Coarctation Hypertensive Rats Research Articles

STRUCTURAL CHANGES IN THE KIDNEY INDUCED BY COARCTATION HYPERTENSION

We investigated structural alterations in renal tissue identifying the morphological and histological changes in the non-ischemic kidney (NIK) and their potential significance in aortic coarctation-hypertensive rats (HR). HR's mean arterial pressure (MAP) was higher compared with sham operated rats (SR). An oral 10 mg/kg/day losartan (LOS) dose diminished but not reverted MAP. Hypertrophy was noted in HR NIK's with significant weight increase (p< 0.01). The ratio IK/NIK in HR's decreased 22% (p<0.01). LOS proved to cause no ischemic kidney (IK) modification nor did it revert NIK hypertrophy. NIK in HR's presented glomerulosclerosis, mesangial proliferation and arteriolar thickening reverted by LOS. The stereological study of afferent NIK arterioles showed hypertrophy and an increase in the wall/lumen ratio without lumen modification. LOS diminished wall thickness. LOS-induced decrease of NIK alterations might result from arteriosclerosis regression, the media/lumen ratio, glomerulosclerosis and mesangial proliferation dependent on angiotensin II.

Angiotensin-converting Enzyme: Induction by Hypertension-induced Vessel Distension

Vascular angiotensin-converting enzyme (ACE) activity is increased in a variety of disorders representing both early and late stages of atherosclerosis. The factors governing the induction of vascular ACE are poorly understood. We hypothesized that vascular ACE activity might be increased by hypertension-induced vessel distension. Hypertension was induced in rats by suprarenal coarctation of the aorta. Analyses were performed 5 days and 4 weeks post-operation. ACE activity and ACE mRNA level were increased in thoracic aortae from coarctation hypertensive rats that had been exposed to elevated blood pressure, whereas they remained at normal level in abdominal aortae from those rats that had been exposed to normal blood pressure. The degree of aortic ACE induction correlated well with the degree of the trans-stenotic blood pressure gradient. An increase in ACE transcript level was also observed in carotid arteries from coarctation hypertensive rats that had been exposed to elevated blood pressure. In contrast, ACE activity and ACE mRNA expression were not altered in tissues that did not contain any large arteries from coarctation hypertensive rats, although these tissues had been exposed to elevated blood pressure. These results demonstrate an induction of ACE in large arteries that had been exposed to elevated blood pressure, and they imply that the induction of vascular ACE is due to hypertension-induced vessel distension.

Atrial natriuretic peptide relaxes arterial basal tone induced by coarctation hypertension

We have investigated the vasorelaxant effect of atrial natriuretic peptide (ANP) on isolated non-contracted aorta from coarctation hypertensive rats (HR) and the role of endothelium in this vasorelaxant action. After 7–14 days of surgery, mean blood pressure was higher ( P < 0.01) in HR compared with sham operated rats (SR), used as the control. ANP (10 −6 mol/l) significantly lowered basal tone in previously unstimulated HR thoracic aortic rings; however, it had no effect in HR abdominal aorta or in SR abdominal and thoracic aorta. Endothelial destruction potentiated the vasorelaxant effect of ANP on basal tone in HR thoracic aorta. A similar potentiation of the ANP-response was observed by pre-treatment with N G-nitro- l-arginine methyl ester ( l-NAME, 3 × 10 −4 mol/l) or methylene blue (2 × 10 −5 mol/l) in unrubbed HR thoracic aorta. Treatment with calcium-free Krebs + EGTA (2 × 10 −3 mol/l) + sodium nitroprusside (10 −5 mol/l) or calcium-free Krebs significantly decreased basal tone and abolished ANP-response. These effects were observed only in HR thoracic aorta. Similarly, staurosporine (10 −7 mol/l) and calphostin C (10 −6 mol/l), inhibitors of protein kinase C (PKC), diminished basal tone and abolished the ANP-response in HR thoracic aorta. Acetylcholine (10 −6 mol/l) had a small but significant action on the basal tone of unrubbed HR thoracic aorta. These results demonstrate that ANP has a vasorelaxant effect on aortic basal tone when the vessel is exposed to high blood pressure. Inhibition of ANP effects on basal tone by calcium-free Krebs and PKC antagonists suggests that the HR aorta increases Ca 2+-active tone, that modifies the response of vascular smooth muscle to the vasodilating hormone ANP.

