Blood pressure response to hyperinsulinemia in salt-sensitive and salt-resistant rats

Insulin Resistance and High Blood Pressure: Mechanistic Insight on the Role of the Kidney

The metabolic effects of insulin predominate in skeletal muscle, fat, and liver where the hormone binds to its receptor, thereby priming a series of cell-specific and biochemically diverse intracellular mechanisms. In the presence of a good secretory reserve in the pancreatic islets, a decrease in insulin sensitivity in the metabolic target tissues leads to compensatory hyperinsulinemia. A large body of evidence obtained in clinical and experimental studies indicates that insulin resistance and the related hyperinsulinemia are causally involved in some forms of arterial hypertension. Much of this involvement can be ascribed to the impact of insulin on renal sodium transport, although additional mechanisms might be involved. Solid evidence indicates that insulin causes sodium and water retention, and both endogenous and exogenous hyperinsulinemia have been correlated to increased blood pressure. Although important information was gathered on the cellular mechanisms that are triggered by insulin in metabolic tissues and on their abnormalities, knowledge of the insulin-related mechanisms possibly involved in blood pressure regulation is limited. In this review, we summarize the current understanding of the cellular mechanisms that are involved in the pro-hypertensive actions of insulin, focusing on the contribution of insulin to the renal regulation of sodium balance and body fluids.

High Fructose-Induced Hypertension and Renal Damage Are Exaggerated in Dahl Salt-Sensitive Rats via Renal Renin-Angiotensin System Activation

Background

High‐fructose diet (HFr) induces hypertension and renal damage. However, it has been unknown whether the HFr‐induced hypertension and renal damage are exaggerated in subjects with salt sensitivity. We tested impacts of HFr in Dahl salt‐sensitive (DS) and salt‐resistant (DR) rats.

Methods and Results

Male DS and DR rats were fed control diet or HFr (60% fructose) with normal‐salt content. After 12 weeks, plasma and urinary parameters, renal histological characteristics, and renal expression of renin‐angiotensin system components were examined. Furthermore, effects of renin‐angiotensin system inhibitors were also examined in DS rats fed the HFr. HFr elevated blood pressure in DS rats but not in DR rats. HFr increased urinary albumin and liver type fatty acid binding protein excretions in both rats, but the excretions were exaggerated in DS rats. HFr increased plasma lipids and uric acid in both rats, whereas HFr increased creatinine clearance in DS rats but not DR rats. Although HFr decreased plasma renin activity in DS rats, HFr‐induced glomerular injury, afferent arteriolar thickening, and renal interstitial fibrosis were exaggerated in DS rats. HFr increased renal expression of angiotensinogen, renin, (pro)renin receptor, angiotensin‐converting enzyme, and angiotensin II type 1 receptor in DS rat, whereas HFr increased only angiotensin‐converting enzyme expression and decreased renin and angiotensin II type 1 receptor expressions in DR rats. Enalapril and candesartan attenuated the HFr‐induced hypertension, albuminuria, glomerular hyperfiltration, and renal damage in DS rats.

Conclusion

HFr‐induced hypertension and renal damage are exaggerated in DS rats via renal renin‐angiotensin system activation, which can be controlled by renin‐angiotensin system inhibitors.

Carotid Body and Metabolic Syndrome: Mechanisms and Potential Therapeutic Targets

The carotid body (CB) is responsible for the peripheral chemoreflex by sensing blood gases and pH. The CB also appears to act as a peripheral sensor of metabolites and hormones, regulating the metabolism. CB malfunction induces aberrant chemosensory responses that culminate in the tonic overactivation of the sympathetic nervous system. The sympatho-excitation evoked by CB may contribute to the pathogenesis of metabolic syndrome, inducing systemic hypertension, insulin resistance and sleep-disordered breathing. Several molecular pathways are involved in the modulation of CB activity, and their pharmacological manipulation may lead to overall benefits for cardiometabolic diseases. In this review, we will discuss the role of the CB in the regulation of metabolism and in the pathogenesis of the metabolic dysfunction induced by CB overactivity. We will also explore the potential pharmacological targets in the CB for the treatment of metabolic syndrome.

Carotid body, insulin, and metabolic diseases: unraveling the links

The carotid bodies (CB) are peripheral chemoreceptors that sense changes in arterial blood O2, CO2, and pH levels. Hypoxia, hypercapnia, and acidosis activate the CB, which respond by increasing the action potential frequency in their sensory nerve, the carotid sinus nerve (CSN). CSN activity is integrated in the brain stem to induce a panoply of cardiorespiratory reflexes aimed, primarily, to normalize the altered blood gases, via hyperventilation, and to regulate blood pressure and cardiac performance, via sympathetic nervous system (SNS) activation. Besides its role in the cardiorespiratory control the CB has been proposed as a metabolic sensor implicated in the control of energy homeostasis and, more recently, in the regulation of whole body insulin sensitivity. Hypercaloric diets cause CB overactivation in rats, which seems to be at the origin of the development of insulin resistance and hypertension, core features of metabolic syndrome and type 2 diabetes. Consistent with this notion, CB sensory denervation prevents metabolic and hemodynamic alterations in hypercaloric feed animal. Obstructive sleep apnea (OSA) is another chronic disorder characterized by increased CB activity and intimately related with several metabolic and cardiovascular abnormalities. In this manuscript we review in a concise manner the putative pathways linking CB chemoreceptors deregulation with the pathogenesis of insulin resistance and arterial hypertension. Also, the link between chronic intermittent hypoxia (CIH) and insulin resistance is discussed. Then, a final section is devoted to debate strategies to reduce CB activity and its use for prevention and therapeutics of metabolic diseases with an emphasis on new exciting research in the modulation of bioelectronic signals, likely to be central in the future.

Selective alpha(1)-adrenoceptor blockade prevents fructose-induced hypertension

The purpose of this study was to investigate the effect of chronic treatment with prazosin, a selective α1-adrenoceptor antagonist, on the development of hypertension in fructose-fed rats (FFR). High-fructose feeding and treatment with prazosin (1 mg/kg/day via drinking water) were initiated simultaneously in male Wistar rats. Systolic blood pressure, fasted plasma parameters, insulin sensitivity, plasma norepinephrine (NE), uric acid, and angiotensin II (Ang II) were determined following 9 weeks of treatment. FFR exhibited insulin resistance, hyperinsulinemia, hypertriglyceridemia, and hypertension, as well as elevations in plasma NE and Ang II levels. Treatment with prazosin prevented the rise in blood pressure without affecting insulin levels, insulin sensitivity, uric acid, or Ang II levels, while normalizing plasma NE levels in FFR. These data suggest that over-activation of the sympathetic nervous system, specifically α1-adrenoceptors, contributes to the development of fructose-induced hypertension, however, this over-activation does not appear to an initial, precipitating event in FFR.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

The effects of phentolamine on fructose-fed rats

Metabolic syndrome (MS) is a combination of medical disorders that increase the risk of developing cardiovascular disease and diabetes. MS is associated with obesity, increased blood pressure, hyperlipidemia, and hyperglycemia. This study was designed to investigate the pharmacological profile of phentolamine, a nonselective α adrenergic receptor antagonist, in the prevention of increased blood pressure in fructose-fed rats. Phentolamine prevented the fructose-induced increase in systolic blood pressure without affecting insulin sensitivity and major metabolic parameters. The levels of plasma noradrenaline and angiotensin II, 2 proposed contributors to the development of fructose-induced elevated blood pressure, were examined. Neither noradrenaline nor angiotensin II levels were affected by phentolamine treatment. Since overproduction of nitric oxide has been shown to lead to an elevation in peroxynitrite, the role of oxidative stress, a proposed mechanism of fructose-induced elevated blood pressure and insulin resistance, was examined by measuring plasma levels of total nitrate/nitrite. Plasma nitrate/nitrite was significantly elevated in all fructose-fed animals, regardless of treatment with phentolamine. Another proposed contributor toward fructose-induced MS is an elevation in uric acid levels. In this experiment, plasma levels of uric acid were found to be increased by dietary fructose and were unaffected by phentolamine treatment.

