Abnormalities of carbohydrate and lipid metabolism in Dahl rats

Antecedent protein restriction and high-fat feeding interactively sensitise the leptin response to elevated insulin

Using a rat model of moderate (8 vs. 20% protein) isocaloric protein restriction initiated in early life (low protein, LP), we examined the possible basis for the association between impaired early growth and elevated leptin levels in later life in man by examining the acute leptin response to insulin and its relationship with glucose utilisation. We placed subsets of LP rats on a high-saturated-fat (HF) diet containing 20% protein for 4 weeks (LP-4HF) or 8 weeks (LP-8HF), making comparison with age-matched control (C) groups (C, C-4HF, C-8HF). At ambient insulin concentrations, LP was not associated with altered leptinaemia compared with C, despite a more active lipolytic programme as inferred from increased adipocyte sensitivity to norepinephrine. HF feeding led to insulin resistance with respect to whole-body glucose disposal (R(d)) (measured using [3-(3)H] glucose at steady state) in both LP and C in vivo and impaired suppression of agonist-stimulated lipolysis by insulin in LP but not C in vitro. Whereas insulin infusion for 2 h (while maintaining euglycaemia) only modestly increased plasma leptin levels in vivo in C, C-4HF, C-8HF and LP groups, the leptin response to insulin was greatly enhanced in the HF-fed LP groups. A close positive correlation (r = 0.96) existed between plasma leptin levels and R(d) in the C groups (viz. C, C-4HF, C-8HF) whereas a close inverse correlation (r = 0.95) existed between plasma leptin levels and insulin-stimulated R(d) in the LP groups (viz. LP, LP-4HF, LP-8HF). Glucose utilisation (estimated from 2-deoxy-D-[1-(3)H] glucose 6-phosphate accumulation) in vivo in two intra-abdominal and two superficial adipose-tissue depots was consistently higher in the LP group. After HF feeding, glucose utilisation by the superficial adipose-tissue depots was threefold higher in the LP than in the C group. We conclude that protein restriction from conception to adulthood followed by high-fat feeding sensitizes the acute leptin response to insulin, an adaptation associated with enhanced glucose utilisation by adipose tissue. This effect is observed despite impaired insulin sensitivity, both at the level of whole-body glucose disposal and adipocyte anti-lipolysis, and increased lipolytic activity (although the latter is not in itself sufficient to influence the leptin response). We propose that associations between a low birthweight and elevated leptin concentrations in later life may reflect long-term modulation of adipocyte glucose handling.

Workshop: hypertension and cardiovascular risk factors: role of the angiotensin II-nitric oxide interaction

Vascular upregulation of nitric oxide (NO) is an adaptive response to increased blood pressure that may help in the prevention of end-organ damage. Differences in cardiovascular and renal morbidity and mortality in hypertensive patients may result, at least in part, from individual variations in endothelial function in response to the hemodynamic workload of hypertension. A functional feedback balance exists between both angiotensin (Ang) II and NO under normal conditions. The NO-Ang II imbalance may not explain all the vascular pathophysiology of hypertension, but it certainly appears to be an important component. In hypertension, salt sensitivity, whether primary (ie, certain populations in the United States and Japan) or secondary (ie, aging, type II diabetes), appears to be a marker of increased cardiovascular and renal risk that is often linked to a decreased bioactivity of NO. In diabetes and atherosclerosis, NO-dependent vascular relaxation is impaired and can be restored by decreasing the synthesis and/or blocking the action of Ang II. An understanding of the relations between hypertension, cardiovascular risk factors, end-organ damage, and the NO-Ang II axis leads one to believe that the combination of therapeutic agents capable of reinstating the homeostatic balance of these vasoactive molecules within the vessel wall would be most effective in preventing or arresting end-organ disease.

Brown Norway chromosome 13 confers protection from high salt to consomic Dahl S rat

Consomic rats (SS.BN13), in which chromosome 13 from normotensive inbred Brown Norway rats from a colony maintained at the Medical College of Wisconsin (BN/Mcw) was introgressed into the background of Dahl salt-sensitive (SS/Mcw) rats, also maintained in a colony at the Medical College of Wisconsin, were bred. The present studies determined the mean arterial pressure (MAP) responses to salt and renal and peripheral vascular responses to norepinephrine and angiotensin II; 24-hour protein excretion and histological analyses were used to assess renal pathology in rats that received a high salt (4% NaCl) diet for 4 weeks. MAP of rats measured daily during the fourth week averaged 170+/-3.3 mm Hg in SS/Mcw rats, 119+/-2.1 mm Hg in SS.BN13 rats, and 103+/-1.3 mm Hg in BN/Mcw rats. After salt depletion, MAP fell an average of 27+/-4.5 mm Hg in SS/Mcw rats, 9+/-2.6 mm Hg in SS.BN13 rats, and 11+/-3.0 mm Hg in BN/Mcw rats. Protein excretion of SS/Mcw rats on a high salt diet averaged 189+/-30 mg/24 h, 63+/-18 mg/24 h in SS.BN13 rats, and 40+/-6.4 mg/24 h in BN/Mcw rats. Compared with SS.BN13 and BN/Mcw rats, SS/Mcw rats exhibited significantly greater increases of renal vascular resistance in response to intravenous norepinephrine and angiotensin II. Severe medullary interstitial fibrosis and tubular necrosis after a high salt diet were found consistently in SS/Mcw rat kidneys but were largely absent in the SS.BN13 and BN/Mcw rat kidneys. A similar degree of glomerular sclerosis was found in both SS/Mcw and SS.BN13 rats. In rats fed a 0.4% salt diet, the glomerular filtration rate of SS/Mcw rats was significantly less than that of BN/Mcw and SS.BN13 rats. These results reveal a powerful gene, or set of genes, within chromosome 13 of BN/Mcw rats that confers protection from the detrimental effects of high salt to the SS/Mcw rats.

