Enhanced glucose tolerance in spontaneously hypertensive rats. Pancreatic beta-cell hyperfunction with normal insulin sensitivity

Rapid Gastric emptying in spontaneously hypertensive rats

OBJECTIVE

To assess the rate of gastric emptying in spontaneously hypertensive rats (SHR) and to evaluate rapid gastric emptying as a possible predisposing factor for hypertension. Rapid gastric emptying of carbohydrates, known to elevate postprandial serum glucose, has been reported to occur in many insulin-resistant states, including hypertension. SHR exhibit insulin resistance similar to human hypertensive patients. No prior studies have assessed gastric emptying of an oral glucose solution in SHR as compared with control Wistar Kyoto rats (WKY).

METHODS

Using scintigraphic imaging, gastric emptying of a physiologic, orally consumed glucose solution was assessed in 12 SHR and 12 control WKY at 5 weeks of age, prior to the development of hypertension, and at 12 weeks of age after hypertension was fully established.

RESULTS

At 5 weeks, the gastric half-emptying time (GHET) was 67.8 ± 9.8 min for the SHR vs. 109.3 ± 18 ( P  = 0.042) minutes for the WKY controls. At 12 weeks, the GHET was 37.29 ± 10.3 min for the SHR vs. 138.53 ± 37.6 ( P  = 0.016) min for the WKY controls.

CONCLUSION

Gastric emptying was significantly more rapid in the SHR before and after the development of hypertension. Even though SHR are known to have increased sympathetic activity associated with their development of hypertension, this increased sympathetic activity does not inhibit gastric emptying. SHR are a promising animal model for investigating therapeutic agents for treating hypertension aimed at slowing the rate of gastric emptying.

Effect of supplementing chromium histidinate and picolinate complexes along with biotin on insulin sensitivity and related metabolic indices in rats fed a high-fat diet

SCOPE

To investigate the effects of chromium histidinate (CrHis) and chromium picolinate (CrPic) complex along with biotin to a high-fat diet (HFD) fed to rats on the insulin sensitivity and the anti-obesity properties.

METHODS

Forty-two Sprague-Dawley male rats were divided into six groups. The rats were fed either (a): a standard diet (Control) or (b): a HFD or (c): a HFD with biotin (HFD+B) or (d): a combination of HFD and biotin along with CrPic (HFD + B + CrPic) or (e): a combination of HFD and biotin along with CrHis (HFD + B + CrHis) or (f): a combination of HFD and biotin along with CrHis and CrPic (HFD + B + CrHis + CrPic).

RESULTS

Adding biotin with chromium to HFD improved the glucose, insulin, HOMA-IR, leptin, lipid profile, with HFD+B+CrHis treatment being the most effective (p = 0.0001). Serum, liver, and brain tissue Cr concentrations increased upon Cr supplementations (p = 0.0001). Supplementing CrHis along with biotin to a HFD (HFD + B + CrHis) provided the greatest levels of GLUT-1, GLUT-3, PPAR-γ, and IRS-1, but the lowest level of NF-κB in the brain and liver tissues.

CONCLUSION

Biotin supplementation with chromium complexes, CrHis in particular, to a HFD pose to be a potential therapeutic feature for the treatment of insulin resistance.

Effects of metformin on endothelial health and erectile dysfunction

Erectile dysfunction (ED) affects approximately 18 million American men. ED may be attributed to several etiologies, including arteriogenic, psychogenic, neurogenic, hormonal, drug-induced, and systemic disease or aging related factors. Specific to arteriogenic ED, three major mechanisms have been identified: (I) endothelium-dependent vasodilatory impairment; (II) sympathetic nerve activity elevation; (III) atherosclerotic luminal narrowing. Additionally, these insults have been linked to the insulin resistant state, which in turn is comorbid with obesity, dyslipidemia, diabetes, and hypertension. In this review, we summarize the evidence regarding the impact of metformin—an insulin sensitizer—on the three mechanisms of arteriogenic ED. We report that metformin treatment positively affects two of three pathways, specifically through enhanced endothelium-dependent vasodilation and sympathetic nerve activity attenuation, but does not seem to have a significant impact on hypertension regulation. Given the encouraging data found in both animal and clinical studies, we advocate for further studies on metformin use in ED.

Chronic fructose intake accelerates non-alcoholic fatty liver disease in the presence of essential hypertension

BACKGROUND

The growing epidemic of metabolic syndrome has been related to the increased use of fructose by the food industry. In fact, the use of fructose as an ingredient has increased in sweetened beverages, such as sodas and juices. We thus hypothesized that fructose intake by hypertensive rats would have a worse prognosis in developing metabolic disorder and non-alcoholic fatty liver disease.