Matsuda-DeFronzo insulin sensitivity index is a better predictor than HOMA-IR of hypertension in Japanese: the Tanno-Sobetsu study

Here we examined whether the Matsuda-DeFronzo insulin sensitivity index (ISI-M) is more efficient than the homeostasis model assessment of insulin resistance (HOMA-IR) for assessing risk of hypertension. Cross-sectional and longitudinal analyses were conducted using normotensive subjects who were selected among 1399 subjects in the Tanno-Sobetsu cohort. In the cross-sectional analysis (n=740), blood pressure (BP) level was correlated with HOMA-IR and with ISI-M, but correlation coefficients indicate a tighter correlation with ISI-M. Multiple linear regression analysis adjusted by age, sex, body mass index (BMI) and serum triglyceride level (TG) showed contribution of ISI-M and fasting plasma glucose, but not of HOMA-IR. In the longitudinal analysis (n=607), 241 subjects (39.7%) developed hypertension during a 10-year follow-up period, and multiple logistic regression indicated that age, TG, systolic BP and ISI-M, but not HOMA-IR, were associated with development of hypertension. In subjects <60 years old, odds ratio of new-onset hypertension was higher in the low ISI-M group (ISI-M, less than the median) than in the high ISI-M group for any tertile of BMI. In conclusion, ISI-M is a better predictor of hypertension than is HOMA-IR. Non-hepatic IR may be a determinant, which is independent of TG, BP level and BMI, of the development of hypertension.

Chronic sodium-retaining action of insulin in diabetic dogs

Insulin-mediated sodium retention is implicated as a mechanism for hypertension in metabolic syndrome and type II diabetes. However, there is no direct experimental evidence for a sustained antinatriuretic effect of insulin outside of rodents, and all previous studies in dogs have been negative. This study used a novel approach to test for a chronic sodium-retaining action of insulin in dogs, by testing the hypothesis that natriuresis in type I diabetes is dependent on the decrease in insulin, rather than being due solely to osmotic actions of hyperglycemia. Dogs were chronically instrumented and housed in metabolic cages. Fasting blood glucose in alloxan-treated dogs was maintained at ~65 mg/dl by continuous intravenous insulin infusion. Then, a 6-day diabetic period was induced by either 1) decreasing the insulin infusion to induce type I diabetes (D; blood glucose = 449 ± 40 mg/dl) or 2) clamping the insulin infusion and infusing glucose continuously (DG; blood glucose = 470 ± 56 mg/dl). Control urinary sodium excretion (UnaV) averaged 70 ± 5 (D) and 69 ± 5 (DG) meq/day and increased on day 1 in both groups. UnaV remained elevated in the D group (115 ± 15 meq/day days 2-6), but it returned to control in the DG group (69 ± 11 meq/day days 2-6) and was accompanied by decreased lithium clearance. Thus, insulin had a sustained antinatriuretic action that was triggered by increased glucose, and it was powerful enough to completely block the natriuresis caused by hyperglycemia. These data may reveal an unrecognized physiologic function of insulin as a protector against hyperglycemia-induced salt wasting in diabetes.

Role of the IRS-1 and/or -2 in the Pathogenesis of Insulin Resistance in Dahl Salt-Sensitive (S) Rats

Insulin resistance is a common finding in hypertensive humans and animal models. The Dahl salt-sensitive (S) rat is an ideal model of genetically predetermined insulin resistance and salt-sensitive hypertension. Along the insulin signaling pathway, the insulin receptor substrates 1 and 2 (IRS-1 and -2) are important mediators of insulin signaling. IRS-1 and/or IRS-2 genetic variant(s) and/or enhanced serine phosphorylation correlate with insulin resistance. The present commentary was designed to highlight the significance of IRS-1 and/or -2 in the pathogenesis of insulin resistance. An emphasis will be given to the putative role of IRS-1 and/or -2 genetic variant(s) and serine phosphorylation in precipitating insulin resistance.

Important genetic checkpoints for insulin resistance in salt-sensitive (S) Dahl rats

Despite the marked advances in research on insulin resistance (IR) in humans and animal models of insulin resistance, the mechanisms underlying high salt-induced insulin resistance remain unclear. Insulin resistance is a multifactorial disease with both genetic and environmental factors (such as high salt) involved in its pathogenesis. High salt triggers insulin resistance in genetically susceptible patients and animal models of insulin resistance. One of the mechanisms by which high salt might precipitate insulin resistance is through its ability to enhance an oxidative stress-induced inflammatory response that disrupts the insulin signaling pathway. The aim of this hypothesis is to discuss two complementary approaches to find out how high salt might interact with genetic defects along the insulin signaling and inflammatory pathways to predispose to insulin resistance in a genetically susceptible model of insulin resistance. The first approach will consist of examining variations in genes involved in the insulin signaling pathway in the Dahl S rat (an animal model of insulin resistance and salt-sensitivity) and the Dahl R rat (an animal model of insulin sensitivity and salt-resistance), and the putative cellular mechanisms responsible for the development of insulin resistance. The second approach will consist of studying the over-expressed genes along the inflammatory pathway whose respective activation might be predictive of high salt-induced insulin resistance in Dahl S rats.

Variations in genes encoding the insulin receptor substrates -1 and/or -2 (IRS-1, -2) and/or genes encoding the glucose transporter (GLUTs) proteins have been found in patients with insulin resistance. To better understand the combined contribution of excessive salt and genetic defects to the etiology of the disease, it is essential to investigate the following question:

Question 1: Do variations in genes encoding the IRS -1 and -2 and/or genes encoding the GLUTs proteins predict high salt-induced insulin resistance in Dahl S rats?

A significant amount of evidence suggested that salt-induced oxidative stress might predict an inflammatory response that upregulates mediators of inflammation such as the nuclear factor- kappa B (NF-kappa B), the tumor necrosis factor-alpha (TNF-α) and the c-Jun Terminal Kinase (JNK). These inflammatory mediators disrupt the insulin signaling pathway and predispose to insulin resistance. Therefore, the following question will be thoroughly investigated:

Question 2: Do variations in genes encoding the NF-kappa B, the TNF-α and the JNK, independently or in synergy, predict an enhanced inflammatory response and subsequent insulin resistance in Dahl S rats in excessive salt environment?

Finally, to better understand the combined role of these variations on glucose metabolism, the following question will be addressed:

Question 3: What are the functional consequences of gene variations on the rate of glucose delivery, the rate of glucose transport and the rate of glucose phosphorylation in Dahl S rats?

The general hypothesis is that "high-salt diet in combination with defects in candidate genes along the insulin signaling and inflammatory pathways predicts susceptibility to high salt-induced insulin resistance in Dahl S rats".

Hyperinsulinemic rats are normotensive but sensitized to angiotensin II

The effect of insulin on blood pressure (BP) is debated, and an involvement of an activated renin-angiotensin aldosterone system (RAAS) has been suggested. We studied the effect of chronic insulin infusion on telemetry BP and assessed sympathetic activity and dependence of the RAAS. Female Sprague-Dawley rats received insulin (2 units/day, INS group, n = 12) or insulin combined with losartan (30 mg·kg−1·day−1, INS+LOS group, n = 10), the angiotensin II receptor antagonist, for 6 wk. Losartan-treated (LOS group, n = 10) and untreated rats served as controls ( n = 11). We used telemetry to measure BP and heart rate (HR), and acute ganglion blockade and air-jet stress to investigate possible control of BP by the sympathetic nervous system. In addition, we used myograph technique to study vascular function ex vivo. The INS and INS+LOS groups developed euglycemic hyperinsulinemia. Insulin did not affect BP but increased HR (27 beats/min on average). Ganglion blockade reduced mean arterial pressure (MAP) similarly in all groups. Air-jet stress did not increase sympathetic reactivity but rather revealed possible blunting of the stress response in hyperinsulinemia. Chronic losartan markedly reduced 24-h-MAP in the INS+LOS group (−38 ± 1 mmHg P < 0.001) compared with the LOS group (−18 ± 1 mmHg, P ≤ 0.05). While insulin did not affect vascular function per se, losartan improved endothelial function in the aorta of insulin-treated rats. Our results raise doubt regarding the role of hyperinsulinemia in hypertension. Moreover, we found no evidence that insulin affects sympathetic nervous system activity. However, chronic losartan treatment revealed an important interaction between insulin and RAAS in BP control.

Insulin's impact on renal sodium transport and blood pressure in health, obesity, and diabetes

Insulin has been shown to have antinatriuretic actions in humans and animal models. Moreover, endogenous hyperinsulinemia and insulin infusion have been correlated to increased blood pressure in some models. In this review, we present the current state of understanding with regard to the regulation of the major renal sodium transporters by insulin in the kidney. Several groups, using primarily cell culture, have demonstrated that insulin can directly increase activity of the epithelial sodium channel, the sodium-phosphate cotransporter, the sodium-hydrogen exchanger type III, and Na-K-ATPase. We and others have demonstrated alterations in the expression at the protein level of many of these same proteins with insulin infusion or in hyperinsulinemic models. We also discuss how this regulation is perturbed in type I and type II diabetes mellitus. Finally, we discuss a potential role for regulation of insulin receptor signaling in the kidney in contributing to sodium balance and blood pressure.

