[Modification of nitrate tolerance using antioxidants].
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Antioxidants↗
Biomedical subjects
G Nickenig
Publications and source records attributed to G Nickenig.
Insulin induces upregulation of vascular AT1 receptor gene expression by posttranscriptional mechanisms.
BACKGROUND: An interaction of insulin with angiotensin II effects could be pathophysiologically important for the pathogenesis of atherosclerosis and hypertension. METHODS AND RESULTS: We examined the effect of insulin on AT1 receptor gene expression in cultured vascular smooth muscle cells (VSMCs). A 24-hour incubation with insulin (100 nmol/L) produced a 2-fold increase in AT1 receptor density on VSMCs, as assessed by radioligand binding assays. This enhanced AT1 receptor expression was caused by a time- and concentration-dependent upregulation of the AT1 receptor mRNA levels, as assessed by Northern analysis. The maximal effect was detected after a 24-hour incubation of cells with 100 nmol/L insulin (270+/-20%). AT1 receptor upregulation was caused by a stabilization of the AT1 receptor mRNA, because the AT1 receptor mRNA half-life was prolonged from 5 hours under basal conditions to 10 hours after insulin stimulation. In contrast, insulin had no influence on AT1 receptor gene transcription, as assessed by nuclear run-on assays. The insulin-induced AT1 receptor upregulation was followed by an increased functional response, because angiotensin II evoked a significantly elevated intracellular release of calcium in cells that were preincubated with 100 nmol/L insulin for 24 hours. The insulin-induced AT1 receptor upregulation was dependent on tyrosine kinases, as assessed by experiments with the tyrosine kinase inhibitor genistein. Furthermore, experiments using the intracellular calcium chelator bis(2-amino-5-methylphenoxy)ethane-N, N,N',N'-tetraacetic acid tetraacetoxymethyl ester suggest that intracellular calcium release may be involved in AT1 receptor regulation. CONCLUSIONS: Insulin-induced upregulation of the AT1 receptor by posttranscriptional mechanisms may explain the association of hyperinsulinemia with hypertension and arteriosclerosis, because activation of the AT1 receptor plays a key role in the regulation of blood pressure and fluid homeostasis.
Angiotensin II↗
Estrogen modulates AT1 receptor gene expression in vitro and in vivo.
BACKGROUND: The AT1 receptor has been implicated in the pathogenesis of hypertension and atherosclerosis. Estrogen deficiency is also associated with cardiovascular diseases. Therefore, we examined the AT1 receptor gene expression in ovariectomized rats with and without estrogen replacement therapy and the influence of estrogen on AT1 receptor expression in cultured vascular smooth muscle cells. METHODS AND RESULTS: Rat aortic tissue was examined 5 weeks after ovariectomy. In one group, estrogen (1.7 mg estradiol) was administered during the 5-week period. Functional experiments assessed angiotensin II-induced contraction of aortic rings. AT1 receptor mRNA levels were measured by quantitative polymerase chain reaction and Northern blotting. AT1 receptor density was assessed by radioligand binding assays. These techniques were also applied in cultured vascular smooth muscle cells. The efficacy of angiotensin II on vasoconstriction was significantly increased in aortas from ovariectomized rats. As assessed by radioligand binding assays, AT1 receptor density was increased to 160% without changes in receptor affinity during estrogen deficiency. AT1 receptor mRNA levels were consistently increased to 187% in ovariectomized rats compared with sham-operated animals. Estrogen substitution therapy in ovariectomized rats reversed this AT1 receptor overexpression. To explore the underlying mechanisms, the direct influence of estradiol on AT1 receptor expression was investigated in VSMCs. Estradiol (1 micromol/L) led to a time-dependent downregulation of AT1 receptor mRNA, with a maximum of 33.3% at 12 hours. There was a correlative decrease in AT1 receptor density. CONCLUSIONS: This novel observation of estrogen-induced downregulation of AT1 receptor expression could explain the association of estrogen deficiency with hypertension and atherosclerosis, because activation of the AT1 receptor plays a key role in the regulation of blood pressure, fluid homeostasis, and vascular cell growth.
