chow diet and 4% fructose in drinking water); WKY-Fructose+NAC (1.5% NAC in chow diet and 4% fructose in drinking water). After 11 weeks, systolic blood pressure, platelet [Ca2+]i and kidney aldehyde conjugates were all significantly higher in fructose treated rats. NAC treatment prevented these changes. These results suggest that aldehydes may be the cause of fructose induced hypertension and elevated cytosolic free calcium.

Type I and II models of diabetes produce different modifications of K+ currents in rat heart: role of insulin

1. Several K+ currents were measured and compared in enzymatically dispersed ventricular myocytes from control and diabetic rats. 2. Diabetic conditions were established either with a single intravenous injection of streptozotocin (STZ, 100 mg kg-1; 6-14 days duration) or by feeding with a fructose-enriched diet for 4-10 weeks. Both groups became hyperglycaemic, with the former having decreased and the latter having elevated levels of plasma insulin. These conditions therefore mimic type I (insulin-dependent) and type II (non-insulin-dependent) diabetes mellitus, respectively. 3. As reported previously, a Ca(2+)-independent transient outward K+ current, I(t), was attenuated in the type I model. This was not observed in the type II model. The two models differed greatly in the changes observed in a quasi-steady-state K+ current denoted Iss. In the STZ model Iss was substantially attenuated, whereas in the fructose-fed model it was augmented. In both models, the background inwardly rectifying current, IK1, was unchanged. Concomitantly, there was a substantial prolongation of the action potential in the STZ model but not in the fructose-fed model. 4. Incubation of control myocytes with insulin (100 nM) for 5-9 h caused a significant augmentation of Iss, with no effect on I(t) or on IK1. Incubation of myocytes from STZ-diabetic rats with insulin reversed the attenuation of I(t), but not of Iss. 5. The effect of insulin was not blocked by wortmannin, an inhibitor of phosphatidylinositol 3-kinase. However, inhibition of the mitogen-activated protein kinase pathway with PD98059 prevented restoration of I(t). Insulin action on I(t) may therefore involve changes in transcription or expression of channel proteins, rather than changes in cellular metabolism.

Sugar-induced blood pressure elevations over the lifespan of three substrains of Wistar rats

OBJECTIVE

Since the majority of studies concerned with sugar-induced blood pressure elevation have principally been short-term, the present investigation followed the effects of heavy sucrose ingestion on systolic blood pressure (SBP) and related parameters over the lifespan of three substrains of Wistar rats.

METHODS

Two hundred twenty-five rats (75 spontaneously hypertensive rats (SHR), 75 Wistar Kyoto rats (WKY), 75 Munich Wistar rats (WAM) were given one of five diets. The baseline diet in terms of calories derived 32% from sucrose, 33% from protein, and 35% from fat. The remaining four diets derived their calories as follows: a high sugar-low protein diet--52% of calories from sucrose, 15% from protein, and 33% from fat; a high sugar-low fat diet--53% of calories from sucrose, 37% from protein, and 10% from fat; a low sugar-high protein diet--11% calories from sucrose, 56% from protein, and 33% from fat, and a low sugar-high fat--13% of calories from sucrose, 32% from protein, and 55% from fat.

RESULTS

All substrains showed the highest systolic blood pressure when ingesting the two diets highest in sucrose. The highest sugar-induced SBP elevation, which remained over the lifespan of all substrains, was found in SHR. WKY had an intermediate elevation. WAM showed the lowest responses, although the average elevation of 6-8 mm Hg was statistically significant. The following parameters could not be correlated with long-term elevation of SBP; body weight, catecholamine excretion, renal function, and plasma renin activity. Only insulin concentrations correlated: insulin concentrations were consistently higher in the two groups of WKY and WAM consuming the high sucrose diets.

CONCLUSIONS

High dietary sucrose can chronically increase SBP in three substrains of Wistar rats. Increased concentrations of circulating insulin were found in WKY and WAM suggesting that the glucose/insulin system was involved, at least in these two substrains, in the maintenance of high SBP levels during chronic, heavy sugar ingestion.

