Role of angiotensin II and free radicals in blood pressure regulation in a rat model of renal hypertension.

One-kidney, 1-clip rats (1K1C) or uninephrectomized controls were treated with either the superoxide dismutase mimetic tempol (0.5 mmol. kg(-1). d(-1)), angiotension type 1 receptor inhibitor losartan (50 mmol. L(-1). kg(-1). d(-1)), or both (n=6 per group) for 2 weeks. At the end of the study, systolic blood pressure (BP) decreased on average by 21% in tempol-treated and 29% in losartan-treated versus untreated 1K1C (217+/-4.4 mm Hg) and was normalized in the losartan plus tempol group. Mean BP also decreased from 159+/-3.7 mm Hg in 1K1C to 93+/-2.8 mm Hg in the losartan plus tempol group. Also, aortic wall area was reduced by 18% in losartan- or tempol-treated 1K1C and by 30% in losartan plus tempol rats compared with untreated 1K1C. Plasma renin activity was increased from 4.8+/-0.3 in untreated 1K1C to 15.9+/-0.9 ng. mL(-1). h(-1) in losartan-treated but not tempol-treated 1K1C. Superoxide generation by the isolated aortic rings assessed by lucigenin chemiluminescence was significantly decreased (by approximately 40%) in all losartan, tempol, and losartan plus tempol groups compared with untreated 1K1C. Nitrotyrosine ELISA in the kidney displayed a significant reduction, from 59+/-13 ng/mg of protein in 1K1C to 12.5+/-5 ng/mg of protein in the losartan plus tempol 1K1C. Western blotting for nNOS in kidney cortex and medulla showed a protein increase in both fractions of 1K1C versus controls and was normalized by losartan plus tempol treatment. Collectively, data show a synergistic effect of losartan and tempol on BP reduction in 1K1C rats. The mechanism may involve reduced superoxide production and nitrotyrosine formation in kidney and decreased kidney neuronal-type NO synthase expression in treated animals. This status in the oxidative balance seems to affect BP in the renal hypertensive rats.

Interleukin-10 prevents loss of tone of rat skeletal muscle arterioles exposed to endotoxin.

BACKGROUND: The anti-inflammatory cytokine interleukin-10 (IL-10) is known to inhibit the development of septic shock in animal models. This study was conducted to investigate the effect of IL-10 on the loss of vascular tone during exposure to endotoxin. Unlike numerous proinflammatory cytokines, the effects of IL-10 at the level of the microvasculature have not been previously studied. MATERIALS AND METHODS: First-order rat cremasteric arterioles (n = 27) were placed in an isolated vessel preparation and allowed to achieve spontaneous tone. An opened segment of thoracic aorta was then placed upstream from the arteriole in the superfusion line. The resistance arteriole of this in-series model, in contrast to the isolated arteriole alone, demonstrates a significant loss of tone when exposed to endotoxin. Following 1 h of equilibration in the presence or absence of IL-10 (20 ng/ml), the aorta and arteriole were then superfused with 2.5 microg/ml endotoxin or physiologic buffer for 60 min and serial arteriolar diameter measurements were recorded. Group 1 was exposed to endotoxin only, Group 2 was a time control, and Group 3 was pretreated with IL-10 prior to endotoxin exposure, while Group 4 was a control pretreated with IL-10 only. RESULTS: After the 60-min equilibration period there were no differences among the four groups in arteriolar tone. At t = 120 min, the percentage of tone in the control group was 43.6 +/- 3% (mean +/- SEM) and this was not changed by treatment with IL-10 (47.0 +/- 7% tone). Endotoxin alone caused arteriolar tone to fall to 31.4 +/- 3% (P < 0.05). However, endotoxin applied to arterioles pretreated with IL-10 was without effect (47.5 +/- 2%). CONCLUSIONS: Resistance arterioles pretreated with IL-10 maintain vascular tone during endotoxin exposure. We conclude that IL-10 pretreatment prevents loss of vascular tone of isolated arterioles exposed to endotoxin.

Oxidative stress in a rat model of obesity-induced hypertension.

