Atherosclerosis in ApoE-deficient mice progresses independently of the NLRP3 inflammasome.

The interleukin-1 (IL-1) family of cytokines has been implicated in the pathogenesis of atherosclerosis in previous studies. The NLRP3 inflammasome has recently emerged as a pivotal regulator of IL-1ß maturation and secretion by macrophages. Little is currently known about a possible role for the NLRP3 inflammasome in atherosclerosis progression in vivo. We generated ApoE-/- Nlrp3-/-, ApoE-/- Asc-/- and ApoE-/- caspase-1-/- double-deficient mice, fed them a high-fat diet for 11 weeks and subsequently assessed atherosclerosis progression and plaque phenotype. No differences in atherosclerosis progression, infiltration of plaques by macrophages, nor plaque stability and phenotype across the genotypes studied were found. Our results demonstrate that the NLRP3 inflammasome is not critically implicated in atherosclerosis progression in the ApoE mouse model.

Connexin40 regulates renin production and blood pressure.

Renin secretion is regulated by coordinated signaling between the various cells of the juxtaglomerular apparatus. The renin-secreting cells (RSC), which play a major role in the control of blood pressure, are coupled to each other and to endothelial cells by Connexin40 (Cx40)-containing channels. In this study, we show that Cx40 knockout (Cx40-/-) mice, but not their heterozygous littermates, are hypertensive due to the increase in the number of RSC, renin biosynthesis, and plasma renin. Treatment with the angiotensin II receptor AT1 antagonist candesartan or the angiotensin II-converting enzyme inhibitor ramipril reduced the blood pressure of the Cx40-/- mice to the same levels seen in wild-type (WT) mice. The elevated blood pressure of the knockout mice was not affected by clipping one renal artery (2K1C, renin-dependent model of hypertension) or after a high salt diet. Under these conditions, however, Cx40-/- mice showed an altered production and release of renin. The renin mRNA ratio between the clipped and the non-clipped kidney was lower in the knockout than in the WT 2K1C mice. This indicates that the response to a change in blood pressure was altered. The RSC of the Cx40-/- mice did not have a compensatory increase in the levels of either Cx43 or Cx37. Our data show that renin secretion is dependent on Cx40 and suggest the Cx40-/- mice may be a genetic model of renin-dependent hypertension.

Dilated cardiomyopathy and impaired cardiac hypertrophic response to angiotensin II in mice lacking FGF-2.

FGF-2 has been implicated in the cardiac response to hypertrophic stimuli. Angiotensin II (Ang II) contributes to maintain elevated blood pressure in hypertensive individuals and exerts direct trophic effects on cardiac cells. However, the role of FGF-2 in Ang II-induced cardiac hypertrophy has not been established. Therefore, mice deficient in FGF-2 expression were studied using a model of Ang II-dependent hypertension and cardiac hypertrophy. Echocardiographic measurements show the presence of dilated cardiomyopathy in normotensive mice lacking FGF-2. Moreover, hypertensive mice without FGF-2 developed no compensatory cardiac hypertrophy. In wild-type mice, hypertrophy was associated with a stimulation of the c-Jun N-terminal kinase, the extracellular signal regulated kinase, and the p38 kinase pathways. In contrast, mitogen-activated protein kinase (MAPK) activation was markedly attenuated in FGF-2-deficient mice. In vitro, FGF-2 of fibroblast origin was demonstrated to be essential in the paracrine stimulation of MAPK activation in cardiomyocytes. Indeed, fibroblasts lacking FGF-2 expression have a defective capacity for releasing growth factors to induce hypertrophic responses in cardiomyocytes. Therefore, these results identify the cardiac fibroblast population as a primary integrator of hypertrophic stimuli in the heart, and suggest that FGF-2 is a crucial mediator of cardiac hypertrophy via autocrine/paracrine actions on cardiac cells.

Interaction between sodium intake, angiotensin II, and blood pressure as a cause of cardiac hypertrophy.