METHODS

Male Wistar and SHR rats aged 6weeks were given water or fructose (10%) for 6weeks. Blood glucose was measured every two weeks, and insulin and glucose sensitivity tests were assessed at the end of the follow-up. Systolic blood pressure was measure by plethysmography. Lean mass and abdominal fat mass were collected and weighed. Liver tissue was analyzed to determine interstitial fat deposition and fibrosis.

RESULTS

Fasting glucose increased in animals that underwent a high fructose intake, independent of blood pressure levels. Also, insulin resistance was observed in normotensive and mostly in hypertensive rats after fructose intake. Fructose intake caused a 2.5-fold increase in triglycerides levels in both groups. Fructose intake did not change lean mass. However, we found that fructose intake significantly increased abdominal fat mass deposition in normotensive but not in hypertensive rats. Nevertheless, chronic fructose intake only increased fat deposition and fibrosis in the liver in hypertensive rats.

CONCLUSIONS

We demonstrated that, in normotensive and hypertensive rats, fructose intake increased triglycerides and abdominal fat deposition, and caused insulin resistance. However, hypertensive rats that underwent fructose intake also developed interstitial fat deposition and fibrosis in liver.

A low-carbohydrate/high-fat diet reduces blood pressure in spontaneously hypertensive rats without deleterious changes in insulin resistance

Previous studies reported that diets high in simple carbohydrates could increase blood pressure in rodents. We hypothesized that the converse, a low-carbohydrate/high-fat diet, might reduce blood pressure. Six-week-old spontaneously hypertensive rats (SHR; n = 54) and Wistar-Kyoto rats (WKY; n = 53, normotensive control) were fed either a control diet (C; 10% fat, 70% carbohydrate, 20% protein) or a low-carbohydrate/high-fat diet (HF; 20% carbohydrate, 60% fat, 20% protein). After 10 wk, SHR-HF had lower (P < 0.05) mean arterial pressure than SHR-C (148 ± 3 vs. 159 ± 3 mmHg) but a similar degree of cardiac hypertrophy (33.4 ± 0.4 vs. 33.1 ± 0.4 heart weight/tibia length, mg/mm). Mesenteric arteries and the entire aorta were used to assess vascular function and endothelial nitric oxide synthase (eNOS) signaling, respectively. Endothelium-dependent (acetylcholine) relaxation of mesenteric arteries was improved (P < 0.05) in SHR-HF vs. SHR-C, whereas contraction (potassium chloride, phenylephrine) was reduced (P < 0.05). Phosphorylation of eNOSSer1177 increased (P < 0.05) in arteries from SHR-HF vs. SHR-C. Plasma glucose, insulin, and homoeostatic model of insulin assessment were lower (P < 0.05) in SHR-HF vs. SHR-C, whereas peripheral insulin sensitivity (insulin tolerance test) was similar. After a 10-h fast, insulin stimulation (2 U/kg ip) increased (P < 0.05) phosphorylation of AktSer473 and S6 in heart and gastrocnemius similarly in SHR-C vs. SHR-HF. In conclusion, a low-carbohydrate/high-fat diet reduced blood pressure and improved arterial function in SHR without producing signs of insulin resistance or altering insulin-mediated signaling in the heart, skeletal muscle, or vasculature.

Strain differences in self-administration of methylphenidate and sucrose pellets in a rat model of attention-deficit hyperactivity disorder

Despite its abuse potential, methylphenidate (MPH) is widely prescribed for treatment of attention-deficit hyperactivity disorder (ADHD). The purpose of the present study was to examine MPH self-administration in a rat model of ADHD. Experiment 1 examined the acquisition of MPH self-administration and assessed the MPH dose-effect curve in spontaneously hypertensive rats (SHR), an inbred rat model of ADHD, Wistar Kyotos (WKY), the progenitor strain for SHR, and Sprague-Dawley (SD), an outbred control strain. Experiment 2 replicated Experiment 1, but replaced MPH infusions with sucrose pellets. Initial acquisition of MPH self-administration was greater in SHR and SD than WKY. However, with extended training using an incrementing fixed ratio schedule SHR and WKY did not differ in responding for MPH using the training dose (0.3 mg/kg/infusion) or other unit doses, except that SHR showed greater responding than WKY at 0.1 mg/kg/infusion. SHR also showed greater acquisition and maintenance of sucrose-reinforced responding compared with both WKY and SD. Greater initial acquisition of MPH self-administration in SHR than WKY may not be due to a strain-specific difference in sensitivity to the reinforcing effect of MPH.