The mechanisms underlying the development of hypertension in obesity are not yet fully understood. We recently reported the development of hypertension in a rat model of diet-induced obesity. When Sprague-Dawley rats (n=60) are fed a moderately high fat diet (32 kcal% fat) for 10 to 16 weeks, approximately half of them develop obesity (obesity-prone [OP] group) and mild hypertension (158+/-3.4 mm Hg systolic pressure), whereas the other half (obesity-resistant [OR] group) maintains a body weight equivalent to that of a low fat control group and is normotensive (135.8+/-3.8 mm Hg). We examined the potential role of oxidative stress in the development of hypertension in this model. Lipid peroxides measured as thiobarbituric acid-reactive substances showed a significant increase in the LDL fraction of OP rats (2.8+/-0.32 nmol malondialdehyde/mg protein) compared with OR and control rats (0.9+/-0.3 nmol malondialdehyde/mg protein). Also, aortic and kidney thiobarbituric acid-reactive substances showed a significant (3- and 5- fold) increase in OP rats after 16 weeks of diet. In addition, superoxide generation by aortic rings, measured by lucigenin luminescence, showed a 2-fold increase in the OP group compared with both the OR and control groups. In addition, free isoprostane excretion and nitrotyrosine in the kidney showed an increase in OP rats only. The urine and plasma nitrate/nitrite measured by the LDH method showed a 1.8-fold decrease in OP rats compared with OR rats. However, endothelial NO synthase expression in the kidney cortex and medulla assessed by reverse transcriptase-polymerase chain reaction showed a strong increase in the OP rats versus OR and control rats (endothelial NO synthase/beta-actin ratio 1.3+/-0.04 in OP rats versus 0.44+/-0.02 in OR rats), suggesting a possible shift toward superoxide production by the enzyme. Collectively, the data show a decreased NO bioavailability in OP animals that is due in part to the increased oxidative stress.

Src tyrosine kinases and extracellular signal-regulated kinase 1/2 mitogen-activated protein kinases mediate pressure-induced c-fos expression in cannulated rat mesenteric small arteries.

Chronic hypertension is associated with remodeling of small arteries. There is evidence that the high pressure itself may cause these structural changes, but the responsible mechanisms are not clearly defined. Previously we showed that pressure-induced c-fos expression in intact cannulated rat mesenteric small arteries was inhibited by genistein, a general tyrosine kinase inhibitor. The purpose of this study was to further unravel the underlying signal transduction mechanisms, and we particularly tested the involvement of src tyrosine kinases and extracellular signal-regulated kinase (ERK). Rat mesenteric small arteries were cannulated in a dual-vessel chamber. After a 60-minute equilibration period, the pressure in 1 artery was increased to 140 mm Hg, while the other artery remained at 90 mm Hg. Semiquantitative reverse transcriptase-polymerase chain reaction was used to determine c-fos expression, and Western blotting was used to examine levels of ERK phosphorylation. The involvement of src and ERK was tested with the inhibitors herbimycin A (1 micromol/L), PP1 (10 micromol/L), PP2 (10 micromol/L), and PD98059 (30 micromol/L). One-hour exposure to 140 mm Hg increased the c-fos/cyclophilin ratio 3.6-fold, from 0.29+/-0.07 to 1.06+/-0.25. All the tested inhibitors suppressed the pressure-induced increase of c-fos expression. A 5-minute exposure period to 140 mm Hg increased ERK phosphorylation, and this was abolished in the presence of PP1. The results suggest that pressure-induced c-fos expression in intact cannulated rat mesenteric small arteries may be mediated, at least in part, by src tyrosine kinases and ERK.

Isolated resistance artery preparation.

The blood vessels that contribute most to precapillary resistance are known as resistance arteries, consisting of arterioles and small arteries with diameters of less than 500 µm (1). These vessels regulate the vascular resistance, and thus the blood supply, through the adjustment of their lumen diameter, which is accomplished by modulation of the level of tone in the vascular smooth muscle cells. The smooth muscle and endothelial cells in the blood vessel wall are sensitive to a great diversity of stimuli including distending pressure, shear stress, neurohumoral factors, and metabolites. All of these different signals are sensed, integrated, and eventually lead to a response.

Development of hypertension in a rat model of diet-induced obesity.