Angiotensin II↗
Interaction between insulin and AT1 receptor. Relevance for hypertension and arteriosclerosis.
The AT1 receptor mediates many biological effects of the renin angiotensin system such as vasoconstriction and cell proliferation. The expression level of the AT1 receptor is subjected to various pathophysiological influences. Insulin, which is elevated in the metabolic syndrome, induces a overexpression of vascular AT1 receptors leading to an enhanced biological efficacy of angiotensin II. This heterologous regulation of the AT1 receptor by insulin may explain the fact that the metabolic syndrome is frequently associated with hypertension and atherosclerosis.
Animals↗
Alteration of intracellular Ca2(+)-handling and receptor regulation in hypertensive cardiac hypertrophy: insights from Ren2-transgenic rats.
Abnormal intracellular Ca2(+)-handling appears to be a major cause of systolic and diastolic dysfunction in animals and humans with cardiac hypertrophy due to pressure overload and heart failure. However, the precise mechanisms which cause alteration of Ca2(+)-handling remain unclear. Several lines of evidence suggest that activation of neurohormonal systems may play a central role. In particular, widespread awareness of the importance of the renin-angiotensin system (RAS) has occurred since experimental and clinical studies have detailed the efficacy of angiotensin-converting enzyme inhibitors in reducing morbidity and mortality in patients with left ventricular dysfunction. To evaluate in vivo the role of activated RAS in the regulation of (a) cardiac receptor expression and signal transduction mechanisms and (b) Ca2+ homeostasis, transgenic TG(mREN2)27 rats harbouring the murine renin Ren2d gene were chosen. These animals develop fulminant hypertension and cardiac hypertrophy at an early age despite low levels of renin in the plasma. High expression of the transgene in the vasculature and the heart is associated with increased local formation of angiotensin II. In the Ren2-transgenic model alterations of beta-adrenergic neuroeffector mechanisms, Ca2(+)-handling and alpha-adrenergic signal transduction are observed which are very similar to those observed in the myocardium of patients with end-stage heart failure. Moreover, treatment with specific inhibitors of the RAS, such as angiotensin-converting enzyme inhibitors or angiotensin II-receptor antagonists, largely reversed these defects. Studies on TG(mREN2)27 rats may provide new insights into the pathogenesis of hypertensive heart disease and mechanisms which promote disease progression to end-stage heart failure and also may have important implications with regard to therapeutics of heart failure in man.
Animals↗
Salt induces vascular AT1 receptor overexpression in vitro and in vivo.
The molecular events governing salt-sensitive hypertension are currently unknown. Because the renin-angiotensin system plays a central role in blood pressure regulation, as well as electrolyte balance, it may be closely involved in the phenomenon of salt sensitivity. Therefore, we examined the effect of a high salt diet (8%) on aortic angiotensin type 1 (AT1) receptor expression in Sprague-Dawley rats by means of radioligand binding assays and quantitative polymerase chain reaction. High salt intake caused an increase of AT1 receptor mRNA and AT1 receptor density to approximately 160% compared with control levels. Northern analysis revealed that incubation of vascular smooth muscle cells (VSMCs) with an increased concentration of sodium chloride (by 10 mmol/L) caused a time-dependent elevation of AT1 receptor mRNA levels, with a maximum of 241+/-28% after 24 hours. There was a similar increase in AT1 receptor density in VSMCs in response to sodium chloride, as assessed by radioligand binding assays. The salt-induced AT1 receptor upregulation led to an enhanced functional response of VSMCs on stimulation with angiotensin II, since the angiotensin II-elicited intracellular calcium response was significantly increased in cells preincubated for 24 hours with the high salt concentration. Thus, sodium chloride may directly induce AT1 receptor upregulation in vitro as well as in vivo; this suggests a potential mechanism participating in salt-induced hypertension because the AT1 receptor activation is tightly coupled to blood pressure regulation.
