The hypertriglyceridemic rat as a genetic model of hypertension and diabetes

Research on Experimental Hypertension in Prague (1966-2009)

The study of ontogenetic aspects of water and electrolyte metabolism performed in the Institute of Physiology (Czechoslovak Academy of Sciences) led to the research on the increased susceptibility of immature rats to salt-dependent forms of hypertension since 1966. Hemodynamic studies in developing rats paved the way to the evaluation of hemodynamic mechanisms during the development of genetic hypertension in SHR. A particular attention was focused on altered renal function and kidney damage in both salt and genetic hypertension with a special respect to renin-angiotensin system. Renal damage associated with hypertension progression was in the center of interest of several research groups in Prague. The alterations in ion transport, cell calcium handling and membrane structure as well as their relationship to abnormal lipid metabolism were studied in a close cooperation with laboratories in Munich, Glasgow, Montreal and Paris. The role of NO and oxidative stress in various forms of hypertension was a subject of a joint research with our Slovak colleagues focused mainly on NO-deficient hypertension elicited by chronic L-NAME administration. Finally, we adopted a method enabling us to evaluate the balance of vasoconstrictor and vasodilator mechanisms in BP maintenance. Using this method we demonstrated sympathetic hyperactivity and relative NO deficiency in rats with either salt-dependent or genetic hypertension. At the end of the first decennium of this century we were ready to modify our traditional approach towards modern trends in the research of experimental hypertension. Keywords: Salt-dependent hypertension o Genetic hypertension o Body fluids o Hemodynamics o Ion transport o Cell membrane structure and function o Renal function o Renin-angiotensin systems.

Metabolic Syndrome in Hypertriglyceridemic Rats: Effects of Antioxidants

Long-lasting disturbances in lipid and glucose metabolism present in metabolic syndrome (MetS) lead to serious cardiovascular diseases. The study was aimed to evaluate the effect of natural antioxidant vitamin E (VitE, 100 mg/kg/day, p.o.) on basal biochemical and physiological parameters characterizing MetS and on the changed function of the heart. Furthermore, the possible potentiation of VitE effect by synthetic pyridoindole antioxidant SMe1EC2 (SMe, 15 mg/kg/day, p.o.) was also tested. MetS was induced in hereditary hypertriglyceridemic rats (HTG) by the 5 weeks administration of high-fat fructose diet (HFFD: 1 % cholesterol, 7.5 % pork lard, 10 % fructose). The heart function was tested using Langendorff preparation under constant pressure. The functional parameters of isolated heart, dysrhythmias and evoked fibrillations were evaluated in conditions of ischemia-reperfusion. The HFFD increased body weight gain and serum levels of total cholesterol, low-density lipoproteins and blood glucose. The HFFD significantly increased heart flow and force of contraction, compared to standard diet (SD). During the reperfusion, the HFFD caused the increase of the ventricular premature beats number at the expense of decreasing the duration of serious dysrhythmias (ventricular tachycardias and fibrillations). The addition of VitE, SMe or their combination to the HFFD decreased body weight gain, depressed blood pressure, improved particular biochemical parameters. The combination of VitE and SMe suppressed the occurrence of serious dysrhythmias. Our data indicate that the HFFD-related disturbances led to alterations within pathophysiology in HTG rats. The results showed that a combination of antioxidants might have the potential to amend disorders accompanying MetS.

Impact of improving eating habits and rosmarinic acid supplementation on rat vascular and neuronal system in the metabolic syndrome model

Decreasing high fat and high carbohydrate intake, together with the administration of natural bioactive drugs, is assumed to have a protective effect in the prevention and amelioration of the metabolic syndrome (MetS). The aim of the study was to evaluate effects of diet improvement and/or a phenolic compound (rosmarinic acid; RA) administration (100 mg/kg per d) on metabolic as well as functional changes of vessels and hippocampus caused by the MetS-like conditions. The MetS-like conditions were induced by a high-fat-fructose diet (HFFD) in Prague hereditary hypertriacylglycerolaemic (HTG) rats. The effect of diet improvement and RA administration was studied using biochemical and functional measurements. Consumption of HFFD by HTG rats resulted in the development of conditions like the MetS. The fat and fructose restriction from the diet led to amelioration of basic indicators of metabolic state in rats fed HFFD and to amendment parameters of glucose tolerance test and reduction of the IL-1β serum levels. Moreover, aortic endothelial function was improved with an impact on blood pressure. The functional measurement of electrophysiology of the hippocampus showed that long-term potentiation of neuronal transmission course deteriorated after HFFD was improved by energy restriction. Oral administration of RA had a supporting effect not only on lipid and glucose metabolism but also on the vascular endothelium. Combination of both types of therapy induced beneficial effect on glucose tolerance and lipid peroxidation. Thus, combined improvement of diet habits and treatment with natural bioactive drugs is assumed to have protective effect in prevention and amelioration of the MetS.