Nutritional factors that can favorably influence the glucose/insulin system: vanadium

A growing body of experimental and clinical research indicates that the trace element, vanadium, exerts potent insulin-mimetic effects in vitro and in vivo when used in pharmacological doses. Since our first demonstration of the anti-diabetic and cardioprotective effects of vanadium in vivo, impressive advances have been made in our understanding of its mechanism of action, pharmacokinetics and pharmacodynamics. A major advance in the use of vanadium as an insulin-mimetic has been the development of organic vanadium complexes which are 2 to 3 times as potent as inorganic vanadium and have been extensively studied in our laboratory. There is an emerging role for the use of vanadium in human diabetes and the recently conducted clinical trials support this contention. The present review summarizes some of the key aspects of vanadium biology which exemplify the potent insulin-mimetic, anti-diabetic and antihypertensive effects of this intriguing trace element.

Elevated blood pressure in spontaneously hypertensive rats consuming a high sucrose diet is associated with elevated angiotensin II and is reversed by vanadium

OBJECTIVE

To determine the changes in serum angiotensin II (Ang II) and endothelin-1 levels induced by vanadium treatment of sugar-fed rats in order to investigate the relationship between changes in blood pressure and Ang II and endothelin-1 levels.

METHODS

Male spontaneously hypertensive rats (SHR) were fed starch (control), sucrose, and sucrose plus vanadium compounds at various concentrations. The systolic blood pressure of the rats was estimated by tail-cuff plethysmography. Serum Ang II and endothelin-1 levels were measured by radioimmunoassay.

RESULTS

There were increases in systolic blood pressure (by 8%) and in serum Ang II (by 20%) in sucrose-fed SHR compared with control. In sucrose plus vanadium-fed SHR, the lowering of the systolic blood pressure (by 11-16% of the sucrose-fed value) was accompanied by a significant decrease in Ang II levels (by 25-60% of the sucrose-fed value) and an increase in endothelin-1 level (by 61-76% of the sucrose-fed value).

CONCLUSION

That Ang II levels are elevated in sucrose-induced hypertension and decreased after vanadium therapy suggests that the renin-angiotensin system plays a role in the induction of hypertension in this model. On the other hand, the elevation of endothelin-1 levels associated with a decreased systolic blood pressure might be secondary to vanadium stimulation of endothelial cells. The data suggest that endothelin-1 is not involved in sugar-induced elevations of the blood pressure.

Effects of bis(maltolato)oxovanadium(IV) are distinct from food restriction in STZ-diabetic rats

In association with the insulin-mimetic properties, vanadium and related compounds have been shown to normalize hyperphagia associated with diabetes mellitus. The objective of this study was to clarify the effects of an organic vanadium compound, bis(maltolato)oxovanadium(IV) (BMOV), vs. food restriction on the metabolic abnormalities that occur in diabetes. BMOV was administered daily in drinking water to streptozotocin (STZ)-diabetic rats for 6 wk. Pair-fed groups were fed based on the intake for their respective counterparts from the previous day. Plasma parameters were measured weekly after a carefully controlled 5-h fasting period. BMOV reduced plasma glucose (diabetic = 31.2 +/- 1.9, diabetic treated = 10.2 +/- 1.8, and diabetic pair fed = 34.2 +/- 1.1 mM), triglyceride, and cholesterol levels to normal without a concomitant increase in plasma insulin levels. There was no body weight gain in the diabetic pair-fed group compared with all other groups. BMOV but not pair feeding was effective in preventing the decreased cardiac function observed in STZ-diabetic rats. These data suggest that the glucose-lowering properties of BMOV are independent of the effects of dietary restriction and reinforce the efficacy of BMOV as an effective antihyperglycemic agent.