Analysis of Variance↗
Elevated serum concentrations of soluble adhesion molecules in coronary artery disease and acute myocardial infarction.
OBJECTIVES: Adhesion molecules are involved in a number of chronic conditions and diseases like rheumatoid arthritis, tumor growth and wound repair. Soluble adhesion molecules (SAM) play an important role in angiogenesis which is a common aspect of the conditions mentioned above and atherosclerosis. The aim of the present study was to assess the prevalence and the impact of elevated soluble adhesive molecule plasma concentrations in patients with atherosclerosis. DESIGN AND SUBJECTS: In this study, we measured the soluble forms of intercellular adhesive molecule (sICAM), endothelial adhesive molecule (sELAM) and vascular adhesive molecule (sVCAM) using a sandwich ELISA technique in plasma of patients with acute myocardial infarction (AMI), coronary heart disease (CHD) and in healthy subjects (HS). RESULTS: Patients suffering from CHD and AMI showed significant higher plasma concentrations of sICAM (p <0. 05 and p <0.005), sELAM (p <0.01 and p <0.001) and sVCAM (p <0.001 and p <0.005) than HS. In patients with fatal outcome of myocardial infarction the plasma concentrations of sICAM, sELAM and sVCAM were significantly elevated compared to surviving patients (p <0.005; p <0.005; p <0.05). In patients undergoing thrombolytic therapy there were no significant differences of plasma adhesive molecule concentrations. The levels of SAM were not related to other risk factors like diabetes, nicotin abuse, hyperlipidemia, hypertension and a familiary history of cardiovascular disease. CONCLUSIONS: Elevated levels of SAMs are found in patients with coronary heart disease. High SAM levels in plasma seem to be a prognostic factor in acute myocardial infarction. This effect is independent from other concomitant risk factors. Our results suggest that SAMs are involved both in acute phase of myocardial infarction and chronic process of atherosclerosis. It seems that similiar to other chronic inflammatory diseases, atherosclerosis seems to be modulated by soluble forms of adhesive molecules.
Adult↗
[Significance of the angiotensin-II receptor AT1 in the pathogenesis of arterial hypertension and arteriosclerosis].
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Animals↗
Regulation of the angiotensin AT1 receptor expression by hypercholesterolemia.
The AT1 receptor mediates most of the biological effects of angiotensin II and has therefore been implicated in the pathogenesis of hypertension as well as arteriosclerosis. Hypercholesterolemia is a prominent risk factor for the development of these cardiovascular diseases. Since experimental results from hyperlipidemic animal models suggested an interaction of the renin-angiotensin system and hypercholesterolemia, the effects of low-density lipoprotein (LDL) and hypercholesterolemia has recently been investigated in vitro and in vivo. LDL causes in cultured vascular smooth muscle cells an up-regulation of AT1 receptor gene expression which is followed by an enhanced functional response upon stimulation with angiotensin II. This effect is also evident in vivo, as assessed in hypercholesterolemic rabbits. The vascular AT1 receptor expression is increased approximately twofold in hypercholesterolemic rabbits in comparison to normocholesterolemic animals. This leads ultimately to an enhanced angiotensin II-induced vasoconstriction. Thus, hypercholesterolemia and elevated concentrations of LDL enhance AT1 receptor expression in the vasculature causing an enhanced biological effectiveness of the renin-angiotensin system. This interaction may explain that blockade of the renin-angiotensin system attenuates the progress of arteriosclerosis in the hypercholesterolemic organismen and indicates a pathophysiologically important role for the lipid-induced AT1 receptor up-regulation.
Animals↗
Cardiac angiotensin II receptors: studies on functional coupling in Sprague-Dawley rats and TGR(alphaMHC-hAT1) transgenic rats.