Dipeptidyl peptidase IV inhibition upregulates GLUT4 translocation and expression in heart and skeletal muscle of spontaneously hypertensive rats

The purpose of the current study was to test the hypothesis that the dipeptidyl peptidase IV (DPPIV) inhibitor sitagliptin, which exerts anti-hyperglycemic and anti-hypertensive effects, upregulates GLUT4 translocation, protein levels, and/or mRNA expression in heart and skeletal muscle of spontaneously hypertensive rats (SHRs). Ten days of treatment with sitagliptin (40 mg/kg twice daily) decreased plasma DPPIV activity in both young (Y, 5-week-old) and adult (A, 20-week-old) SHRs to similar extents (~85%). However, DPPIV inhibition only lowered blood pressure in Y-SHRs (119 ± 3 vs. 136 ± 4 mmHg). GLUT4 translocation, total protein levels and mRNA expression were decreased in the heart, soleus and gastrocnemius muscle of SHRs compared to age-matched Wistar Kyoto (WKY) normotensive rats. These differences were much more pronounced between A-SHRs and A-WKY rats than between Y-SHRs and Y-WKY rats. In Y-SHRs, sitagliptin normalized GLUT4 expression in the heart, soleus and gastrocnemius. In A-SHRs, sitagliptin increased GLUT4 expression to levels that were even higher than those of A-WKY rats. Sitagliptin enhanced the circulating levels of the DPPIV substrate glucagon-like peptide-1 (GLP-1) in SHRs. In addition, stimulation of the GLP-1 receptor in cardiomyocytes isolated from SHRs increased the protein level of GLUT4 by 154 ± 13%. Collectively, these results indicate that DPPIV inhibition upregulates GLUT4 in heart and skeletal muscle of SHRs. The underlying mechanism of sitagliptin-induced upregulation of GLUT4 in SHRs may be, at least partially, attributed to GLP-1.

Enhanced sympathetic reactivity associates with insulin resistance in the young Zucker rat

Somatosympathetic reflexes were studied in young hyperinsulinemic, insulin-resistant (Zucker fatty) rats (ZFR) and a related control (Zucker lean) strain (ZLR). Glucose metabolism was characterized by minimal model analysis of intravenous glucose tolerance test data. Seven-week-old ZFR (n=18) and ZLR (n=17) were studied under pentobarbital anesthesia. Mean body weight and plasma glucose and insulin concentration were significantly greater (P<0.05) in ZFR than in ZLR, whereas basal values of mean arterial pressure (MAP) and heart rate (HR) were not significantly different. Increments of MAP (DeltaMAP) and HR (DeltaHR) elicited by electrical stimulation of the sciatic nerve (5-s trains of 100 pulses, 0.5-ms pulse duration, 100- to 400-microA pulse intensity) were significantly higher (ANOVA, P<0.05) in ZFR at each level of stimulus intensity. Regression analysis showed a linear increase in DeltaMAP and DeltaHR with increasing sciatic nerve stimulus intensity. Pressor responses to phenylephrine after ganglionic blockade demonstrated that vascular reactivity to adrenergic stimulation is not increased in ZFR compared with ZLR. Thus this factor does not contribute to enhancement of somatosympathetic reflexes observed in this strain. Insulin sensitivity in ZFR was one-fourth (P<0.05) that in ZLR. These results suggest that stronger sympathetic nervous reactivity in ZFR is associated with a severe insulin-resistant state before the onset of hypertension and support the hypothesis that insulin-mediated stimulation of the sympathetic nervous system is involved in the development of cardiovascular diseases related to alterations of glucose metabolism.

Disappearance of Glibenclamide-Induced Hypoglycemia in Wistar-Kyoto Rats

The aim of the present study is to investigate difference in sensitivity to glibenclamide, a sulfonylurea oral antidiabetic agent, among Wistar rats, Spontaneously Hypertensive rats (SHR/Izm) and Wistar-Kyoto rats (WKY/Izm). We examined the effect of glibenclamide on blood levels of glucose and insulin in these rat strains. Under anesthesia with pentobarbital sodium (50 mg/kg, i.p.), blood samples were collected before and 5-120 min after administration of glibenclamide (10 mg/kg, i.p.). Blood levels of glucose and insulin in each sample were measured by glucose oxidase method and radioimmunoassay, respectively. In 8 week-old rats of all strains tested, blood levels of glucose were decreased by glibenclamide. In 12-20-week-old rats, although blood levels of glucose in Wistar and SHR/Izm were decreased after glibenclamide administration, those of WKY/Izm were not decreased. In rats of this age, time-course and extent of increases in blood insulin levels observed after administration of glibenclamide in WKY/Izm was almost the same as that of SHR/Izm, however, smaller than that of Wistar. Both insulin secretions induced via inactivation of ATP-sensitive K+ channel and sensitivity of pancreatic beta-cells to insulin seems to be decreased in WKY/Izm after 12 weeks of age. This phenomenon may explain the mechanism of glucose intolerance previously reported in WKY/Izm.

