Sex- and age-related differences in the chronic pressure-natriuresis relationship: role of the angiotensin type 2 receptor.

Sex hormones regulate the renin-angiotensin system. For example, estrogen enhances expression of the angiotensin type 2 receptor. We hypothesized that activation of the angiotensin type 2 receptor shifts the chronic pressure-natriuresis relationship leftward in females compared with males and that this effect is lost with age. Mean arterial pressure was measured by radiotelemetry in adult (4 mo old) and aged (14 mo old) wild-type and angiotensin type 2 receptor knockout male and female mice. Chronic pressure-natriuresis curves were constructed while mice were maintained on a normal-salt (0.26%) diet and following 6 days of high salt (5.0%) diet. Mean arterial pressure was lower in adult wild-type females than males (88 ± 1 and 97 ± 1 mmHg, respectively), a difference that was maintained with age, but was absent in adult knockout mice. In wild-type females, the chronic pressure-natriuresis relationship was shifted leftward compared with knockout females, an effect that was lost with age. In males, the chronic pressure-natriuresis relationship was not influenced by angiotensin type 2 receptor deficiency. Compared with age-matched females, the chronic pressure-natriuresis relationships in male mice were shifted rightward. Renal expression of the angiotensin type 2 receptor was fourfold greater in adult wild-type females than males. With age, the angiotensin type 2 receptor-to-angiotensin type 1 receptor balance was reduced in females. Conversely, in males, angiotensin receptor expression did not vary significantly with age. In conclusion, the angiotensin type 2 receptor modulates the chronic pressure-natriuresis relationship in an age- and sex-dependent manner.

Reduced sensitivity of the renal vasculature to angiotensin II in young rats: the role of the angiotensin type 2 receptor.

BACKGROUND: The angiotensin type-2 receptor (AT2R) opposes the vasoconstrictor actions of angiotensin II (AngII) mediated through the angiotensin type-1 receptor (AT1R). Renal AT2R levels are high during fetal life, but decrease significantly during postnatal maturation. To provide insight into the functional role of the AT2R in the kidney during postnatal development, we investigated the effects of AT2R antagonism on cardiovascular responses to AngII in young and adult male rats. METHODS: In anesthetized 3- and 6-wk-old male Sprague-Dawley rats, mean arterial pressure (MAP) and renal blood flow (RBF) were measured in response to AngII in the presence of vehicle treatment or AT2R blockade with PD123319. RESULTS: The pressor effect of AngII and associated reduction in RBF were significantly less in 3-wk- than 6-wk-old rats. AT2R blockade potentiated the reduction in RBF in response to AngII in 3-wk-old rats only. CONCLUSION: In young rats, the AT2R modulates the response to AngII, blunting renal vasoconstriction. This effect is attenuated with age in association with a developmental reduction in renal AT2R expression. These findings may have implications for the development of novel therapies that target the renin-angiotensin system for the improvement of renal function in term and, in particular, preterm infants.

Unmasking the potential of the angiotensin AT2 receptor as a therapeutic target in hypertension in men and women: what we know and what we still need to find out.

Major sex differences exist in the development and progression of hypertension and cardiovascular disease. Prior to menopause, women have lower arterial pressure and, furthermore, are protected from hypertension and cardiovascular disease relative to age-matched men. However, after menopause this cardiovascular protection in women is lost. These sex differences have been linked to sexual dimorphism in the physiological mechanisms that regulate arterial pressure, including the renin-angiotensin system (RAS), which can also impact on the male and female response to different therapeutic approaches. This suggests that antihypertensive regimens need to be tailored according to sex. Newly discovered components of the RAS have emerged in recent years, allowing us to look beyond the classical RAS for novel therapeutic targets for hypertension. In this context, it is now well established that the angiotensin AT2 receptor (AT2 R) elicits depressor and natriuretic effects and that these effects are greater in females due to enhanced AT2 R levels modulated by oestrogen. In light of knowledge that AT2 R expression is regulated by oestrogen and that the prevalence of hypertension and cardiovascular risk is greater in women after menopause, AT2 R agonist therapy may represent an innovative therapeutic approach to treat hypertension. Consequently, understanding how ageing and changes in the sex hormone balance influence the RAS is vital if we are to evaluate the potential of the AT2 R as a therapeutic target in women and also in men.

Placental Regulation of Inflammation and Hypoxia after TNF-α Infusion in Mice.

PROBLEM: Increased levels of inflammatory cytokines are demonstrated in the serum of women with pre-eclampsia. TNF-α infusion in animal models induces proteinuric hypertension similar to human pre-eclampsia. The effect of TNF-α on regulation of the immune and hypoxic pathways in the developing placenta and their relationship with experimental pre-eclampsia remains unexamined. METHOD OF STUDY: TNF-α was infused into pregnant mice, and the effects on maternal hypertension, proteinuria, circulating levels of sFlt-1 and corresponding placental changes in molecules responding to inflammation (TLR-3 and TLR-4) and hypoxia (HIF-1α) were examined. RESULTS: TNF-α infusion resulted in maternal hypertension and proteinuria. Molecular changes in the placenta involved upregulation of TLR-3, TLR-4 and HIF-1α. Serum levels of sFlt-1 were high in pregnant animals, but not further upregulated by TNF-α infusion. CONCLUSION: A role for maladaptive regulation of TLR and HIF-1α induced by an imbalance in inflammatory cytokines is implicated in the pathogenesis of pre-eclampsia.