The renin-angiotensin system plays an important role in the pathogenesis of cardiac hypertrophy and chronic heart failure as angiotensin II has been shown to induce cardiac hypertrophy and fibrosis. Besides these structural alterations, functional effects on cardiomyocytes have been reported in different mammalian species. Angiotensin II is known to produce a positive inotropic effect in some species, and differences in atrial and ventricular myocardium have been described. So far, the molecular events which govern angiotensin II-mediated changes in cardiac contractility are not completely understood. In order to study the dependency of the angiotensin II-induced positive inotropic effect on receptor density, we examined the effect of angiotensin II on cardiac function in atria, papillary muscles and isolated ventricular cardiomyocytes from adult Sprague-Dawley rats and TGR(alphaMHC-hAT1) transgenic rats, which expressed the human angiotensin AT1 receptor (hAT1) specifically in the heart. In atrial myocardium from adult Sprague-Dawley rats, angiotensin II (30 micromol/l) produced an AT1-mediated positive inotropic effect (38.5% of control), whereas in papillary muscles and isolated ventricular myocytes, no inotropic response was observed. As shown by polymerase chain reaction (PCR) and radioligand binding, the human angiotensin AT1 receptor was exclusively expressed in transgenic animals, which markedly overexpressed the angiotensin AT1 receptor. However, in transgenic rats the positive inotropic effect in atrial preparations was similar to the controls, and neither in papillary muscles nor in isolated cardiomyocytes the increase in receptor density led to an inotropic effect induced by angiotensin II. These data suggest that the existence of functionally uncoupled receptors rather than the low density of receptors at the ventricular site is responsible for the inability of ventricular myocardium to respond to angiotensin II.
1-Sarcosine-8-Isoleucine Angiotensin II↗
Angiotensin type-1 (AT1) receptor gene expression in primarily cultured human arterial umbilical endothelial cells.
The aim of this study was to investigate the interaction of angiotensin II (Ang II) and human umbilical arterial endothelial cells (HUAEC). Specific binding of 125I-ANG II to primarily cultured HUAEC showed a K(D) of 1.98 +/- 0.53 x 10(-9) M (n = 5) with a maximum binding site of 2.84 +/- 1.07 x 10(-13) mol/mg protein (n = 5). In later passages (third and fifth subculture), this binding site was no longer detectable. Gene expression analysis revealed a strong expression of the angiotensin type-I receptor (AT1-R) in the primarily cultured HUAEC, with a decrease in additional passages. In primarily cultured HUAEC, Ang II (10(-10)-10(-6) M) induced a concentration-dependent increase in intracellular free calcium ([Ca2+]i) that could be blocked by a preincubation with candesartan (TCV-112) but not by PD123319. These data show the expression of the AT1-R in primary cultures of HUAEC. The Ang II-induced increase in [Ca2+]i seems to be mediated by this receptor.
Angiotensin II↗
Upregulation of vascular angiotensin II receptor gene expression by low-density lipoprotein in vascular smooth muscle cells.
BACKGROUND: Because LDL and the angiotensin II type 1 (AT1) receptor are thought to be involved in the pathogenesis of chronic vascular disease, we studied possible interactions between these two biological systems in cultured rat vascular smooth muscle cells. METHODS AND RESULTS: Incubation of vascular smooth muscle cells with 100 micrograms/mL LDL profoundly increased AT1 receptor mRNA to approximately 250% of control levels as assessed by Northern hybridization analysis. This effect is maximal 12 hours after addition of LDL to the culture medium and is sustained for up to 24 hours. The LDL-induced upregulation is dose dependent, with a maximal effect obtained with 100 micrograms/mL LDL. There is a correlative increase of cell surface-associated AT1 receptors as assessed by saturation radioligand binding assays. The half-life of AT1 receptor mRNA is increased substantially by LDL compared with that of cells treated only with 5,6-dichlorobenzimidazole to block transcription. Angiotensin II-induced elevation of cytosolic calcium concentration is significantly increased in vascular smooth muscle cells pretreated with LDL to 368 +/- 41 nmol/L compared with control cells pretreated with vehicle (248 +/- 33 nmol/L). Moreover, angiotensin II-induced DNA synthesis is markedly enhanced when cells are coincubated with 100 micrograms/mL LDL. CONCLUSIONS: These data reveal a significant upregulation of AT1 receptor gene expression by LDL in vascular smooth muscle cells through mechanisms that involve posttranscriptional mRNA stabilization. Ultimately, this AT1 receptor upregulation leads to an elevated functional response of vascular smooth muscle cells on angiotensin II stimulation.