Increased systolic blood pressure associated with hypertriglyceridemia in female Sprague-Dawley rats

The effect of hyperlipidemia on the cardiovascular system is uncertain in females. The aim of the present study was to determine whether administration of a lipogenic diet alters cardiovascular parameters in female rats. Fifty female Sprague-Dawley rats were assigned into 2 groups of rats receiving a standard or a high-fat, high-sucrose diet (HFHS) for 6 weeks (n = 25 per group). Body mass, blood lipids concentrations, triglycerides clearance, blood pressures (BPs), systolic and diastolic functions, as well as vascular reactivity were assessed at the end of the diet intervention. At termination, body mass was similar between the 2 groups. Fasting blood triglycerides concentration (BTG) was greater in the HFHS group. Triglycerides clearance was impaired in the HFHS group. High-density lipoprotein (HDL) cholesterol concentration was lower in the HFHS group. The early-to-late diastolic filling velocity ratio (E/A) was lower in the HFHS group and negatively associated with BTG. The sensitivity (EC₅₀) of mesenteric arteries to phenylephrine was greater in HFHS and was negatively associated with BTG, but not HDL. Systolic BP was higher in the HFHS group and was positively associated with BTG and HDL. The association between systolic BP and BTG was independent of other lipids measured. In conclusion, hypertriglyceridemia may have increased resistance arteries responsiveness to alpha-agonist and systolic BP in female rats.

Rat Models of Metabolic Syndrome

Metabolic syndrome is a complex disorder that comprises several other complex disorders, including obesity, hypertension, dyslipidemia, and diabetes. There are several rat models that encompass component features of MetS. Some models are inbred strains selected for one or more traits underlying MetS; others are population models with genetic risk for MetS traits, are induced by environmental stressors such as diet, are spontaneous monogenic mutant models, or are congenic strains derived from a combination of these models. Together they can be studied to identify the genetic and physiological underpinnings of MetS to identify candidate genes or mechanisms for study in human MetS subjects.

Effect of high-fat-fructose diet on synaptic plasticity in hippocampus and lipid profile of blood serum of rat: pharmacological possibilities of affecting risk factors

The aim of this study was to determine pharmacological possibilities of influencing the risk factors of metabolic syndrome (MetS). Hypertriacylglycerolemic (HTG) rats fed with high-fat-fructose diet (HFFD) were used as a model of the MetS. Wistar rats fed with standard diet were used as negative control group. HTG rats fed with HFFD for 8 weeks were used as positive control group. The effects of atorvastatin and SMe1EC2 were tested. The compounds were administered to the HTG rats after 5 weeks of HFFD, once a day for 3 weeks. After 8 weeks, the blood serum lipid profile and electrophysiology of neurotransmission in hippocampal sections were evaluated in vitro. SMe1EC2 and atorvastatin had a significant effect on total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-cholesterol) and atorvastatin had a significant effect on triacylglycerols (TGs). SMe1EC2 improved the long-term potentiation (LTP) course in the hippocampus.

Positive Effects of Different Drug Forms of Silybin in the Treatment of Metabolic Syndrome

Silymarin and silybin are widely used for their hepatoprotective properties. Our previous studies confirm positive effect of silymarin on lipoprotein profile and lipid homeostasis. Advanced drug forms may improve the bioavailability of these compounds. In this study, we investigate the effects of silybin in different drug forms (standardized silybin, micronized silybin, and silybin in form of phytosomes) on dyslipidemia and glucose metabolism in hereditary hypertriglyceridemic (HHTg) rats. Male HHTg rats were divided into four groups of seven animals and were fed by experimental diets. Silybin significantly decreased serum level of triglycerides in groups of rats fed by standardized silybin and silybin in form of phytosomes compared to control group. Results show that silybin did not affect the total cholesterol level, but significantly increased the levels of HDL cholesterol in all groups of animals. Silybin in a standardized form had the highest hypotriglyceridemic effect. On the other hand, the micronized form has caused the highest increase of protective HDL and most significantly decreased glucose and insulin levels. Our results suggest that silybin is probably responsible for some positive properties of silymarin. Subsequent dose-dependent studies of silybin action may reveal the intensity of its positive effects on lipid and glucose parameters.