Although obesity is a risk factor for hypertension, the relationship between these 2 conditions is not well understood. Therefore, we examined some parameters of hypertension and cardiovascular disease in a dietary model of obesity. Male Sprague-Dawley rats were provided either a control diet (C) or a diet containing 32% kcal as fat (similar to a Western diet) for 1, 3, or 10 weeks. Rats in the latter group diverged based on body weight gain into obesity-prone (OP) and obesity-resistant (OR) groups. Systolic blood pressure in OP rats was significantly higher after 10 weeks of the diet (149+/-4. 8 mm Hg) compared with both OR and C groups (131+/-3.7 and 129+/-4.5 mm Hg, respectively). The aortic wall area of OP rats was significantly increased, indicating arterial hypertrophy, and a 2-fold increase in plasma renin activity was found in OP rats compared with OR and C rats. The lipid profile showed a significant increase in plasma and VLDL triglycerides of OP versus OR and C groups as early as 3 weeks on the diet. Plasma and LDL-cholesterol levels were increased in the OP group versus the OR and C groups after 3 weeks of the diet, but the difference was blunted after 10 weeks. Lipid peroxidation (thiobarbituric acid-reactive substances) in OP rats was increased 2-fold in LDL and 1.5-fold in aortic wall compared with OR rats, suggesting an increased oxidative stress in these animals. Periodic acid-Schiff staining of the kidney showed mesangial expansion and focal sclerosis that were more prominent in OP rats than in OR rats. The results suggest that hypercholesterolemia, but not hypertriglyceridemia, is linked to the diet; that hypertension and renin-angiotensin system activation are associated with obesity; and that lipid peroxidation and renal damage are the results of both factors.

AT(1) receptor inhibition does not reduce arterial wall hypertrophy or PDGF-A expression in renal hypertension.

To separate the role of ANG II from pressure in hypertrophy of the vascular wall in one-kidney, one-clip (1K1C) hypertension, experimental and sham-operated rats were given the AT(1)-receptor antagonist losartan (20 mg x kg(-1) x day(-1)) or tap water for 14 days. Mean arterial pressure was elevated in both experimental groups compared with controls. Rats were anesthetized with pentobarbital sodium, and the thoracic aorta and carotid, small mesenteric, and external spermatic arteries were harvested and embedded in paraffin. Tissue sections were used for morphological analysis, immunohistochemistry for 5-bromo-2'-deoxyuridine (BrdU) and platelet-derived growth factor (PDGF)-AA, stereological measurements, and in situ hybridization with a (35)S-labeled riboprobe for PDGF-A mRNA. Elevated cross-sectional areas of thoracic, carotid, and small mesenteric artery in 1K1C rats were not reduced by losartan. The internal diameter of the external spermatic artery and microvascular density of the cremaster muscle were reduced in 1K1C rats. The number of BrdU-positive nuclei per cross section did not differ between 1K1C and control arteries. PDGF-A mRNA was elevated in the arterial walls of 1K1C rats compared with controls and was hardly changed by losartan. PDGF-A protein stained strongly in the media of 1K1C arteries and was not inhibited by losartan; it appeared in the adventitia of all aortas and carotid arteries. These observations demonstrate that effects of ANG II mediated through the AT(1) receptor are not necessary for hypertrophy of the vascular wall during 1K1C hypertension or expression of PDGF-A.

Genistein inhibits pressure-induced expression of c-fos in isolated mesenteric arteries.

We have previously demonstrated that elevating intraluminal pressure from 90 to 140 mm Hg in isolated mesenteric arteries increases the expression of proto-oncogenes. These proto-oncogenes encode nuclear transcription factors that regulate the expression of target genes during various stages of the cell cycle. Thus, pressure-induced proto-oncogene expression may represent a mechanism by which pressure can induce growth and/or proliferation of vascular smooth muscle. The purpose of this study was to determine the intracellular signals that contribute to the pressure-induced increase in c-fos expression. Small mesenteric arteries were isolated from male Wistar rats and transferred to a dual-vessel chamber. The arteries were cannulated and slowly equilibrated to initial conditions (90 mm Hg, 37 degrees C) while being continuously superfused with a HEPES-bicarbonate-buffered Krebs' solution. After the equilibration period, the intraluminal pressure in 1 artery was increased to 140 mm Hg for 1 hour. In experiments designed to determine the intracellular signals involved in the pressure-induced increase in c-fos expression, specific inhibitors were introduced to the superfusate reservoir of both arteries before the pressure increase. The arteries were then fixed in phosphate-buffered formalin and embedded in paraffin blocks. Sections of paraffin-embedded arteries were fixed on slides, and the expression of c-fos was determined by in situ hybridization with the use of (35)S-labeled riboprobes. The pressure-induced expression of c-fos was not inhibited by nitrendipine (10 micromol/L), a calcium-free Krebs' solution containing EGTA (1 to 2 mmol/L), calphostin C (0.1 micromol/L), or cytochalasin D (0.4 micromol/L) but was inhibited by genistein (30 micromol/L). The results suggest that activation of a tyrosine kinase is required for pressure-induced c-fos expression, but the signaling pathway does not require extracellular calcium entry, intact actin filaments, or protein kinase C. As we have shown previously, the expression of c-fos correlated with wall stress.