Evaluation of Insulin Resistance and Sodium Sensitivity in Normotensive Offspring of Hypertensive Individuals

BACKGROUND

Sodium sensitivity (SS) and insulin resistance (IR) might be a common link in the pathogenesis of essential hypertension. The aim of the present study is to investigate the relationship, if any, between SS and IR in a population with a family predisposition to develop hypertension.

METHODS

Twenty normotensive subjects aged 20 to 40 years with a family history of hypertension (1 or both parents hypertensive) and no other risk factor (group A) and 10 normotensive subjects aged 20 to 40 years without a family history of hypertension (group B) were enrolled. SS and IR were estimated using the euglycemic clamp technique and correlated in both groups. Blood pressure, mean blood pressure (MAP), and biochemical control were recorded.

RESULTS

The frequency of SS was equal in offspring of hypertensive subjects and the control group. Individuals with a family history of hypertension had a tendency to develop IR (45%) compared with the control group (20%). This group had a greater MAP at both salt-loading and salt-deprivation periods. Increased IR was associated with increased MAP. No significant relationship was found between SS and IR in either the entire sample or the subgroup of individuals with a family history of hypertension. Serum urea and total cholesterol levels were significantly greater in group A. Age, sex, and body mass index were not related to the presence of IR or SS in either group.

CONCLUSION

SS and IR do not relate in young normotensive adults or offspring of hypertensive parents. However, the latter may comprise a high-risk group, currently normotensive, who have an increased possibility to present or develop IR in early adolescent life. Moreover, the increased IR is related to the greater MAP in group A. Thus, they are subject to increased cardiovascular risk because of the subsequent disturbed glucose and lipid metabolism.

Dietary salt loading exacerbates the increase in sympathetic nerve activity caused by intravenous insulin infusion in rats

Obesity and type 2 diabetes mellitus frequently produce chronic elevations in blood insulin levels. Importantly, hyperinsulinemia stimulates increases in sympathetic nerve activity that may predispose to hypertension, atherosclerosis, and end-organ damage. Because depletion of dietary salt (NaCl) increases angiotensin II levels, which has been shown to enhance sympathetic responses to excitatory stimuli such as thermal stimulation and bicuculline in the hypothalamus, we predicted that insulin-induced elevations in lumbar sympathetic activity would be augmented by low NaCl and suppressed by high dietary NaCl. Adult male Sprague-Dawley rats were randomized into groups receiving low (0.0 mEq/d, n = 10), normal (2.0 mEq/d, n = 10), and high (5.7 mEq/d, n = 10) NaCl for a period of 8 days. After this, the animals were anesthetized for measurement of heart rate, mean arterial pressure, and lumbar sympathetic nerve activity during 110 minutes of intravenous insulin infusion (15 mU/kg per minute) with euglycemic clamp. Insulin administration caused modest blood pressure decreases accompanied by heart rate increases that were similar across the 3 dietary groups. Unexpectedly, sympathetic increases to insulin were lowest in the low-NaCl group (100%-135% +/- 24%), moderate in the normal-NaCl group (100%-170% +/- 23%), and greatest in the high-NaCl group (100%-252% +/- 39%). Dietary NaCl level did not affect baseline blood glucose or insulin sensitivity as assessed by euglycemic clamp. These findings indicate that dietary salt loading exacerbates the lumbar sympathoexcitatory response to intravenous insulin infusion in rats.

The antinatriuretic effect of insulin: an unappreciated mechanism for hypertension associated with insulin resistance?

Insulin resistance is proposed to be causally related to the metabolic syndrome disorders, but a direct cause-and-effect relationship between insulin resistance and hypertension was not originally obvious. Previous data suggested that insulin promotes sodium retention from the kidney, and thus research efforts focused on this action among several other possible pathways connecting insulin resistance and hyperinsulinemia with hypertension. A review of numerous studies provides evidence that this antinatriuretic effect of insulin is preserved in states of metabolic insulin resistance, representing a major mechanism for blood pressure elevation. More recent experimental and clinical studies have added data about the exact tubular sites of this insulin action, its relation with the respective insulin action on potassium handling, its possible role in the development of salt sensitivity in essential hypertension, as well as the involvement of oxidant stress in these associations. This review summarizes the current state of knowledge in this area and attempts to highlight an important but rather overlooked pathway for hypertension development in the metabolic syndrome, the influence of high insulin levels leading to volume expansion.

Consomic strategies to localize genomic regions related to vascular reactivity in the Dahl salt-sensitive rat

Chromosomal substitution strains afford the opportunity to discover regions of the rat genome that contain genes related to cardiovascular traits with the long-range goal of linking these genes to physiological function. PhysGen (Programs for Genomic Applications) created a consomic panel of rats derived from the introgression of a single chromosome (> or =95% of the BN chromosome, one at a time) of the Brown Norway (BN/NHsdMcwi) rat onto the homogeneous genetic background of the Dahl salt-sensitive rat (SS/JrHsdMcwi). For 3 wk before the experiment, the rats were maintained on a low-salt diet (0.4% NaCl). The dose response of aortic rings from each strain of rat to phenylephrine, acetylcholine, sodium nitroprusside, and three different levels of tissue bath hypoxia (10, 5, and 0% O2) was measured and compared with the parental SS rat. To maximize the possibility that differences among the strains would become apparent, each strain of rat including the parental SS and BN was also studied after being maintained on a high-salt diet (4.0% NaCl) for 3 wk. If the response of the aortic ring from a consomic strain to these vasoactive substances was different from that of the SS parental strain, it was concluded that the introgressed chromosome contained a gene or genes that contributed to that difference. Because the BN chromosome is removed from its native background and the SS rat loses a native chromosome, it is also necessary to consider the contribution of changes in gene-to-gene interaction.

Regulation of blood pressure, the epithelial sodium channel (ENaC), and other key renal sodium transporters by chronic insulin infusion in rats

Hyperinsulinemia is associated with hypertension. Dysregulation of renal distal tubule sodium reabsorption may play a role. We evaluated the regulation of the epithelial sodium channel (ENaC) and the thiazide-sensitive Na-Cl cotransporter (NCC) during chronic hyperinsulinemia in rats and correlated these changes to blood pressure as determined by radiotelemetry. Male Sprague-Dawley rats (∼270 g) underwent one of the following three treatments for 4 wk ( n = 6/group): 1) control; 2) insulin-infused plus 20% dextrose in drinking water; or 3) glucose water-drinking (20% dextrose in water). Mean arterial pressures were increased by insulin and glucose (mmHg at 3 wk): 98 ± 1 (control), 107 ± 2 (insulin), and 109 ± 3 (glucose), P < 0.01. Insulin (but not glucose) increased natriuretic response to benzamil (ENaC inhibitor) and hydrochlorothiazide (NCC inhibitor) on average by 125 and 60%, respectively, relative to control rats, suggesting increased activity of these reabsorptive pathways. Neither insulin nor glucose affected the renal protein abundances of NCC or the ENaC subunits (α, β, and γ) in kidney cortex, outer medulla, or inner medulla in a major way, as determined by immunoblotting. However, insulin and to some extent glucose increased apical localization of these subunits in cortical collecting duct principal cells, as determined by immunoperoxidase labeling. In addition, insulin decreased cortical “with no lysine” kinase (WNK4) abundance (by 16% relative to control), which may have increased NCC activity. Overall, insulin infusion increased blood pressure, and NCC and ENaC activity in rats. Increased apical targeting of ENaC and decreased WNK4 expression may be involved.

Hypothalamic PI3K and MAPK differentially mediate regional sympathetic activation to insulin

The action of insulin in the central nervous system produces sympathetic nervous system activation (also called sympathoactivation), although the neuronal intracellular mechanisms that mediate this are unclear. We hypothesized that PI3K and MAPK, the major pathways involved in insulin receptor signaling, mediate sympathetic nerve responses to insulin. Intracerebroventricular administration of insulin in rat increased multifiber sympathetic nerve activity to the hindlimb, brown adipose tissue (BAT), adrenal gland, and kidney. Ex vivo biochemical studies of mediobasal hypothalamic tissue revealed that insulin stimulated the association of insulin receptor substrate-1 with the p85alpha subunit of PI3K and also tyrosine phosphorylation of p42 and p44 subunits of MAPK in the hypothalamus. In order to determine whether PI3K and/or MAPK were involved in insulin-mediated sympathoactivation, we tested the effect of specific inhibitors of PI3K (LY294002 and wortmannin) and MAPK (PD98059 and U0126) on regional sympathetic responses to insulin. Interestingly, regional sympathoactivation to insulin was differentially affected by blockade of PI3K and MAPK. Inhibition of PI3K specifically blocked insulin-induced sympathoactivation to the hindlimb, while inhibition of MAPK specifically blocked insulin-induced sympathoactivation to BAT. Sympathoactivation to corticotrophin-releasing factor, however, was not affected by inhibition of PI3K and MAPK. These data demonstrate that PI3K and MAPK are specific and regionally selective mediators of the action of insulin on the sympathetic nervous system.