Local metabolism of glucocorticoids in Prague hereditary hypertriglyceridemic rats--effect of hypertriglyceridemia and gender

11β-Hydroxysteroid dehydrogenase type 1 (11HSD1) is a microsomal NADPH-dependent oxidoreductase which elevates intracellular concentrations of active glucocorticoids. Data obtained from mouse strains with genetically manipulated 11HSD1 showed that local metabolism of glucocorticoids plays an important role in the development of metabolic syndrome. Tissue specific dysregulation of 11HSD1 was also found in other models of metabolic syndrome as well as in a number of clinical studies. Here, we studied local glucocorticoid action in the liver, subcutaneous adipose tissue (SAT) and skeletal muscles of male and female Prague hereditary hypertriglyceridemic rats (HHTg) and their normotriglyceridemic counterpart, the Wistar rats. 11HSD1 bioactivity was measured as a conversion of [(3)H]11-dehydrocorticosterone to [(3)H]corticosterone or vice versa. Additionally to express level of active 11HSD1 protein, enzyme activity was measured in tissue homogenates. mRNA abundance of 11HSD1, hexoso-6-phosphate dehydrogenase (H6PDH) and the glucocorticoid receptor (GR) was measured by real-time PCR. In comparison with normotriglyceridemic animals, female HHTg rats showed enhanced regeneration of glucocorticoids in the liver and the absence of any changes in SAT and skeletal muscle. In contrast to females, the glucocorticoid regeneration in males of HHTg rats was unchanged in liver, but stimulated in SAT and downregulated in muscle. Furthermore, SAT and skeletal muscle exhibited not only 11-reductase but also 11-oxidase catalyzed by 11HSD1. In females of both strains, 11-oxidase activity largely exceeded 11-reductase activity. No dramatic changes were found in the mRNA expression of H6PDH and GR. Our data provide evidence that the relationship between hypertriglyceridemia and glucocorticoid action is complex and gender specific.

Occurrence of lipids in the liver of the hypertriglyceridemic rats

AIMS

The purpose of this study was to demonstrate the accumulation and distribution of lipids in the liver of the adult Prague hereditary hypertriglyceridemic (HHTg) rats. They reveal an increased expression of 11beta-hydroxysteroid dehydrogenase 1 (11HSD1), which locally increases concentration of corticosterone in the liver. We studied the effect of the 11HSD1 inhibition on the lipid content.

METHODS

Samples of liver of three groups of adult female rats--HHTg, HHTg treated for 14 days with 50 mg/kg/day carbenoxolone (HHTg+CBX) and control Wistar rats, were examined histochemically. Cryosections of the samples were stained with Oil red O or Sudan black B to demonstrate different kinds of lipids. Extent and intensity of staining was evaluated semiquantitatively.

RESULTS

The orientational analysis showed a higher extent and intensity of the staining of the liver of HHTg and HHTg+CBX rats (equal in both hypertriglyceridemic groups) than that of the control Wistar rats. Oil red O stained unsaturated fatty acids and neutral fats, mainly triglycerides. The difference was on average 30 per cent. Staining of phospholipids with Sudan black B showed similarly the higher positivity in the hypertriglyceridemic groups than in controls.

CONCLUSIONS

Staining for triglycerides and phospholipids demonstrated a higher amount of lipids in the liver of HHTg and HHTg+CBX female rats than in controls. The inhibition of 11HSD1 activity had no effect on the lipid content in the liver of the HHTg rats.

