Blood hyperviscosity in spontaneously hypertensive rats

The effects of physical exercise on the serum iron profile in spontaneously hypertensive rats

The purpose of this study was to evaluate the profile of serum iron in spontaneously hypertensive rats after an aerobic physical exercise. To accomplish this, 12 normotensive Wistar rats and 12 spontaneously hypertensive rats were distributed into "physical exercise" and "no physical exercise" groups. The animals in the physical exercise group underwent to an aerobic exercise for a total of 4 weeks. Blood was collected for the analysis of iron. Our results indicate that rats of the physical exercise group had significantly lower serum iron levels after the aerobic exercise protocol compared to the spontaneously hypertensive rats no physical exercise group (F ((3,16)) = 4.4915, p < 0.01). No significant difference was found between no physical exercise groups. The results indicated that the difference in iron may be due to an increased demand for iron, prompted by chronic physical exercise. In addition, erythrocytosis has been associated with increased blood pressure in spontaneously hypertensive rats, suggesting that iron reduction may be related to decreased blood pressure in these animals.

Losartan inhibits experimental venous thrombosis in spontaneously hypertensive rats

The potential antithrombotic action of losartan, the AT1 receptor antagonist, in an experimental model of venous thrombosis in spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) was tested. The involvement of nitric oxide and prostacyclin in this effect was also studied. Venous stasis was induced by ligation of the vena cava. Losartan, after administration of a single, hypotensive dose (10 mg/kg, p.o.), significantly reduced the thrombus weight in SHR but not in WKY. The antithrombotic activity of losartan in SHR was abolished by NG-nitro L-arginine methyl ester (L-NAME) (30 mg/kg s.c.) but not by indomethacin (2.5 mg/kg s.c.). No changes in primary hemostasis, platelet aggregation, coagulation parameters such as activated partial thromboplastin time, prothrombin time, euglobulin clot lysis time, and fibrinogen level, either in SHR or in WKY rats, were found. Our results indicate the NO-dependent mechanism in the antithrombotic effect of losartan on venous thrombosis in SHR.

Alterations to the nitric oxide pathway in the spontaneously hypertensive rat

OBJECTIVES

To examine the role of nitric oxide in the cardiovascular system in spontaneous hypertension. In particular, we wanted to know whether the production of nitric oxide in the cardiovascular system of the spontaneously hypertensive rat is different from that of the normotensive Wistar-Kyoto rat and whether nitric oxide is biologically effective in this system.

DESIGN

We studied various aspects of the L-arginine-nitric oxide pathway in the cardiovascular system of spontaneously hypertensive rats and Wistar-Kyoto rats.

METHODS

To address the first objective we analysed the expression of endothelial nitric oxide synthase in the heart by Western blotting and the activity of constitutive nitric oxide synthase in resistance microvessels obtained from the mesenterium, both from spontaneously hypertensive rats and Wistar-Kyoto rats aged 14-18 weeks. We also analysed the concentration of the oxidative product of nitric oxide, nitrate, in plasma from these rats. To address the second objective, that is, to assess the bioactivity of nitric oxide, we studied the accumulation in tissue of cyclic guanosine 3',5'-monophosphate (GMP), as well as the acute and chronic effects of withdrawing the nitric oxide vasodilatory tone with the inhibitor of nitric oxide synthesis NG-nitro-L-arginine methyl ester on Wistar-Kyoto rats and spontaneously hypertensive rats.

RESULTS

We found that the expression of endothelial nitric oxide synthase in the heart, the activity of constitutive nitric oxide synthase in resistance microvessels and the concentration of nitrate in plasma were all significantly higher in the spontaneously hypertensive rats. In contrast, neither cyclic GMP levels nor the effects of NG-nitro-L-arginine methyl ester were greater in the spontaneously hypertensive rat than they were in the Wistar-Kyoto rat.

CONCLUSIONS

The nitric oxide pathway is upregulated in the resistance circulation and the heart of the spontaneously hypertensive rat by a mechanism involving induction of the constitutive nitric oxide synthase and overproduction of nitric oxide. However, nitric oxide is not sufficiently bioactive to stimulate the formation of cyclic GMP and to maintain an adequate nitric oxide-dependent vasodilatory tone.