Cardiac hypertrophy is common in hypertension but its development is influenced by angiotensin II, sodium intake aldosterone, and the time of day blood pressure (BP) is elevated. This study examined and compared cardiac hypertrophy in the 2 kidney-1 clip (2K-1C) and 1 kidney-clip (1K-1C) Goldblatt models of hypertension. Blood pressure was measured by telemetry in a selected group of rats. Rats were placed on a high (4%) or reduced (0.2%) salt intake and were given captopril (75 mg/kg per day) or losartan (10 mg/kg per day). Appropriate sham-operated and untreated controls were used. Cardiac hypertrophy was greater in the IK-1C than in the 2K-1C model. Day and sleep BP were also higher. In the 2K-1C model BP was lower on the reduced salt intake and BP decreased with captopril in both reduced and high salt groups. Cardiac weight and index decreased significantly only in the reduced salt and captopril group and was less than the size before treatment. In the 1K-1C model captopril caused all BP measures to decrease in the reduced salt group but had no significant effect in the high salt group. Cardiac weight and index were reduced only in the reduced salt + captopril group and cardiac weight was less than the pretreatment control. Losartan had a similar effect in the lK-1C model to that achieved with captopril. The responses achieved correlated in part with renin status and dependency level. There is no prime determinant of cardiac hypertrophy. Blood pressure, sodium intake, and hormonal status are all important. It is postulated that the common pathway may be alterations in cell composition that signal the nucleus to increase cell growth.

Angiotensin II-induced cardiac hypertrophy is associated with different mitogen-activated protein kinase activation in normotensive and hypertensive mice.

OBJECTIVE: In addition to its haemodynamic effects, angiotensin II (AngII) is thought to contribute to the development of cardiac hypertrophy via its growth factor properties. The activation of mitogen-activated protein kinases (MAPK) is crucial for stimulating cardiac growth. Therefore, the present study aimed to determine whether the trophic effects of AngII and the AngII-induced haemodynamic load were associated with specific cardiac MAPK pathways during the development of hypertrophy. Methods The activation of the extracellular-signal-regulated kinase (ERK), the c-jun N-terminal kinase (JNK) and the p38 kinase was followed in the heart of normotensive and hypertensive transgenic mice with AngII-mediated cardiac hypertrophy. Secondly, we used physiological models of AngII-dependent and AngII-independent renovascular hypertension to study the activation of cardiac MAPK pathways during the development of hypertrophy. RESULTS: In normotensive transgenic animals with AngII-induced cardiac hypertrophy, p38 activation is associated with the development of hypertrophy while ERK and JNK are modestly stimulated. In hypertensive transgenic mice, further activation of ERK and JNK is observed. Moreover, in the AngII-independent model of renovascular hypertension and cardiac hypertrophy, p38 is not activated while ERK and JNK are strongly stimulated. In contrast, in the AngII-dependent model, all three kinases are stimulated. CONCLUSIONS: These data suggest that p38 activation is preferentially associated with the direct effects of AngII on cardiac cells, whereas stimulation of ERK and JNK occurs in association with AngII-induced mechanical stress.

Cardiac hypertrophy depends upon sleep blood pressure: a study in rats.

OBJECTIVE: The objective of this study was to determine whether cardiac hypertrophy in hypertensive rats could be reduced and normalized by intermittent reduction of blood pressure, and to determine whether left ventricular hypertrophy was related to 24 h workload or peak blood pressure responses. METHODS: Hypertension was created by the application of a 0.20 mm clip to the left renal artery. Blood pressure response was monitored using a telemetry system (Data Science International). Blood pressure was reduced for varying periods of the day by giving different doses of captopril in the drinking water or by intra-peritoneal administration. Cardiac size was measured by weighing the ventricles and factoring by the body weight to obtain a cardiac index. RESULTS: Captopril 75 mg/kg per day and 25 mg/kg per day in the drinking water administered between 1800 and 2000 h lowered the 24 h blood pressure more than captopril 15 mg/kg per day or 5 mg/kg per day intra-peritoneally given at 0800 h. Captopril 75 mg/kg per day and captopril 15 mg/kg per day (intra-peritoneal) caused regression of cardiac hypertrophy whereas the other doses had no effect The best predictor of the cardiac hypertrophy response was the blood pressure between 0800 and 1200 h (i.e. the sleeping blood pressure). Twenty-four hour cardiac work did not correlate with the response. CONCLUSION: Cardiac hypertrophy can be reduced by intermittent treatment of elevated blood pressure. It is also caused by intermittent elevation of blood pressure. It appears that the crucial factor is when these alterations in blood pressure take place. An elevated blood pressure during the sleeping hours causes left ventricular hypertrophy, whereas a normal blood pressure during the sleeping hours allows reduction. It is suggested that acute wall stress is the signal to initiate the events that lead to cardiac hypertrophy but this only occurs if the hormonal milieu is appropriate.

Role of endothelin receptor subtypes in volume-stimulated ANF secretion.