Abnormal Igf2 gene in Prague hereditary hypertriglyceridemic rats: its relation to blood pressure and plasma lipids

Prague hypertriglyceridemic (HTG) rats represent a suitable model of metabolic syndrome. We have established the set of F(2) hybrids derived from HTG and Lewis progenitors to investigate the relationship between respective polymorphism(s) of Igf2 gene and blood pressure (BP) or other cardiovascular phenotypes. HTG rats had elevated systolic BP and plasma triglycerides but lower plasma cholesterol compared to Lewis rats of both genders. In males, there was higher mean arterial pressure, diastolic BP and relative heart weight in HTG than in Lewis rats. The results obtained in the total population of F(2) hybrids indicated strong segregation of Igf2 genotype with plasma triglycerides. There was no segregation of Igf2 genotype with any BP component except BP changes occurring after the blockade of either renin-angiotensin system (RAS) or NO synthase. When F(2) population was analyzed according to gender, male F(2) progeny homozygous for HTG Igf2 allele had significantly higher plasma triglycerides and greater BP changes after NO synthase blockade than those homozygous for Lewis allele. On the contrary, male F(2) progeny homozygous for HTG Igf2 allele had significantly lower plasma cholesterol and smaller BP changes after RAS blockade. PCR analysis of Igf2 gene by using of microsatelite D1Mgh22 has shown polymorphism between HTG and Lewis rats. Sequence analysis of cDNA revealed insertion of 14 nucleotides in HTG gene. In conclusion, polymorphism in Igf2 gene may be responsible for differences in lipid metabolism between HTG and Lewis rats. It remains to determine how these abnormalities could be involved in BP regulation by particular vasoactive systems.

Rat model of familial combined hyperlipidemia as a result of comparative mapping

Total genome scan was carried out in 266 F2intercrosses from the Prague hypertriglyceridemic (HTG) rat that shares several clinical characteristics with human metabolic syndrome. Two loci for plasma triglycerides (TG) were localized on chromosome 2 (Chr 2) (LOD 4.4, 3.2). The first locus overlapped with the rat syntenic region of the human locus for the metabolic syndrome and for small, dense LDL, while the second overlapped with the syntenic region of another locus for small, dense LDL in humans by the comparative mapping approach. Loci for TG on rat Chr 13 (LOD 3.3) and Chr 1 (LOD 2.7) overlapped with the syntenic region of loci for human familial combined hyperlipidemia (FCHL) in Finnish and Dutch populations, respectively. The concordances of loci for TG localized in this study with previously reported loci for FCHL and its related phenotypes are underlying the generalized importance of these loci in dyslipidemia. These data suggest the close relationship between dyslipidemia in HTG rats and human FCHL, establishing a novel animal model for exploration of pathophysiology and therapy based on genomic determinants.

Cyclic nucleotides in platelets of genetically hypertriglyceridemic and hypertensive rats. Thrombin and nitric oxide responses are unrelated to plasma triglyceride levels

Prague hereditary hypertriglyceridemic (HTG) rats constitute a genetic model of hypertension associated with hyperlipidemia and insulin resistance. Various cell alterations, including changes in membrane dynamics, ion transport, and decreased platelet responses to thrombin have been observed in this strain. As hypertriglyceridemia appears to be associated with reduced endothelium-dependent vasodilation and platelet aggregation, we examined whether triglycerides could modulate cell responsiveness through changes in cyclic nucleotides in platelets of HTG rats. From the age of 6 weeks, these hypertensive animals were subjected for 10 weeks to interventions that modified circulating triglycerides levels (2.17+/-0.09 mmol/l), leading to their reduction (gemfibrozil treatment, 0.87+/-0.05 mmol/l) or elevation (high fructose intake, 3.23+/-0.07 mmol/l). Basal cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) contents were 15% and 48% lower in isolated platelets of HTG rats than in those of Lewis controls. cAMP level was further reduced in HTG rats subjected to high fructose intake. Irrespective of their plasma triglyceride levels, the thrombin-induced increase in platelet cGMP levels present in Lewis rats was absent in platelets of HTG rats. In contrast, no strain- or treatment-related differences were observed in the magnitude or kinetics of cGMP response to exogenous nitric oxide (NO). NO-induced cGMP and cAMP changes were associated in an opposite manner with trimethylamino-diphenylhexatriene (TMA-DPH) anisotropy, a biophysical parameter that reflects the microviscosity of the outer part of the cell membrane. Our results indicate that the attenuation of platelet responsiveness to thrombin in HTG rats represents a strain difference that cannot merely be due to a difference in plasma triglyceride levels. Platelet hyporesponsiveness to agonists such as thrombin in HTG rats cannot be explained by a change in levels of inhibitory cyclic nucleotides, since they were actually found to be low and not high.

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.