Effects of age and anesthetic on plasma glucose and insulin levels and insulin sensitivity in spontaneously hypertensive and Wistar rats

We examined the effects of anesthetic, age, and strain on oral glucose tolerance tests (OGTT, 1 g/kg body weight) and intraperitoneal glucose tolerance tests (IPGTT, 2 g/kg body weight) in spontaneously hypertensive (SH) and Wistar rats. Pentobarbital anesthesia caused an elevation in basal glucose and insulin levels in Wistar rats at 9 and 16 weeks of age and in SH rats at 9 weeks. Anesthesia increased the insulin output during an OGTT in both strains of rats while glucose was unchanged. Anesthesia reduced the insulin sensitivity index calculated from the OGTT but not from the IPGTT data. The age of the rats (9-11 vs. 16-18 weeks) had no effect on the basal glucose or insulin levels, but older Wistar rats had a greater insulin output following oral glucose and older SH rats had a greater insulin output following intraperitoneal glucose. On the basis of the insulin sensitivity index, SH rats were clearly more insulin resistant than age-matched Wistar rats. The SH rats also had higher basal insulin levels, as well as higher insulin output, following both glucose challenges. In summary, SH rats are more insulin resistant than Wistar rats, and anesthesia, which elevated basal glucose and insulin levels and increased the insulin output in response to a glucose challenge, may increase insulin resistance.

Cardiovascular and renal responses to a high-fat diet in Osborne-Mendel rats

This study examined the cardiovascular, renal, and hormonal responses of dietary-induced obesity in Osborne-Mendel (OM) rats. Male OM rats were fed either a low (LF; n = 10)- or high-fat (HF; n = 11) diet for 17 wk. During week 15 of the study, arterial pressure was measured directly, 24 h/day, from chronically indwelling catheters. Body and kidney weights were 46 +/- 5 and 33 +/- 5% greater, respectively, in rats fed HF vs. LF diet. Left and right ventricular weights were also greater in rats fed HF diet (21 +/- 7 and 36 +/- 6%, respectively). Direct measurement of arterial pressure revealed only a slight increase in mean arterial pressure (88 +/- 1 in rats fed HF diet vs. 85 +/- 1 mmHg in rats fed LF diet), whereas there was no difference in resting heart rate between the two groups. Consumption of HF diet was also associated with a 3.5-fold increase in plasma insulin, a 16 +/- 4% higher blood glucose, and a 40 +/- 6% reduction in plasma renin activity compared with LF-fed rats. Thus feeding OM rats HF diet led to obesity, cardiac and renal hypertrophy, and hyperinsulinemia but only a slight increase in mean arterial pressure.

Effects of high-fat diet and exercise training on intracellular glucose metabolism in rats

We examined the effects of high-fat diet (HFD) and exercise training on insulin-stimulated whole body glucose fluxes and several key steps of glucose metabolism in skeletal muscle. Rats were maintained for 3 wk on either low-fat (LFD) or high-fat diet with or without exercise training (swimming for 3 h per day). After the 3-wk diet/exercise treatments, animals underwent hyperinsulinemic euglycemic clamp experiments for measurements of insulin-stimulated whole body glucose fluxes. In addition, muscle samples were taken at the end of the clamps for measurements of glucose 6-phosphate (G-6-P) and GLUT-4 protein contents, hexokinase, and glycogen synthase (GS) activities. Insulin-stimulated glucose uptake was decreased by HFD and increased by exercise training (P < 0.01 for both). The opposite effects of HFD and exercise training on insulin-stimulated glucose uptake were associated with similar increases in muscle G-6-P levels (P < 0.05 for both). However, the increase in G-6-P level was accompanied by decreased GS activity without changes in GLUT-4 protein content and hexokinase activities in the HFD group. In contrast, the increase in G-6-P level in the exercise-trained group was accompanied by increased GLUT-4 protein content and hexokinase II (cytosolic) and GS activities. These results suggest that HFD and exercise training affect insulin sensitivity by acting predominantly on different steps of intracellular glucose metabolism. High-fat feeding appears to induce insulin resistance by affecting predominantly steps distal to G-6-P (e.g., glycolysis and glycogen synthesis). Exercise training affected multiple steps of glucose metabolism both proximal and distal to G-6-P. However, increased muscle G-6-P levels in the face of increased glucose metabolic fluxes suggest that the effect of exercise training is quantitatively more prominent on the steps proximal to G-6-P (i.e., glucose transport and phosphorylation).

Influence of age on contractile response to insulin-like growth factor 1 in ventricular myocytes from spontaneously hypertensive rats

