Genetic deletion of angiotensin AT2 receptor leads to increased cell numbers in different brain structures of mice

Angiotensin II (Ang II) is a potent vasoactive peptide and displays growth factor-like properties. Different high-affinity Ang II receptor subtypes (AT1A, AT1B and AT2) have been cloned. They are expressed in various brain structures. Additionally, it has been assumed that Mas could interact directly or indirectly with the renin-angiotensin system. The AT1 receptor mediates pressor and mitogenic effects of Ang II, whereas physiological function and signaling mechanisms of the AT2 receptor remain poorly understood. Recent reports have shown that Ang II could mediate apoptosis through AT2 receptors. Since the AT1A, AT2 and Mas knockout mice provide new tools for uncovering potential actions of Ang II, the cell number in different brain structures of male adult wild-type mice and mice deficient for AT1A, AT2 or Mas was evaluated to get more insight into the role of Ang II in central nervous system development. In nearly all investigated brain structures (cortex, hippocampus, amygdala, thalamus), the cell number was significantly higher in AT2-deficient mice in comparison to wild-type mice. To the contrary, in AT1A-deficient mice the cell number was significantly less than in controls in the lateral geniculate and the medial amygdaloid nucleus. However, cell numbers were not changed in Mas-knockout mice compared to their wild-types. These results show the contrary effects of both angiotensin receptors on cell growth and represent the first demonstration of their action on neuronal cell development evidenced in the adult mouse brain.     

Effects of angiotensin II microinjected into medial amygdala on male sexual behavior in rats

Research was undertaken to study the role of central angiotensin in the modulation of male sexual behavior, testing the effect of angiotensin II (Ang II) injections into the medial amygdaloid nucleus (MeA). The sexual behavior of adult male Wistar rats was evaluated, 15 min after bilateral intra-amygdaloid microinjection (0.3 microl) of saline and 5 doses of Ang II: 10; 25; 50; 100, and 150 fmol. The effects of the Ang II receptor blockade were also studied. We tested the effect of coinjection of Ang II (50 fmol) with the AT1 antagonist, losartan (20 pmol) and the AT2 antagonist, CGP 42112 (1 pmol). Ang II inhibited sexual behavior and this inhibition was prevented by the coinjection of AT1 antagonist, losartan, or the AT2 antagonist, CGP 42112. Results show that Ang II has a powerful effect on male sexual behavior, which may be mediated by both AT1 and AT2 receptors.     

Excitatory action of angiotensins II and IV on hippocampal neuronal activity in urethane anesthetized rats

The renin–angiotensin system of the mammalian brain seems to interfere with the process of cognition and has been associated with the hippocampal function in relation to mechanisms of learning and memory. In our investigation, the effects of angiotensin II (Ang II) and angiotensin IV (Ang II) on neuronal activity have been studied in the hippocampus of adult rats anesthetized with urethane. Excitatory effects of both angiotensins predominated over inhibitory effects. Angiotensins also induced an enhancement of burst discharges. These angiotensin-induced effects were blocked by the specific angiotensin antagonists. Our findings showed that the different effects of Ang II and Ang IV in behavioral studies are not similarly reflected in a different change of the discharge rate and/or pattern of hippocampal neurons after microiontophoretic administration of both substances.     

Angiotensinergic and noradrenergic neurons in the rat and human heart

Although the physiological and pharmacological evidences suggest a role for angiotensin II (Ang II) with the mammalian heart, the source and precise location of Ang II are unknown. To visualize and quantitate Ang II in atria, ventricular walls and interventricular septum of the rat and human heart and to explore the feasibility of local Ang II production and function, we investigated by different methods the expression of proteins involved in the generation and function of Ang II. We found mRNA of angiotensinogen (Ang-N), of angiotensin converting enzyme, of the angiotensin type receptors AT(1A) and AT? (AT(1B) not detected) as well as of cathepsin D in any part of the hearts. No renin mRNA was traceable. Ang-N mRNA was visualized by in situ hybridization in atrial ganglial neurons. Ang II and dopamine-β-hydroxylase (DβH) were either colocalized inside the same neuronal cell or the neurons were specialized for Ang II or DβH. Within these neurons, the vesicular acetylcholine transporter (VAChT) was neither colocalized with Ang II nor DβH, but VAChT-staining was found with synapses en passant encircle these neuronal cells. The fibers containing Ang II exhibited with blood vessels and with cardiomyocytes supposedly angiotensinergic synapses en passant. In rat heart, right atrial median Ang II concentration appeared higher than septal and ventricular Ang II. The distinct colocalization of neuronal Ang II with DβH in the heart may indicate that Ang II participates together with norepinephrine in the regulation of cardiac functions: produced as a cardiac neurotransmitter Ang II may have inotropic, chronotropic or dromotropic effects in atria and ventricles and contributes to blood pressure regulation.     