Decreased vascular reactivity in metformin-treated fructose-hypertensive rats

We have previously demonstrated that long-term metformin treatment prevents the development of hyperinsulinemia and hypertension in fructose-hypertensive (FH) rats; however, the exact nature of its antihypertensive effects remains elusive. Since hyperinsulinemia has been proposed to be a strong stimulus for norepinephrine (NE) release, the present study examined the effects of long-term metformin treatment (500 mg/kg/d for 10 weeks) on the reactivity of superior mesenteric arteries to NE in FH rats. Metformin treatment prevented the development of hyperinsulinemia and hypertension in FH rats. Mesenteric arteries from FH rats exhibited an increased cross-sectional area ([CSA] 0.45 +/- 0.07 mm2 v 0.32 +/- 0.05 in controls, P < .05), which was prevented by long-term metformin treatment (0.34 +/- 0.04 mm2, p > .05 v untreated FH). Interestingly, mesenteric arteries from metformin-treated fructose and control rats exhibited a reduction in maximum responsiveness to NE both with and without the endothelium. These data suggest that metformin directly reduces catecholamine constrictor responses in resistance arteries of rats, which may contribute to its antihypertensive effects in rats.

Vascular reactivity to phenylephrine and angiotensin II in hypertensive rats associated with insulin resistance

Previous reports suggest that when rats are fed a carbohydrate-enriched diet they develop hyperinsulinemia associated with elevated blood pressure. The purpose of this study was to assess the vascular reactivity of fructose-treated rats to various pressor agents. Male Sprague Dawley rats (n = 24) were used for this study and were divided into two equal groups. One of the groups was fed normal rat chow and served as the control group, whereas the other group was fed a fructose-enriched diet for four weeks. Mean blood pressure was elevated in the fructose-treated rats at the end of the second week of fructose treatment and remained elevated for the remainder of the study. At the end of the second and fourth weeks of fructose treatment, six rats from each group were used to assess both in vivo and subsequently in vitro vascular reactivity to various pressor agents. The jugular vein and carotid artery were cannulated under anesthesia. Twenty four hours after recovery from surgery pressor responses to angiotensin II (AII) and phenylephrine (PE) were determined. Twenty four hours later rats were decapitated and the thoracic aorta was removed, cleaned of adhering fat and cut into ring segments for vascular reactivity studies. Tissues were suspended in muscle baths containing physiological saline solution and maintained at 37 degrees C. Dose-response curves were generated in the aorta in response to potassium chloride (KCl), AII and PE. At the end of the second week of fructose treatment pressor response to AII was significantly increased in the fructose-treated rats compared to the controls whereas there was no significant difference in pressor response to PE. There was no significant difference in pressor response to AII and PE between the two groups at the end of the fourth week of fructose treatment. In vitro contractile response of the aorta to AII and PE were significantly greater in the fructose-fed rats compared to the controls at the end of the second week of fructose treatment; however, there was no change in the EC50 between the two groups. At the end of the fourth week of fructose treatment, the contractile responses to AII and PE were similar in both groups, although the response to AII tended to be lower in the fructose-fed rat. There was no significant difference in the contractile response to potassium chloride or in acetylcholine-induced relaxation throughout the study. These results strongly suggest that hypertension in fructose-treated rats is associated with increased in vitro vascular reactivity to AII and PE in the early stages of hypertension.

Changes in angiotensin AT1 receptor density during hypertension in fructose-fed rats

Feeding carbohydrate-enriched diets to normal rats has been shown to induce insulin resistance and hyperinsulinemia associated with an elevation of blood pressure. Previously we reported that the renin-angiotensin system (RAS) is likely to be involved in the elevation of blood pressure. The purpose of this study was to determine the changes in plasma angiotensin II (AII) and AII receptor density associated with the elevation of blood pressure in fructose-treated rats. Male Sprague-Dawley rats were divided into two groups and were fed either normal rat chow or a 60% fructose-enriched diet for four weeks. Plasma insulin of fructose-treated rats was significantly elevated (p < 0.05) by the end of first week of fructose treatment and remained elevated throughout the study. Plasma AII levels of fructose-fed rats was 3.5 fold greater than the controls at the end of second week and returned to basal levels at the end of the fourth week of dietary treatment. Blood pressure was significantly elevated in the fructose-fed rats within two weeks of fructose treatment. Elevation of blood pressure was associated with left ventricular hypertrophy. Angiotensin II type I receptor (AT1) density was determined in the left ventricle, aorta, adrenal gland and hypothalamus. There was a significant increase in AT1 receptor density in the ventricle at the end of third and fourth weeks of treatment, whereas there was a significant decrease in the receptor density in the aorta at the end of the fourth week of treatment. Receptor density in the adrenal gland and hypothalamus of fructose-fed rats was similar to their respective controls. The results of this study suggest that the RAS plays a role in the elevation of blood pressure of fructose-fed rats and also contributes to the ventricular hypertrophy observed in these rats.