Evidence suggests a pathophysiological role of insulin-like growth factor 1 (IGF-1) in hypertension. Cardiac function is altered with advanced age, similar to hypertension. Accordingly, the effects of IGF-1 on cardiac myocyte shortening and intracellular Ca(2+) were evaluated in hypertension at different ages. Ventricular myocytes were isolated from Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR), aged 12 and 36 weeks. Mechanical and intracellular Ca(2+) properties were examined by edge-detection and fluorescence microscopy. At 12 weeks, IGF-1 (1 to 500 ng/mL) increased peak twitch amplitude (PTA) and FFI changes (DeltaFFI) in a dose-dependent manner in WKY myocytes, with maximal increases of 27.5% and 35.2%, respectively. However, IGF-1 failed to exert any action on PTA and DeltaFFI in the age-matched SHR myocytes. Interestingly, at 36 weeks, IGF-1 failed to exert any response in WKY myocytes but depressed both PTA and DeltaFFI in a dose-dependent manner in SHR myocytes, with maximal inhibitions of 40.5% and 16.1%, respectively. Myocytes from SHR or 36-week WKY were less sensitive to norepinephrine (1 micromol/L) and KCl (30 mmol/L). Pretreatment with nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 micromol/L) did not alter the IGF-1-induced response in 12-week WKY myocytes but unmasked a positive action in 12-week SHR and 36-week WKY myocytes. L-NAME also significantly attenuated IGF-1-induced depression in 36-week SHR myocytes. In addition, the Ca(2+) channel opener Bay K8644 (1 micromol/L) abolished IGF-1-induced cardiac depression in 36-week SHR myocytes. Collectively, these results suggest that the IGF-1-induced cardiac contractile response was reduced with advanced age as well as with hypertension. Alterations in nitric oxide and intracellular Ca(2+) modulation may underlie, in part, the resistance to IGF-1 in hypertension and advanced age.

11beta-hydroxysteroid dehydrogenase and corticosteroid action in lyon hypertensive rats

Adrenocorticosteroid activity in Lyon hypertensive (LH) and low blood pressure (LL) rat strains differ in several respects. Abnormal activity of 11beta-hydroxysteroid dehydrogenase enzymes (11beta-HSD1 and 11beta-HSD2), which interconvert corticosterone and inactive 11-dehydrocorticosterone, might contribute to the LH phenotype by regulating corticosteroid hormone access to receptors. 11beta-HSD2 (expressed in kidney but not liver) prevents endogenous glucocorticoids from binding to mineralocorticoid receptors. 11beta-HSD1 (expressed in liver and kidney) favors active glucocorticoid formation from 11-dehydrocorticosterone. 11beta-HSD properties in LH and LL have been compared by several approaches: (1) 11betaHSD activities have been measured in vitro as corticosterone dehydrogenation and in vivo as interconversion of injected cortisol and cortisone; (2) the effects of cortisol and cortisone on urine electrolytes and volume have been measured; and (3) 11beta-HSD mRNA expression has been measured by in situ hybridization. 11beta-HSD2 enzyme activities in LH and LL rats were similar and urinary cortisone:cortisol ratios were not different after cortisol injection. Cortisol caused a natriuresis and kaliuresis in both strains, with a slightly reduced response in LH rats. Renal 11beta-HSD2 mRNA expression was slightly lower in LH rats. 11beta-HSD1 was less active in LH than LL rats: enzyme activities were lower in tissue extracts; urinary cortisone:cortisol was lower in LL rats after cortisone injections; cortisone increased urine volume in LL but not LH rats; and mRNA levels tended to be lower in LH tissues. We conclude that 11beta-HSD1 is impaired in LH rats. The LH phenotype of heavier adrenals, raised corticosterone, and reduced thymus weight is similar to that described for 11beta-HSD1 knockout mice.

Glucose effectiveness is the major determinant of intravenous glucose tolerance in the rat

To determine the importance of insulin for glucose disposal during an intravenous glucose tolerance test in rats, experiments were performed in four cohorts of conscious unrestrained rats fasted overnight. In cohorts 1-3, a bolus of tracer ([3-3H]glucose, 50 microCi) was given alone, with glucose (0.3 g/kg) to induce an endogenous insulin response (approximately 1,100 pmol/l), or with exogenous insulin to give physiological (1,700 pmol/l) or supraphysiological (12,000 pmol/l) plasma levels. Raising plasma insulin within the physiological range had no effect (P > 0.05), but supraphysiological levels induced hypoglycemia (7.3 +/- 0.2 to 3.6 +/- 0.2 mmol/l) and increased [3H]glucose disappearance rate (P < 0.001). In cohort 4, a primed, continuous tracer infusion was started 120 min before saline or glucose bolus injection. [3H]glucose levels fell 15-20%, and the disappearance rate rose 36% (P < 0.05) after glucose injection. These results indicate that in fasted rats a tracer bolus injection protocol is not sufficiently sensitive to measure the physiological effect of insulin released in response to a bolus of glucose because this effect of insulin is small. Glucose itself is the predominant mediator of glucose disposal after a bolus of glucose in the fasted rat.