PDGF-A expression correlates with blood pressure and remodeling in 1K1C hypertensive rat arteries.

We previously demonstrated remodeling of large and small arteries in angiotensin II-treated rats, paralleled by an increased expression of platelet-derived growth factor (PDGF)-A chain mRNA in large arteries. Both remodeling and PDGF-A expression were associated with elevation of blood pressure rather than a direct effect of angiotensin II. To further delineate the role of PDGF-A and elevated blood pressure, we assessed the level of PDGF-A and -B mRNA and protein in the wall of large as well as small arteries in the one-kidney, one-clip (1K1C) hypertensive rat, a non-renin-dependent model of hypertension. Fourteen days after renal artery stenosis, the thoracic aorta and both femoral arteries were collected from 1K1C rats (n = 8) and uninephrectomized controls (n = 8) and immediately processed for morphological measurement, immunohistochemistry, RT-PCR, and Western blotting. Systolic blood pressure was significantly elevated in hypertensive rats (202 +/- 26 mmHg) compared with control rats (122 +/- 7.9 mmHg) and was accompanied by arterial hypertrophy in both aorta and femoral arteries. The mRNA for PDGF-A chain was increased threefold in the thoracic aorta (P < 0.05) of 1K1C rats, whereas the message for PDGF-B was not significantly changed in hypertensive versus control animals. A higher staining of the intima-media was observed by using an anti-PDGF-A chain polyclonal antibody on paraffin-embedded sections. Western blot results indicated an approximately 2-fold increase in PDGF-A protein in aortic and femoral wall of the 1K1C rats. The results showed that both the mRNA and protein for PDGF-A chain are increased and well correlated with the blood pressure and wall area, suggesting a direct effect of elevated pressure on PDGF synthesis, which, in turn, may affect the onset and progression of vascular hypertrophy.

Teaching vascular adaptations to mechanical stress.

Blood vessels change their number and structure in attempt to meet tissue demands for blood flow while simultaneously controlling mechanical stresses. A great deal of information is emerging in this field, especially concerning the role of the endothelium and signaling pathways for mechanotransduction. While not delving too deeply into the rapidly changing details, the students can be introduced to this exciting field by describing the structural changes that take place and outlining the major theories that are being investigated. The applications to peripheral vascular disease, myocardial infarctions, hypertension and tumor growth are readily apparent.

Impaired arteriolar mechanotransduction in experimental diabetes mellitus.

Decreased arteriolar distensibility in diabetes may impair signal transduction mechanisms that are required for converting a pressure stimulus into smooth muscle contraction. These studies aimed to determine if pressure-induced increases in arteriolar intracellular Ca(2+) are altered in diabetes and whether diabetes is associated with alterations in composition of the extracellular matrix. Studies of mechanical properties used single, isolated, and cannulated cremaster arterioles from streptozotocin (60 mg/kg) diabetic rats and age-matched controls. To measure Ca(2+)(i), arterioles were loaded with Fura 2 (5 microM) after which preparations were examined by fluorescence microscopy and image analysis. Matrix protein (type IV collagen, laminin, fibronectin) deposition was studied by immunohistochemistry. Over a range of 30-120 mm Hg control vessels showed a linear relationship (r = 0.98, p < 0.01) between intraluminal pressure and Ca(2+)(i). Vessels from diabetic animals also showed a linear relationship (r = 0.99, p < 0.01), however, the mean slope was significantly (p < 0.02) less in the diabetic (0.17 +/- 0.05, n = 5) compared to controls (0.51 +/- 0.09, n = 7). Similarly, the slope of the wall tension-Ca(2+)(i) relationship was significantly decreased in vessels from diabetic animals. These differences were ameliorated by treatment of diabetic animals (n = 5) with aminoguanidine. Increased content of type IV collagen, laminin and fibronectin in vessel media was evident after 2 weeks of diabetes and showed a further increase with duration of diabetes. The data suggest that for a given increase in luminal pressure arterioles from diabetic animals response with an attenuated rise in smooth muscle Ca(2+)(i). This mechanotransduction defect may relate to alterations in the composition of the extracellular matrix within the arteriolar wall.