Effect of insulin-induced hypokalemia on lumbar sympathetic nerve activity in anesthetized rats

OBJECTIVE

Acute euglycemic hyperinsulinemia produces sympathoexcitation and a profound fall in plasma potassium levels. Because hypokalemia may activate the renin-angiotensin system to produce the observed increases in sympathetic nerve activity (SNA), the present study was designed to determine whether acute euglycemic-hyperinsulinemia in rats causes decreases in plasma potassium accompanied by increases in plasma renin activity (PRA) as well as elevations in SNA, and whether these alterations would be prevented by maintaining normokalemia with an exogenous potassium infusion.

METHODS

We infused vehicle (control; n = 10) or insulin (10 mU/min) in anesthetized untreated rats (insulin; n = 11), or in rats receiving simultaneous KCl infusion (Insulin + K+; n = 10), while measuring mean arterial pressure (MAP), heart rate (HR), SNA, plasma potassium, and PRA during euglycemic clamp.

RESULTS

As expected, insulin rats had a large fall in plasma potassium (4.6 +/- 0.1 to 3.9 +/- 0.1 mEq/l), contrasting with no change in the control (4.8 +/- 0.2 to 4.8 +/- 0.2 mEq/l) and insulin + K+ (4.4 +/- 0.1 to 4.6 +/- 0.2 mEq/l) groups. However, PRA levels at study completion were not different in the three experimental groups. In addition, insulin rats had large increases in lumbar SNA (194 +/- 11% from 100% baseline) compared with modest elevations in control rats (122 +/- 10%), and prevention of hypokalemia failed to affect sympathetic increases (213 +/- 20%) in insulin + K+ rats. MAP and HR did not change in any of the experimental groups.

CONCLUSIONS

These findings indicate that insulin per se, rather than insulin-induced hypokalemia or hormonal and compensatory adjustments secondary to hypokalemia, is the main mechanism that triggers increases in lumbar SNA.

Dietary salt regulates renal SGK1 abundance: relevance to salt sensitivity in the Dahl rat

Serum and glucocorticoid-induced kinase 1 (SGK1) activates the epithelial sodium channel (eNaC) in tubules. We examined renal SGK1 abundance in salt-adaptation and in salt-sensitive hypertension. Sprague-Dawley and Dahl salt-sensitive rats were placed on either 8% or 0.3% NaCl diets for 10 days. Plasma aldosterone levels were approximately 2.5-fold greater on 0.3% versus 8% NaCl diets in both rat strains. Both serum and glucocorticoid-induced kinase 1 transcript and protein abundance were less (P<0.01) in Sprague-Dawley rats and greater (P<0.01) in Dahl salt-sensitive rats on 8% versus 0.3% NaCl diets. The cDNA sequences of serum and glucocorticoid-induced kinase 1 in both strains of rat were the same. The present results provide evidence that the abundance of serum and glucocorticoid-induced kinase 1 in rat kidney may play a role in salt adaptation and the pathogenesis of hypertension and suggests that aldosterone is not the primary inducer of SGK1 in the Sprague-Dawley rat.

Cardiovascular vagosympathetic activity in rats with ventromedial hypothalamic obesity

OBJECTIVE

Rats with ventromedial hypothalamic lesion (VMH) are massively obese with endogenous hyperinsulinemia, insulin resistance, low sympathetic activity, and high parasympathetic activity, which are likely to induce hypertension. The goal was to follow in this model the long-term hemodynamic changes and to investigate the role of autonomic nervous system and insulin resistance in these changes.

RESEARCH METHODS AND PROCEDURES

Heart rate and blood pressure were monitored for 12 weeks after operation using a telemetric system in VMH and sham rats. Plasma catecholamines and heart beta-adrenoceptors were measured. Glucose tolerance was studied after an intravenous glucose injection and insulin sensitivity during a euglycemic hyperinsulinemic clamp test.

RESULTS

A marked bradycardia and only a mild increase in blood pressure occurred in VMH rats compared with sham animals. Response to autonomic-acting drugs showed an increase in heart vagal tone and responsiveness to a beta-agonist drug. Plasma catecholamine levels were markedly increased, and the density and affinity of heart beta-adrenoceptors were similar in VMH, sham, and control rats. Muscle glucose use was reduced by 1 week after operation in VMH animals.

DISCUSSION

These results show the following in this model of massively obese rats with sympathetic impairment: 1). adrenal medulla secretion is increased, probably as a result of hyperinsulinemia and increased vagal activity; 2). cardiac responsiveness to beta-agonist stimulation is increased; and 3). despite these changes and suspected resistance to the vasodilative effect of insulin, blood pressure does not increase. We conclude that high vagal activity may be protective against hypertension associated with obesity.

Hypertension and insulin disorders

Insulin resistance and/or compensatory hyperinsulinemia are associated with hypertension, obesity, dyslipidemia, and glucose intolerance. Insulin resistance and hyperinsulinemia are considered to increase blood pressure through sympathetic nervous system activation, renin-angiotensin system stimulation, and vascular smooth muscle cell proliferation. Leptin, magnesium ions, nitric oxide, endothelin, peroxisome proliferator-activated receptor gamma, and tumor necrosis factor-alpha also modulate blood pressure. Decreasing insulin resistance by lifestyle modification including diet, weight loss, and physical exercise has been shown to reduce blood pressure. Angiotensin-converting enzyme inhibitors have a beneficial effect on insulin resistance. On the other hand, the angiotensin II antagonist, losartan, does not affect insulin sensitivity. The selective alpha1-blockers have a favorable metabolic profile producing increases in insulin sensitivity. A short-acting type calcium channel blocker seems to decrease insulin sensitivity. On the other hand, long-acting type calcium channel blockers improve insulin sensitivity. Thiazide diuretics and most of the beta-blockers decrease insulin sensitivity. Vasodilatory beta-blockers have been reported to improve insulin sensitivity. Use of low-dose diuretics avoids the adverse effects seen with conventional doses.

Inhibition of nitric oxide synthesis attenuates insulin-mediated sympathetic activation in rats

BACKGROUND AND OBJECTIVES

Infusion of insulin produces sympathoexcitation, nitric oxide (NO) generation and NO-mediated vasodilation. Because central nervous system NO may inhibit sympathetic outflow, the present study was designed to determine whether NO synthase blockade would enhance insulin-mediated sympathetic activation. We additionally aimed to determine whether augmented sympathoexcitation and reduced NO-mediated vasodilation, during combined NO synthase blockade and hyperinsulinemia, would result in a blood pressure increase.

DESIGN AND METHODS

We infused vehicle (Control; n = 7) or insulin (10 mU/min) in anaesthetized rats receiving either no pretreatment (Insulin; n = 7) or after pretreatment with the NO blocker, NG-monomethyl-L-arginine (L-NMMA-insulin; 0.25 mg/kg per min; n = 7), while measuring mean arterial pressure (MAP), heart rate and lumbar sympathetic nerve activity (SNA) during euglycemic clamp. An additional control group received L-NMMA (L-NMMA; n = 7).

RESULTS

Insulin rats had large SNA increases (190 +/- 22% from 100% baseline), contrasting with small increases in the Control (136 +/- 10%) and L-NMMA (135 +/- 20%) groups. Unexpectedly, NO blockade abolished insulin-induced SNA increases in the L-NMMA-insulin group (96 +/- 12%). In agreement with the SNA findings, Insulin rats had heart rate increases while no heart rate changes were observed in the L-NMMA-insulin, Control, or L-NMMA groups. In addition, there was an unexpected was a lack of MAP increase in L-NMMA-insulin rats. MAP also did not change in the Control, L-NMMA or Insulin groups.

CONCLUSIONS

These findings suggest that NO is necessary for insulin to exert its sympathoexcitatory effects, and that insulin-induced NO release may play a role in activating increases in lumbar SNA.