Blood pressure and heart rate development in young spontaneously hypertensive rats

The course of hypertension development in young spontaneously hypertensive rats (SHR) was studied by the measurement of changes in systolic blood pressure (BP), body weight, and heart rate (HR) at 2, 3, 4, and 6 wk of age. To achieve this, we compared inbreeding lines of SHR and Wistar-Kyoto rats (WKY) to determine if differences in BP, body weight, or HR were present among inbreeding lines of the same strain or between strains. The effect of these differences on the eventual level of BP was then assessed. We found that BP began to diverge between SHR and WKY at 4 wk of age. Significant differences in systolic BP (24 mmHg) between SHR inbreeding lines at 4 wk of age did not affect the BP at 8 wk (172 vs. 170 mmHg). Pulse pressure was significantly higher in SHR than in WKY at 4 wk of age. HR was elevated in SHR over age-matched WKY at 3 wk of age and positively correlated to the level of BP attained by individual animals at 6 wk (P = 0.037). Moreover, WKY inbreeding lines showing elevated HR developed higher BP (145 vs. 127 mmHg) at 10-12 and 20 wk of age. The prehypertensive tachycardia in SHR was investigated further and found to result from an increased intrinsic HR. Because HR at 3 wk is a genetic trait that can be partitioned into inbreeding lines, and inbreeding lines most expressive of this trait showed the highest eventual BP, we conclude that prehypertensive tachycardia may be an important first step during hypertension development in SHR. Moreover, early elevations in HR are highly predictive (r = 0.41) of hypertension occurrence in the animal population studied.

Reduced glucose clearance as the major determinant of postabsorptive hyperglycemia in diabetic rats

The relationships between postabsorptive glucose concentration and hepatic glucose output (HGO) and glucose clearance were studied in rats one day after treatment with various doses of streptozotocin (STZ; 0, 15, 30, 40, 50, or 75 mg/kg; n = 6 per dose; study 1). Glucose fluxes were estimated using a prolonged (6-h) infusion of [3-3H]glucose to ensure complete tracer equilibration at hyperglycemia. Postabsorptive glucose was significantly increased at the high doses of STZ (50 and 75 mg/kg; P < 0.01) and was strongly correlated with glucose clearance across all doses (r = -0.85, P < 0.001) but less strongly with HGO (r = 0.46, P < 0.01). In the group treated with 50 mg/kg STZ, postabsorptive glucose was increased twofold compared with the control (i.e., zero dose) group, with no change in HGO and a 45% decrease in glucose clearance, indicating that the hyperglycemia was due to a decrease in glucose clearance. To understand the cellular mechanisms of decreased glucose clearance in STZ diabetic rats, skeletal muscle glucose clearance and intracellular glucose and glucose 6-phosphate (G-6-P) concentrations were determined in normal and STZ (50 mg/kg) diabetic rats at their postabsorptive glucose levels as well as at matched hyperglycemia (12 mM; study 2). Glucose clearance was significantly decreased in soleus (P < 0.05) muscles of the diabetic rats, and this was associated with significantly decreased intracellular glucose and G-6-P levels at matched hyperglycemia (P < 0.05), suggestive of decreased glucose transport. In conclusion, postabsorptive hyperglycemia in STZ diabetic rats was largely due to decreased glucose clearance, although increased HGO may also have been a contributing factor at the highest STZ dose. The decrease in postabsorptive glucose clearance in STZ diabetic rats appeared to be associated with an impairment of glucose transport in soleus (type I) muscles.

Lack of insulin resistance in the Lyon hypertensive rat

Genetically hypertensive rats (LH) of the Lyon strain, compared to their normo-tensive (LN) controls associate, in a unique manner, high blood pressure with increases in body weight and in plasma lipids and insulin/glucose ratio. The present work investigated the development of insulin resistance with age in this model. At the age of 22 and 52 weeks, LH and LN fasted male rats were submitted to an intravenous glucose tolerance test, allowing measurement of the elimination rate of the glucose and the area under the curve of the insulin response. Insulin sensitivity was calculated as the ratio of these two parameters. It was observed that insulin sensitivity coefficient decreased with age in all the animals and that LH rats did not significantly differ from LN controls (from 62.6 +/- 3.3 and 69.1 +/- 4 at 22 weeks to 42.1 +/- 4.4 and 49.5 +/- 12.8 at 52 weeks for LH and LN rats, respectively). It is concluded that 1) elevated plasma insulin/glucose ratio does not mean insulin resistance and 2) hypertension can develop without being associated, even in aged rats, to a true insulin resistance.