Medial and endothelial platelet-derived growth factor A chain expression is regulated by in vivo exposure to elevated flow.

Previous work from this laboratory demonstrated that in vivo exposure to elevated arterial flow stimulates endothelial and smooth muscle cell hyperplasia concomitant with lumen enlargement, medial wall hypertrophy, and increases in medial extracellular connective tissue in rat mesenteric small arteries. In an effort to elucidate the role of growth factors in mediating this arterial remodeling response, in situ hybridization was performed on control and high flow arterial sections using 35S-labeled riboprobes for PDGF-A, PDGF-B, basic FGF, and TGFbeta1 mRNA. Results demonstrate that after exposure to elevated arterial flow for 24 h, expression of PDGF-A mRNA in the media was significantly elevated over basal levels (+215%, p < 0.001). This expression decreased towards control levels by 3 and 7 days. Increased endothelial expression of PDGF-A mRNA over basal levels was evident after 3 and 7 days of elevated flow (+129%, p < 0.01; +182%; p < 0.01, respectively). Acute medial PDGF-A mRNA expression may result from elevated circumferential hoop stress secondary to flow-induced dilation while delayed expression in the endothelium may result from normalization of wall shear stress. Endothelial expression of PDGF-B mRNA was significantly elevated over control levels (+62%, p < 0.01) after exposure to high flow for 7 days. Expression of bFGF and TGFbeta1 mRNA was not significantly different between control and high flow vessels at all times measured. These results suggest a role for PDGF-A in regulating acute arterial remodeling following in vivo exposure to elevated flow.

Pressure mediates angiotensin II-induced arterial hypertrophy and PDGF-A expression.

Angiotensin II (Ang II) may induce arterial hypertrophy either directly or through an increase in arterial pressure. To separate these 2 mechanisms, rats were implanted with osmopumps delivering either Ang II (100 ng x kg-1 x min-1) or saline. 5-Bromo-2'-deoxyuridine (BrdU) was delivered to both groups by osmopump (2.5 microg x kg-1 x min-1). Half of the rats in each group were given minoxidil (9 mg x kg-1 . d-1) in their drinking water. After 14 days, systolic blood pressure was 117+/-2, 124+/-3, and 115+/-2 mm Hg in the control, Ang II-minoxidil, and minoxidil groups, respectively, and 181+/-6 mm Hg in the Ang II group (P<0.05). After perfusion-fixation, the thoracic aorta, carotid artery, small mesenteric artery, external spermatic artery, and kidneys were harvested, paraffin-embedded, and used for morphological measurements, immunohistochemistry for BrdU, and in situ hybridization with a 35S-labeled riboprobe for platelet-derived growth factor-A chain (PDGF-A) mRNA. The walls of the aorta and carotid arteries hypertrophied in the Ang II group only. There were no significant morphological differences in the small arteries. BrdU was negative in all arteries but positive in the renal tubules. Expression of PDGF-A was elevated 8-fold in the thoracic aorta of the Ang II group (P<0.05). These results show that (1) arterial hypertrophy from Ang II infusion occurs in response to elevated arterial pressure, (2) hypertrophy was not associated with hyperplasia or polyploidy of vascular smooth muscle cells, and (3) PDGF-A expression correlated with elevated pressure and arterial wall hypertrophy.

Flow-induced arterial remodeling in rat mesenteric vasculature.