Renal sodium handling and haemodynamics are equally affected by hyperinsulinaemia in salt-sensitive and salt-resistant hypertensives

OBJECTIVE

It is well-known that insulin induces renal sodium retention. It is not yet known whether insulin's renal effects are involved in the development of salt-sensitive hypertension. We assessed the effects of insulin on renal sodium handling and haemodynamics in 10 salt-sensitive (SS) and 10 salt-resistant (SR) essential hypertensives.

DESIGN

After a baseline period of 90 min, all subjects underwent a euglycaemic clamp with sequential infusion of a physiological and supraphysiological dose of insulin (50 and 150 mU/kg per h) during 90 min periods each. Time-control studies were performed in the same subjects. Clearances of 131I-hippuran, 125I-iothalamate, sodium and lithium were used to evaluate renal plasma flow (RPF), CNa/glomerular filtration rate (GFR) and fractional proximal and distal sodium reabsorption.

RESULTS

Plasma insulin levels and insulin-mediated glucose uptake did not differ between both groups. RPF and GFR showed similar increases during both insulin infusions in both groups. During physiological hyperinsulinaemia, fractional sodium excretion decreased 38% (P = 0.009) in the SS group and 36% (P = 0.002) in the SR group. During supraphysiological hyperinsulinaemia, fractional sodium excretion decreased 49% (P = 0.01) in the SS group and 19% (P = 0.2) in the SR group, not statistically different between both groups. Fractional proximal sodium reabsorption was unaffected and fractional distal sodium reabsorption increased to a similar magnitude in both groups.

CONCLUSION

The comparable renal effects of acute exogenous hyperinsulinaemia in SS and SR hypertensives do not support a role for insulin in the development of salt-sensitive hypertension. However, the results do not yet exclude a role for chronic hyperinsulinaemia.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Subpressor dose of L-NAME unmasks hypertensive effect of chronic hyperinsulinemia

We previously found that chronic exogenous hyperinsulinemia without sugar supplementation does not elevate blood pressure. This may be partially explained by the ability of insulin to release nitric oxide and cause vasodilatation. To test this hypothesis, we studied 4 groups of rats: 9 rats (body weight, 213+/-14 g) treated with a gradual increase of a sustained-release subcutaneous insulin pellet; 9 rats (body weight, 213+/-9 g) treated with N:(G)-nitro-L-arginine methyl ester (L-NAME) in drinking water 50 mg/L; 19 rats (body weight, 217+/-11 g) treated with the combination of L-NAME and insulin; and 9 control rats (body weight, 218+/-11 g). Blood pressure was followed weekly for 6 weeks, and then rats were studied in metabolic cages. Weight gain was not different during the 6 weeks. Renal function did not differ between the 4 groups, but 24-hour urinary nitrite/nitrate excretion was lower (P<0.02) in L-NAME-treated and higher in insulin-treated rats. Plasma insulin doubled (P<0.002) in the insulin-treated rats, but there was no hypoglycemia and, by week 6, fructosamine levels were 2.1+/-0.2, 2.1+/-0.2, 2.3+/-0.1, and 2.3+/-0.2 mmol/L in control rats and rats treated with L-NAME, insulin, and L-NAME plus insulin, respectively. Systolic blood pressure, which did not differ at baseline, at week 3 was 122+/-17, 118+/-17, and 118+/-24 mm Hg in the control, L-NAME, and insulin groups and 136+/-14 mm Hg (P<0.03) in the combination group. At week 6, systolic blood pressure was 128+/-14, 127+/-15, and 118+/-13 mm Hg in the control, L-NAME, and insulin groups, respectively, and 150+/-14 mm Hg (P<0.0005) in the combination group. In a subsequent experiment, L-arginine 2 g/L abrogated the effects of L-NAME and insulin combination. In conclusion, chronic exogenous hyperinsulinemia does not affect blood pressure but may cause hypertension when endothelial function is compromised.

Insulin, nitric oxide and the sympathetic nervous system: at the crossroads of metabolic and cardiovascular regulation

Epidemiological studies demonstrate an association between insulin resistance, hypertension and cardiovascular morbidity. Over the past decade, evidence has accumulated indicating that short-term insulin administration, in addition to its metabolic effects, also has important cardiovascular actions. The sympathetic nervous system and the L-arginine-nitric oxide pathway have emerged as central players in the mediation of insulin's cardiovascular actions. The underlying mechanisms and the factors that may govern the interaction between insulin and these two major cardiovascular regulatory systems have been studied extensively in healthy people and insulin-resistant subjects. Here we summarize the current understanding and gaps in knowledge on insulin's cardiovascular actions in humans, and discuss possible pathophysiological consequences of their alteration. Based on recent new insight, we propose that a genetic and/or acquired defect of nitric oxide synthesis could represent a central defect triggering many of the metabolic, vascular and sympathetic abnormalities characteristic of insulin-resistant states, all of which may predispose to cardiovascular disease.

Glycemic modulation of insulin/IGF-1 mediated skeletal muscle blood following sympathetic denervation in normal rats

Both insulin and IGF-1 decrease vascular resistance and increase blood flow in skeletal muscle, and it has been suggested that the mechanistic action for insulin may be by increasing autonomic vasodilatory activity. In this study we evaluated the effects of insulin and IGF-1 on blood flow to denervated and non-denervated skeletal muscle as part of a continuing investigation into the mechanism of regulation of cardiovascular responses by these hormones. Normal rats were prepared for measurements of mean arterial pressure (MAP), heart rate (HR) and vascular flow in the left and right iliac artery. Resection of the left lumbar sympathetic chain increased flow (expressed as conductance, flow/MAP) in the denervated left iliac but not in the intact right artery. Subsequent insulin infusion where hypoglycemia was allowed to occur increased conductance in both arteries, but more so in the denervated artery. Similarly, IGF-1 infusion increased conductances in both intact and denervated iliac arteries, and the effect was slightly greater in the denervated artery. Insulin infusion when euglycemia was maintained increased conductance to a similar extent in denervated and intact iliac arteries. Contrastingly, IGF-1 infusion under euglycemic conditions resulted in a much greater increased conductance in the intact iliac. We conclude that both insulin and IGF-1 increase conductance directly and that glycemic status and sympathetic nerve activity modulate these responses. The insulin-induced increase in conductance in the denervated limb under hypoglycemic conditions suggest that hypoglycemic-stimulated epinephrine release may enhance the dilatory response. while the greater response to IGF-1 in the intact vessel under euglycemic conditions may be due to IGF-1 capacity to decrease sympathetic activity leading to an enhanced conductance.

Insulin's acute effects on glomerular filtration rate correlate with insulin sensitivity whereas insulin's acute effects on proximal tubular sodium reabsorption correlation with salt sensitivity in normal subjects

BACKGROUND

Insulin induces sodium retention by increasing distal tubular sodium reabsorption. Opposite effects of insulin to offset insulin-induced sodium retention are supposedly increases in glomerular filtration rate (GFR) and decreases in proximal tubular sodium reabsorption. Defects in these opposing effects could link insulin resistance to blood-pressure elevation and salt sensitivity.

METHODS

We assessed the relationship between the effects of sequential physiological and supraphysiological insulin dosages (50 and 150 mU/kg/h) on renal sodium handling, and insulin sensitivity and salt sensitivity using the euglycaemic clamp technique and clearances of [131I]hippuran, [125I]iothalamate, sodium, and lithium in 20 normal subjects displaying a wide range of insulin sensitivity. Time-control experiments were performed in the same subjects. Salt sensitivity was determined using a diet method.

RESULTS

During the successive insulin infusions, GFR increased by 5.9% (P = 0.003) and 10.9% (P<0.001), while fractional sodium excretion decreased by 34 and 50% (both P<0.001). Distal tubular sodium reabsorption increased and proximal tubular sodium reabsorption decreased. Insulin sensitivity correlated with changes in GFR during physiological (r = 0.60, P = 0.005) and supraphysiological (r = 0.58, P = 0.007) hyperinsulinaemia, but not with changes in proximal tubular sodium reabsorption. Salt sensitivity correlated with changes in proximal tubular sodium reabsorption (r = 0.49, P = 0.028), but not in GFR, during physiological hyperinsulinaemia. Neither insulin sensitivity or salt sensitivity correlated with changes in overall fractional sodium excretion.

CONCLUSIONS

Insulin sensitivity and salt sensitivity correlate with changes in different elements of renal sodium handling, but not with overall sodium excretion, during insulin infusion. The relevance for blood pressure regulation remains to be proved.

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.

Role of angiotensin II in hyperinsulinemia-induced hypertension in rats

OBJECTIVE

To investigate the role of angiotensin II in the pathogenesis of hyperinsulinemia-induced hypertension in rats.