Reduced insulin receptor signaling in the obese spontaneously hypertensive Koletsky rat

Insulin resistance is associated with both obesity and hypertension. However, the cellular mechanisms of insulin resistance in genetic models of obese-hypertension have not been identified. The objective of the present study was to investigate the effects of genetic obesity on a background of inherited hypertension on initial components of the insulin signal transduction pathway and glucose transport in skeletal muscle and liver. Oral glucose tolerance testing in SHROB demonstrated a sustained postchallenge elevation in plasma glucose at 180 and 240 min compared with lean spontaneously hypertensive rat (SHR) littermates, which is suggestive of glucose intolerance. Fasting plasma insulin levels were elevated 18-fold in SHROB. The rate of insulin-stimulated 3-O-methylglucose transport was reduced 68% in isolated epitrochlearis muscles from the SHROB compared with SHR. Insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit and insulin receptor substrate-1 (IRS-1) in intact skeletal muscle of SHROB was reduced by 36 and 23%, respectively, compared with SHR, due primarily to 32 and 60% decreases in insulin receptor and IRS-1 protein expression, respectively. The amounts of p85 alpha regulatory subunit of phosphatidylinositol-3-kinase and GLUT-4 protein were reduced by 28 and 25% in SHROB muscle compared with SHR. In the liver of SHROB, the effect of insulin on tyrosine phosphorylation of IRS-1 was not changed, but insulin receptor phosphorylation was decreased by 41%, compared with SHR, due to a 30% reduction in insulin receptor levels. Our observations suggest that the leptin receptor mutation fak imposed on a hypertensive background results in extreme hyperinsulinemia, glucose intolerance, and decreased expression of postreceptor insulin signaling proteins in skeletal muscle. Despite these changes, hypertension is not exacerbated in SHROB compared with SHR, suggesting these metabolic abnormalities may not contribute to hypertension in this model of Syndrome X.

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.

Cardiac glucose and fatty acid oxidation in the streptozotocin-induced diabetic spontaneously hypertensive rat

Hypertension intensifies the cardiac dysfunction of diabetes. We investigated the possible role of altered exogenous fuel oxidation in this phenomenon. Diabetes was induced by streptozotocin in spontaneously hypertensive rats and normotensive Sprague-Dawley rats. Two weeks later, mechanical performance and the oxidation of glucose and palmitate were quantified in working hearts ex vivo at intermediate and high workloads. The results showed that the nondiabetic spontaneously hypertensive rat hearts, compared with those of the normotensive controls, oxidized glucose at a higher rate but oxidized palmitate at a much lower rate, as reported previously. The effects of diabetes in the hypertensive rats, compared with its effects in the normotensive strain, were characterized by (1) a more pronounced decrease in heart performance, (2) either a similar or a less marked reduction in the rate of glucose oxidation, depending on the workload, and (3) a relatively greater increase in palmitate oxidation, particularly at the higher workload. These findings suggest that the exaggerated stimulation of fatty acid oxidation by diabetes in the hypertrophic left ventricle may be a more important contributor to the premature mechanical dysfunction than the inhibition of glucose oxidation. Possible mechanisms include antagonism of energetically favorable shifts in fuel oxidation or inhibition of accelerated membrane lipid biosynthesis in left ventricular hypertrophy.

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)

Regulatory effects of glucose on the catalytic activity and cellular content of glucokinase in the pancreatic beta cell. Study using cultured rat islets

Glucose regulates the cellular content of glucokinase in the pancreatic beta cell by altering the level of the enzyme. We investigated the existence of a second regulatory pathway, an alteration in the catalytic activity, by comparing Vmax and protein levels of glucokinase in rat islets cultured under high glucose conditions (16.7 mM) for 6, 14, and 24 h. The Vmax was increased by glucose at all time points. In contrast, glucokinase protein levels on Western blots were unchanged from the control value at 6 h but increased 40% at the later time points (P < 0.0002). Further evidence for a dual regulatory system was obtained with a reversal protocol. After a 6-h incubation at high glucose, an additional 3-h incubation at 5.5 mM glucose restored glucokinase Vmax to normal, but failed to change the Vmax after a 24-h incubation at high glucose. Finally, 10 microM cycloheximide partially prevented the increase in glucokinase Vmax induced by 24 h of high glucose, but had no effect at 6 h, suggesting the early increase in enzymatic activity did not require protein synthesis. In summary, glucose regulates both the catalytic activity and cellular content of glucokinase in the beta cell. Glucose-induced increases in glucokinase activity are an important element of the beta cell adaptive response to hyperglycemia.

Nutritional status modifies insulin-mediated glucose uptake in spontaneously hypertensive rats

In this study we quantified insulin-mediated glucose uptake in weight-matched (260-330 g) fed (6-8 h fast) and fasted (24 h fast) male rats with spontaneous hypertension (SHR) and control Wistar-Kyoto (WKY) rats. To accomplish this goal, rats were infused continuously for 165 min with glucose and insulin. Blood was taken at frequent intervals from 120-165 min, and the values averaged to determine the steady-state plasma glucose (SSPG) and insulin (SSPI) concentrations. In some studies epinephrine and propranolol were added to the infusate in order to suppress endogenous insulin secretion. Steady-state plasma insulin (SSPI) concentrations were similar in SHR and WKY during the three infusion studies (382-483 pmol/L). However, SSPG was significantly higher in fed SHR as compared to fed WKY during infusions performed with (9.4 +/- 0.8 vs 7.0 +/- 0.4 mmol/L, p < 0.05) or without (8.6 +/- 0.2 vs 7.0 +/- 0.6 mmol/L, p < 0.05) epinephrine and propranolol in the infusate. In contrast, SSPG concentrations (mmol/L) were similar in SHR (6.8 +/- 0.3) and WKY rats (6.5 +/- 0.6) when they were studied after a 24 h fast. These results demonstrates that differences in insulin-mediated glucose removal from plasma between SHR and WKY rats will vary as a function of nutritional status.