Amygdalar catecholaminergic input to septal nuclei, relation to clomipramine actions on lateral septal neurons in the rat.

Antidepressants exert mixed actions on serotonergic and catecholaminergic systems. However, it is unknown whether a catecholaminergic blockade impinge on the actions of a tricyclic with serotonergic agonist properties (clomipramine) in limbic structures. The aim of the present study is to explore the effects of a catecholaminergic lesion in the basolateral amygdala on the firing rate of lateral septal, and hippocampal neurons in rats treated with clomipramine. An amygdaline lesion with 6-OHDA resembled the actions of clomipramine on the firing rate in lateral septal neurons, i.e. an increased rate of firing. However, the lesion blocked further effects of clomipramine on septal firing. Clomipramine decreased the firing rate in hippocampal neurons; however, neither the 6-OHDA lesion nor the added treatment with clomipramine modified the firing rate. It is concluded that an intact catecholaminergic amygdaloid input to lateral septal nuclei is necessary for clomipramine actions; however, the initial action of the tricyclic may involve a catecholaminergic blockade.     

Differential response of angiotensin peptides in the urine of hypertensive animals

Urinary excretion rates of angiotensin I (Ang I), angiotensin II (Ang II), and angiotensin-(1-7) [Ang-(1-7)] were determined in normotensive Sprague Dawley (SD), spontaneously hypertensive (SHR), and mRen-2 transgenic hypertensive animals before and following blockade of Ang II synthesis or activity for two weeks. This study was performed to determine for the first time whether inhibition of Ang II alters the excretion of angiotensin peptides in the urine. Rats were given either tap water or water medicated with lisinopril, losartan or both agents in combination. Blood pressure was monitored at regular intervals during the experiment by the tail-cuff method, and once again at the end of the study with a catheter implant into a carotid artery. Metabolic studies and 24 h urinary excretion variables and angiotensin peptides were determined before and during the procedures. While all three treatments normalized the blood pressure of hypertensive animals, therapy with either lisinopril or the combination of lisinopril and losartan had a greater antihypertensive effect in both SHR and [mRen-2]27 transgenic hypertensive rats. In the urine, the concentration of the angiotensins (normalized by 24-h creatinine excretion) was several-fold higher in the untreated hypertensive animals than in normotensive SD rats. In SD rats, lisinopril or lisinopril and losartan produced a sustained rise in urinary levels of Ang-(1-7) without changes in the excretion of Ang I and Ang II. In contrast, Ang I and Ang-(1-7) were significantly elevated in SHR medicated with lisinopril alone or in combination with losartan. Only losartan, however, augmented urinary levels of Ang II in the SHR. The antihypertensive effects of the three separate regimens had no effect on the urinary excretion of angiotensin peptides in [mRen-2]27 transgenic hypertensive rats. These data show that Ang I and Ang-(1-7) are excreted in large amounts in the urine of SD, SHR and [mRen-2]27 hypertensive rats. The unchanged Ang-(1-7) excretion in transgenic hypertensive (Tg+) rats after inhibition of the renin-angiotensin system agrees with the previous finding of a reduced plasma clearance of the peptide in this model of hypertension. The data suggest that this form of hypertension may be associated with increased activity of an endogenous converting enzyme inhibitor.     

Effects of angiotensin II and its selective antagonists on inferior olivary neurones.

On the basis of biochemical and autoradiographic studies it has been shown that the inferior olivary nucleus (ION) contains predominantly angiotensin II (Ang II) receptors of the subtype 2 (AT2). In the present investigation we used microiontophoretic techniques to test the effect of Ang II on the spontaneous firing rate of rat neurones in the ION in vivo. Ang II excited the majority of histologically identified ION neurones. Furthermore, the antagonism of this angiotensin-induced excitation by selective angiotensin receptor blockers of subtype 1 and 2 (AT1 and AT2) was examined. The excitation could be blocked by low doses of the AT2-antagonists PD 123177 and CGP 42112A, whereas the AT1-antagonist DuP 753 was ineffective even at high doses. On a few occasions, however, ejection of the AT1-antagonist resulted in a potentiation of angiotensin-induced excitation. The results suggest that Ang II has an excitatory effect on a considerable number of ION neurones and that this effect is mediated by AT2-receptors.     