Salt water promotes hypertension in Dahl-S rats

Hypertension was induced in Dahl-salt-sensitive (Dahl-S) rats by administering salt in drinking water. Control rats receiving tap water did not show a significant change in blood pressure or abnormalities in the kidney. Rats receiving 0.5% NaCl solution developed moderate hypertension and renal lesions. Rats receiving 1.0% NaCl solution showed prominent and increasing hypertension and severe renal damage. This method of salt administration should be simpler than administration in the diet as a means of promoting renal hypertension. The lower concentration salt water caused chronic mild hypertension in Dahl-S rats, and may serve as a useful model for progressive hypertension.

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.

Effects of quinapril and losartan on insulin sensitivity in genetic hypertensive rats with different metabolic abnormalities

This study was designed to investigate the effects of the angiotensin-converting enzyme (ACE) inhibitor quinapril and the angiotensin II-receptor antagonist losartan on insulin sensitivity in two types of genetic hypertensive rats with insulin resistance. Quinapril (3 mg/kg) and losartan (10 mg/kg) decreased the systolic blood pressure to almost the same extent in both spontaneously hypertensive rats (SHRs) and Dahl salt-sensitive (Dahl S) rats. Quinapril increased the glucose requirement for the euglycemic clamp test in both SHRs and Dahl S rats, whereas losartan increased it in SHRs but not in Dahl S rats. The severity of the metabolic abnormalities may be responsible for the failure of losartan to improve the insulin sensitivity in Dahl S rats because serum insulin, total cholesterol, triglyceride, and free fatty acids (FFAs) were higher in the Dahl S rats than in SHRs. A kinin antagonist, Hoe 140, inhibited the increase in the glucose requirement by quinapril without affecting the depressor effect of quinapril in SHRs. In conclusion, quinapril improved the insulin sensitivity more effectively than did losartan in the genetic hypertensive rats with insulin resistance and relatively severe metabolic abnormalities. Based on our findings, one of the mechanisms underlying the difference between quinapril and losartan may thus be endogenous kinins.

Metformin attenuates salt-induced hypertension in spontaneously hypertensive rats

Metformin, an antihyperglycemic agent used for treatment of type 2 diabetes mellitus, lowers blood pressure in humans and experimental animals. We recently demonstrated that short-term administration of metformin may lower blood pressure by reducing sympathetic neural outflow. The present studies were initiated to determine whether long-term administration of metformin blunts salt-induced hypertension, a condition characterized by elevated sympathetic activity. Male spontaneously hypertensive rats, in which radiotelemeters had been implanted for continuous monitoring of heart rate and blood pressure, were randomly assigned to groups that received vehicle (drinking water) or metformin (500 mg/kg per day) and ate a normal 0.3% NaCl diet and to groups that received vehicle or metformin and ate a high 8.0% NaCl diet for a period of 4 weeks. Although metformin did not affect blood pressure in the animals that ate the normal-salt diet (vehicle, 130+/-3 mm Hg; metformin, 133+/-5 mm Hg; mean+/-SEM), drug treatment blunted the rise in pressure caused by a high-salt diet (vehicle, 153+/-4 mm Hg; metformin, 140+/-5 mm Hg; P<0.001). In agreement, during direct pressure recordings in anesthetized rats, the animals that ate the high-salt diet had higher pressures (136+/-13 mm Hg) than those in the control (98+/-5 mm Hg, P<0.01), metformin (100+/-7 mm Hg, P<0.01), and metformin/high-salt groups (92+/-3 mm Hg, P<0.01). Finally, metformin lowered heart rate in rats that ate the normal- and high-salt diets (310+/-3 and 305+/-4 bpm) compared with rats that ate normal- and high-salt diets given vehicle (332+/-3 and 324+/-2 bpm, P<0.01). These data indicate that the chronic depressor actions of metformin are enhanced in animals with hypertension exacerbated by a high-salt diet.

Insulin resistance and human disease: a short history

The notion that tissue resistance to insulin might play an important role in certain disease states is approximately 60 years old. However, recognition of its central role in this regard is a relatively recent phenomenon. In this review an effort has been made to trace a brief history of insulin resistance from its inception to its current position as the fundamental abnormality in both type 2 diabetes and Syndrome X.