Antihypertensive effects of vanadium compounds in hyperinsulinemic, hypertensive rats

Although considerable evidence lends credence to the association between insulin resistance, hyperinsulinemia and essential hypertension, the precise nature of this relationship remains unexplained. In the present investigation, we examined the proposition that these metabolic defects contribute causally to the development of high blood pressure. If these metabolic abnormalities were responsible for the development of hypertension, then drug interventions that improve these defects should also decrease high blood pressure. Since previous studies have demonstrated that vanadium compounds enhance insulin action and lower plasma insulin levels in nondiabetic rats, we examined the effects of these compounds on insulin sensitivity, plasma insulin concentration and blood pressure in two hyperinsulinemic models of experimental hypertension. The animal models studied were the genetically predisposed spontaneously hypertensive rat and the fructose-hypertensive rat, where hypertension is induced in normotensive rats by feeding them a high fructose diet. Vanadium compounds caused marked and sustained decreases in plasma insulin concentration and blood pressure in both the animal models studied. Furthermore, the effect of the drugs on blood pressure was reversed by restoring plasma insulin levels in the drug-treated rats to those observed in their untreated counterparts. These data suggest that either hyperinsulinemia contributes to the development of hypertension in both the spontaneously hypertensive and the fructose-hypertensive rats or that the underlying mechanism is closely related to the expression of both these disorders.

Increased potency of vanadium using organic ligands

The in vivo glucose lowering effect of orally administered inorganic vanadium compounds in diabetes was first reported in our laboratory in 1985. While both vanadate and vanadyl forms of vanadium are orally active, they are still not well absorbed. We have synthesized several organic vanadium compounds and one compound, bis(maltolato)oxovanadium(lV) or BMOV, has been extensively investigated. BMOV proved effective in lowering plasma glucose and lipids in STZ-diabetic rats when administered in drinking water over a 25 week period. The maintenance dose (0.18 mmol/kg/day) was approximately 50% of that required for vanadyl sulfate (VS). Secondary complications of diabetes were prevented by BMOV and no marked toxicity was noted. Oral gavage of STZ-diabetic rats with BMOV also reduced blood glucose levels. The ED50 for BMOV was 0.5 mmol/kg, while for VS the estimated ED50 was 0.9 mmol/kg. BMOV was also effective by the intraperitoneal route in STZ-diabetic rats. The ED50 was 0.08 mmol/kg compared to 0.22 mmol/kg for VS. Some animals treated p.o. or i.p. remained euglycemic for up to 14 weeks. An i.v. infusion of BMOV of 0.05 mmol/kg over a 30 min period reduced plasma glucose levels by 50% while VS was not effective.

In vivo and in vitro studies of vanadate in human and rodent diabetes mellitus

In vivo vanadate and vanadyl have been shown to mimic the action of insulin and to be effective treatment for animal models of both Type I and Type II diabetes. The molecular mechanism of action of the vanadium salts on insulin sensitivity remains uncertain, and several potential sites proposed for the insulin-like effects are reviewed. In human trials, insulin sensitivity improved in patients with NIDDM, as well as in some patients with IDDM after two weeks of treatment with sodium metavanadate. This increase in insulin sensitivity was primarily due to an increase in non-oxidative glucose disposal, whereas oxidative glucose disposal and both basal and insulin stimulated suppression of hepatic glucose output (HGP) were unchanged. Clinically, oral vanadate was associated with a small decrease in insulin requirements in IDDM subjects. Of additional benefit, there was a decrease in total cholesterol levels in both IDDM and NIDDM subjects. Furthermore, there was an increase in the basal activities of MAP and S6 kinases to levels similar to the insulin-stimulated levels in controls, but there was little or no further stimulation with insulin was seen. Further understanding of the mechanism of vanadium action may ultimately be useful in the design of drugs that improve glucose tolerance.