Increased NPY activity in the PVN contributes to food-restriction induced reductions in blood pressure in aortic coarctation hypertensive rats

We hypothesized that hypothalamic NPYergic mechanisms mediate the blood pressure lowering effect of caloric restriction in hypertensive rats. Aortic coarctation-induced (AC) hypertensive rats ( n=25) were assigned to either an ad libitum fed control group (AL) or food restricted group (FR; 60% of AL consumption) for 3 weeks. Rats were instrumented chronically with vascular catheters and bilateral guide cannulae directed at the paraventricular hypothalamic nuclei (PVN). Blood pressure (BP) and heart rate (HR) responses to bilateral PVN microinjection of saline (200 nl) or the putative NPY receptor antagonists [ d-Trp 32]NPY (1–36) (3.3 μg/200 nl) and [ d-Tyr 27,36 Thr 32]NPY (27–36) ( d-NPY (27–36); 3.3 μg/200 nl) were determined. The FR rats were then refed and cardiovascular responses to PVN injections of NPY receptor antagonists were again determined. FR rats had significantly reduced resting BP (159±4 vs. 129±4 mmHg) and HR (360±11 vs. 326±9 bpm) compared to AL controls. Refeeding restored BP and HR of FR rats to levels similar to AL (BP=153±4 mmHg, HR=359±11 bpm). PVN administration of [ d-Trp 32]NPY produced foraging behavior and concurrent increases in BP and HR in FR, AL and Re-fed rats. The behavioral activation suggests that [ d-Trp 32]NPY (1–36) produced activation of NPY receptors. In contrast, d-NPY (27–36) did not produce any behavioral response or affect BP or HR in AL or Re-fed rats. In FR rats, d-NPY (27–36) produced significant increases in BP (peak=15±3 mmHg) which partially reversed the effect of FR on BP. Thus, in FR rats with reduced BP, PVN administration of an NPY receptor antagonist increases BP. NPY blockade in the PVN accounted for about 50% of the BP effect of food restriction, thus other mechanisms are likely to be involved. These findings are consistent with the hypothesis that NPYergic mechanisms may contribute to the reduction of BP produced by food restriction.

Eflornithine Alters Changes in Vascular Responsiveness Associated with Coarctation Hypertension

This study examined the temporal effects of the polyamine synthesis inhibitor eflornithine (alpha-difluoromethylornithine) on vascular responses to KCI, norepinephrine, sodium nitroprusside and acetylcholine in aortic rings from coarctation hypertensive rats. Coarctation hypertension reduced the contractile response of aortic rings to KCI and norepinephrine, increased sensitivity (reduced the EC50 value) to norepinephrine and attenuated relaxation to acetylcholine by 14 days of hypertension. Treatment of coarctation hypertensive rats with eflornithine resulted in a normalization of the contractile intensity to KCI and norepinephrine and relaxations to acetylcholine by 14 days of hypertension. Responses to sodium nitroprusside were similar in all groups at all time points. Hyperresponsiveness to norepinephrine produced by coarctation of the aorta was not affected by eflornithine. These studies indicate that normalization of vascular function can occur in the presence of significantly elevated blood pressure upon chronic administration of eflornithine. This functional normalization correlates with eflornithine-mediated regression of structural abnormalities normally associated with pressure overload hypertension.