Mechanism of compensatory hyperinsulinemia in normoglycemic insulin-resistant spontaneously hypertensive rats. Augmented enzymatic activity of glucokinase in beta-cells

The cause of compensatory hyperinsulinemia in normoglycemic insulin-resistant states is unknown. Using spontaneously hypertensive rats (SHR), we tested the hypothesis that a lowered beta-cell set-point for glucose causes a hypersecretion of insulin at a normal glucose level. Islets isolated from normoglycemic hyperinsulinemic SHR were compared to age-matched (12 wk old) Wistar-Kyoto (WK) rats. The ED50 for glucose-induced insulin secretion was 6.6 +/- 1.0 mM glucose in SHR versus 9.6 +/- 0.5 mM glucose in WK (P < 0.02). Glucokinase enzymatic activity was increased 40% in SHR islets (P < 0.02) without any change in the glucokinase protein level by Western blot. The level of the beta-cell glucose transporter (GLUT-2) was increased 75% in SHR islets (P < 0.036). In summary, the beta-cell sensitivity for glucose was increased in these normoglycemic insulin resistant rats by an enhanced catalytic activity of glucokinase. We have identified a regulatory system for glucokinase in the beta-cell which entails variable catalytic activity of the enzyme, is modulated in response to variations in whole-body insulin sensitivity, and is not dependent on sustained changes in the plasma glucose level.

Chronic insulin administration elevates blood pressure in rats

To examine the relative contribution of dietary glucose and infused insulin on blood pressure, we administered a 4% glucose supplement (in drinking water) with and without insulin infusion (15.8 nmol [2.2 U]/d via osmotic minipump) to male Sprague-Dawley rats (n = 6). We also tested the effect of the sympatholytic agent clonidine on rats receiving glucose and insulin. Blood pressure and heart rate were recorded via a novel radio telemetry system. Experiments were performed using a crossover design with three animals receiving treatment and three receiving vehicle for 10 days. After a 10-day washout period, the groups were reversed, and the experiment was repeated. Blood samples for insulin and glucose were drawn throughout the study. Systolic and diastolic blood pressures increased (by 6.0 +/- 1.2 and 2.2 +/- 1.3 mm Hg, respectively) in the animals given glucose alone in association with an increase in plasma insulin. However, blood pressure increased more rapidly and to a greater extent, systolic by 8.6 +/- 0.7 mm Hg and diastolic by 2.9 +/- 1.1 mm Hg, during the insulin treatment that raised plasma insulin above the levels observed during glucose feeding alone. Heart rate increased equally during both treatments. The average change in blood pressure and average plasma insulin during the infusion were correlated (r = .72, P = .009). Blood pressure dropped during the week following discontinuation of the insulin infusion. On rechallenge with insulin and glucose, blood pressure again rose and then decreased after termination of the insulin and glucose administration. Clonidine prevented the rise in blood pressure and heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Salt sensitivity in hypertension. Renal and cardiovascular implications

The mechanisms responsible for the increase in blood pressure response to high salt intake in salt-sensitive patients with essential hypertension are complex and only partially understood. A complex interaction between neuroendocrine factors and the kidney may underlie the propensity for such patients to retain salt and develop salt-dependent hypertension. The possible role of vasodilator and natriuretic agents, such as the prostaglandins, endothelium-derived relaxing factor, atrial natriuretic factor, and kinin-kallikrein system, requires further investigation. An association between salt sensitivity and a greater propensity to develop renal failure has been described in certain groups of hypertensive patients, such as blacks, the elderly, and those with diabetes mellitus. Salt-sensitive patients with essential hypertension manifest a deranged renal hemodynamic adaptation to a high dietary salt intake. During a low salt diet, salt-sensitive and salt-resistant patients have similar mean arterial pressure, glomerular filtration rate, effective renal plasma flow, and filtration fraction. On the other hand, during a high salt intake glomerular filtration rate does not change in either group, and effective renal blood flow increases in salt-resistant but decreases in salt-sensitive patients; filtration fraction and glomerular capillary pressure decrease in salt-resistant but increase in salt-sensitive patients. Salt-sensitive patients are also more likely than salt-resistant patients to manifest left ventricular hypertrophy, microalbuminuria, and metabolic abnormalities that may predispose them to cardiovascular diseases. In conclusion, salt sensitivity in hypertension is associated with substantial renal, hemodynamic, and metabolic abnormalities that may enhance the risk of cardiovascular and renal morbidity.