Effects of losartan on blood pressure, metabolic alterations, and vascular reactivity in the fructose-induced hypertensive rat

Fructose feeding induces a moderate increase in blood pressure levels in normal rats that is associated with insulin resistance, hyperinsulinemia, and hypertriglyceridemia. The sympathetic nervous system seems to participate in the alterations of this model. To further explore the mechanisms underlying fructose-induced hypertension, the effects of the AT1 receptor antagonist losartan on blood pressure, insulin resistance, renal function, and vascular reactivity in mesenteric vascular beds were studied. Sprague-Dawley rats were fed for 4 weeks with diets containing 60% fructose or 60% starch (control), and half of each group received losartan (1 mg/kg per day) in the drinking water. Fructose-fed rats showed higher (P < .05) blood pressure levels and plasma concentrations of triglycerides and insulin than those of controls. Losartan treatment prevented both blood pressure elevation and hyperinsulinemia in fructose-fed rats but not elevation of plasma triglycerides. Plasma glucose and insulin levels in response to an oral glucose load were higher (P < .05) in fructose-fed rats than in controls. These exaggerated responses were prevented by losartan treatment. No differences in the constrictor responses of mesenteric vascular beds to KCl (60 mumol), angiotensin II (1 nmol), phenylephrine (10(-5) mol/L), or endothelin-1 (10 pmol) were found between the two groups. Relaxing responses to acetylcholine or sodium nitroprusside in phenylephrine-precontracted mesenteric vascular beds and constrictor response to the nitric oxide synthesis inhibitor NG-nitro-L-arginine methyl ester (100 nmol) were comparable in both groups. Losartan blunted angiotensin II constriction and reduced (P < .05) responses to phenylephrine in all groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Fructose-induced hypertension in rats is concentration- and duration-dependent

The present study determined the most suitable concentration and duration of fructose treatment for inducing hypertension in Wistar rats. The correlation between fructose-induced hypertension and hyperinsulinemia was also evaluated. The rats were treated with 5%, 10%, or 20% fructose in drinking water. The greatest changes, including increases in blood pressure, fluid intake, and plasma levels of insulin, glucose, and triglycerides, and a decrease in food intake following fructose treatment, were observed with the 10% solution. The times of the onset and maximum response differed for the various parameters measured. The increase in blood pressure occurred earlier than the increase in the plasma insulin level. All abnormalities disappeared rapidly after fructose withdrawal. There was no significant correlation between plasma insulin level and systolic blood pressure. In conclusion, treatment with 10% fructose in drinking water (equivalent to a diet containing 48-57% fructose) for one week or longer is appropriate for the rapid production of fructose-induced hypertension in Wistar rats, which is associated with elevated levels of plasma insulin, glucose, and triglycerides.

Tissue-dependent activation of protein kinase C in fructose-induced insulin resistance

Rats fed a fructose-enriched diet develop increases in blood pressure and resistance to insulin-mediated glucose disposal, but the underlying biochemical alterations have not been clearly defined. Since protein kinase C (PKC) has been implicated in the pathogenesis of insulin resistance, as well as blood pressure (BP) regulation, the present study was initiated to see whether changes in PKC signaling are present in rats with fructose-induced insulin resistance and hypertension. Consequently, liver, muscle, and adipose tissues were collected from fructose (n = 13) and chow (n = 12) fed Sprague-Dawley rats. PKC enzyme activity, and expression of classical PKC isozymes, were measured in cytosol and membrane fractions, and 1, 2-diacylglycerol (DAG), an endogenous stimulator of PKC, was measured by radio-enzymatic assay. Fructose feeding was associated with significant increases in fasting plasma insulin (140%) and triglyceride (400%) levels, and increased BP (20 mmHg). PKC activity was increased in the membrane fraction of adipose tissue (234 ± 38 (SE)vs 85 ± 30 pmol/min/mg protein,P< 0.007), without evidence of increased translocation or activation by DAG. Thus, fructose-induced insulin resistance has no effect on conventional PKC activity and subcellular distribution in liver and muscle, but the 3-fold increase in membraneassociated kinase activity in fat may be relevant to the mechanism of hypertriglyceridemia associated with fructose feeding.