The role of endothelin (ET) receptors was tested in volume-stimulated atrial natriuretic factor (ANF) secretion in conscious rats. Mean ANF responses to slow infusions (3 x 3.3 ml/8 min) were dose dependently reduced (P < 0.05) by bosentan (nonselective ET-receptor antagonist) from 64.1 +/- 18.1 (SE) pg/ml (control) to 52.6 +/- 16.1 (0.033 mg bosentan/rat), 16.1 +/- 7.6 (0. 33 mg/rat), and 11.6 +/- 6.5 pg/ml (3.3 mg/rat). The ET-A-receptor antagonist BQ-123 (1 mg/rat) had no effect relative to DMSO controls, whereas the putative ET-B antagonist IRL-1038 (0.1 mg/rat) abolished the response. In a second protocol, BQ-123 (>/=0.5 mg/rat) nonsignificantly reduced the peak ANF response (106.1 +/- 23.0 pg/ml) to 74.0 +/- 20.5 pg/ml for slow infusions (3.5 ml/8.5 min) but reduced the peak response (425.3 +/- 58.1 pg/ml) for fast infusions (6.6 ml/1 min) by 49.9% (P < 0.001) and for 340 pmoles ET-1 (328.8 +/- 69.5 pg/ml) by 83.5% (P < 0.0001). BQ-123 abolished the ET-1-induced increase in arterial pressure (21.8 +/- 5.2 mmHg at 1 min). Changes in central venous pressure were similar for DMSO and BQ-123 (slow: 0.91 and 1.14 mmHg; fast: 4.50 and 4.13 mmHg). The results suggest 1) ET-B receptors mainly mediate the ANF secretion to slow volume expansions of <1.6%/min; and 2) ET-A receptors mainly mediate the ANF response to acute volume overloads.

Unresponsiveness to cannabinoids and reduced addictive effects of opiates in CB1 receptor knockout mice.

The function of the central cannabinoid receptor (CB1) was investigated by invalidating its gene. Mutant mice did not respond to cannabinoid drugs, demonstrating the exclusive role of the CB1 receptor in mediating analgesia, reinforcement, hypothermia, hypolocomotion, and hypotension. The acute effects of opiates were unaffected, but the reinforcing properties of morphine and the severity of the withdrawal syndrome were strongly reduced. These observations suggest that the CB1 receptor is involved in the motivational properties of opiates and in the development of physical dependence and extend the concept of an interconnected role of CB1 and opiate receptors in the brain areas mediating addictive behavior.

Vasopressin dilates the rat carotid artery by stimulating V1 receptors.

The acute effects of various vasopressor agents on the diameter of the common carotid artery were studied in halothane-anesthetized normotensive rats. The animals were infused intravenously for 60 min with equipressor doses of angiotensin II (10 ng/min), the alpha1-stimulant methoxamine (5 microg/min), lysine vasopressin (5 mU/min), or vehicle. The arterial diameter was measured by using a high-resolution ultrasonic echo-tracking device. The three vasoconstrictors increased the carotid artery diameter, but this effect was significantly more pronounced with lysine vasopressin. Even a nonpressor dose of lysine vasopressin (1 mU/min) caused a significant increase in the arterial diameter. The lysine vasopressin-induced vasodilatation could be prevented by the administration of d(CH2)5Tyr(Me)AVP (10 microg, i.v.), a selective V1-vasopressinergic receptor antagonist. These data therefore suggest that a short-term increase in blood pressure induces in rats a distention of the carotid artery. The increase in arterial diameter seems to involve an active mechanism with lysine vasopressin caused by the stimulation of V1-vasopressinergic receptors.

Sodium, angiotensin II, blood pressure, and cardiac hypertrophy.

Blood pressure (BP) in rats was elevated intermittently by i.p. injections of angiotensin II (Ang II; 200 microg/kg), and the effect on cardiac index was determined. The BP response was assessed in selected rats by telemetry. Elevation of BP between 8:00 and 12:00 produced cardiac enlargement similar to that produced by continuous Ang II infusion, and the response correlated better with the acute BP elevation than with 24-hour cardiac work. A high-sodium diet also increased left-ventricular hypertrophy (LVH) without a major effect on BP. The addition of Ang II intensified this response. A low-sodium diet had no significant effect on BP or on cardiac size, but prevented the cardiac hypertrophy produced by Ang II without altering the BP response. These results suggest that acute BP elevation, probably working through increased wall tension, is a more potent stimulus for cardiac hypertrophy than 24-hour workload. The sodium intake of the rat plays an important role influencing the cardiac but not the BP response to Ang II. These results infer that it is important to avoid episodes of acute BP elevation.