This study was designed to characterize in vivo arterial remodeling of male Wistar rat small mesenteric arteries exposed to varying levels of elevated blood flow in the presence of normal arterial pressure. Through a series of arterial ligations, respective ileal artery and second-order branch blood flows acutely increased approximately 36 and approximately 170% over basal levels. Their respective diameters increased 12 and 38% and their wall area increased 58 and 120% in a time-dependent fashion between 1 and 7 days postlitigation compared with same-animal control vessels. Medical extracellular connective tissue increased concomitantly with medical wall hypertrophy. Immunostaining for proliferating cell nuclear antigen and nuclear profile analyses suggests that both smooth muscle and endothelial cell hyperplasia contribute to flow-induced vascular remodeling. The initial stimulus in this model is flow-mediated shear stress, with possible augmentation by hoop stress, which is increased approximately 7% by the resultant vasodilation. Stable wall thickness-to-lumen diameter ratios at 1, 3, and 7 days, however, suggest chronic hoop stress is tightly regulated and remains constant. The model described herein allows analyses of two arteries with different degrees of flow elevation within the same animal and demonstrates that the magnitude of vessel remodeling in vivo is directly dependent on the duration of flow elevation after abrupt arterial occlusion.

Myogenic tone attenuates pressure-induced gene expression in isolated small arteries.

This study was designed to determine whether pressure-induced expression of early response genes in the arterial wall is dependent on an increase in cell stretch or an increase in wall stress. Mesenteric arteries (245 to 385 microm in diameter) were isolated from Wistar rats and subjected to static pressures of either 90 mm Hg (control), 140 mm Hg, or 165 mm Hg for a period of 3 hours. Arteries developed a range of myogenic tone such that wall stresses in the 140 and 165 mm Hg arteries (1.60 to 4.44x10(6) dynes/cm2) were equivalent in some cases to those of controls (1.76 to 2.63x10(6) dynes/cm2). Vessels subjected to 140 or 165 mm Hg intraluminal pressure had diameters ranging from 74% to 104% of their relaxed diameter at 90 mm Hg, whereas control vessel diameters ranged from 88% to 100%. At the end of each experiment, vessels were fixed in 10% formalin, embedded in paraffin, and sectioned for in situ hybridization. Wall stress significantly correlated with c-myc mRNA and 18S rRNA expression. Gene expression did not correlate with vessel diameter, expressed as a percentage of the relaxed diameter at 90 mm Hg, ie, cell stretch. The expression of beta-actin mRNA did not differ between vessels and showed no correlation with wall stress, suggesting that the induction of c-myc mRNA and 18S rRNA was part of a specific response. These findings show that in an isolated artery, a pressure stimulus can be perceived as an increase in wall stress, independently of cell stretch. Therefore, wall stress may be the signaling parameter in hypertension where arteries are tonically constricted. The inhibition of gene expression by myogenic constriction may explain why hypertrophy takes place in large arteries during hypertension but not in arterioles where increased tone reduces wall stress.

Elevated pressure stimulates protooncogene expression in isolated mesenteric arteries.

The aim of this study was to determine whether an increase in pressure alone is a sufficient stimulus in isolated small arteries to induce the immediate early genes that are associated with vascular wall growth. Mesenteric arteries (303-506 microm diam) were isolated from Wistar rats and subjected to static pressure of 90 mmHg (control) or 140 mmHg (hypertensive). The arteries possessed little active tone or myogenic response to pressure elevation; therefore, both sets of vessels were stretched by similar amounts, but wall stress in the hypertensive vessels was 60-80% above that of controls. After 30, 60, 180, and 360 min, the arteries were fixed in Formalin, embedded in paraffin, and sectioned for in situ hybridization. The levels of mRNA for c-fos increased in the hypertensive arteries 2.33-fold at 30 min and 6.64-fold at 60 min. mRNA for c-myc increased 5.13-fold at 60 min and 5.25-fold at 180 min. After this early response gene induction, 18S rRNA increased in hypertensive vessels: 3.35-fold at 180 min and 4.2-fold at 360 min. These changes were not the result of a nonspecific activation of total gene expression in hypertensive vessels, inasmuch as levels of mRNA for beta-actin did not differ from controls; however, hypertensive and control vessels showed increases at 60 min. These results indicate that increased pressure is a sufficient stimulus for protooncogene induction and rRNA production in vascular smooth muscle cells in the arterial wall and suggest that the mechanical signal is wall stress. Therefore, this model represents a unique tool to complement cultured cells for the study of the signaling pathways in the mechanotransduction of a pressure stimulus.