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

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

Chronic physiologic hyperinsulinemia impairs suppression of plasma free fatty acids and increases de novo lipogenesis but does not cause dyslipidemia in conscious normal rats

Type 2 diabetes mellitus and obesity are characterized by fasting hyperinsulinemia, insulin resistance with respect to glucose metabolism, elevated plasma free fatty acid (FFA) levels, hypertriglyceridemia, and decreased high-density lipoprotein (HDL) cholesterol. An association between hyperinsulinemia and dyslipidemia has been suggested, but the causality of the relationship remains uncertain. Therefore, we infused eight 12-week-old male catheterized conscious normal rats with insulin (1 mU/min) for 7 days while maintaining euglycemia using a modification of the glucose clamp technique. Control rats (n = 8) received vehicle infusion. Baseline FFAs were 1.07+/-0.13 mmol/L, decreased to 0.57+/-0.10 (P < .05) upon initiation of the insulin infusion, and gradually increased to 0.95+/-0.12 by day 7 (P = NS vbaseline). On day 7 after a 6-hour fast, plasma insulin, glucose, and FFA levels in control and chronically hyperinsulinemic rats were 32+/-5 versus 116+/-21 mU/L (P < .005), 122+/-4 versus 129+/-8 mg/dL (P = NS), and 1.13+/-0.18 versus 0.95+/-0.12 mmol/L (P = NS); total plasma triglyceride and cholesterol levels were 78+/-7 versus 66+/-9 mg/dL (P = NS) and 50+/-3 versus 47+/-2 mg/dL (P = NS), respectively. Very-low-density lipoprotein (VLDL) + intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and HDL2 and HDL3 subfractions of plasma triglyceride and cholesterol were similar in control and hyperinsulinemic rats. Plasma FFA correlated positively with total (r = .61, P < .005) triglycerides. On day 7 after an 8-hour fast, hyperinsulinemic-euglycemic clamps with 3-3H-glucose infusion were performed in all rats. Chronically hyperinsulinemic rats showed peripheral insulin resistance (glucose uptake, 15.8+/-0.8 v 19.3+/-1.4 mg/kg x min, P < .02) but normal suppression of hepatic glucose production (HGP) compared with control rats (4.3+/-1.0 v 5.6+/-1.4 mg/kg x min, P = NS). De novo tissue lipogenesis (3-3H-glucose incorporation into lipids) was increased in chronically hyperinsulinemic versus control rats (0.90+/-0.10 v 0.44+/-0.08 mg/kg x min, P < .005). In conclusion, chronic physiologic hyperinsulinemia (1) causes insulin resistance with regard to the suppression of plasma FFA levels and increases lipogenesis; (2) induces peripheral but not hepatic insulin resistance with respect to glucose metabolism; and (3) does not cause an elevation in VLDL-triglyceride or a reduction in HDL-cholesterol.

Involvement of angiotensin II in development of spontaneous nephrosis in Dahl salt-sensitive rats

We investigated the effect of angiotensin-converting enzyme inhibition on spontaneous nephrosis in Dahl salt-sensitive (Dahl/S) rats. Dahl/S rats fed on a normal sodium diet spontaneously developed nephrosis and mild hypertension from a young age. In young Dahl/S rats, an angiotensin-converting enzyme inhibitor, imidapril, attenuated the development of proteinuria accompanied by a decrease in blood pressure. Methylprednisolone, a potent therapeutic agent for proteinuria, did not affect the development of nephrosis. An angiotensin AT1 receptor antagonist, losartan, but not a Ca2+ channel blocker, verapamil, inhibited the development of nephrosis while both agents decreased blood pressure to a similar extent as imidapril. In mature Dahl/S rats, imidapril suppressed not only the development of proteinuria but also the glomerular lesions. It is concluded that the development of spontaneous nephrosis in Dahl/S rats is mediated by angiotensin II.

Endothelial dysfunction precedes hypertension in diet-induced insulin resistance

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

Insulinemia and pancreatic alpha2-adrenoceptors in salt-sensitive (SBH) and salt-resistant (SBN) Sabra rats: effect of high salt diet

Studies in humans have suggested that hyperinsulinemia might play an important role in salt sensitivity and in the later development of high blood pressure. This possibility has been tested in this study on Sabra rats, an animal model of salt-induced hypertension. Salt-sensitive (SBH) and salt-resistant (SBN) Sabra rats have been submitted to either a normal (0.2% NaCl) or a high salt (8% NaCl) diet for 6 weeks. Comparisons of blood pressure, basal glucose and insulin levels, and insulin response to glucose overload (1 g/kg) have been made. As pancreatic alpha2-adrenergic receptors are implicated in the control of insulin release, their densities have been determined on plasma membranes by saturation studies with [3H]-RX-821002 as the specific radioligand. Under normal diet, blood pressures were respectively 133 +/- 9 and 108 +/- 10 mm Hg (n = 6) in SBH and SBN. Basal glucose and insulin levels and insulin response to glucose overload were found to be significantly higher in SBH than in SBN. In contrast, alpha2-adrenergic receptor densities were lower (P < .001) in SBH when compared to SBN. High salt diet increased (P < .01) blood pressure, decreased basal glucose (P < .01) and insulin (P < .001) levels only in SBH. However, when compared to SBN the insulin response to glucose overload was maintained higher in SBH. Alpha2-adrenergic receptor densities and difference between SBH and SBN did not differ from those found in normal diet. In conclusion, the salt-induced high blood pressure of salt-sensitive Sabra rats is not associated with hyperinsulinemia and insulin resistance. Indeed, an improvement in the insulin sensitivity appears to be induced by either a high salt diet or high blood pressure.