Cardiovascular response, feeding behavior and locomotor activity in mice lacking the NPY Y1 receptor.

Neuropeptide Y (NPY) is a 36-amino-acid neurotransmitter which is widely distributed throughout the central and peripheral nervous system. NPY involvement has been suggested in various physiological responses including cardiovascular homeostasis and the hypothalamic control of food intake. At least six subtypes of NPY receptors have been described. Because of the lack of selective antagonists, the specific role of each receptor subtype has been difficult to establish. Here we describe mice deficient for the expression of the Y1 receptor subtype. Homozygous mutant mice demonstrate a complete absence of blood pressure response to NPY, whereas they retain normal response to other vasoconstrictors. Daily food intake, as well as NPY-stimulated feeding, are only slightly diminished, whereas fast-induced refeeding is markedly reduced. Adult mice lacking the NPY Y1 receptor are characterized by increased body fat with no change in protein content. The higher energetic efficiency of mutant mice might result, in part, from the lower metabolic rate measured during the active period, associated with reduced locomotor activity. These results demonstrate the importance of NPY Y1 receptors in NPY-mediated cardiovascular response and in the regulation of body weight through central control of energy expenditure. In addition, these data are also indicative of a role for the Y1 receptor in the control of food intake.

Blood pressure-independent cardiac hypertrophy induced by locally activated renin-angiotensin system.

Cardiac hypertrophy is frequent in chronic hypertension. The renin-angiotensin system, via its effector angiotensin II (Ang II), regulates blood pressure and participates in sustaining hypertension. In addition, a growing body of evidence indicates that Ang II acts also as a growth factor. However, it is still a matter of debate whether the trophic effect of Ang II can trigger cardiac hypertrophy in the absence of elevated blood pressure. To address this question, transgenic mice overexpressing the rat angiotensinogen gene, specifically in the heart, were generated to increase the local activity of the renin-angiotensin system and therefore Ang II production. These mice develop myocardial hypertrophy without signs of fibrosis independently from the presence of hypertension, demonstrating that local Ang II production is important in mediating the hypertrophic response in vivo.

Quantitation of megakaryocytopoiesis by computerized automatic in culture image analysis.

A computer-assisted automatic image procedure was karyocytopoiesis in culture. This analysis system was based on acetylcholinesterase staining, a specific staining for murine bone marrow megakaryocytes, and an image capturing instrument with a computer program. Two kinds of routine megakaryocyte culture methods were used, the plasma clot and the serum-free agar systems. A comparison between manual counting and the instrument was made. The image analysis software was able to distinguish between megakaryocytes (MK) at different stages of maturation. The results show that this analysis system can simultaneously detect not only the number of megakaryocytes and their colonies in each dish, but also the surface area of individual megakaryocytes. In addition, this analysis system functions automatically 24 hours a day and the results obtained are reproducible. Using this system, we have confirmed previous observations that thrombopoietin (TPO) and heparin stimulate both proliferation and maturation of megakaryocytes. In addition, we found that platelet factor 4 (PF-4) significantly reduced the number of megakaryocytes but not their cell surface area, whereas TGFbeta1 decreased both number and surface area of megakaryocytes, suggesting that PF4 and TGFbeta1 negatively regulate megakaryocytopoiesis by different mechanisms. We noticed that megakaryocytes grown under agar culture conditions regularly had an increased size in comparison with those grown in a plasma clot system, which may be an indication that the plasma clot culture media contains an inhibitor(s) of megakaryocyte maturation. Our data indicate that this image analysis system, in addition to its automatic and reproducible features, is more efficient and allows detection of more parameters than routine manual microscopic detection.

Decreased blood pressure response in mice deficient of the alpha1b-adrenergic receptor.

