Prevention by blockade of angiotensin subtype1-receptors of the development of genetic hypertension but not its heritability

A proposed mechanism for the Berecek phenomenon with implications for cardiovascular reprogramming

Berecek et al reported in the 1990s that when spontaneously hypertensive rat (SHR) mating pairs were treated with captopril and the resulting pups were continued on the drug for 2 months followed by drug discontinuation, the pups did not develop full blown hypertension, and the cardiovascular structural changes associated with hypertension in SHR were mitigated. The offspring of the pups also displayed diminished hypertension and structural changes, suggesting that the drug therapy produced a heritable amelioration of the SHR phenotype. This observation is reviewed. The link between cellular renin angiotensin systems and epigenetic histone modification is explored, and a mechanism responsible for the observation is proposed. In any case, the observations of Berecek are sufficiently intriguing and biologically important to merit re-exploration and definitive explanation. Equally important is determining the role of renin angiotensin systems in epigenetic modification.

Epigenetics and a New Look on Metabolic Syndrome

The incidence of metabolic syndrome increases in the developed countries, therefore biomedical research is focused on the understanding of its etiology. The study of exact mechanisms is very complicated because both genetic and environmental factors contribute to this complex disease. The ability of environmental factors to promote phenotype changes by epigenetic DNA modifications (i.e. DNA methylation, histone modifications) was demonstrated to play an important role in the development and predisposition to particular symptoms of metabolic syndrome. There is no doubt that the early life, such as the fetal and perinatal periods, is critical for metabolic syndrome development and therefore critical for prevention of this disease. Moreover, these changes are visible not only in individuals exposed to environmental factors but also in the subsequent progeny for multiple generations and this phenomenon is called transgenerational inheritance. The knowledge of molecular mechanisms, by which early minor environmental stimuli modify the expression of genetic information, might be the desired key for the understanding of mechanisms leading to the change of phenotype in adulthood. This review provides a short overview of metabolic syndrome epigenetics.

Gluten exorphin B5 stimulates prolactin secretion through opioid receptors located outside the blood-brain barrier

Gluten exorphin B5 (GE-B5) is a food-derived opioid peptide identified in digests of wheat gluten. We have recently shown that GE-B5 stimulates prolactin (PRL) secretion in rats; this effect is abolished by preadministration of the opioid receptor antagonist naloxone. However, since the structure of naloxone allows it to cross the blood-brain barrier (BBB) and antagonize opioid effects centrally as well as peripherally, it could not established, on the basis of those data, if GE-B5-induced PRL release is exerted through sites located inside or outside the BBB. In this study, we sought to determine the site of action of GE-B5 on PRL secretion, by pretreating male rats with naloxone methobromide (NMB), an opioid antagonist that does not cross the BBB. Four groups of rats were given the following treatments: 1) intravenous vehicle; 2) intravenous GE-B5 (3 mg kg(-1) body weight); 3) intraperitoneal NMB (5 mg kg(-1) body weight), followed by vehicle; 4) NMB, followed by GE-B5. Blood samples for PRL were taken at intervals for 40 minutes after vehicle or GE-B5 administration. GE-B5 stimulated PRL secretion; the effect was statistically significant at time 20. NMB preadministration completely abolished PRL response. Our experiment indicates that GE-B5 stimulates PRL secretion through opioid receptors located outside the BBB. Since opioid peptides do not exert their effect on PRL secretion directly, but via a reduced dopaminergic tone, our data suggest that GE-B5 can modify brain neurotransmitter release without crossing the BBB.

AT2 receptors cross talk with AT1 receptors through a nitric oxide- and RhoA-dependent mechanism resulting in decreased phospholipase D activity

ANG II activation of phospholipase D (PLD) is required for ERK and NAD(P)H oxidase activation, both of which are involved in hypertension. Previous findings demonstrate that ANG II stimulates PLD activity through AT(1) receptors in a RhoA-dependent mechanism. Additionally, endogenous AT(2) receptors in preglomerular smooth muscle cells attenuate ANG II-mediated PLD activity. In the present study, we examined the signal transduction mechanisms used by endogenous AT(2) receptors to modulate ANG II-induced PLD activity through either PLA(2) generation of lysophosphatidylethanolamine or Galpha(i)-mediated generation of nitric oxide (NO) and interaction with RhoA. Blockade of AT(2) receptors, Galpha(i) and NO synthase, but not PLA(2), enhanced ANG II-mediated PLD activity in cells rich in, but not poor in, AT(2) receptors. Moreover, NO donors, a direct activator of guanylyl cyclase and a cGMP analog, but not lysophosphatidylethanolamine, inhibited ANG II-mediated PLD activity, whereas an inhibitor of guanylyl cyclase augmented ANG II-induced PLD activity. AT(2) receptor- and NO-mediated attenuation of ANG II-induced PLD activity was completely lost in cells transfected with S188A RhoA, which cannot be phosphorylated on serine 188. Therefore, our data indicate that AT(2) receptors activate Galpha(i), subsequently stimulating NO synthase and leading to increased soluble guanylyl cyclase activity, generation of cGMP, and activation of a protein kinase, resulting in phosphorylation of RhoA on serine 188. Furthermore, because AT(2) receptors inhibit AT(1) receptor signaling to PLD via modulating RhoA activity, AT(2) receptor signaling can potentially regulate multiple vasoconstrictive signaling systems through inactivating RhoA.