The kidney: an unwilling accomplice in syndrome X

The ability of insulin to stimulate glucose disposal by muscle varies widely within the population at large. Individuals with muscle insulin resistance develop type 2 diabetes if they cannot compensate for this defect by secreting large amounts of insulin. Although this philanthropic effort on the part of the pancreatic B-cell may prevent gross decompensation of glucose homeostasis, it renders such individuals at increased risk to develop a cluster of abnormalities (syndrome X) associated with coronary heart disease. Although the kidney is not considered to be an insulin sensitive tissue, two features of syndrome X, hyperuricemia and hypertension, are likely to be dependent on the retention of normal insulin action on the kidney. More specifically, there is evidence to support the hypothesis that elevated plasma insulin concentrations may enhance renal sodium retention and decrease urinary uric acid clearance. As such, it is possible that a normal kidney response to the compensatory hyperinsulinemia associated with insulin resistance in nondiabetic subjects contributes to the development of hyperuricemia and hypertension in such individuals.

Cytosolic pH and calcium in Dahl salt-sensitive and salt-resistant rats: the relationship to plasma lipids

OBJECTIVE

To search for alterations of cytosolic pH and cell calcium handling in platelets and erythrocytes of Dahl rats susceptible and resistant to salt-induced hypertension.

DESIGN AND METHODS

Blood pressure, plasma lipids, platelet cytosolic calcium concentration ([Ca2+]i) and pH (pHi) together with thrombin-induced changes in these parameters as well as erythrocyte [Ca2+]i and 45Ca influx were determined in Dahl salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats aged 9, 15 and 24 weeks, which were fed a low-salt diet (0.3% NaCl), and in animals fed high-salt diet (4% NaCl) for 5-10 weeks since weaning.

RESULTS

With a low salt intake platelet pHi was lower in SS/Jr than it was in SR/Jr rats, whereas basal platelet [Ca2+]i was similar in rats of both strains. The difference in basal pHi between SS/Jr and SR/Jr rats increased progressively with age of animals. A high salt intake from youth did not influence platelet [Ca2+]i in rats of either strain but it caused an earlier decrease in pHi in SR/Jr than it did in SS/Jr rats. Thrombin stimulation induced similar elevations of pHi and [Ca2+]i in rats of both strains, irrespective of age, salt intake and response of blood pressure to salt intake. Erythrocyte 45Ca influx and [Ca2+]i were greater for SS/Jr rats but only the latter parameter was correlated positively to blood pressure. Both regulation of platelet pHi and erythrocyte Ca2+ handling were significantly related to plasma lipid levels.

CONCLUSIONS

Platelets of SS/Jr rats fed a low-salt diet were characterized by a lower basal cytosolic pHi but unchanged [Ca2+]i relative to those of SR/Jr rats. Hypertension induced by high salt intake was associated with increased erythrocyte [Ca2+]i but not with elevation of platelet [Ca2+]i or alteration of response to stimulation with thrombin.

Reduction of insulin resistance attenuates the development of hypertension in sucrose-fed SHR

We examined the effect of pioglitazone, a thiazolidinedione derivative that increases insulin sensitivity without increasing insulin secretion, on the development and maintenance of hypertension in sucrose-fed SHR. Nine-week-old male SHR received 12% sucrose dissolved in tap water as drinking water. For 5 weeks, half of the rats were given regular rat chow, and the rest were fed with rat chow containing 0.03% pioglitazone. In week 6, blood glucose and plasma insulin levels were examined before and after oral glucose administration by gavage. Sucrose treatment elicited a significant elevation of systolic blood pressure 3 weeks after the beginning of treatment; pioglitazone treatment attenuated this elevation. The insulin resistance and hyperinsulinemia observed in sucrose-fed SHR were prevented by pioglitazone treatment. Pioglitazone treatment also significantly reduced the urinary excretion of catecholamines and plasma renin activity, both of which were significantly greater in sucrose-fed SHR than in control SHR. Along with improving insulin sensitivity, pioglitazone treatment also attenuated the development of hypertension in SHR fed the regular rat chow, but not in WKY rats. These results indicate that insulin resistance and hyperinsulinemia play an important role in the development of hypertension in SHR probably through the activation of the renin-angiotensin system and sympathetic nervous outflow. This study also shows that chronic sucrose treatment exacerbated the development of hypertension through these mechanisms, precipitating insulin resistance.

Quantitative trait loci influencing cholesterol and phospholipid phenotypes map to chromosomes that contain genes regulating blood pressure in the spontaneously hypertensive rat

The frequent coincidence of hypertension and dyslipidemia suggests that related genetic factors might underlie these common risk factors for cardiovascular disease. To investigate whether quantitative trait loci (QTLs) regulating lipid levels map to chromosomes known to contain genes regulating blood pressure, we used a genome scanning approach to map QTLs influencing cholesterol and phospholipid phenotypes in a large set of recombinant inbred strains and in congenic strains derived from the spontaneously hypertensive rat and normotensive Brown-Norway (BN.Lx) rat fed normal and high cholesterol diets. QTLs regulating lipid phenotypes were mapped by scanning the genome with 534 genetic markers. A significant relationship (P < 0.00006) was found between basal HDL2 cholesterol levels and the D19Mit2 marker on chromosome 19. Analysis of congenic strains of spontaneously hypertensive rat indicated that QTLs regulating postdietary lipid phenotypes exist also on chromosomes 8 and 20. Previous studies in the recombinant inbred and congenic strains have demonstrated the presence of blood pressure regulatory genes in corresponding segments of chromosomes 8, 19, and 20. These findings provide support for the hypothesis that blood pressure and certain lipid subfractions can be modulated by linked genes or perhaps even the same genes.