Intralipid enhances alpha1-adrenergic receptor mediated pressor sensitivity

The dyslipidemia in obese hypertensive persons may contribute to their increased vascular alpha-adrenergic receptor reactivity and tone. To further examine this notion, we conducted 2 studies of pressor sensitivity to phenylephrine, an alpha1-adrenergic receptor agonist, in lean normotensive subjects. In the first study (n=6), pressor responses to phenylephrine were obtained before and during a saline and heparin infusion. On another day, pressor reactivity to phenylephrine was measured before and during infusion of 20% Intralipid at 0.5 mL . m-2 . min-1 with heparin at 1000 U/h to increase lipoprotein lipase activity and raise nonesterified fatty acids (NEFAs). In the second study (n=8), baseline reactivity to phenylephrine was obtained on 2 separate days and repeated after raising NEFAs and triglycerides either with 0.8 mL . m-2 . min-1 of 20% Intralipid alone or together with heparin. The infusion of saline and heparin did not significantly change plasma NEFAs from baseline (516+/-90 versus 512+/-108 micromol/L, respectively; P=NS) or the dose of phenylephrine required to raise mean blood pressure by 20 mm Hg ([PD20PE]; 1.00+/-0.14 versus 0. 95+/-0.10 microg . kg-1 . min-1, respectively, P=NS). Intralipid at 0.5 mL . m-2 . min-1 with heparin raised plasma NEFAs to 793+/-30 micromol/L per liter (P<0.05 versus baseline) and reduced PD20PE from 1.01+/-0.10 to 0.80+/-0.09 microg . kg-1 . min-1 (P<0.05). Compared with baseline, Intralipid alone increased plasma NEFAs to 946+/-80 micromol/L (P<0.05), and NEFAs increased further with the addition of heparin to 2990+/-254 micromol/L (P<0.01). Despite an apparently greater increase of plasma NEFAs with Intralipid and heparin, Intralipid alone and together with heparin similarly reduced PD20PE. Across all study conditions, changes in levels of triglycerides and NEFAs correlated with changes in mean arterial pressure responses to phenylephrine, especially at the 0.4- microg . kg-1 . min-1 infusion rate of phenylephrine (r=0.64, P<0.01 and r=0. 54, P<0.01, respectively). These data suggest that raising levels of plasma NEFAs and/or triglycerides enhances alpha1-adrenoceptor mediated pressor sensitivity. The findings suggest that lipid abnormalities in obese hypertensives, which include elevated NEFAs and triglycerides, contribute to greater vascular alpha1-adrenergic reactivity.

Platelet hypoaggregability in hereditary hypertriglyceridemic rats: relation to plasma triglycerides

To define better the relationships between lipid metabolism disturbances and platelet aggregation we have examined these parameters in hereditary hypertriglyceridemic and control Lewis rats. Hereditary hypertriglyceridemic rats are hypertensive and have high plasma triglycerides but not elevated plasma total cholesterol. In the present study, we have demonstrated that platelets from hereditary hypertriglyceridemic rats have lowered initial rate and maximal aggregation after stimulation with thrombin or ADP in comparison with controls. These two strains did not differ significantly in the inhibition of platelet aggregation by the thromboxane A2 receptor inhibitor, SQ 29 548. In hereditary hypertriglyceridemic rats, the thrombin response, as well as the contribution of the thromboxane A2-sensitive pathway, were positively associated with the plasma level of triglycerides. Similar trend was found in Lewis rats. However, the slopes of these relationships were reduced in hereditary hypertriglyceridemic rats. These alterations of the aggregatory responses in hereditary hypertriglyceridemic rats were independent of blood pressure and plasma cholesterol level. In conclusion, our results showed a clear-cut platelet hypoaggregability to both thrombin and ADP in hypertensive hypertriglyceridemic rats. This hypoaggregability was not due to an impaired function of the thromboxane A2 pathway but could be connected with disturbances of lipid metabolism.