Nitric oxide mediates renal vasodilation during erythropoietin-induced polycythemia

The renal blood flow (RBF) of patients with polycythemia rubra vera is increased despite the high hematocrit (Hct) which elevates the whole blood viscosity. Since blood viscosity determines the shear force on the endothelium which is a major stimulus to nitric oxide (NO) release, we investigated the hypothesis that renal vasodilation during erythropoietin-induced erythrocytosis is mediated by the L-arginine-NO pathway. Groups of Sprague-Dawley rats received thrice weekly injections of erythropoietin (E) for two to five weeks; responses were contrasted with normal rats (N) which received sham injections. The first group was studied after five weeks of erythropoietin injections which led to sharp increases in Hct (E: 72 +/- 3 vs. N: 44 +/- 1%) and mean arterial pressure (MAP: 126 +/- 3 vs. 107 +/- 3 mm Hg). These rats had an elevated basal RBF whether measured by the clearance and renal extraction of PAH or by a transit-time renal blood flow meter. Subsequent groups were studied after two to three weeks of erythropoietin which raised the Hct more modestly to 59 +/- 2%. In this group, the basal MAP was similar in E and N rats. Graded doses of the NO synthase inhibitor, N omega-monomethyl-L-arginine (L-NMA) led to a steeper rise in MAP in E than N; at the highest doses, the MAP had increased by 36 +/- 2 in E and 23 +/- 3 mm Hg in N (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

The role of post-ischaemic reperfusion in the development of microvascular incompetence and ultrastructural damage in the myocardium

To determine the contribution of oxygenated reperfusion to the development of myocardial microvascular incompetence and ultrastructural damage following ischaemia, isolated buffer perfused rat hearts were subjected to either temporary (n = 15) or permanent (n = 15) ischaemia for 15, 30 or 45 minutes. The temporarily ischaemic hearts were reperfused for 5 min with oxygenated Krebs Henseleit buffer. All hearts were then fixed by perfusion fixation with nitrogen-bubbled glutaraldehyde. The transmural development of microvascular incompetence was determined quantitatively by scanning electron microscopy using nuclear track photographic emulsion as an intravascular marker of competent capillaries, and ultrastructural damage was examined by transmission electron microscopy. Thirty or more minutes of ischaemia where required to significantly reduce the mean density of competent capillaries in the subendocardial third of the left-ventricular wall. Such ischaemic myocardium contained relatively normal, open unobstructed vessels, indicating that the microvascular incompetence arising during ischaemia per se was not due to ultrastructural change in the capillaries. Subendocardial myocardium reperfused following 15 min ischaemia also showed little ultrastructural change, but did show a significant reduction in the density of competent capillaries. However, reperfusion of more severely ischaemic myocardium resulted in obvious ultrastructural damage as well as significant further reduction in capillary competence. These findings demonstrate that oxygenated reperfusion of ischaemic myocardium paradoxically results in the further development of microvascular incompetence and, in severely ischaemic myocardium, also to additional ultrastructural damage.

Megakaryocytopoiesis in spontaneously hypertensive rats (SHR)

Bone marrow megakaryocytes and their progenitors were studied in SHR in order to obtain more information about megakaryocytopoiesis in hypertension since it is known that various anomalies of platelet function occur in hypertension. Megakaryocytopoiesis under steady state conditions and following stimulated erythropoiesis and thrombocytopenia was not found to be significantly different in SHR from that in normotensive Wistar controls.

Leukocyte adherence initiation in skeletal muscle capillaries and venules

In vivo leukocyte adherence has many physiological implications but the effect of vessel size and hemodynamic parameters on adherence initiation remains unclarified. The early phases of adherence, defined to include attachment of previously freely moving leukocytes to endothelium or leukocytes which roll along the vessel wall, are the focus of this report. Blood velocity and diameter in 121 capillaries and venules in the cremaster of 13 rats were measured. The hemodynamic and vessel size difference characterizing vessels in which either initial leukocyte wall attachment or leukocyte rolling occurred (termed "adherent vessels"), as distinguished from vessels in which neither transient stoppage nor cell rolling was observed (termed "nonadherent vessels"), was then determined. Fifty-seven percent of the vessels observed were adherent vessels. These vessels were characterized by a larger diameter, a smaller blood velocity, and a smaller calculated wall shear rate than the nonadherent vessels. The results show that leukocyte adherence can be initiated in capillaries and postcapillary vessels with diameters that are equal to and less than the leukocyte size. Within vessels less than 11 micron, adherence or nonadherence appears dependent on local hemodynamics with the possibility of a critical shear rate threshold of about 400 sec-1. In vessels with shear rates greater than this value only 7% were observed to have adherence. In vessels greater than 11 micron the absence of hemodynamic differences between adherent and nonadherent vessels suggests the presence of other adherence-initiating mechanisms. It is thought that in these larger size vessels both local erythrocyte effects and adherence initiation within upstream capillaries affect observed adherence.