Role of EGFR transactivation in angiotensin II signaling to extracellular regulated kinase in preglomerular smooth muscle cells

Angiotensin (Ang) II promotes the phosphorylation of extracellular regulated kinase (ERK); however, the mechanisms leading to Ang II-induced ERK phosphorylation are debated. The currently accepted theory involves transactivation of epidermal growth factor receptor (EGFR). We have shown that generation of phosphatidic acid (PA) is required for the recruitment of Raf to membranes and the activation of ERK by multiple agonists, including Ang II. In the present report, we confirm that phospholipase D-dependent generation of PA is required for Ang II-mediated phosphorylation of ERK in Wistar-Kyoto and spontaneously hypertensive rat preglomerular smooth muscle cells (PGSMCs). However, EGF stimulation does not activate phospholipase D or generate PA. These observations indicate that EGF recruits Raf to membranes via a mechanism that does not involve PA, and thus, Ang II-mediated phosphorylation of ERK is partially independent of EGFR-mediated signaling cascades. We hypothesized that phosphoinositide-3-kinase (PI3K) can also act to recruit Raf to membranes; therefore, inhibition of PI3K should inhibit EGF signaling to ERK. Wortmannin, a PI3K inhibitor, inhibited EGF-mediated phosphorylation of ERK (IC50, approximately 14 nmol/L). To examine the role of the EGFR in Ang II-mediated phosphorylation of ERK we utilized 100 nmol/L wortmannin to inhibit EGFR signaling to ERK and T19N RhoA to block Ang II-mediated ERK phosphorylation. Wortmannin treatment inhibited EGF-mediated but not Ang II-mediated phosphorylation of ERK. Furthermore, T19N RhoA inhibited Ang II-mediated ERK phosphorylation, whereas T19N RhoA had significantly less effect on EGF-mediated ERK phosphorylation. We conclude that transactivation of the EGFR is not primarily responsible for Ang II-mediated activation of ERK in PGSMCs.

Renal phenotype of low kallikrein rats

BACKGROUND

Renal kallikrein has been linked with inheritance of arterial hypertension and with sensitivity to drug nephrotoxicity. Identification of a cause--effect relationship between low kallikrein and intermediate phenotypes has been hampered by the lack of adequate animal models.

METHODS

Kallikrein was measured in tissues obtained from rats inbred for low urinary kallikrein excretion (LKR) and wild-type controls. Blood pressure and indices of myocardial contractility were recorded via an intraventricular cannula connected to a transducer. The functional relevance of endogenous angiotensin II (Ang II) in LKR was explored by determining the effect of Ang II subtype 1 (AT(1)) receptor blockade on glomerular filtration rate, renal blood flow, and urinary sodium excretion. In addition, sensitivity to gentamycin-induced nephrotoxicity was evaluated.

RESULTS

Kallikrein activity was reduced by 60% in the kidney of LKR (P < 0.01), whereas it was increased in the heart (P < 0.05) and was unaltered in the pancreas, liver, and salivary glands. Heart rate and myocardial contractility were reduced, and the mean blood pressure (MBP) was increased in LKR as compared with controls (P < 0.05). LKR exhibited polydipsia, polyuria, glomerular hyperfiltration, and reduced fractional sodium excretion under basal conditions and impaired renal vasodilation in response to volume expansion. These functional alterations were significantly attenuated by AT(1) receptor blockade. Gentamycin reduced the glomerular filtration rate in LKR, but not in controls.

CONCLUSIONS

In LKR, unopposed activity of Ang II appears to be responsible for increased glomerular hydrostatic pressure and augmented tubular reabsorption. Balance between the kallikrein-kinin and renin-angiotensin systems is essential for normal renal function.