Animals↗
Characterization of angiotensin receptors on human skin fibroblasts.
Angiotensin II is involved in blood pressure regulation, cell growth and angioneogenesis. The angiotensin receptors which mediate the intracellular effects of angiotensin II are expressed in numerous tissues and cell types. We studied the expression of angiotensin II receptors in cultured human skin fibroblasts derived from a skin biopsy. Angiotensin II binding characteristics were analyzed by radioligand binding assays. The DNA synthesis was assessed by [H]thymidine incorporation assays. Intracellular calcium concentrations were measured by fura-2 spectrofluorometry, and mRNA expression levels were analyzed by northern blot technology. Two distinct angiotensin receptors were detectable on human skin fibroblasts: the AT1 receptor with Kd = 1.0 +/- 0.7 nmol/l and Bmax = 17.9 +/- 0.9 fmol/mg protein, and an angiotensin(1-7) binding site with Kd = 26 +/- 6.6 nmol/l and Bmax = 80.4 +/- 3.5 fmol/mg protein, as shown by competition binding assays using selective angiotensin II receptor antagonists and the heptapeptide angiotensin(1-7). The angiotensin AT1 receptor mRNA was substantially expressed in human skin fibroblasts and was subjected to homologous downregulation. In human skin fibroblasts angiotensin II caused a profound increase in intracellular calcium which was blocked by angiotensin AT1 receptor antagonists such as Exp-3174. Furthermore, both angiotension II and angiotensin(1-7) led to increased DNA synthesis in human skin fibroblasts. In conclusion, cultured human skin fibroblasts express angiotensin AT1 receptors and a putatively new angiotensin receptor activated by angiotensin(1-7), both coupled to signaling pathways involved in DNA synthesis.
Angiotensin II↗
Low density lipoprotein enhances the thrombin-induced growth of vascular smooth muscle cells.
OBJECTIVE: In the present study we investigated whether low density lipoprotein is able to enhance the growth promoting effects of thrombin in vascular smooth muscle cells. METHODS: DNA synthesis was examined by measurement of the [3H]thymidine incorporation into the cell DNA. Cell count was measured with a Neubauer cell box. Thrombin receptor mRNA was determined by Northern blotting. Ca2+ was measured by the fura 2-method. RESULTS: Thrombin (5 nmol/l), thrombin receptor activating protein (3 mumol/l) and low density lipoprotein (33 nmol/l) induce a 652 +/- 80%, 593 +/- 80% and a 316 +/- 60% increase in [3H]thymidine incorporation into DNA (mean +/- SD, n = 3), respectively. A coincubation of thrombin or thrombin receptor activating protein with low density lipoprotein led to a 1245 +/- 160% or 1200 +/- 40% increase of DNA synthesis (mean +/- SD, n = 3). Thus, coincubation of low density lipoprotein and thrombin causes a synergistic rather than an additive mitogenic effect on smooth muscle cells. Thrombin and low density lipoprotein induced a 22 +/- 8.4% and a 29% +/- 6% increase in cell number, respectively. Simultaneous treatment of vascular smooth muscle cells with thrombin and low density lipoprotein caused a 63 +/- 14% increase in cell number (mean +/- SD, n = 3). To further elucidate the underlying mechanism, we studied the effect of low density lipoprotein on the expression of thrombin receptor mRNA. Low density lipoprotein caused a 2.5-fold increase of thrombin receptor mRNA within 24 h, as assessed by Northern analysis. Preincubation of cells for 24 h with 33 nmol/l low density lipoprotein resulted in an elevation of the thrombin-induced increase in cytosolic free Ca2+ concentration from 538 +/- 54 to 923 +/- 75 nmol/l (mean +/- SD, n = 4). CONCLUSION: In summary, low density lipoprotein may enhance the mitogenic effect of thrombin probably by an up-regulation of thrombin receptor gene expression in vascular smooth muscle cells or by an elevation of the thrombin-induced increase in cytosolic free Ca2+ concentration.