MATERIALS AND METHODS

Chronic hyperinsulinemia was achieved by infusing insulin (3 mU/kg per min) subcutaneously by an osmotic minipump for 6 weeks. An angiotensin converting enzyme inhibitor (fosinopril, 10 mg/kg per day) was added in drinking water and the angiotensin II subtype 1 receptor antagonist losartan (3.5 microg/kg per min) was co-infused via the minipump. Control rats were administered the vehicle only. The rats were housed in individual metabolic cages and fed a sodium-controlled diet. Food and water intake and urine output were measured daily. Systolic blood pressure and heart rate were measured by the tail-cuff method twice a week.

RESULTS

By the end of weeks 4 and 6 of the sustained insulin infusion, systolic blood pressure had increased significantly (P < 0.05), from 134+/-1 to 157+/-2 and 158+/-2 mmHg, respectively, and the heart rate had increased significantly (P< 0.05), from 380+/-9 to 423+/-7 and 426+/-6 beats/min, respectively. The plasma insulin concentration increased by 2-2.5 times but no significant changes in plasma glucose and triglyceride levels were noted. Concomitant treatment with fosinopril prevented the rises in systolic blood pressure and heart rate in the insulin-infused rats. When the insulin-induced hypertension had become established (systolic blood pressure increased from 132+/-3 to 155+/-2 mmHg 4 weeks after the infusion, P< 0.05 ), subsequent fosinopril or losartan treatment for 2 weeks reversed the elevated systolic blood pressure and heart rate to the control levels. There were no significant differences in water intake, urine flow, sodium gain and body weight gain between the control and the insulin-infused rats.

CONCLUSIONS

Angiotensin converting enzyme inhibition or angiotensin II type 1 receptor antagonism can prevent and reverse insulin-induced hypertension in rats, suggesting that angiotensin II itself or an angiotensin II-dependent mechanism has an etiological influence in the pathogenesis of this hypertension model.

Angiotensin receptor blockade blunts hyperinsulinemia-induced hypertension in rats

The study was conducted to examine the effects of the angiotensin subtype 1 and 2 receptor antagonists (losartan and PD123319, respectively) on blood pressure (BP) and renal excretory function in chronic hyperinsulinemia-induced hypertension in rats. Hyperinsulinemia was achieved by insulin infusion (21.5 pmol/kg per minute) via osmotic minipump for 6 weeks. Losartan or PD 123319 was coinfused either at the beginning or after 4 weeks of insulin infusion. The results showed that insulin infusion significantly increased the plasma insulin concentration from 259.0+/-22.2 to 646.5+/-33.0 and 713.9+/-26.5 pmol/L (P<0.05) by the end of the fourth and sixth weeks, respectively, after insulin infusion. There were no significant changes in plasma glucose and triglyceride concentrations. Systolic BP increased from 139+/-3 to 156+/-1 and 157+/-2 mm Hg (P<0.05) at the corresponding time points. Combined losartan (3.5 microg/kg per minute) and insulin infusion prevented the rise in BP and improved insulin resistance. When hypertension had been established after 4 weeks of insulin infusion, superimposed infusion of losartan on insulin reversed the elevated BP to control levels within 1 week. In contrast, administration of PD123319 (0.5 and 10 microg/kg per minute) failed to alter insulin-induced hypertension. Combined PD123319 with losartan did not alter the losartan-induced hypotensive effect in insulin-infused rats. There were no significant differences in water intake, urine flow, body weight gain, and sodium gain before and after antagonist administration among groups. These results indicate that angiotensin type 1 receptors play a determinant role in the pathogenesis of insulin-induced hypertension in rats.

Renal denervation prevents and reverses hyperinsulinemia-induced hypertension in rats

Experiments were performed to evaluate the role of the renal nerves in hyperinsulinemia-induced hypertension. Male Sprague-Dawley rats were made hyperinsulinemic by insulin infusion via osmotic minipumps implanted subcutaneously (3.0 mU/kg per minute for 6 weeks). Rats with vehicle infusion served as controls. Bilateral renal denervation was performed either at the beginning of or 4 weeks after insulin infusion. The systolic blood pressure was measured by the tail-cuff method twice a week. Food and water intake and urine flow were measured daily. The results showed that sustained insulin infusion significantly increased plasma insulin concentrations from 277.7+/-25.8 pmol/L to 609.9+/-22.2 and 696.7+/-23.0 pmol/L by the end of weeks 4 and 6, respectively (P<0.05). Systolic blood pressure was significantly increased from 135+/-3 to 157+/-3 and 159+/-2 mm Hg (P<0.05) at the corresponding time points. There was a significant increase in the plasma norepinephrine concentration after insulin infusion, whereas no significant changes in plasma triglyceride and glucose concentrations, water intake, urine flow, sodium excretion, sodium gain, and body weight gain were observed. Bilateral renal denervation depleted renal norepinephrine stores and prevented the development of hyperinsulinemia-induced hypertension. After hyperinsulinemia-induced hypertension had been fully established (from 134+/-2 to 157+/-2 mm Hg), bilateral renal denervation reversed the elevated systolic blood pressure to normotensive levels within 2 weeks. Transient denervated diuresis and natriuresis were observed. These results indicate that chronic hyperinsulinemia-induced hypertension requires the presence of intact renal nerves in rats.

Regional blood flow dynamics in response to insulin and IGF-1 in diabetic animals

Vascular changes in diabetes characterized by increased contractile or decreased dilator responses have been demonstrated in isolated blood vessels as well as in vivo systems. Previous studies in our laboratory have demonstrated that insulin and insulin like growth factor-1 (IGF-1) can decrease mean arterial pressure (MAP) and increase blood flow in vascular beds. In this study we evaluated the regional hemodynamic responses to insulin and IGF-1 in normal and diabetic rats. Normal male rats were made diabetic with streptozotocin (55 mg/kg) i.v. and maintained 60 to 70 days. On the day of the study the rats were anesthetized with urethane/chloralose, the femoral artery and vein cannulated for blood pressure monitoring and blood sampling or infusion, respectively. Pulsed-Doppler flow probes were placed around the iliac artery, renal artery and superior mesenteric artery to monitor blood flow. Insulin (16 nmol/kg) was infused as a bolus via the femoral vein and it decreased the MAP approximately 17% in both normal and diabetic rats. Insulin enhanced vascular flow (expressed as conductance) in the iliac and renal vascular bed but not the superior mesenteric vascular bed in normals. In diabetic rats the flow response to insulin compared to normals was attenuated in the iliac and renal vascular beds and increased in the superior mesenteric vascular bed. A bolus infusion of IGF-1 (16 nmol/kg) also decreased the MAP in normals and diabetics. IGF-1 increased vascular flow in all three vascular beds in normals but in the diabetics the response to IGF-1 was attenuated in the iliac, increased in the renal vascular bed and suppressed in the superior mesenteric vasculature. From these studies we conclude that diabetes is associated with an attenuated vascular response to insulin and IGF-1 in the iliac and renal vessels while insulin augments and IGF-1 decreases blood flow in the superior mesenteric vasculature.

Is insulin resistance linked to hypertension?

1. The volume of work reporting insulin resistance in multiple forms of chronic hypertension has generated tremendous interest in whether this abnormality is an important factor in causing hypertension. Insulin resistance, however, is an imprecise term used interchangeably to describe widely disparate types of impairment in insulin action throughout the body and the type of insulin resistance has major ramifications regarding its potential for inducing long-term increases in blood pressure (BP). 2. Hepatic insulin resistance (impaired insulin-mediated suppression of hepatic glucose output) is the primary cause of fasting hyperinsulinaemia and is a cardinal feature of obesity hypertension. Evidence from chronic insulin infusion studies in rats suggests hyperinsulinaemia can increase BP under some conditions; however, conflicting evidence in humans and dogs leaves in question whether hyperinsulinaemia is a factor in hypertension induced by obesity. 3. Peripheral insulin resistance (impaired insulin-mediated glucose uptake, primarily of an acute glucose load in skeletal muscle) also present in obesity hypertension, but now reported in lean essential hypertension as well, is linked most notably to impaired insulin-mediated skeletal muscle vasodilation. This derangement has also been proposed as a mechanism through which insulin resistance can cause hypertension. 4. The present review will discuss the lack of experimental or theoretical support for that hypothesis and will suggest that a direct link between insulin resistance and BP control may not be the best way to envision a role for insulin resistance in cardiovascular morbidity and mortality.