Vanadyl sulfate lowers plasma insulin and blood pressure in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) are hyperinsulinemic compared with their Wistar-Kyoto (WKY) controls. Since previous studies have demonstrated that vanadyl sulfate lowers insulin levels in nondiabetic rats, we used vanadyl to explore the relation between hyperinsulinemia and hypertension. In a prevention study, 5-week-old SHR and WKY rats were started on long-term vanadyl sulfate treatment. Vanadyl in doses of 0.4 to 0.6 mmol/kg per day lowered plasma insulin (252 +/- 22.8 versus 336 +/- 12.6 pmol/L, treated versus untreated, P < .01) and systolic blood pressure (158 +/- 2 versus 189 +/- 1 mm Hg, P < .001) in SHR without causing any change in plasma glucose. No changes were seen in the treated WKY rats. At 11 weeks of age, a group of untreated rats from the prevention study was started on vanadyl treatment as before. Again, vanadyl caused significant and sustained decreases in plasma insulin (264 +/- 12.6 versus 342 +/- 6.6 pmol/L, treated versus untreated, P < .001) and blood pressure (161 +/- 1 versus 188 +/- 1 mm Hg, P < .001) in SHR but had no effect in the normotensive WKY controls. Furthermore, restoration of plasma insulin in the vanadyl-treated SHR to pretreatment levels (subcutaneous insulin, 14,000 pmol/kg per day) reversed the effects of vanadyl on blood pressure (vanadyl with insulin, 190 +/- 3.0 mm Hg versus vanadyl without insulin, 152 +/- 3.0 mm Hg, P < .001). Since vanadyl treatment resulted in decreased weight gain, treated SHR were compared with a corresponding pair-fed group.(ABSTRACT TRUNCATED AT 250 WORDS)

Modified method for the performance of glucose insulin clamp studies in conscious rats

A modified method for performance of insulin-glucose clamp studies in rats was developed via catheterization of the tail vessels, after preconditioning of animals to limited restraint. The procedure is performed in conscious animals under local anesthesia and employs a specially designed foam rubber jacket which allows the animal mobility of the limbs and forward vision. In addition, a table utilizing a belt system allows easy positioning of the animal in the left and right lateral and supine positions during surgery. After initial development of the procedure, its use in 123 animals is reported. Line placement was successfully achieved in all cases with insignificant blood loss or morbidity and zero mortality. We note that 11% of animals did not complete the subsequent insulin-glucose clamp study due to either one of the vascular cannulae leaving the vessel (one animal), venous rupture (12 animals), or cannula blockage unrelated to surgical technique (one animal). Studies on Wistar Kyoto, Spontaneously Hypertensive, and Sprague-Dawley rats showed a fall in catecholamines after animals were replaced in cages, with stabilization within 30 min. In comparison to traditional techniques, this method is, therefore, proposed as a less traumatic and rapid way of performing infusion studies in conscious rats with a high success rate and minimization of loss of animal life due to procedural problems.

Effect of chronic losartan potassium treatment on fructose-induced hypertension

Carbohydrate enriched diets have been shown to elevate blood pressure in the rat. The precise mechanism by which carbohydrate feeding elevates blood pressure is not known. We evaluated the role of the renin-angiotensin system in the etiology of fructose-induced hypertension. Losartan potassium, an angiotensin II (AII) Type 1 (AT1) receptor antagonist, was utilized to assess the blood pressure response to fructose treatment. Male Sprague-Dawley rats were divided into 3 groups. Rats in the control group were fed regular chow. The other two groups were fed 60% fructose diet for 4 weeks. One of these groups was chronically treated with losartan potassium in drinking water. Throughout the study there was no significant difference in body weight between the three groups. There was a significant increase in blood pressure of fructose-treated rats within one week of treatment which remained elevated for the remainder of the study. Chronic losartan treatment significantly attenuated the rise in blood pressure. Within two weeks both the dipsogenic response and the pressor response to AII demonstrated complete blockage of AII receptors. These results suggest that the renin-angiotensin system plays a role in the development of fructose-induced hypertension.