Animals↗
Down-regulation of aortic and cardiac AT1 receptor gene expression in transgenic (mRen-2) 27 rats.
1. Transgenic(TG) (mRen-2) rats overexpressing the mouse renin gene develop fulminant hypertension and cardiac hypertrophy. Since the activation of AT1 receptor by angiotensin II is involved in blood pressure regulation, cardiac performance and myocardial growth, we investigated the biological effects of angiotensin II and the regulation of the AT1 receptor in the heart and aorta of TGR (mRen-2)27 rats in comparison to control animals. 2. Contraction studies on isolated cardiac muscle strips reveal that angiotensin II exerts no positive inotropic effect on the left ventricular myocardium of both, transgenic and control rats. In contrast, angiotensin II leads via AT1 receptor activation in the left atrium of control rats to a significant contraction (130 +/- 5% of basal contraction) which is not detectable in left atrium preparations of the transgenic animals. Furthermore, AT1 receptor activation causes a profound contraction of aortic rings isolated from control rats amounting to 1.39 +/- 0.2 mN mg-1 wet weight, whereas aortic rings from TGR (mRen-2)27 rats contract only minimally upon angiotensin II stimulation (0.2 +/- 0.02 mN mg-1 wet weight). 3. These altered physiological responses of angiotensin II in the transgenic rats are in part due to a marked down-regulation of the AT1 receptor in atrial, ventricular and aortic tissue of these transgenic animals in comparison to control Sprague-Dawley rats, as shown by radioligand binding assays and quantitative polymerase chain reaction (PCR) experiments. The AT1 receptor density Bmax in the left atrium was 1.3 +/- 0.08 fmol mg-1 protein in control rats (KD 1.1 +/- 0.18 nmol l-1) and 0.94 +/- 0.15 fmol mg-1 protein (KD 2.1 +/- 0.3 nmol l-1. In the aorta Bmax values were 15.1 +/- 0.5 fmol mg-1 protein (KD 1.9 +/- 0.27 nmol l-1) for control rats and 11.3 +/- 0.76 fmol mg-1 protein (KD 1.9 +/- 0.27 nmol l-1) for the TGR(mRen-2)27 rats AT1 receptor mRNA was reduced in the transgenic animals to 46 +/- 3% in the left atrium, 50 +/- 11% in the left ventricle and 40 +/- 3% in the aorta, respectively. 4. Together, the AT1 receptor is down-regulated in TGR (mRen-2)27 rats in comparison to wildtype Sprague Dawley rats leading to a profoundly decreased response of cardiac and aortic tissue upon stimulation with angiotensin II.
Analysis of Variance↗
Influence of oxidized low-density lipoprotein on vascular angiotensin II receptor expression.
OBJECTIVES: To investigate the effect of oxidized low-density lipoprotein (LDL) on angiotensin type 1 (AT1) receptor expression and on angiotensin-induced mitogenesis in vascular smooth muscle cells. DESIGN: Since both LDL and the AT1 receptor are thought to be involved in the pathogenesis of chronic vascular disease, we studied possible interactions between these two biological systems in cultured rat vascular smooth muscle cells. Recent studies have demonstrated that native LDL profoundly increases the AT1 receptor gene expression in vascular smooth muscle cells. The present study was designed to show whether an oxidative modification of LDL, which has been implicated as a major participant in atherogenesis, has similar effects on vascular smooth muscle cells. MATERIALS AND METHODS: Vascular smooth muscle cells isolated from rat thoracic aorta were cultured with oxidized LDL and assessed for their ability to synthesize DNA, and messenger (m)RNA expression, using Northern blot technology. RESULTS: Oxidized LDL had no significant effect on AT1 receptor mRNA steady-state levels when incubated for 0-24 h. Moreover, oxidized LDL itself had no stimulatory effect on DNA synthesis in vascular smooth muscle cells, nor was the angiotensin II-induced increase in DNA synthesis influenced by oxidized LDL. CONCLUSIONS: These data show that in contrast to native LDL, oxidized LDL has no effect on AT1 receptor mRNA expression and no effect on angiotensin II-induced mitogenesis in vascular smooth muscle cells.