Evidence for a role of AT(2) receptors at the CVLM in the cardiovascular changes induced by low-intensity physical activity in renovascular hypertensive rats

In the present study, we evaluated the involvement of the rennin-angiotensin system (RAS) in the control of the blood pressure (BP), baroreceptor-mediated bradycardia and the reactivity of caudal ventrolateral medulla (CVLM) neurons to Ang II and to AT(2) receptor antagonist in sedentary or trained renovascular hypertensive rats. Physical activity did not significantly change the baseline mean arterial pressure (MAP), heart rate (HR) or the sensitivity of the baroreflex bradycardia in normotensive Sham rats. However, in 2K1C hypertensive rats, physical activity induced a significant fall in baseline MAP and HR and produced an improvement of the baroreflex function (bradycardic component). The microinjections of Ang II into the CVLM produced similar decreases in MAP in all groups, Sham and 2K1C, sedentary and trained rats. The hypotensive effect of Ang II at the CVLM was blocked by previous microinjection of the AT(2) receptors antagonist, PD123319, in all groups of rats. Unexpectedly, microinjection of PD123319 at the CVLM produced a depressor effect in 2K1C sedentary that was attenuated in 2K1C trained rats. No significant changes in MAP were observed after PD123319 in Sham rats, sedentary or trained. These data showed that low-intensity physical activity is effective in lowering blood pressure and restoring the sensitivity of the baroreflex bradycardia, however these cardiovascular effects are not accompanied by changes in the responsiveness to Ang II at CVLM in normotensive or hypertensive, 2K1C rats. In addition, the blood pressure changes observed after AT(2) blockade in 2K1C rats suggest that hypertension may trigger an imbalance of AT(1)/AT(2) receptors at the CVLM that may be restored, at least in part, by low-intensity physical activity.     

Role of angiotensin II receptor subtypes in porcine basilar artery: functional, radioligand binding, and cell culture studies

We aimed to clarify responsiveness to angiotensin (Ang) II in the porcine basilar artery and the role of Ang II receptor subtypes by functional, radioligand binding, and cell culture studies. Ang II induced more potent contractions in the proximal part than in the distal part of isolated porcine basilar arteries. The contraction induced by Ang II was inhibited by the Ang II type 1 (AT1) receptor antagonist losartan, but the Ang II type 2 (AT2) receptor antagonist PD123319 enhanced it. After removal of the endothelium, the effect of losartan remained but the effect of PD123319 was abolished. The specific binding site of [3H]Ang II on the smooth muscle membrane was inhibited by losartan, but not by PD123319. Stimulation of angiotensin II increased nitric oxide (NO) production in cultured basilar arterial endothelial cells. This production was inhibited by PD123319 and the NO synthase inhibitor L-NG-nitroarginine. These results suggest that the contraction induced by Ang II might be mediated via the activation of AT1 receptors on the basilar arterial smooth muscle cells and be modulated via the activation of AT2 receptors on the endothelial cells, followed by NO production.     

Effects of angiotensin II on the spontaneous activity of rostral ventrolateral medullary cardiovascular neurons and blood pressure in spontaneously hypertensive rats

The interactive role of rostral ventrolateral medulla (RVL) cardiovascular neurons and brain angiotensin II (Ang II) in regulating the arterial blood pressure was examined by recording simultaneously the spontaneous activity of these spinal projecting neurons and the arterial blood pressure in the pentobarbital-anesthetized spontaneously hypertensive rat (SHR) and its normotensive control, the Wistar Kyoto rat (WKY). It was found that Ang II elicited dose-dependent excitatory responses in a subpopulation of RVL cardiovascular neurons, followed by a subsequent increase in blood pressure. These effects of Ang II were significantly greater in SHR than in WKY. The effects were attenuated or abolished by co-administration of Ang II antagonist, [Sar¹, Ile⁸]-Ang II. Pre-administration of [Sar¹, Ile⁸]-Ang II to RVL using bilateral microinjection attenuated the blood pressure effects of intracerebroventricularly administered Ang II by as much as 70%. These results indicated that spinal projecting RVL cardiovascular neurons are important in mediating the pressor action of Ang II. The enhanced sensitivity and responsiveness of RVL cardiovascular neurons to Ang II may be pertinent to the genesis of hypertension in adult SHR.     