To investigate the functional role of different alpha1-adrenergic receptor (alpha1-AR) subtypes in vivo, we have applied a gene targeting approach to create a mouse model lacking the alpha1b-AR (alpha1b-/-). Reverse transcription-PCR and ligand binding studies were combined to elucidate the expression of the alpha1-AR subtypes in various tissues of alpha1b +/+ and -/- mice. Total alpha1-AR sites were decreased by 98% in liver, 74% in heart, and 42% in cerebral cortex of the alpha1b -/- as compared with +/+ mice. Because of the large decrease of alpha1-AR in the heart and the loss of the alpha1b-AR mRNA in the aorta of the alpha1b-/- mice, the in vivo blood pressure and in vitro aorta contractile responses to alpha1-agonists were investigated in alpha1b +/+ and -/- mice. Our findings provide strong evidence that the alpha1b-AR is a mediator of the blood pressure and the aorta contractile responses induced by alpha1 agonists. This was demonstrated by the finding that the mean arterial blood pressure response to phenylephrine was decreased by 45% in alpha1b -/- as compared with +/+ mice. In addition, phenylephrine-induced contractions of aortic rings also were decreased by 25% in alpha1b-/- mice. The alpha1b-AR knockout mouse model provides a potentially useful tool to elucidate the functional specificity of different alpha1-AR subtypes, to better understand the effects of adrenergic drugs, and to investigate the multiple mechanisms involved in the control of blood pressure.

The relationship of blood pressure to cardiac hypertrophy: experimental studies in rats.

Cardiac hypertrophy is an independent predictor of morbidity and mortality. The exact relationship between blood pressure, hormones and cardiac hypertrophy is unclear. This study was undertaken to determine if intermittent elevation of blood pressure could cause left ventricular hypertrophy. Blood pressure was elevated intermittently by intraperitoneal injections of angiotensin II, noradrenalin and methoxamine. Blood pressure was acutely elevated by 60 mmHg or more for periods lasting up to 1 hour on up to 4 occasions each day. Cardiac index was measured 2 and 4 weeks after the experiment started. The cardiac index was increased by all procedures. The results were complicated by a retardation of growth in some experimental groups, meaning that the cardiac weight did not increase though the index did. In a study looking at the interaction of sodium and angiotensin II high sodium intake caused left ventricular hypertrophy and injections of angiotensin II caused further left ventricular hypertrophy. This study indicated that acute intermittent elevation of blood pressure could cause left ventricular hypertrophy and suggests that wall stress rather than 24 hour workload is the important triggering mechanism.

Time course changes of the mechanical properties of the carotid artery in renal hypertensive rats.

Distensibility of the carotid artery is not altered 2 weeks after renal artery clipping despite adaptive vascular hypertrophy related to hypertension. The purpose of this study was to assess arterial wall behavior with hypertension persisting for a longer period. Male Wistar rats were examined 1, 5, 9, and 24 weeks after renal artery clipping (two-kidney, one clip renal hypertension; n = 40) or after sham operation (n = 39). Mean blood pressure increased significantly to 132 +/- 4, 143 +/- 4, 153 +/- 4, and 144 +/- 4 versus 98 +/- 2, 107 +/- 2, 115 +/- 3, and 108 +/- 3 mm Hg, respectively, in 1-, 5-, 9-, and 24-week hypertensive rats and age-matched controls. Cardiac and vascular hypertrophy increased in parallel and were correlated to mean blood pressure. Wall stress at mean blood pressure did not differ between the hypertensive and normotensive groups (3.79 +/- 0.24, 4.60 +/- 0.34, 4.49 +/- 0.27, and 4.14 +/- 0.28 versus 3.15 +/- 0.12, 4.14 +/- 0.25, 4.80 +/- 0.28, and 4.69 +/- 0.32 10(3) dyne/cm2, respectively, in 1-, 5-, 9-, and 24-week hypertensive rats and age-matched controls). Distensibility-pressure data from the two groups fell on a common curve for all study periods. The intrinsic properties of the wall constituents were similar in controls and hypertensive rats at 1 and 5 weeks. However, the arteries became stiffer in the 9- and 24-week hypertensive rats, as illustrated by a shift to higher levels of the incremental elastic modulus-stress curve. Wall stress remains constant at mean blood pressure as a result of the increase in wall tissue mass. With time, even though the distensibility-pressure curve is not shifted downward, the thickened wall becomes stiffer in the hypertensive rats, which may predispose them to accelerated alterations of the wall material.

Hypertension increases connexin43 in a tissue-specific manner.