Abnormalities of insulin receptors in spontaneously hypertensive rats

Insulin resistance is present in some strains of rats with genetic hypertension. To determine whether this abnormality is present at the level of the insulin receptor, we compared insulin sensitivity, insulin receptor binding, and mRNA levels in tissues of 10-week-old spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto (WKY) controls. Because we have previously demonstrated an inverse relationship between dietary sodium intake and renal insulin receptor density and mRNA levels in normal Sprague-Dawley rats, the two rat strains in the current experiment were fed either low salt (0.07% NaCl) or high salt (7.5% NaCl) chow until the SHR became hypertensive. Fasting plasma glucose and plasma insulin levels did not differ between SHR and WKY and were not affected by salt intake. When the rats were maintained on the low salt diet, the rate of glucose infusion required to main euglycemia during a hyperinsulinemic clamp was significantly lower in SHR than WKY. High salt diet decreased the rate of glucose utilization during the hyperinsulinemic clamp in WKY but not SHR. During the low salt diet, insulin infusion decreased sodium excretion in both WKY and SHR. When the rats were maintained on the high salt diet, the antinatriuretic response to insulin was blunted in WKY but not SHR. Both the density and mRNA levels of insulin receptor were comparable in the kidney of WKY and SHR, but only WKY had the previously demonstrated decrease in receptor number and mRNA levels when fed the high salt chow. Hepatic insulin receptor mRNA levels were significantly lower in SHR than WKY fed the low salt diet. High salt diet decreased significantly insulin receptor mRNA levels in the liver of WKY but not of SHR. Thus, SHR appear to have lost the feedback mechanism that normally limits insulin-induced sodium retention when extracellular volume is expanded. A decreased expression of insulin receptor in the liver of SHR provides a possible explanation for the insulin resistance and decreased insulin clearance present in this strain.

Malonyl coenzyme A and adiposity in the Dahl salt-sensitive rat: effects of pioglitazone

These studies were designed to assess the effects of pioglitazone, a new oral antidiabetic agent that acts by improving insulin sensitivity, on blood pressure, plasma and tissue lipids, and insulin resistance in the Dahl salt-sensitive (Dahl-S) rat. Reaven et al had reported that male Dahl-S rats are moderately hyperinsulinemic and insulin-resistant. This was of particular interest since these rats are not obese but are hypertriglyceridemic, and on a high-salt diet they become hypertensive. In the current study, male Sprague-Dawley control and Dahl-S rats were compared when fed standard chow of high-fat, high-sucrose (HFHS) diets with or without pioglitazone (20 mg/kg body weight/d) for 3 weeks. On the standard chow diet, Dahl-S rats were hypertriglyceridemic and had high tissue levels of malonyl coenzyme A ([CoA] Dahl-S 5.0 v control 3.3 nmol/g in muscle, and Dahl-S 15.6 v control 10.7 nmol/g in liver); however, they were not hyperinsulinemic. Pioglitazone therapy decreased both malonyl CoA and plasma triglycerides toward control values, but had no effect on plasma insulin levels. On the HFHS diet, both groups became glucose-intolerant and hyperinsulinemic; however, the hyperinsulinemia was greater and more sustained in Dahl-S rats. In addition, the HFHS diet appeared to increase the mass of retroperitoneal fat in the Dahl-S but not in the control group. Treatment with pioglitazone decreased retroperitoneal fat, but as reported previously, it increased the mass of the epididymal fat pad. The results suggest that the hypertriglyceridemia of the Dahl-S rat is associated with an increase in the concentration of malonyl CoA in both liver and muscle. They also show that pioglitazone reverses both of these abnormalities independently of its effect on plasma insulin. Whether these high levels of malonyl CoA predispose the Dahl-S rat to hyperinsulinemia and possibly obesity when placed on a HFHS diet remains to be determined.

Insulin and insulin-like growth factor-I: their role as risk factors in the development of diabetic cardiovascular disease

Experimental data support a role for insulin and insulin-like growth factor-I (IGF-I) in the pathophysiology of vascular complications in diabetes. Clinical data for both hormones are less convincing, mainly because the various studies vary in methodologies, sample sizes, and populations. So far, by epidemiological means, insulin's vascular toxicity has been shown only in middle-aged non-diabetic men. Furthermore, serious methodological problems hamper the clear understanding of IGF-I's significance in this context. Definitive determination of the role of insulin, IGFs and other growth factors in the development of diabetic vascular complications needs considerably more work. In any case, hyperinsulinemia is associated with a cluster of other accepted risk factors for cardiovascular disease which altogether resemble the entire insulin-resistance syndrome.