Angiotensin II type 1 receptor-modulated signaling pathways in neurons

Mammalian brain contains high densities of angiotensin II (Ang II) type 1 (AT1) receptors, localized mainly to specific nuclei within the hypothalamus and brainstem regions. Neuronal AT1 receptors within these areas mediate the stimulatory actions of central Ang II on blood pressure, water and sodium intake, and vasopressin secretion, effects that involve the modulation of brain noradrenergic pathways. This review focuses on the intracellular events that mediate the functional effects of Ang II in neurons, via AT1 receptors. The signaling pathways involved in short-term changes in neuronal activity, membrane ionic currents, norepinephrine (NE) release, and longer-term neuromodulatory actions of Ang II are discussed. It will be apparent from this discussion that the signaling pathways involved in these events are often distinct.     

Expression of angiotensin II type 1 (AT(1)) receptor in the rostral ventrolateral medulla in rats.

In the present study, the changes of amino acids release in the spinal cord after the application of angiotensin II (ANG II) in the rostral ventrolateral medulla (RVLM) and the distribution of ANG receptors on neurons of the RVLM were investigated. A microdialysis experiment showed that microinjection of angiotensin II into the RVLM significantly (P < 0.01) increased the release of aspartate and glutamate in the intermediolateral column of the spinal cord. Immunofluorescence technique combined with confocal microscopy demonstrated that most of the glutamatergic and GABAergic neurons in the RVLM of both Wistar and spontaneously hypertensive rats (SHR) were double labeled with ANG type 1 (AT1) receptor. Immunocytochemical studies demonstrated that the mean optic density of AT1 receptor of the cell surface as well as the whole cell was higher (P < 0.05) in SHR than that in Wistar rats, indicating that the higher expression of AT1 receptors in the RVLM may contribute to the higher responsiveness of SHR to ANG II stimulation. Immunogold staining and electronmicroscopic study demonstrated that AT1 receptor in the RVLM was distributed on the rough endoplasmic reticulum, cell membrane, and nerve processes. The results suggest that effects evoked by ANG II in the RVLM are closely related to glutamatergic and GABAergic pathways. These results indirectly support the hypothesis that ANG II in the RVLM may activate vasomotor sympathetic glutamatergic neurons, leading to an increase in sympathetic nerve activity and arterial blood pressure.     

Adrenal, kidney, and heart angiotensins in female murine ren-2 transfected hypertensive rats

We analyzed by high-performance liquid chromatography and radioimmunoassay angiotensin I (Ang I), Ang II, Ang-(1–7), and metabolites in the adrenal, kidney and heart of normotensive female Sprague–Dawley (SD) and transgenic hypertensive [TGR(mRen-2)27] rats carrying the murine Ren-2^d renin gene. The monogenetic model of hypertensive rats had significant increases in adrenal Ang II; whereas in the kidney Ang II was unchanged, but Ang I and Ang-(1–7) were significantly lower. Cardiac Ang I, Ang II, and Ang-(2–10) were significantly reduced in transgenic rats, while Ang-(2–7) was increased. In SD and transgenic rats kidney and adrenal angiotensins increased primarily during estrus or proestrus. In female transgenic rats the increased adrenal Ang II and the sustained renal Ang II may contribute to the established phase of hypertension.     

Adrenomedullin inhibits angiotensin AT1A receptor expression and function in cardiac fibroblasts

Adrenomedullin (AM) is a multifunctional peptide hormone with wide-ranging actions related to cardiovascular homeostasis. AM receptors are highly expressed in the heart and AM has antihypertrophic and antiproliferative effects on cardiac myocytes and fibroblasts, respectively. We have investigated the interaction between AM and angiotensin II (Ang II) signalling in neonatal cardiac fibroblast cultures to determine whether the antagonistic effects of AM are mediated via the modulation of Ang II receptors. Cardiac fibroblasts exclusively expressed the Ang II type 1 receptor (AT(1)R) and binding to this site was downregulated by 35% following an 18-h incubation with 100 nM AM. Levels of AT(1A)R mRNA were dose-dependently lowered by AM, with a maximal 40-50% inhibition by 6 h. The decreases in both AT(1)R binding and AT(1A)R mRNA levels were mimicked by 8-Br-cAMP or forskolin, suggesting that the effects of AM were mediated via an elevation of cAMP. In cardiac fibroblasts pretreated with AM, the Ang II induction of collagen biosynthesis was attenuated, although basal collagen synthesis was unaffected. These data suggest that AM mediates the heterologous downregulation of AT(1)R expression via a relatively rapid decrease in AT(1A)R mRNA pools. This interaction may represent a relevant pathophysiological mechanism for modulating Ang II responsiveness in the diseased heart.     