Effect of chronic high pressure on transient and tonic vascular contractions to serotonin in hypertension

This study examined whether vascular contractions related to the extracellular or intracellular pools of calcium activated by serotonin are altered by high arterial pressure in chronic hypertension. Coarctation hypertensive (CH), sham normotensive control (C), and one-kidney, one-clip hypertensive rats (1K1C) were used. Tail systolic, carotid, and femoral arterial pressures were measured. Thoracic aortas from 1K1C and CH rats, as well as abdominal aortas from 1K1C rats, but not abdominal aortas from CH rats were exposed chronically to elevated arterial pressure. Isolated rings of thoracic and abdominal aortas from all groups were suspended in muscle baths for isometric force recordings. After cellular calcium depletion, dose-response curves to extracellular calcium, in the presence of 10(-5) mol/L serotonin, were unchanged in thoracic or abdominal aortas among the three groups. In the presence of submaximum levels of serotonin (2 x 10(-6) mol/L), thoracic and abdominal rings without endothelium and abdominal rings with endothelium from the three groups showed similar responses to extracellular calcium. Such responses were significantly depressed in thoracic rings with endothelium from CH and 1K1C rats. Transient contractions attributable to release of an intracellular calcium pool by 10(-5) mol/L serotonin were enhanced significantly in hypertensive thoracic and abdominal aortas from 1K1C rats, when the pool was loaded with 0.25, 0.5, 1.0, 2.0, or 4.0 mmol/L CaCl2, as well as in hypertensive thoracic aortas from CH rats when the pool was loaded with 1.0, 2.0, or 4.0 mmol/L CaCl2, but not in normotensive abdominal aortas from CH rats at any calcium-loading concentration. Similar results were observed in aortas from C, CH, and 1K1C rats loaded with 1.0 or 2.0 mmol/L CaCl2 and stimulated with 2 x 10(-6) mol/L serotonin. Endothelial removal had no effect on these calcium-release contractions in abdominal aortas from any group, but enhanced contractions in thoracic aortas of 1K1C rats. In rings loaded with 2.0 mmol/L CaCl2, 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate inhibited contractions attributable to release of cellular calcium stores to a similar extent in C, CH, and 1K1C rats. This study suggests that serotonin-stimulated intracellular calcium-dependent aortic contractions are enhanced by elevated pressure in renal hypertensive rats.

Vascular smooth muscle responses to endothelial autacoids in rats with chronic coarctation hypertension.

To examine whether elevated intravascular pressure in chronic hypertension alters responses of vascular smooth muscle to agents of endothelial origin. Coarctation hypertensive, sham normotensive control, and one-kidney, one clip hypertensive (1K1C) rats were used. Tail systolic, carotid and femoral arterial pressures were measured. Responses to histamine, endothelin-1 and the prostacyclin analog iloprost were evaluated in isolated helically cut strips of thoracic and abdominal aortas, with and without endothelium, from all groups. Responses to nitroglycerin were also evaluated in strips of abdominal aortas. Thoracic aortas from 1K1C and coarctation hypertensive rats, as well as abdominal aortas from 1K1C rats, but not abdominal aortas from coarctation hypertensive rats were exposed chronically to elevated arterial pressure. Endothelium-dependent maximal relaxation by histamine was significantly depressed in thoracic aortas from both groups of rats, as well as in abdominal aortas from 1K1C rats. Maximal relaxation and sensitivity to histamine were normal in abdominal aortas from coarctation hypertensive rats. Sensitivity to nitroglycerin was impaired in abdominal aortas from 1K1C rats but not in those from coarctation hypertensive rats; maximal relaxation to nitroglycerin was similar in all groups. Relaxation to iloprost was independent of the endothelium, observed only in thoracic aortas and impaired in hypertensive rats. Responses to endothelin-1 were similar in the groups. Vasorelaxation by histamine, iloprost and nitroglycerin are impaired in hypertension. The impaired relaxation by histamine results from exposure of the vascular endothelium to chronically elevated pressure. This impairment may be related to effects of high pressure in reducing the ability of the endothelium to produce endothelium-derived relaxing factor and inhibit cyclic GMP-dilator mechanisms.