Structural changes in the aorta of the hereditary hypertriglyceridemic rat

Structural changes in the ascending thoracic aorta of hereditary hypertriglyceridemic (hHTG), insulin-resistant, and hypertensive rats were studied using transmission electron microscopy. Normotensive Wistar rats were used as controls. The most-pronounced morphological changes were observed in the tunica intima. Endothelial cells of hHTG rats formed a continual layer around the whole circumference. Subendothelial space was enlarged. Some endothelial cells were delaminated from the subendothelial space by big lipid droplets that were often present in the subendothelial space, and the endothelial cells bulged out towards the lumen. Big electron-lucent lipid droplets were present in the majority of the endothelial cells and occupied the main part of the cytoplasm. Degenerative microvesicular and membranous material was present in the cytoplasm. Increased numbers of vesicles of Golgi apparatus and cisternae of endoplasmic reticulum were found. Similar morphological alterations, but in less-extended form, were observed in smooth muscle cells. The organization and orientation of smooth muscle cells were essentially intact. In muscle cells, lipid droplets were localized in close relation to Golgi complex and in dilated cisternae of the sarcoplasmic reticulum. Lipid droplets, degenerative material, myelin figures, myelinoid membranes, and vesicular components were also sporadically found in the intercellular space among muscle cells. This pilot morphological investigation provides further arguments for a thorough and more-focused electron microscopy study of conductance arteries of the hHTG rats.

Plasma triglycerides and red cell ion transport alterations in genetically hypertensive rats

Ion transport abnormalities in essential hypertension are often associated with concomitant changes of lipid metabolism, but this information is missing in rats with genetic hypertension. We therefore studied the alterations of red cell Na+ and K+ transport and their relationship to blood pressure and plasma lipids (cholesterol and triglycerides) in Prague hereditary hypertriglyceridemic (HTG) rats, Lyon hypertensive (LH) rats, and HTG x Lewis F2 hybrids. In both hypertensive models and F2 hybrids, red cell Na+ content (Na+(i)) was positively related to plasma triglycerides but not to plasma cholesterol levels. Na+(i) elevation was more pronounced in HTG than in LH rats, probably due to higher plasma triglycerides in the former strain. The two hypertensive strains differed in bumetanide-sensitive Na+ transport, which was augmented in HTG rats with low plasma cholesterol but suppressed in LH rats characterized by high cholesterol levels. In the two genetic models, there was a positive association of blood pressure with Na+ leak, and this was also confirmed by the cosegregation of these parameters in F2 hybrids. We conclude that the enhancement of Na+ leak represents the major ion transport abnormality in rats with genetic hypertension. The alterations in plasma lipids are important determinants of abnormal red cell ion transport in hypertensive models studied. Although the detailed mechanism of their participation in ion transport regulation is still not completely understood, triglyceride-dependent changes in membrane microviscosity seem to be responsible for the modulation of particular ion transport pathways.

Glycation of collagen in hypertriglyceridemic rats

Nonenzymatic collagen glycation and modification with lipid derived metabolites was studied in rat skin and tail tendon collagen of control and hypertriglyceridemic (HTG) rats. Age-dependent changes typical for lipid and sugar derived adducts were evaluated by measuring fluorescence of these collagens at wavelengths typical for sugar (335/385 and 370/440 nm) and lipid derived adducts (356/460 and 390/460 nm). In addition pentosidine assay (corresponding to the fluorescence parameters 335/385 nm) was performed as well. It was found that pentosidine concentration as well as fluorescence intensities in skin collagen was the same for control and HTG rats and significantly increased with age. On the other hand, no significant age-dependent changes in fluorescence intensities were observed in tail tendon collagen. Pentosidine concentration in tail tendon collagen was much lower than that in skin and it was decreased in young HTG rats compared to control ones. It increased with age, more distinctly in HTG rats than in their control counterparts, in such a way that at the age of 19 months the pentosidine levels were undistinguishible in both rat strains. Possible mechanisms underlying these results are discussed.

Cell calcium handling and intracellular pH regulation in hereditary hypertriglyceridemic rats: reduced platelet response to thrombin stimulation

Multiple cell membrane alterations have been described in humans and animals with various genetic forms of hypertension and/or dyslipidemia. The aim of our study was to characterize some properties of platelets and/or erythrocytes (cytosolic calcium handling, intracellular pH regulation and thrombin responsiveness) in a new model of genetic hypertension associated with hyperlipidemia-Prague hereditary hypertriglyceridemic (HTG) rats. There were no differences in basal cytosolic Ca2+ values in platelets or erythrocytes of HTG rats and control Wistar rats. Ca2+ influx into erythrocytes was also similar in HTG and control rats. In both strains Ca2+ influx correlated positively with plasma triglycerides. The slope of this relationship was less steep in HTG than in Wistar rats. Cytosolic Ca2+ response to thrombin stimulation was smaller in HTG platelets, which were also characterized by a major reduction of thrombin-induced Mn2+ entry through receptor-operated Ca2+ channels. Platelets of HTG rats had the same basal intracellular pHi values and similar buffering capacity as control rats but their pHi response to thrombin stimulation was substantially reduced. It can be concluded that reduced responsiveness to thrombin stimulation is a major alteration found in platelets of hypertensive hereditary hypertriglyceridemic rats.