Insulin modulation of vascular reactivity is already impaired in prehypertensive spontaneously hypertensive rats

Hyperinsulinemia reduces the vasoconstrictive response to norepinephrine in Wistar-Kyoto rats (WKY) but not in spontaneously hypertensive rats (SHR). It has been hypothesized that this difference in the vascular effect of insulin could be a hallmark of the hypertensive state. To test this hypothesis we studied SHR before (5 weeks old, n = 10) and after (15 weeks old, n = 10) the establishment of hypertension as well as two groups of age- and sex-matched WKY (5 weeks old, n = 14; 15 weeks old, n = 13). Blood pressure was significantly higher in SHR compared with WKY (181 +/- 5 versus 118 +/- 6 mm Hg, respectively, P < .001) in the 15-week-old rats but not in the 5-week-old rats (121 +/- 5 versus 117 +/- 3 mm Hg, P < NS). We tested vascular reactivity using increasing amounts of norepinephrine (from 10(-10) to 10(-5) mmol/L) on isolated aortic rings in control conditions and after 30 minutes of exposure to 715 pmol/L insulin. In WKY insulin reduced the vascular response to norepinephrine in both the 5-week-old (repeated-measures ANOVA with grouping factor: F = 2.443, P < .05) and 15-week-old (F = 9.667, P < .01) groups. In SHR at both ages insulin failed to modify the vascular response to norepinephrine (5 weeks: F = 0.107, P < NS; 15 weeks: F = 0.075, P < NS). Sodium nitroprusside was able to attenuate the vascular response to norepinephrine in WKY and SHR at 5 and 15 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Impaired glucose and insulin metabolism in borderline hypertension

This study investigated glucose and insulin metabolism in borderline hypertension (BHT) defined as repeated diastolic blood pressures (DBP) of 85-94 mmHg. Seventy-five BHT and 75 age-matched normotensive (NT, DBP < or = 80 mmHg) men were recruited from a population screening programme. Plasma lipoproteins were determined and an oral glucose tolerance test was performed (WHO criteria). Fasting insulin was significantly higher in the BHT group (17.2 vs 14.2 mU/ml, p < 0.001), whereas fasting blood glucose levels were similar in the two groups, indicating a reduced insulin sensitivity. The BHT group had significantly lower levels of HDL cholesterol and higher levels of plasma triglycerides, VLDL cholesterol and VLDL triglycerides. When adjusted for BMI these differences disappeared, whereas the basal insulin levels remained significantly elevated (F = 10.7, p < 0.001). These results indicate that an altered glucose and insulin metabolism is present already in the early stages of hypertension. They also suggest that these disturbances are only partly dependent on BMI. This supports the hypothesis that reduced insulin sensitivity could be of importance in the early phases of essential hypertension.

Antihypertensive effect of pioglitazone is not invariably associated with increased insulin sensitivity

Hypertension is often associated with insulin resistance, and several chemically diverse agents that increase insulin sensitivity attenuate the development of experimental hypertension. We undertook the present study to determine whether attenuation of hypertension by pioglitazone, a thiazolidinedione derivative that increases insulin sensitivity without increasing insulin secretion, is specifically related to its effect on insulin-mediated glucose uptake. Pioglitazone administered daily by oral gavage (20 mg/kg per day) for 3 weeks attenuated the development of hypertension in both the Dahl salt-sensitive (DS) rat (an insulin-resistant model of hypertension) and the one-kidney, one clip rat (a model of hypertension not associated with insulin resistance). Based on euglycemic insulin clamp studies in conscious animals, the glucose clearance rate was increased (P < .05) in pioglitazone-treated DS rats (36 +/- 3 mg/kg per minute) compared with control DS rats (27 +/- 1 mg/kg per minute). However, pioglitazone did not affect the glucose clearance rate in one-kidney, one clip hypertensive rats. Metformin, an unrelated agent that also improves glucose tolerance, had no significant effect on blood pressure or glucose clearance rate in either DS or one-kidney, one clip rats. Thus, the hypotensive effect of pioglitazone is not invariably associated with its capacity to improve insulin-induced glucose utilization.

Lipid abnormalities in tissues of the KKAy mouse: effects of pioglitazone on malonyl-CoA and diacylglycerol

Insulin resistance is present in liver and muscle of subjects with type 2 diabetes and obesity. Recent studies suggest that such insulin resistance could be related to abnormalities in lipid-mediated signal transduction; however, the nature of these abnormalities is unclear. To examine this question further, tissue levels of diacylglycerol (DAG), malonyl-CoA, and triglyceride (TG) were determined in liver and soleus muscle of obese insulin-resistant KKAy mice and lean C57 BL control mice. In addition, the effects of treatment with pioglitazone, an antidiabetic agent that acts by increasing insulin sensitivity in muscle, liver, and other tissues, were assessed. The KKAy mice were hyperglycemic (407 vs. 138 mg/dl), hypertriglyceridemic (337 vs. 109 mg/dl), hyperinsulinemic (631 vs. 15 mU/ml), and weighed more (42 vs. 35 g) than the control mice. They also had 1.5- to 2.0-fold higher levels of malonyl-CoA in both liver and muscle, higher DAG (twofold) and TG (1.3-fold) levels in muscle, and higher TG (threefold), but not DAG, levels. Treatment of the KKAy mice with pioglitazone for 4 days decreased plasma glucose, TGs, and insulin by approximately 50% and restored hepatic and muscle malonyl-CoA levels to control values. In contrast, pioglitazone increased hepatic and muscle DAG levels two- or threefold. It has no effect on muscle or hepatic TG content, and it slightly increased hepatic TGs in the control group. The results indicate that abnormalities in tissue lipids occur in both liver and muscle of the KKAy mouse and that they are differentially altered when insulin sensitivity is enhanced by treatment with pioglitazone.(ABSTRACT TRUNCATED AT 250 WORDS)