Insulin as a vascular and sympathoexcitatory hormone: implications for blood pressure regulation, insulin sensitivity, and cardiovascular morbidity

The past several years have witnessed a major surge of interest in the cardiovascular actions of insulin. This interest has stemmed on the one hand from epidemiological studies that demonstrated an association between obesity, insulin resistance, and hypertension, leading to the so-called insulin hypothesis of hypertension. On the other hand, this interest has been stimulated by experimental evidence suggesting that the vascular actions of insulin may play a role in its main action, namely the promotion of glucose uptake in skeletal muscle tissue. Two tenets have emerged about how insulin may exert its cardiovascular actions. First, it is now firmly established that acute insulin administration stimulates sympathetic nerve activity in both animals and humans. Second, there is increasing evidence that insulin stimulates muscle blood flow, an effect that appears to be mediated at least in part by an endothelium-dependent mechanism. This review summarizes the current understanding and gaps in knowledge on cardiovascular actions of insulin in humans and pathophysiological consequences of derangements of such actions.

Sodium, blood pressure, and arterial distensibility in insulin-dependent diabetes mellitus

We investigated 24-hour ambulatory blood pressure and arterial distensibility, a marker of biophysical vessel wall properties, in 32 normoalbuminuric type I diabetic patients and 32 healthy control subjects on diets containing 50 mmol and 200 mmol sodium per day. The increase in daytime diastolic blood pressure from 50 to 200 mmol sodium was significantly higher in the diabetic patients than in the control subjects (2.3+/-4.9 versus 0.2+/-3.7 mm Hg, P<.05). On a high sodium regimen, femoral artery distensibility was decreased in the diabetic patients compared with the control subjects (19.2+/-7.6 versus 24.1+/-9.3 10[-3]/kPa, P<.05). Angiotensin-converting enzyme inhibition in the diabetic patients on a high sodium diet decreased daytime diastolic blood pressure and increased femoral artery distensibility. The blood pressure decrease in response to angiotensin-converting enzyme inhibition correlated significantly with the blood pressure increase to sodium (for 24-hour systolic and diastolic blood pressure, r=.72, P<.001 and r=.76, P<.001). In addition, we found that in the diabetic patients on a high sodium diet, the renal blood flow response to exogenous angiotensin II was not bimodally distributed, as is the case in essential hypertension, in which a subgroup of the patients are characterized by sodium sensitivity of the blood pressure and an abnormal renal blood flow response to exogenous angiotensin II ("nonmodulator phenotype"). These results show that blood pressure in insulindependent diabetes mellitus is sodium sensitive, but that this is not related to the nonmodulator phenotype, and suggest that in IDDM a relatively high sodium intake may be a factor that predisposes to the development of diabetic vascular disease.

Lipid metabolism and renal protection by chronic cicletanine treatment in Dahl salt-sensitive rats with salt-induced hypertension

We investigated the role of lipid metabolism in renal protection by chronic cicletanine treatment in Dahl salt-sensitive (Dahl S) rats with salt-induced hypertension. Forty-four 6-week old Dahl S rats were divided into four groups: (1) low-salt (0.3% NaCl) control group: (2) high-salt (4% NaCl) control group; (3) low-dose (10 mg/kg/day) cicletanine (CICL)-treated group given a high-salt diet; and (4) high-dose (30 mg/kg/day) cicletanine-treated group given a high-salt diet. The rats were treated for 6 weeks; blood pressure was measured by the tail-cuff method. Cicletanine significantly reduced the systolic blood pressure in a dose-dependent manner (223 mmHg in the high-salt controls vs 195 mmHg in the high-dose, high-salt group, p < 0.01). Cicletanine treatment did not affect plasma concentration of total cholesterol or triglyceride or free fatty acid; in contrast, it significantly decreased low-density lipoprotein (LDL) cholesterol and increased high-density lipoprotein (HDL) cholesterol. Morphological examination demonstrated that glomerulosclerosis in the kidney was significantly improved by 15% with high-dose cicletanine (p < 0.01). Multivariate analysis revealed that glomerular sclerosis was determined independently by LDL cholesterol levels and arterial injury score, but not by total cholesterol or HDL cholesterol levels or blood pressures. LDL cholesterol was also an independent predictor of urinary excretion of protein. Thus, it is suggested that cicletanine treatment lowers the levels of LDL cholesterol in Dahl salt-sensitive rats, and that besides blood pressure reduction, this decrease in LDL cholesterol level contributes, in part, to regression of glomerular injury in salt-induced hypertension.

Renal function in spontaneously hypertensive rats with insulin-exacerbated hypertension

This study tests the hypothesis that in spontaneously hypertensive rats (SHR), insulin decreases natriuresis and diuresis and thereby contributes importantly to the hypertensive response to exogenous insulin administration. Seven week old SHR were given daily subcutaneous injections of either insulin (mixture of 5 U/Kg regular and 10 U/Kg NPH) or vehicle (isotonic saline). Within one week of treatment, systolic arterial pressure (SAP) was significantly higher in the insulin, compared to saline, treated SHR (184.2 +/- 2.5 vs. 158.3 +/- 4.0 mm Hg). However, twenty-four hour sodium and fluid excretion and the natriuretic and diuretic responses to an intravenous saline load were not affected either before or after the insulin-induced exacerbation of hypertension in SHR. Insulin treatment did not affect glomerular filtration rate, effective renal blood flow, or fractional excretion of Na+ or fluid. Therefore, our data do not support a major role for sodium and fluid retention in the insulin-induced exacerbation of hypertension in SHR.

Contribution of the sympathetic nervous system to hypertensive response to insulin excess in spontaneously hypertensive rats

Our previous studies demonstrate that chronic insulin administration exacerbates hypertension in spontaneously hypertensive rats (SHR). In the present study, we tested the hypothesis that the pressor effect of insulin in SHR is medicated by sympathetic nervous system overactivity. Male SHR (7 weeks old) were given daily subcutaneous injection of insulin or vehicle for 3 days, after which each rat received an intravenous infusion of the peripheral ganglionic blocker hexamethonium. Two days later, in a second experiment, the infusion protocol was repeated with the alpha 2-adrenoceptor agonist clonidine, which more selectively inhibits sympathetic (as compared with parasympathetic) nervous system activity. Insulin treatment for 3 days caused a significant increase in mean arterial pressure (MAP; 164 +/- 2 mm Hg vs. saline control 148 +/- 3 mm Hg), but ganglionic blockade with hexamethonium eliminated the difference in blood pressure (BP) between the insulin-treated and control SHR. Infusion of clonidine significantly reduced MAP in the insulin-treated group to the level of the untreated control SHR, but the infusion did not reduce MAP in the latter group. In a second group of rats, acute administration of prazosin also eliminated the difference in MAP between insulin-treated and control SHR. We conclude that in SHR the sympathetic nervous system contributes importantly to the pressor effect of insulin administration and that this effect may be mediated by the central nervous system.

Chronic exogenous hyperinsulinaemia accelerates the development of hypertension in spontaneously hypertensive rats

1. An association between hyperinsulinaemia, insulin resistance and hypertension was previously described in spontaneously hypertensive rats (SHR). We therefore tested whether chronic exogenous hyperinsulinaemia, which did not affect blood pressure of normotensive rats, may aggravate hypertension in young SHR. 2. Insulin was administered for 4 weeks by a graded increase of a sustained release insulin implant, without carbohydrate supplementation. 3. Initial bodyweight of seven SHR and five sham-implanted control SHR, aged 6-8 weeks, was not different between the groups or by week 4. 4. Glucose levels decreased in the treated rats [2-way ANOVA F(1:10) = 18.7. P < 0.005] and were 7.3 +/- 0.1 mmol/L in the controls and 4.4 +/- 0.7 mmol/L in the treated SHR, respectively. Insulin levels were comparable at baseline and increased to 1002 +/- 978 pmol/L in treated rats at week 4 while remaining 270 +/- 78 pmol/L in the controls [F(1:10) = 6.1, P < 0.05]. The systolic blood pressure (tail-cuff) was significantly increased in insulin treated SHR in weeks 1-3[F(1:10) = 5.1, P < 0.05] though it was comparable at baseline and week 4. 5. In the presence of a hypertensive predisposition, chronic exogenous hyperinsulinaemia accelerates the time course of the development of hypertension without affecting its severity.