Angiotensin II↗
The vascular smooth muscle type I angiotensin II receptor mRNA is destabilized by cyclic AMP-elevating agents.
Although processes involved in mRNA degradation play a significant role in dictating steady state mRNA levels, the influence of cell surface signaling on mRNA stability control is understood incompletely. In this study, the effects of cAMP-elevating agents on type I angiotensin II receptor (AT1-R) mRNA levels were assessed in cultured rat aortic vascular smooth muscle cells (VSMCs). AT1-R mRNA levels are rapidly reduced by forskolin treatment, in which the maximal effect yields an 80% reduction in AT1-R mRNA levels after 6 hr of treatment. The rate of AT1-R mRNA decay in response to forskolin is greater than its apparent intrinsic decay, as assessed in the presence of the transcriptional inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, suggesting forskolin treatment destabilizes the AT1-R mRNA. Nuclear run-on analysis indicates forskolin treatment does not affect transcription of the AT1-R gene in VSMCs, implying induced AT1-R mRNA destabilization accounts for the entire effect of forskolin in decreasing AT1-R mRNA levels. Dose-effect studies that assessed AT1-R mRNA levels and cAMP production were conducted using forskolin and the beta-adrenergic receptor agonist isoproterenol as agonists. Isoproterenol is almost 3 orders of magnitude more potent at eliciting the reduction in AT1-receptor mRNA levels than it is at stimulating cAMP production. Similarly, forskolin elicits reductions in AT1-R mRNA, which occur at concentrations that fail to elicit a detectable production of cAMP. However, protein kinase A activity is stimulated maximally by isoproterenol and forskolin concentrations that do not stimulate detectable cAMP production. These data provide evidence that the mechanism for down-regulation of AT1-R mRNA levels by cAMP-elevating agents in VSMCs occurs via a PKA-regulated mRNA destabilization pathway.
Adrenergic beta-Agonists↗
Hypercholesterolemia is associated with enhanced angiotensin AT1-receptor expression.
Low-density lipoprotein increases the AT1-receptor gene expression in vascular smooth muscle cells. To elucidate whether elevated cholesterol serum levels upregulate the AT1 receptor and its functional response to angiotensin II in vivo, we compared 1) the vasoconstrictive effect of angiotensin II and 2) the level of expression of the vascular AT1 receptor in aortas of normocholesterolemic and hypercholesterolemic rabbits. Contraction experiments on isolated aortic rings showed that the angiotensin II-induced vasoconstriction was increased in hypercholesterolemic New Zealand White rabbits compared with normocholesterolemic New Zealand White rabbits. This difference in the angiotensin II-induced vasoconstriction was caused by a twofold increase in the density of cell surface AT1 receptors in hypercholesterolemic rabbits, as assessed by radioligand binding assays. The enhanced expression of AT1 receptors on the surface of these vascular cells was caused by elevated steady-state levels of the AT1-receptor mRNA to 220 +/- 35% in aortas excised from hypercholesterolemic rabbits compared with levels in aortas from normocholesterolemic rabbits, as measured by Northern blot analysis. These data indicate that hypercholesterolemia is associated with upregulation of expression and function of vascular AT1 receptors in vivo. This suggests a novel mechanism by which hypercholesterolemia could be involved in the onset and progression of chronic vascular diseases such as hypertension and arteriosclerosis if the phenomenon is confirmed in humans.
Angiotensin II↗