BACKGROUND: Connexin43 (Cx43), a membrane protein involved in the control of cell-to-cell communication, is thought to play a role in the contractility of the vascular wall and in the electrical coupling of cardiac myocytes. The aim of this study was to investigate the effects of experimental hypertension on Cx43 expression in rat aorta and heart. METHODS AND RESULTS: Rats were made hypertensive after one renal artery was clipped (two kidney, one-clip renal model) or after the administration of deoxycorticosterone and salt (DOCA-salt model). After 4 weeks, all rats showed a similar increase in intra-arterial mean blood pressure and in the thickness of both the aortic wall and the heart. Northern blot analysis of aorta mRNA and immunolabeling for Cx43 showed that hypertensive rats expressed twice as much Cx43 in aorta as the control animals. In contrast, no difference in Cx43 mRNA or in the immunolabeled protein was observed in heart. CONCLUSIONS: The results show that rats exhibiting a similar degree of blood pressure elevation, as the result of different mechanisms, feature a comparable increase in Cx43 gene expression, which was observed in the aortic but not in the cardiac muscle. These data suggest that localized mechanical forces induced by hypertension are major tissue-specific regulators of Cx43 expression.

Hypertension differentially affects the expression of the gap junction protein connexin43 in cardiac myocytes and aortic smooth muscle cells.

Electrical and mechanical coupling of myocytes in heart and of smooth muscle cells in the aortic wall is thought to be mediated by intercellular channels aggregated at gap junctions. Connexin43 (Cx43) is one of the predominant membrane proteins forming junctional channels in the cardiovascular system. This study was undertaken to assess its expression during experimental hypertension. Rats were made hypertensive by clipping one renal artery (two-kidney, one-clip renal hypertension) or by administering deoxycorticosterone and salt (DOCA-salt hypertension). After four weeks, rats from both models showed a similar increase in intra-arterial mean blood pressure, as well as in the thickness of both aorta and heart walls. Northern blot analysis showed that, compared to controls, hypertensive rats expressed twice more Cx43 in aorta, but not in heart. These results suggest that localized mechanical forces induced by hypertension are major tissue-specific regulators of Cx43 expression.

Effects of neuropeptide Y on intrarenal hemodynamics, plasma renin activity and urinary sodium excretion in rats.

Neuropeptide Y (NPY) is a vasoconstrictor peptide possibly involved in the regulation of renal sodium handling and renin release. This investigation was undertaken to assess in conscious normotensive rats the acute effects of a non-pressor dose of NPY on renal plasma flow, glomerular filtration rate, sodium excretion and plasma renin activity. Experiments were also performed during concomitant beta-adrenoceptor stimulation with isoproterenol. NPY per se had no effect on the studied parameters. Renal plasma flow was increased by isoproterenol and was significantly higher when the beta-adrenoceptor stimulant was infused alone (13.4 +/- 2.1 ml/min, p < 0.05, mean +/- SEM) that when administered together with NPY (7.2 +/- 2.0 ml/min). This was also true for glomerular filtration rate (3.3 +/- 0.3 vs. 1.8 +/- 0.3 ml/min, p < 0.01) and plasma renin activity (6.3 +/- 1.7 vs. 2.1 +/- 0.4 ng Ang I/ml/h, p < 0.05). Our data however do not allow to deduce whether the inhibitory effect of NPY on isoproterenol-induced renin release is mediated by changes in intrarenal hemodynamics or a direct effect on juxtaglomerular cells.

Renin and angiotensin II receptor gene expression in kidneys of renal hypertensive rats.

The aim of this investigation was to examine the interrelation between renal mRNA levels of renin and angiotensin II receptor type 1 (AT1) in a renin-dependent form of experimental hypertension. Rats were studied 4 weeks after unilateral renal artery clipping. Mean blood pressure and plasma renin activity were significantly higher in the hypertensive rats (n = 10 206 +/- mm Hg and 72.4 +/- 20.9 ng/mL-1/h-1, respectively) than in sham-operated controls (n = 10, 136 +/- 3 mm Hg and 3.3 +/- 0.5 ng/mL-1/h, respectively). Northern blot analysis of polyA+ RNA obtained from the kidneys of renal hypertensive rats showed increased levels of renin mRNA in the clipped kidney, whereas a decrease was observed in the unclipped kidney. Plasma renin activity was directly correlated with renin mRNA expression of the poststenotic kidney (r = .94, P < .01). AT1 mRNA expression was lower in both kidneys of the hypertensive rats. This downregulation was specific for the AT1A subtype since the renal expression of the AT1B subtype remained normal in hypertensive rats. The downregulation of the renal AT1A receptor may be due to high circulating angiotensin II levels. This is supported by the significant inverse correlation (r = .71, P < .01) between plasma renin activity and AT1A mRNA expression measured in the clipped kidney of the hypertensive rats.