Wistar fatty rat is obese and spontaneously hypertensive

The purpose of this study was to determine whether genetically obese Wistar fatty rats have higher blood pressure than their lean littermates and if so to elucidate the mechanism of this obesity-related hypertension. We measured blood glucose and plasma insulin levels, blood pressure, and catecholamine and sodium excretions in age-matched female Wistar fatty and lean rats. After 12 weeks of age, the body weight of Wistar fatty rats was significantly greater than that of their lean counterparts. Fasting blood glucose and plasma insulin concentrations were higher in the fatty than the lean rats throughout the observation period (8 to 24 weeks of age). Systolic blood pressure of fatty rats measured by the tail-cuff method was similar to that of lean rats at 8 weeks of age (135 +/- 2 [mean +/- SEM] versus 134 +/- 3 mm Hg) but significantly higher at 16 (158 +/- 2 versus 136 +/- 3 mm Hg, P < .01) and 24 (166 +/- 5 versus 142 +/- 2 mm Hg, P < .01) weeks of age. Urinary norepinephrine excretion was significantly increased in the fatty rats at both 16 (1755 +/- 173 versus 977 +/- 128 ng/24 h, P < .05) and 24 (1907 +/- 283 versus 737 +/- 173 ng/24 h, P < .01) weeks of age. The ratio of urinary norepinephrine excretion to body weight was also significantly increased in the fatty rats. These results show that with increasing body weight Wistar fatty rats develop hypertension, which may be attributable to an increased sympathetic nerve activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of insulin action on sympathetic nerve activity

Insulin resistance and hyperinsulinemia may contribute to the development of arterial hypertension. Although insulin may elevate arterial pressure, in part, through activation of the sympathetic nervous system, the sites and mechanisms of insulin-induced sympathetic excitation remain uncertain. While sympathoexcitation during insulin may be mediated by the baroreflex, or by modulation of norepinephrine release from sympathetic nerve endings, it has been shown repeatedly that insulin increases sympathetic outflow by actions on the central nervous system. Previous studies employing norepinephrine turnover have suggested that insulin causes sympathoexcitation by acting in the hypothalamus. Recent experiments from our laboratory involving direct measurements of regional sympathetic nerve activity have provided further evidence that insulin acts in the central nervous system. For example, administration of insulin into the third cerebralventricle increased lumbar but not renal or adrenal sympathetic nerve activity in normotensive rats. Interestingly, this pattern of regional sympathetic nerve responses to central neural administration of insulin is similar to that seen with systemic administration of insulin. Further, lesions of the anteroventral third ventricle hypothalamic (AV3V) region abolished increases in sympathetic activity to systemic administration of insulin with euglycemic clamp, suggesting that AV3V-related structures are critical for insulin-induced elevations in sympathetic outflow.

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)

Anteroventral third ventricle lesions abolish lumbar sympathetic responses to insulin

Insulin has been shown to increase sympathetic nerve activity. Because evidence shows that insulin acts within the central nervous system, we hypothesized that lesions of the anteroventral third ventricle region, an area rich in insulin receptors, would abolish sympathetic responses to hyperinsulinemia. We measured mean arterial pressure and lumbar sympathetic nerve activity in fasted, anesthetized sham-lesioned (n = 8) and lesioned (n = 8) rats before and after intravenous insulin infusion at 0.13 U/h during euglycemic clamp. Additional sham-lesioned (n = 10) and lesioned (n = 5) rats received vehicle infusion. Insulin-infused sham-lesioned rats had substantially greater increases in lumbar sympathetic nerve activity (+83 +/- 18%) than vehicle-infused sham-lesioned rats (+27 +/- 4%). Most importantly, insulin-infused lesioned rats had increases in sympathetic activity (+32 +/- 11%) that were no greater than lesioned rats receiving vehicle (+23 +/- 16%). Blood pressure was not altered by insulin or vehicle. To test the possibility that lesions of the anteroventral third ventricle region nonspecifically suppress sympathetic excitatory responses, we evaluated reflex increases in lumbar sympathetic activity to nitroglycerin in sham-lesioned (n = 5) and lesioned (n = 8) rats. Rats with lesions and sham lesions showed comparable increases in lumbar nerve activity during nitroglycerin-induced hypotension. In summary, increases in sympathetic nerve activity to intravenous insulin infusion are abolished by anteroventral third ventricle lesions. These data indicate that the integrity of this brain region is necessary for activation of lumbar sympathetic nerve activity by systemic administration of insulin.

Louis K. Dahl Memorial Lecture. Renal and cardiovascular mechanisms of hypertension in obesity

In all forms of hypertension, including human essential hypertension, pressure natriuresis is reset to higher blood pressures. Because human essential hypertension is a heterogeneous disease, it is likely that there are multiple neurohumoral and intrarenal causes of abnormal pressure natriuresis and increased blood pressure. Weight gain is recognized to be an important contributor to essential hypertension, although the mechanisms that link obesity with altered renal function and high blood pressure have not been fully elucidated. In obese dogs and humans, the shift of pressure natriuresis to higher blood pressures appears to be due mainly to increased tubular reabsorption, as glomerular filtration rate and renal plasma flow are increased compared with normal. Multiple causes of increased tubular reabsorption and hypertension in obesity have been postulated, including insulin resistance and hyperinsulinemia, activation of the sympathetic nervous and renin-angiotensin systems, and physical changes within the kidney itself. Support for the insulin resistance-hyperinsulinemia link between obesity and hypertension has been inferred mainly from acute and epidemiologic studies showing a correlation between insulin and blood pressure. Recent studies suggest that chronic hyperinsulinemia, comparable to that found in obesity, cannot account for obesity hypertension in dogs or humans. Activation of the sympathetic nervous system may play a role in obesity-induced hypertension, and there is evidence for a role of altered intrarenal physical forces caused by histological changes within the renal medulla. The quantitative importance of each of these abnormalities in altering renal function and raising blood pressure in obesity remains to be determined but is an important area of research for understanding human essential hypertension.

Sucrose does not raise blood pressure in rats maintained on a low salt intake

Diets high in sucrose or fructose have been shown by others to induce a modest elevation of blood pressure in rats. The present experiments were conducted to determine whether the sucrose-induced increase of blood pressure is dependent on the intake of sodium chloride. Four groups of Sprague-Dawley rats were studied: 1) a group maintained on a low salt diet and distilled water (0.45% sodium chloride, no added sucrose), 2) a low salt-high sucrose group (0.45% sodium chloride diet and 7% sucrose in distilled water), 3) a high salt group (4% sodium chloride diet and distilled water), and 4) a high salt-high sucrose group on a diet adjusted daily to maintain the same high intakes of sodium chloride and sucrose as those of groups 2 and 3. Systolic blood pressures were measured by tail-cuff plethysmography during weeks 1-3 of treatment, and direct mean arterial blood pressures were recorded in conscious animals during week 4. Animals on the high salt diet gained weight more slowly than those on the low salt intake. On the low sodium chloride intake, blood pressures were not affected by high dietary sucrose (group 1 versus 2). In contrast, on the high sodium chloride intake, blood pressures were 10-14 mm Hg higher in sucrose-drinking animals than in water-drinking animals (group 3 versus 4). The increments in blood pressures of the high sodium chloride-high sucrose group were not accompanied by greater increments in body weight compared with the animals on the high sodium chloride intake alone.(ABSTRACT TRUNCATED AT 250 WORDS)

The inter-relationship between insulin resistance and hypertension

Insulin resistance and compensatory hyperinsulinaemia commonly occur in patients with untreated essential hypertension. The coexistence of insulin resistance and hypertension can be viewed as a cause-effect relationship (insulin resistance as a cause of hypertension or vice versa) or as a noncausal association. Insulin can increase blood pressure via several mechanisms: increased renal sodium reabsorption, activation of the sympathetic nervous system, alteration of transmembrane ion transport, and hypertrophy of resistance vessels. Conversely, hypertension can cause insulin resistance by altering the delivery of insulin and glucose to skeletal muscle cells, resulting in impaired glucose uptake. For example, hypertension can impair vasodilation of skeletal muscle as a result of vascular structural changes and rarefaction, and increased response to vasoconstrictor stimuli. Also, the prevalence of muscle type 2b fibres (fast twitch fibres) may contribute to the development of insulin resistance. The common pathogenetic mechanism for both insulin resistance and hypertension could be activation of the sympathetic nervous system. This results in vasoconstriction, and may contribute to the genesis of vascular structural changes and increase the number of fast twitch fibres. Finally, hypertension and insulin resistance can be viewed as a noncausal association, according to the following hypotheses: 1) they may represent 2 independent consequences of the same metabolic disorder (intracellular free calcium accumulation), or 2) insulin resistance is a genetic marker and/or a pathogenetic mechanism of multiple metabolic abnormalities frequently associated with hypertension.