Salt-sensitivity is associated with a hyperinsulinaemic and hyperglycaemic response to atrial natriuretic peptide infusion in human essential hypertension

To evaluate the influence of salt-sensitivity on the plasma insulin and glucose response to infusion of ANP, we studied 22 men with essential hypertension, who were between 40 and 60 years old. After 1 month under normal Na+ intake (120 mmol Na+ per day), patients were randomly assigned to receive either ANP (0.04 micrograms.kg-1.min-1) (n = 15) or vehicle (50 ml saline) (n = 7) over a 60-min period, while in the supine position. Plasma insulin and glucose were measured at time -60, 0, 20, 40, 60, 120, 180, 240 min. Ten days after ANP infusion, blood pressure sensitivity to changes in dietary salt intake was assessed according to a randomized double-blind crossover protocol. Patients were classified into two groups either salt-sensitive (n = 8) or salt-resistant (n = 7). Our results showed that plasma insulin and glucose did not change during ANP infusion in both groups. However, both plasma insulin (from 75.6 +/- 45.1 pmol/l at 60 min to 121.2 +2- 48.6 pmol/l at 240 min, p < 0.05 vs time 0) and glucose levels (from 4.86 +/- 0.73 mmol/l at 60 min to 6.56 +/- 1.03 mmol/l at 240 min, p < 0.01 vs time 0) rose after discontinuation of ANP in salt-sensitive patients, but did not change at all in salt-resistant patients. In conclusion, this randomized vehicle-controlled study demonstrates that plasma insulin and glucose levels increase in salt-sensitive hypertensive patients after the infusion of ANP. The increase of plasma insulin levels observed after ANP discontinuation, if occurring under physiologic conditions, could influence the blood pressure sensitivity to dietary Na+ intake.

Insulin and hypertension: are they related?

It has been postulated that insulin resistance and the concomitant compensatory hyperinsulinemia contribute to the pathogenesis of hypertension, possibly by stimulating the sympathetic nervous system, promoting renal sodium reabsorption, modulating cation transport, and/or stimulating vascular smooth muscle hypertrophy. The purpose of this article is to present a comprehensive up-to-date review of the literature and critically examine the insulin resistance-hyperinsulinemia-hypertension hypothesis.

Insulin resistance and risk factors for coronary heart disease

In this presentation an effort has been made to review the impact of resistance to insulin-mediated glucose uptake and/or hyperinsulinaemia on various metabolic end-points and clinical syndromes. Insulin resistance is present in the great majority of patients with states of glucose intolerance, but frank decompensation of glucose homoeostasis does not occur if individuals can maintain a state of compensatory hyperinsulinaemia. Although compensatory hyperinsulinaemia may prevent the development of NIDDM in insulin-resistant individuals, there is substantial evidence that insulin resistance and/or hyperinsulinaemia is associated with higher plasma concentrations of triglyceride, uric acid and plasminogen activator inhibitor 1 and with lower HDL cholesterol concentrations. Obesity, decreased physical activity and possibly cigarette smoking accentuate the degree of insulin resistance and its manifestations, and a genetic basis is also involved. Resistance to insulin-mediated glucose uptake and/or hyperinsulinaemia have been shown to be associated with high blood pressure, microvascular angina and CHD. Thus, resistance to insulin-mediated glucose uptake is a common phenomenon, which makes a major contribution to the aetiology and clinical course of common and serious diseases. Based on the above considerations, it is difficult to over-emphasize the health-related implication of a defect in insulin-mediated glucose uptake.

Renal acid excretion and intracellular pH in salt-sensitive genetic hypertension

Acid-base status and renal acid excretion were studied in the Dahl/Rapp salt-sensitive (S) rat and its genetically salt-resistant counterpart (R). S rats developed hypertension while on a very high salt diet (8%) and while on a more physiological salt diet (1%) and remained normotensive while on a very low salt diet (0.08%). Under the high salt diet, intracellular pH measured in freshly isolated thymic lymphocytes using 2',7'-bis (carboxyethyl)-5 (6)-carboxyfluorescein acetomethyl ester, a pH-sensitive dye, was lower in S than in R rats both when measured in the presence of HCO3/CO2 (7.32 +/- 0.02 vs. 7.38 +/- 0.02, respectively, P < 0.05) and in its absence (7.18 +/- 0.04 vs. 7.27 +/- 0.02, respectively, P < 0.05). Under the high salt diet, net acid excretion was higher in S than R rats (1,777 +/- 111 vs. 1,017 +/- 73 muEq/24 h per 100 g body wt, respectively, P < 0.001), and this difference was due to higher rates of both titratable acid and ammonium excretion. Directionally similar differences in intracellular pH and net acid excretion between S and R rats were also observed in salt-restricted animals. In S and R rats placed on a normal salt intake (1%) and strictly pair-fed to control food intake as a determinant of dietary acid, net acid excretion was also higher in S than in R rats (562 +/- 27 vs. 329 +/- 21 muEq/24 h per 100 g, respectively, P < 0.01). No significant difference in either blood pH or bicarbonate levels were found between S and R rats on either the 0.08%, 1%, or 8% salt diets. We conclude that renal acid excretion is augmented in the salt-sensitive Dahl/Rapp rat. Enhanced renal acid excretion may be a marker of increased acid production by cells from subjects with salt-sensitive hypertension.