Vascular response to angiotensin II is exaggerated through an upregulation of AT1 receptor in AT2 knockout mice

Blood pressure is elevated and pressor response to angiotensin II (Ang II) is exaggerated in AT2 null mice. The purpose of the present study was to elucidate the mechanism for the increased responsiveness to Ang II in the mice. The contraction of aortic strips generated by Ang II was significantly greater in the AT2 gene-deleted mice than the control, which was completely abolished by AT1 antagonist losartan. The aortic content of AT1 receptor was significantly increased (P < 0.05, n = 5) in the AT2 null mice (212 +/- 58.2 fmol/mg protein) compared with the control (98.2 +/- 55.9 fmol/mg protein). While both AT1 and AT2 mRNAs were expressed in the aorta of the control mice, only AT1 mRNA was expressed in the AT2 knockout mice. The expression of AT1 mRNA in the AT2 knockout mice was significantly higher (1.5-fold, P < 0.05, n = 5) than that in the control. The present study clearly demonstrated that the increased vascular reactivity to Ang II in AT2 knockout mice is at least partly due to an increased vascular AT1 receptor expression and suggested that AT2 counteracts AT1-mediated vascular action of Ang II through downregulation of AT1 receptor by a crosstalk between these receptors by some as yet unknown mechanisms.     

Changes in cardiac structure and function in rats immunized by angiotensin type 1 receptor peptides

Angiotensin II (Ang II) is known to induce cardiomyocyte hypertrophy by activating the Ang II type 1 (AT1) receptor. Some studies have demonstrated that the autoantibodies against angiotensin AT1 receptor (AT1-AAs) cause functional effects, which is similar to those observed for the natural agonist Ang II. In this study, we investigated the effects of AT1-AAs on cardiomyocytes' structure and function. Male Wistar rats were immunized with synthetic peptides corresponding to the second extracellular loop of AT1 receptor and Freund's adjuvant. The titers of AT1-AAs in rat serum were detected by enzyme-linked immunosorbent assay every week. Hemodynamic analysis and heart weight (HW) indices were measured on the 4th and 8th months after initial immunization, respectively. Cultured neonatal rat cardiomyocytes were used to observe the hypertrophic effects of AT1-AAs. Results showed that systolic blood pressure and heart rate were significantly increased, the titers of AT1-AAs were also increased after 4 weeks of initial immunization. Compared with control group, the HW/body weight (BW) and left ventricular weight/BW of immunized rats were increased significantly and cardiac function was enhanced compensatively. The cultured neonatal rat cardiomyocytes respond to AT1-AAs stimulation with increased (3)H-leucine incorporation and cell surface area in a dose-dependent manner. These results suggest that the AT1-AAs have an agonist effect similar to Ang II in hypertrophy of cardiomyocytes in vivo and in vitro. AT1-AAs are involved in the pathogenesis of cardiovascular diseases and hypertension.     

Angiotensin II inhibits inward rectifier K+ channels in rabbit coronary arterial smooth muscle cells through protein kinase Calpha

We investigated the effects of the vasoconstrictor angiotensin (Ang) II on the whole cell inward rectifier K(+) (Kir) current enzymatically isolated from small-diameter (<100 microm) coronary arterial smooth muscle cells (CASMCs). Ang II inhibited the Kir current in a dose-dependent manner (half inhibition value: 154 nM). Pretreatment with phospholipase C inhibitor and protein kinase C (PKC) inhibitors prevented the Ang II-induced inhibition of the Kir current. The PKC activator reduced the Kir currents. The inhibitory effect of Ang II was reduced by intracellular and extracellular Ca(2+) free condition and by G?6976, which inhibits Ca(2+)-dependent PKC isoforms alpha and beta. However, the inhibitory effect of Ang II was unaffected by a peptide that selectively inhibits the translocation of the epsilon isoform of PKC. Western blot analysis confirmed that PKCalpha, and not PKCbeta, was expressed in small-diameter CASMCs. The Ang II type 1 (AT(1))-receptor antagonist CV-11974 prevented the Ang II-induced inhibition of the Kir current. From these results, we conclude that Ang II inhibits Kir channels through AT(1) receptors by the activation of PKCalpha.     