Increased proteoglycan synthesis by the cardiovascular system of coarctation hypertensive rats

Proteoglycan (PG) synthesis in the cardiovascular system of coarctation hypertensive rats was examined by in vivo and in vitro labeling of glycosaminoglycans with 35SO4 in rats made hypertensive for short (4 days) and longer (14 days) durations. With in vivo labeling, only tissues directly exposed to elevated pressure (left ventricle, LV and aorta above the clip, AOR increases) exhibited elevated PG synthesis after 4 days of hypertension. By 14 days, tissues both exposed to (LV and AOR increases) and protected from elevated pressure (right ventricle and kidney) exhibited elevated PG synthetic rates. Slight elevations in the proportion of galactosaminoglycans were observed with a concurrent proportional decrease in heparan sulfate PGs. Using the in vitro labeling procedure, no significant increases in PG synthesis were observed in any tissue at either 4 days or 14 days of hypertension. These data indicate that: 1) coarctation hypertension stimulates PG production that is dependent initially on increased pressure and later, on additional non-pressure related factors, 2) these other factors are responsible for enhanced PG production in tissues not directly exposed to pressure overload, 3) pressure and/or these other factors are essential for enhanced PG production in coarctation hypertension, and 4) synthesis of all GAG types appears to be affected.

Increased Ca2+ signaling after alpha-adrenoceptor activation in vascular hypertrophy.

In an effort to explain the increased sensitivity to agonists of hypertrophic vascular muscle, intracellular Ca2+ concentration ([Ca2+]i)-signaling mechanisms were studied in normal and hypertrophic rat aortas from normotensive and coarctation-hypertensive rats. Based on both fura 2 fluorescence and aequorin luminescence measurements, qualitatively different patterns of Ca2+ mobilization occur in normal and hypertrophic rat aortic muscle. Normal rat aortic muscle contracts to phenylephrine with little or no increase in [Ca2+]i, whereas the angiotensin II-induced contraction is accompanied by a marked [Ca2+]i transient. In contrast, hypertrophic rat aortic muscle shows a dramatic increase in Ca2+ signaling after phenylephrine stimulation. Moreover, both the amplitude of the angiotensin-induced [Ca2+]i transient and the contractile sensitivity to this agonist are decreased in the hypertrophic muscle. Our results strongly suggest that the amplitude of the [Ca2+]i transient after agonist stimulation determines the contractile sensitivity and that there is an altered coupling of the alpha-adrenoceptor in the hypertrophic vascular muscle.

Endothelium in functional aortic changes of coarctation hypertension

Eleven coarctation hypertensive (CH), twelve sham control (C), and seven one-kidney, one-clip (1K,1C) rats were used to examine the role of pressure and the endothelium in vascular sensitivity changes to acetylcholine (ACh), serotonin (5-HT), and norepinephrine (NE) in chronic arterial hypertension. Terminal mean carotid artery pressures were CH = 156 +/- 5 mmHg, C = 99 +/- 3 mmHg, and 1K,1C = 159 +/- 5 mmHg. Femoral artery pressures were CH = 100 +/- 3 mmHg, C = 98 +/- 4 mmHg, and 1K,1C = 154 +/- 4 mmHg, respectively. Isometric tension recordings were made from helically cut strips of thoracic and abdominal aortas, with and without functional endothelium, from the three groups of rats. Sensitivity to relaxation by acetylcholine, expressed as -log of 50% effective dose, was significantly depressed in thoracic aortas from CH and 1K,1C rats and abdominal aortas from 1K,1C rats but not from abdominal aortas of CH rats. A similar relationship between the groups was seen for 5-HT contractions. Sensitivity to NE was enhanced in thoracic and abdominal aortas of hypertensive rats. Inactivation of aortic endothelia abolished ACh responses, did not alter 5-HT relationships between the three groups, and abolished the differences in sensitivity to NE in thoracic aortas. The data suggest that pressure and the endothelium may play a role in vascular sensitivity changes in hypertension.