Hereditary hypertriglyceridemic rat: a new animal model of metabolic alterations in hypertension

Hereditary hypertriglyceridemic rats (hHTg) were developed as a new genetic model for the study of relationships between blood pressure (BP) and metabolic abnormalities. This strain has been produced by selective inbreeding from Wistar rats according to the rise of plasma triglycerides induced by a high-sucrose diet. Though hHTg rats display hypertriglyceridemia, impaired glucose tolerance, hyperinsulinemia, insulin resistance and increased BP even without nutritional stimuli, high sucrose feeding further aggravates these symptoms. High plasma triglycerides levels in hHTg rats seem to be a consequence of their hyperproduction. Impaired insulin action is responsible for the defective glucoregulation in this strain. The loss of insulin responsiveness might be due to a reduction in the number of glucose transporters. Highly significant relationships among plasma triglycerides, ouabain-resistant Na+ transport and BP were demonstrated in the hHTg rats. Segregating populations (F2 hybrids) should be used for genetic analysis of the primary role of lipid and/or ion transport abnormalities in the pathogenesis of this form of genetic hypertension.

Effect of a low salt diet on blood pressure and vasoactive hormones in the hereditary hypertriglyceridemic rat

In hereditary HTG rats, basal systolic blood pressure using tail-cuff sphygmomanometry was significantly higher (122.1 +/- 2.1 mm Hg; n = 16) than that in NTG animals (107.1 +/- 1.52; n = 16). A low salt diet did not influence blood pressure in NTG rats during the consecutive 4 weekly periods. However, in the second week blood pressure in HTG rats rose significantly in both the control rats on a normal salt diet and those on a low salt diet (132.5 +/- 1.89, n = 8, and 132.6 +/- 1.93, n = 8). No further changes were registered in the third and fourth week in control HTG rats. On the other hand, blood pressure fell significantly in HTG rats on a low salt diet in the third week in comparison with the second week (119.5 +/- 3.2, n = 8), and it increased again in the fourth week (123.0 +/- 2.35, n = 8). Hormones in plasma were determined at the end of the experiment. Plasma levels of norepinephrine were not influenced by differences in salt intake and were significantly higher by about 45% in HTG than in NTG animals. The lowest concentration of corticosterone in plasma was found in control HTG rats (1.2 +/- 0.2 vs 4.6 +/- 0.8 micrograms/100 ml in control NTG rats). Nevertheless, corticosterone concentration increased in HTG rats on a low salt diet at comparable values found in NTG rats on a low salt diet (3.1 +/- 0.8 vs 4.3 +/- 1.5). Plasma renin activity and plasma aldosterone concentrations were not different in the NTG and HTG groups and were uninfluenced by the diets (Table 1). We conclude that the elevated blood pressure in HTG rats and its variations during the experiment may reflect more pronounced sympathetic activity in HTG rats rather than blood pressure dependency on different salt intake.

Adrenergic overactivity and insulin resistance in nonobese hereditary hypertriglyceridemic rats

Nonobese, hereditary hypertriglyceridemic (HTG) rats provide an interesting model of hypertriglyceridemia, glucose intolerance, and hypertension. In age-matched 15 HTG and 16 control Wistar rats fed on a high sucrose diet (70 cal%) for 6 weeks, we measured insulin sensitivity in vivo and some parameters of sympatoadrenal system. Using euglycemic clamps with administration of 2-deoxy[1-3H]glucose, we found whole body insulin resistance and decreased glucose metabolic index Rg' in soleus muscle, epitrochlearis muscle, diaphragm, and white adipose tissue in HTG rats. We found higher levels of plasma epinephrine and higher excretion of vanilmandelic and homovanilic acids in HTG rats. The binding of [3H]-dihydroalprenol to the heart membrane fraction was similar in both groups, but the dissociation constant Kd was increased by 75% in the heart of HTG rats.