Estrogen prevents intestinal inflammation after trauma-hemorrhage via downregulation of angiotensin II and angiotensin II subtype I receptor

Although angiotensin II (Ang II) plays a key role in development of organ ischemia-reperfusion injury, it remains unclear whether it is involved in development of intestinal injury following trauma-hemorrhage (T-H). Studies have shown that 17beta-estradiol (E2) administration following T-H improves small intestinal blood flow; however, it is unclear whether Ang II plays a role in this E2-mediated salutary effect. Male Sprague-Dawley rats underwent laparotomy and hemorrhagic shock (removal of 60% total blood volume, fluid resuscitation after 90 min). At onset of resuscitation, rats were treated with vehicle, E2, or E2 and estrogen receptor antagonist ICI 182,780 (ICI). A separate group of rats was treated with Ang II subtype I receptor (AT1R) antagonist losartan. At 24 h after T-H, plasma Ang II, IL-6, TNF-alpha, intercellular adhesion molecule (ICAM)-1, cytokine-induced neutrophil chemoattractant (CINC)-1 and CINC-3 levels, myeloperoxidase (MPO) activity, and AT1R expression were determined. T-H significantly increased plasma and intestinal Ang II, IL-6, TNF-alpha levels, intestinal ICAM-1, CINC-1, CINC-3 levels, MPO activity, and AT1R protein compared with shams. E2 treatment following T-H attenuated increased intestinal MPO activity, Ang II level, and AT1R protein expression. ICI administration abolished the salutary effects of E2. In contrast, losartan administration attenuated increased MPO activity without affecting Ang II and AT1R levels. Thus Ang II plays a role in producing small intestine inflammation following T-H, and the salutary effects of E2 on intestinal inflammation are mediated in part by Ang II and AT1R downregulation.     

Selective down-regulation of AT2 receptors in uterine arteries from pregnant ewes given 24-h intravenous infusions of angiotensin II

Previously, we showed that uterine arteries from late gestation pregnant ewes infused intravenously with angiotensin II (Ang II) for 24 h, displayed heightened responsiveness to Ang II in vitro. Furthermore, we found that a small population of ewes with a "preeclampsia-like" disorder also displayed this. Therefore, we have investigated the density and affinity of Ang II receptor subtypes in the uterine arteries from these groups. Ang II receptor binding was measured using 125I [Sar1Ile8] Ang II. Proportions of AT1 and AT2 receptors were determined by inhibiting 125I [Sar1Ile8] Ang II with losartan (AT1 antagonist) or PD 123319 (AT2 antagonist). Uterine arteries from 24-h Ang II-infused ewes had a lower proportion of AT2 receptors (56.2+/-2.3%) than control (saline-infused) ewes (84.1+/-1.0%; P<0.05). The density of AT2 receptors was reduced (P<0.05) while the density of AT1 receptors was not different. Thus, 24-h infusions of Ang II selectively down-regulated AT2 receptors in the uterine artery, resulting in heightened Ang II reactivity. By contrast, the binding properties of Ang II receptor subtypes in uterine arteries from ewes with the "preeclampsia-like" disorder were not different from control ewes.     

Differential response of human cardiac fibroblasts to angiotensin I and angiotensin II

The vasoactive peptide angiotensin II (Ang II) has been implicated as a mediator of myocardial fibrosis. We carried out a comparative investigation of the effects of Ang II and its precursor Ang I on collagen metabolism and proliferation in cultured human cardiac fibroblasts. Cardiac fibroblasts responded to both Ang I and Ang II with concentration-dependent increases in collagen synthesis but no proliferation. The stimulatory effect of Ang II was abolished by the AT(1) receptor antagonist losartan but not the AT(2) receptor antagonist PD123319. The response to Ang I was not affected by either antagonist, nor by the angiotensin-converting enzyme (ACE) inhibitor captopril. In conclusion, Both Ang I and Ang II stimulate collagen synthesis of human cardiac fibroblasts, the effect of Ang II occurring via the AT(1) receptor whilst Ang I appears to exert a direct effect through non-Ang II-dependent mechanisms. These results suggest distinct roles for angiotensin peptides in the development of cardiac fibrosis.