Pressor responsiveness to angiotensin II in female mice is enhanced with age: role of the angiotensin type 2 receptor.

BACKGROUND: The pressor response to angiotensin II (AngII) is attenuated in adult females as compared to males via an angiotensin type 2 receptor (AT2R)-dependent pathway. We hypothesized that adult female mice are protected against AngII-induced hypertension via an enhanced AT2R-mediated pathway and that in reproductively senescent females this pathway is no longer operative. METHODS: Mean arterial pressure was measured via telemetry in 4-month-old (adult) and 16-month-old (aged) and aged ovariectomized (aged-OVX) wild-type and AT2R knockout (AT2R-KO) female mice during baseline and 14-day infusion of vehicle (saline) or AngII (600 ng/kg/min s.c.). Real-time reverse transcription polymerase chain reaction (RT-PCR) was used to determine renal gene expression of angiotensin receptors and angiotensin-converting enzyme 2 in response to 14-day treatment with vehicle or AngII. RESULTS: Basal mean arterial pressure was similar between the groups. The pressor response to AngII was augmented in adult AT2R-KO compared to adult wild-type mice (29 ± 3 mmHg versus 10 ± 4 mmHg, respectively, on day 14 as compared to basal mean arterial pressure, P = 0.002). In wild-type mice, pressor responsiveness to AngII was augmented with age, such that the pressor response to AngII was similar between aged AT2R-KO and wild-type female mice (31 ± 4 mmHg versus 34 ± 3 mmHg, respectively, on day 14, P = 0.9). There were no significant differences in pressor responsiveness to AngII between aged and aged-OVX mice. Vehicle-treated aged wild-type mice had a lower renal AT2R/AT1R balance as compared to adult counterparts. In response to AngII, the renal AT2R/AT1R balance in aged wild-type females was greater than that observed in vehicle-treated aged wild-type females and adult wild-type females, yet the protective effects of AT2R activation were not restored. CONCLUSIONS: The protective role of the AT2R depressor pathway is lost with age in female mice. Therefore, targeting deficits in AT2R expression and/or signaling may represent a novel anti-hypertensive approach in aged females.

Role of inflammation and the angiotensin type 2 receptor in the regulation of arterial pressure during pregnancy in mice.

During normal pregnancy the renin-angiotensin system is activated, yet pregnant women are resistant to the pressor effects of angiotensin II. Our aim was to determine the role of the angiotensin type 2 receptor (AT2R) in the regulation of arterial pressure, natriuresis, and immune cell infiltration during pregnancy. Mean arterial pressure was measured via telemetry, and flow cytometry was used to enumerate immune cell infiltration in 14-week-old wild-type and AT2R knockout mice during gestation. In wild-type mice, mean arterial pressure decreased during gestation, reaching a nadir at gestational day 9 (-6±2 mm Hg) and returned to near preconception levels during late gestation. In AT2R-deficient mice, the midgestational decrease in mean arterial pressure was absent. Furthermore, mean arterial pressure was significantly increased during late gestation compared with wild-type mice (≈10 mm Hg). As expected, circulating immune cell activation was suppressed during pregnancy. However, this response was absent in AT2R-deficient mice. While renal immune cell infiltration was similar between the genotypes, there was a significant T cell phenotypic switch toward a proinflammatory T-helper 1 phenotype in AT2R-deficient mice. These data indicate that the AT2R plays an important role in arterial pressure regulation and may modulate T cell activation and renal cytokine production during pregnancy. Therefore, deficits in AT2R expression may contribute to pregnancy-induced hypertension and thus represents a potential therapeutic target.

Angiotensin type 2 receptor stimulation increases renal function in female, but not male, spontaneously hypertensive rats.

Accumulating evidence suggests that the protective pathways of the renin-angiotensin system are enhanced in women, including the angiotensin type 2 receptor (AT2R), which mediates vasodilatory and natriuretic effects. To provide insight into the sex-specific ability of pharmacological AT2R stimulation to modulate renal function in hypertension, we examined the influence of the AT2R agonist, compound 21 (100-300 ng/kg per minute), on renal function in 18- to 19-week-old anesthetized male and female spontaneously hypertensive rats. AT2R stimulation significantly increased renal blood flow in female hypertensive rats (PTreatment<0.001), without influencing arterial pressure. For example, at 300 ng/kg per minute of compound 21, renal blood flow increased by 14.3±1.8% from baseline. Furthermore, at 300 ng/kg per minute of compound 21, a significant increase in urinary sodium excretion was observed in female hypertensive rats (+180±59% from baseline; P<0.05 versus vehicle-treated rats). This was seen in the absence of any major change in glomerular filtration rate, indicating that the natriuretic effects of AT2R stimulation were likely the result of altered renal tubular function. Conversely, we did not observe any significant effect of AT2R stimulation on renal hemodynamic or excretory function in male hypertensive rats. Finally, gene expression studies confirmed greater renal AT2R expression in female than in male hypertensive rats. Taken together, acute AT2R stimulation enhanced renal vasodilatation and sodium excretion without concomitant alterations in glomerular filtration rate in female hypertensive rats. Chronic studies of AT2R agonist therapy on renal function and arterial pressure in hypertensive states are now required to establish the suitability of AT2R as a therapeutic target for cardiovascular disease, particularly in women.