Expression and intracellular distribution of stress fibers in aortic endothelium

Immunofluorescence microscopy was used to determine the number of endothelial cells with stress fibers for three age groups, and for three distinct anatomical locations within the descending thoracic aorta of both normotensive and spontaneously hypertensive rats. For each age group examined, hypertensive rats consistently demonstrated greater stress fiber expression than did normotensive rats. Neither age nor blood pressure was the predominant influence on stress fiber expression in aortic endothelium. In the normotensive rats, stress fiber expression remained unchanged for all age groups examined. For both strains, however, more endothelial cells with stress fibers were found in those regions where fluid shear stresses are expected to be high, when compared with those regions where the fluid shear stresses are expected to be low. This observation suggests that anatomical location, with its implied differences in fluid shear stress levels, is a major influence on stress fiber expression within this tissue. Electron microscopy was used to determine the intracellular distribution of stress fibers for both strains. Most stress fibers in both strains were located in the abluminal portion of the endothelial cells. This result is consistent with a role for stress fibers in cellular adhesion. However, the hypertensive rats had a higher proportion of stress fibers in the luminal portion of their cytoplasm than the normotensive rats. This increased presence of stress fibers in the luminal portion of the cell may be important in maintaining the structural integrity of the endothelial cell in the face of elevated hemodynamic forces in situ.

Hemodynamic effects of chronic alteration in hematocrit in spontaneously hypertensive rats

This study describes the effect of a chronic decrease in hematocrit on blood pressure, cardiac output (CO), total peripheral resistance (TPR), and plasma volume in spontaneously hypertensive rats (SHR), and all but plasma volume in normotensive Wistar rats (NWR). Hematocrit was decreased by treatment with either heparin, vitamin K inhibitor ( pelentan ), or by repeated blood letting (BL). The results show that in SHR, a decrease in hematocrit, regardless of how produced, was associated with a significant decrease (p less than 0.01) in blood pressure. Prevention of heparin-induced decrease in hematocrit by repeated transfusions of red blood cells abolished the blood-pressure-lowering effect of heparin. By using combined data on hematocrit and systolic blood pressure in all five SHR groups, a significantly positive correlation and linear regression between hematocrit and blood pressure were obtained. When compared to control untreated SHR, heparin- or pelentan -treated SHR showed a significant (p less than 0.001) decrease in TPR and a significant increase in CO, while in SHR BL, no difference in TPR or CO was found. Plasma or blood volume did not differ among the groups. In NWR, heparin treatment resulted in significantly decreased hematocrit, decreased TPR, and increased CO compared to control normotensive rats. However, blood pressure did not change. Results confirming the authors' previous study and those of other investigators indicate a direct association between hematocrit and systemic hypertension. Lowering the hematocrit can effectively lower an elevated blood pressure. Moreover, the data suggest that heparin or pelentan induces a vasodilator effect that cannot be attributed to a decrease in hematocrit alone.

Cardiovascular and sympathetic effects of leukotriene D4 in renovascular and spontaneously hypertensive rats

Leukotriene D4 (LTD4), a constituent of slow-reacting substance of anaphylaxis (SRS-A), elicits a pressor response followed by prolonged hypotension in spontaneously hypertensive (SHR) but not in Wistar-Kyoto (WKY) rats. In order to investigate whether the depressor response to LTD4 in SHR rats is related to hypertension itself, we have studied the cardiovascular and sympathetic effects of LTD4 in 1-kidney, 1-clip hypertensive rats and 1-kidney, normotensive rats. In all groups of conscious rats, intra-arterial administration of LTD4 (0.2-20 micrograms/kg) caused dose-dependent pressor responses of similar degree except in WKY rats, which responded less. Only SHR rats developed a significant and progressive hypotension (-58 +/- 5 mm Hg) following pressor phase. In all but WKY rats, the response phase was attended by an increase in plasma levels of norepinephrine and epinephrine. Only SHR rats showed marked and persistent hemoconcentration following pressor effect. Thus, depressor response of SHR rats to systemic administration of LTD4 does not appear to be merely due to the magnitude of blood pressure elevation and may in part result from microcirculatory changes not present in other hypertensive rats.