Regulation of PDGF-A: a possible mechanism for angiotensin II-induced vascular growth.

This study was designed to determine the effects of angiotensin II infusion on structure of conduit and resistance arteries and to see if the effects correlate with changes in platelet-derived growth factor A chain (PDGF-A) gene and protein expression. Wistar rats were subcutaneously infused by osmotic minipump with either angiotensin II (ANG II) at 200 ng.kg-1.min-1 or physiological saline (control) for 14 days. Tail-cuff systolic blood pressure was significantly higher in ANG II compared with control rats beginning the second day of infusion and continuing to the end of 2 wk. Both aorta and external spermatic artery (first-order arteriole of the cremaster muscle) developed increased wall-to-lumen ratios in the ANG II rats, but this occurred by hypertrophy of the wall in the aorta and reduction of the lumen in the arteriole. Digoxigenin-labeled cRNA probes were used for in situ hybridization of vascular sections to identify PDGF-A mRNA. Gene expression of PDGF-A in ANG II rats was upregulated in the hypertrophied aorta and the nonhypertrophied arteriole. With the use of immunocytochemistry techniques, PDGF-A and proliferating cell nuclear antigen were increased in the aorta but not in the arterioles of ANG II rats compared with control rats. These results suggest that the difference in growth response between the aorta and the arteriole induced by ANG II may lie in posttranscriptional modification of PDGF-A mRNA, differential control of transition, or turnover of PDGF-A protein.

Effect of NG-monomethyl-L-arginine on regional vascular resistance in rats.

This study investigated the effects of intravenous injection of the specific inhibitor of nitric oxide (NO) formation, NG-monomethyl-L-arginine (L-NMMA), on mean arterial pressure and regional vascular resistance in rats (n = 7). Regional vascular resistances of brain, heart, left kidney, intestine (jejunum), left testis and right testis were measured using radiolabelled microspheres before and after injection of 12.5 mg/kg of L-NMMA. Injection of L-NMMA raised mean arterial pressure significantly (p < 0.05) from 119 +/- 4 (mean +/- SEM) mmHg to 139 +/- 7 mmHg, accompanied by a significant (p < 0.05) decrease in both heart rate and cardiac index and an increase in total peripheral resistance. Regional vascular resistances in brain, heart, kidney and intestine increased significantly (p < 0.05) with L-NMMA, but no change was observed in the testes. The results indicate that the resistance of vascular beds is affected by NO synthesis, while the extent of regulation may differ among the various vascular beds.

Pressure-flow curves reflect arteriolar responses in perfused rat hindquarters.

Results from studies using pump-perfused rat hindquarters are consistent with increased wall-to-lumen ratios in resistance vessels of spontaneously hypertensive rats (SHR). However, in vivo measurements of cremaster arterioles have not shown increased wall-to-lumen ratios in SHR. To investigate this discrepancy, we studied three groups of male SHR and Wistar-Kyoto rats at 12 weeks of age. In the first two groups, the cremaster muscle was prepared to allow microscopic observation while the hindquarters were pump-perfused with increasing concentrations of norepinephrine in oxygenated Tyrode's solution. Both groups of SHR showed an increase in vasodilated resistance and elevated maximal vasoconstrictor response. In the first group, arterioles showed dose-dependent constriction that was greater in smaller arterioles but did not differ between hypertensive and normotensive rats. Vasodilated diameters of second-order arterioles were significantly smaller in the hypertensive rats. In the second group, servo-null pressures in the first-order arteriole showed that the microvessels contributed proportionally to the elevation in resistance in both SHR and normotensive rats. In the third group, first- and second-order arterioles were measured in vivo and histologically. Arteriolar diameters did not differ between SHR and normotensive rats with either method. In fixed sections the cross-sectional area of the media-intima was greater in the SHR. Therefore, data from the pump-perfused rat hindquarters accurately reflect vasoconstrictor responses of the arterioles, and in deference to in vivo measurements on arteriolar walls that include the adventitia, the increased response in the SHR can be explained by hypertrophy of the arteriolar medial-intimal area.