Mechanism of hypertriglyceridemia in Dahl rats

Plasma triglyceride concentrations were shown to be higher in hypertensive (153 +/- 2 mm Hg) male Dahl salt-sensitive rats than in control Sprague-Dawley rats (122 +/- 2 mm Hg). These differences in triglyceride concentrations were seen when blood was drawn at 9 AM from unfasted animals (229 +/- 27 versus 111 +/- 8 mg/dL), at 1 PM after a 4-hour fast (186 +/- 13 versus 88 +/- 4 mg/dL), or at 9 AM after a 13-hour fast (151 +/- 6 versus 90 +/- 6 mg/dL), all p < 0.001. Total triglyceride secretion was also compared in groups of rats by determining the increment in plasma triglyceride concentration for 2 hours after blocking triglyceride removal from plasma by injecting Triton. Studies performed at 1 PM and 9 AM, after the 4- and 13-hour fast, demonstrated that total triglyceride secretion was greater (p < 0.05) in Dahl rats only when studied at 1 PM. Direct estimates of hepatic triglyceride secretion at 1 PM also demonstrated a significant (p < 0.02) increase in secretion rate by perfused livers from Dahl rats, due in part to their increased liver size. In addition, removal of prelabeled very low density lipoprotein-triglyceride in the intact rat was significantly (p < 0.05) decreased in Dahl rats. Lipoprotein lipase activity measured in skeletal muscle, heart, and adipose tissue was also significantly decreased at 9 AM and 1 PM (after 0 and 4 hours of fasting) in tissue from Dahl rats. These data confirm that Dahl rats have higher plasma triglyceride concentrations than Sprague-Dawley rats. Since both total and hepatic triglyceride secretion were somewhat greater in Dahl rats, in association with a decrease in both removal of very low density lipoprotein from plasma and decreased muscle and adipose tissue lipoprotein lipase activity, it seems likely that hypertriglyceridemia in Dahl rats results from a combination of increased triglyceride secretion and decreased triglyceride removal.

Modification of cardiovascular risk factors during antihypertensive treatment: a multicentre trial with quinapril

A large multicentre study involving 6003 [3044 males, 2959 females; mean (+/- SD) age 59 +/- 11 years] mild-to-severe hypertensive patients was carried out to evaluate the effects of the angiotension converting enzyme inhibitor quinapril on blood pressure and on metabolic cardiovascular risk factors during 3-6 months' treatment (mean follow-up 90.4 days). The study population included 551 elderly [mean (+/- SD) age 71.9 +/- 9.3 years] patients, 1314 subjects with diabetes mellitus and 154 non-diabetic patients with hyperlipaemia; 4% of patients were lost to follow-up. Diastolic blood pressure decreased from 102 to 87 mmHg (intent-to-treat analysis) and 62% of patients were normalized (diastolic blood pressure below 90 mmHg) at the last evaluable visit. Overall, serum lipids were favourably affected during quinapril treatment; when corrected for changes in body weight, a significant improvement in total, low- and high-density lipoprotein cholesterol, and triglycerides was detected. Quinapril treatment in elderly patients was efficacious and well tolerated, and quinapril appears to be an effective antihypertensive drug devoid of untoward effects on metabolic risk factors for cardiovascular disease.

Red cell sodium-proton exchange is increased in Dahl salt-sensitive hypertensive rats

To investigate the relationship between red blood cell Na+/H+ exchange (EXC) and genetic factors in hypertension, we studied the maximal rate of the antiporter (mmol/liter cell x hr; flux units = FU) in three strains of genetically hypertensive rats. Salt-resistant Dahl rats (DR) were normotensive under low (0.02%) and high (8%) NaCl diets, while salt-sensitive Dahl rats (DS) became markedly hypertensive after four weeks on the high-NaCl diet. Na+/H+ exchange did not differ between DR and DS rats when both were fed with the low-NaCl diet (mean +/- SE, 31 +/- 3, N = 15, vs. 29 +/- 3 FU, N = 14). On the high-NaCl diet, the DR strain did not exhibit significant changes in blood pressure and antiporter activity, but the DS rats significantly increased their blood pressure and Na+/H+ exchange (57 +/- 4 FU, N = 13) versus DR rats (38 +/- 3 FU, N = 15, P < 0.02). DS rats also significantly increased blood pressure and antiporter activity when fed with high-NaCl diet for one week. These data indicate that high NaCl intake per se does not increase Na+/H+ EXC because the control DR strain did not exhibit transport and blood pressure alterations as observed in the DS strain. Milan hypertensive and spontaneously hypertensive rats (Charles River substrain) had higher blood pressures than Milan and Wistar-Kyoto normotensive rats when they were maintained for four weeks on a 1.5% NaCl diet; however, no differences were seen among normotensive and hypertensive strains in Na+/H+ exchange activity.(ABSTRACT TRUNCATED AT 250 WORDS)