Intracellular free Ca2+ is elevated in hypertrophic aortic muscle from hypertensive rats

Vessels from hypertensive animals have been shown to have increased medial thickness and also to exhibit increased sensitivity to agonists. We tested the hypothesis that changes in intracellular Ca2+ concentration [( Ca2+]i) may be associated with the growth response and the altered contractility of the aortic smooth muscle of modified coarctation-hypertensive rats. Freshly isolated single cells, loaded with fura-2, showed significantly higher [Ca2+]i both at rest and after depolarization compared with normal rat aortic cells. These cells also exhibited at rest a significant nuclear-cytoplasmic [Ca2+] gradient. Furthermore, the increased [Ca2+]i was associated with increased nuclear volumes, suggesting the presence of polyploid nuclei, as determined by ethidium dimer fluorescence imaging. Intact vascular rings from hypertrophic rat aortas, loaded with aequorin, also showed significantly higher [Ca2+]i at rest and after depolarization compared with normal rat aortas. In addition, the maximal force per cross-sectional area generated by the hypertrophic muscle was less than normal, consistent with a change in the contractile phenotype of the growing smooth muscle cells. This is the first report to directly indicate an increase in [Ca2+]i in hypertrophic vascular smooth muscle in hypertensive animals.

Vascular responsiveness to phorbol esters in coarctation-hypertensive rats

Recent observations suggest that a phospholipid-sensitive, calcium-dependent protein kinase affects the contractile responses of vascular smooth muscle. Protein kinase C activators such as the tumor-promoting phorbol esters have been used as tools to study protein kinase C function in various intact cells. The present study characterizes vascular reactivity to protein kinase C activation in rats made hypertensive by coarctation of the abdominal aorta. Thoracic aortic strips from hypertensive rats developed greater force than arteries from normotensive rats in response to the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA). Thoracic aortae from hypertensive rats were more responsive (lower threshold dose) to the phorbol ester than those from normotensive rats. Additionally, arteries from hypertensive rats were more responsive to the contractile effects of mezerein, a non-phorbol ester activator of protein kinase C. Removal of the endothelium did not eliminate the difference in responsiveness to TPA in thoracic aortae from normotensive and hypertensive rats. The threshold dose of TPA in abdominal aortae from hypertensive rats was not different from that in normotensive rats. However, the maximal response to 10(-6) mol/l TPA after 60 min in abdominal aortae from hypertensive rats was significantly less than that in aortae from normotensive rats. Thus, contractile responses to TPA appear to be influenced by arterial pressure per se. The inhibitory effects of the calcium antagonist, verapamil, in thoracic aortae from hypertensive rats were greater than in those from normotensive rats. Verapamil inhibited TPA-induced contractions in abdominal aortae from hypertensive rats to the same extent as in those from normotensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-dependent basilar and aortic vascular responses in normotensive and coarctation hypertensive rats

The responsiveness of acetycholine (ACh), nitroglycerin (NG) and norepinephrine (NE) (aorta only) in both basilar arteries (BA) and thoracic aortic (TA) rings from coarctation hypertensive rats (CHR) were studied and compared to their sham-operated normotensive control rats (SNR). The effects of these agents were also evaluated in TA or BA with and without endothelium from naive normotensive rats (NNR). Blood pressure (BP) and plasma renin activity (PRA) of CHR were significantly higher than their time-matched SNR. Endothelium removal from TA of NNR significantly enhanced NE and NG sensitivity and reduced the maximum ACh relaxation. Removal of BA endothelium of NNR abolished ACh-induced relaxation but had no effect on NG-induced relaxation. In BA from CHR at any stage of hypertension studied, the sensitivity and maximum relaxation induced by ACh or NG were not significantly different than their respective time-matched SNR. ACh sensitivity of TA did not change in 1 Day CHR but decreased in 4 and 14 Day CHR. NG sensitivity increased, did not change and decreased in 1, 4 and 14 Day CHR, respectively. NE sensitivity increased in all stages of hypertension. These data suggest that in coarctation-induced hypertension there is a complex progression of events in TA which is modulated by different mechanisms as evidenced by the changes in the effects of NE, ACh and NG at various stages of hypertension. The results also suggest that the vascular endothelium of TA but not of BA may provide an acute protective mechanism to counteract the imbalance created by the increased sensitivity of smooth muscle cells to contractile agonists in the early stage of hypertension. However, persistent hypertension appears to override this mechanism.