Changes of polyunsaturated fatty acids in serum lipids of spontaneously hypertensive rats during the onset of high blood pressure

The fatty acid pattern of serum triglycerides and cholesterol esters has been estimated in spontaneously hypertensive rats (SHR) and normotensive Wistar rats (WR) at 4, 5, 6, 7, and 8 weeks of age. The percentage of linoleic acid (LA) was lower in SHR when compared with WR, but increased with age in both strains. Arachidonic acid (AA) showed an abrupt fall in SHR and WR at 5 weeks of age in comparison to 4 week old rats. The fall was more pronounced in serum triglycerides of the former and in cholesterol esters of the latter. The changes in AA occurred prior to the onset of hypertension in SHR. From the data it can be concluded that the formation of AA from LA might be dependent on age being quantitatively different in SHR and WR. Furthermore, eicosapentaenoic acid (EPA) appeared lower in SHR than in age-related WR. Docosapentaenoic and docosahexaenoic acids also revealed a marked decrease in rats between 4 and 5 weeks of age indicating that individual polyunsaturated fatty acids (PUFA) change in a different manner which might be relevant to the development of high blood pressure in SHR.

Noise effects on the cochlear vasculature in normotensive and spontaneously hypertensive rats

The cochlear vessels and the distribution of red blood corpuscles were studied in normotensive (N) and spontaneously hypertensive (SH) rats with and without noise exposure. The evaluation was made in a double blind manner. Ten N-rats and ten SH-rats were exposed to noise for 8 weeks and terminated immediately thereafter. The noise consisted of a 2 kHz broad noise band swept from 3 kHz to 30 kHz with a resultant equivalent sound level of 100 dB (lin) 10 h. Seven of 84 vascular parameters differed statistically at the 1% level between N- and SH-rats, and 8 and 10 parameters respectively differed for N- and SH-rats as a result of noise exposure. The observations were interpreted as indicative of a precapillary sphincter constriction in the radiating arterioles of the external wall with an upstream relative stasis and a downstream relative lack of blood, both in N- and SH-rats. In the spiral lamina the observations were interpreted as signs of a decreased blood flow after noise exposure, both in the N- and SH-animals, and most marked in the SH-animals after noise.

Capillary pressure gradients in cremaster muscle of normotensive and spontaneously hypertensive rats

The purpose of this investigation is to test the hypothesis that capillary pressure gradients are elevated in spontaneously hypertensive rats (SHR) and to determine the mechanism for the elevation. The cremaster muscle was prepared for microscopic examination under chloralose-urethane anesthesia in seven SHR and eight Wistar-Kyoto (WKY) rats 4-6 weeks of age. Capillary hematocrit, diameter, and red cell velocity were measured. Capillary flow induced by a time-varying pressure gradient was treated mathematically. A finite Hankel transformation was applied to the Navier-Stokes equation for capillary vessels. The solution was expressed as a Fourier-Bessel series, and the fluctuation of capillary flow induced by a time-varying pressure gradient was studied. It was shown that if the velocity fluctuation depended only on the pressure gradient, then the velocity fluctuation would be diminished almost instantly after the capillary started to flow. Capillary pressure gradient and shear stress were evaluated according to two different flow models, Newtonian and Casson. The capillary viscosity was obtained from the capillary hematocrit based on the empirical correlation of viscosity vs hematocrit. Calculations based on both flow models indicate that the capillary pressure gradient and shear stress of SHR is higher than in WKY, especially in vessels near 6 microns in diameter. The elevated pressure gradient is due to a combination of reduced capillary density, causing a higher red cell velocity and a tendency toward smaller capillary diameters in the SHR. Capillary hematocrit and viscosity were not elevated in the SHR.

Enhanced platelet turnover and prostaglandin production in spontaneously hypertensive rats

The anti-hypertensive effect of ketanserin, a selective 5-HT2 antagonist, both in experimental and human pathology (1,2) could implicate the blood platelets as a source for peripherally available 5-hydroxytryptamine (5-HT), acting to increase peripheral vascular resistance (3). Therefore we examined various blood platelet parameters in spontaneously hypertensive rats in comparison with normotensive Wistar rats.