Angiotensin II signaling to phospholipase D in renal microvascular smooth muscle cells in SHR

Angiotensin II (Ang II)-induced phospholipase D (PLD) activity is greater in aortic smooth muscle from spontaneously hypertensive rats (SHR) versus normotensive Wistar-Kyoto rats (WKY). Whether and how this signaling pathway is altered in preglomerular microvascular smooth muscle cells (PGSMCs), a cell type that may participate in genetic hypertension, is unknown. The goals of the present study were to determine in SHR and WKY PGSMCs the following: (1) whether Ang II induces PLD activity; (2) whether the effect of Ang II on PLD activity is greater in SHR; (3) which PLD isoform is stimulated by Ang II; (4) what signaling pathway mediates Ang II-induced PLD stimulation; and (5) whether the signaling pathways mediating Ang II-induced PLD activity are different in SHR and WKY. The EC(50) for Ang II-induced PLD stimulation in SHR was 10-fold lower than the EC(50) in WKY, and both were inhibited by L-158,805, an AT(1) antagonist. Inhibitors of phosphoinositol-3-kinase and protein kinase C did not block Ang II-induced PLD activity in SHR and WKY PGSMCs. Catalytically-inactive constructs of PLD2 and RhoA, but not PLD1, ADP ribosylation factor 1 (ARF1), ARF6, or ADP ribosylation factor nucleotide exchange factor (ARNO) blocked Ang II-induced PLD activity in SHR and WKY PGSMCs. Brefeldin A completely blocked Ang II-induced PLD activity in SHR but only slightly reduced Ang II-induced PLD activity in WKY PGSMCs. Therefore, we conclude that in PGSMCs, the effect of Ang II on PLD activity is (1) greater in SHR; (2) mediated by AT(1) receptors signaling to PLD2; (3) transduced primarily by Rho proteins; and (4) inhibited in SHR by brefeldin A.

Persistent improvement of cardiovascular risk factors in spontaneously hypertensive rats following early short-term captopril treatment

This study was designed to determine whether an improvement in cardiovascular risk factors persists in spontaneously hypertensive rats (SHR) following withdrawal of angiotensin converting enzyme inhibitor (ACE-I) treatment. SHR were given deionized drinking water or captopril solution from four to sixteen weeks of age. At twelve weeks of age, rats from each group were instrumented with radiotelemetry devices for continuous monitoring of blood pressure. Mean arterial blood pressure was significantly lower in captopril-treated SHR during treatment (92+/-2 vs 147+/-1 mm Hg), and at twelve weeks after treatment withdrawal (131+/-2 vs 158+/-2 mm Hg). In addition, proteinuria, renal vascular resistance, plasma triglyceride levels, fasting glucose levels, post-prandial insulin levels, and heart weights were significantly reduced in the treated SHR compared to control SHR, at time-points between three to seven months after captopril withdrawal. Our findings indicate that short-term administration of an ACE-I during the developmental phase of hypertension in the SHR results in a long-term overall improvement of cardiovascular risk factors.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Young SHR express increased type 1 angiotensin II receptors in renal proximal tubule

A potential role for the renin-angiotensin system (RAS) in the development and/or maintenance of hypertension in the genetic model of rat hypertension, spontaneously hypertensive rats (SHR), has been suggested by studies indicating that treatment of immature animals with angiotensin-converting enzyme (ACE) inhibitors prevents subsequent development of hypertension. Because young SHR also demonstrate RAS-dependent increased sodium retention, we examined proximal tubule type 1 angiotensin II receptor (AT1R) mRNA expression in young (4 wk) or adult (14 wk) SHR compared with age-matched Wistar-Kyoto (WKY) rats. Proximal tubules were isolated by Percoll gradient centrifugation, and AT1R mRNA expression was measured by quantitative reverse transcription-polymerase chain reaction (RT-PCR). At 14 wk, when SHR had established hypertension [mean arterial blood pressure (MAP) of SHR vs. WKY: 145 +/- 6 vs. 85 +/- 5 mmHg, n = 14-15], there were no differences in proximal tubule AT1R mRNA levels [SHR vs. WKY: 79 +/- 14 vs. 72 +/- 14 counts/min (cpm) per cpm mutant AT1R per cpm beta-actin x 10(-6), n = 6; not significant (NS)]. In contrast, in 4 wk SHR, at a time of minimal elevations in blood pressure (MAP: 70 +/- 8 vs. 63 +/- 3), SHR proximal tubule AT1R mRNA levels were 263 +/- 30% that of WKY (143 +/- 18 vs. 60 +/- 11 cpm per cpm of mutant AT1R per cpm beta-actin x 10(-6), n = 8; P < 0.005). We have recently shown that chronic ACE inhibition decreases proximal tubule AT1R expression and have also shown that chronic L-3,4-dihydroxyphenylalamine (L-DOPA) administration inhibits AT1R expression in adult Sprague-Dawley proximal tubule and cultured proximal tubule, and this inhibition is mediated via Gs-coupled DA1 receptors. When 3-wk-old animals were given L-DOPA or captopril for 1 wk, MAP was not altered (70 +/- 8 vs. 60 +/- 4 or 61 +/- 5 mmHg), but proximal tubule AT1R mRNA was no longer significantly different between SHR and WKY (68 +/- 9 vs. 38 +/- 7 or 20 +/- 3 vs. 47 +/- 15 cpm per cpm of mutant AT1R per cpm beta-actin x 10(-6)), due to a significant decrease in proximal tubule AT1R expression in SHR (P < 0.005, compared with untreated SHR). Immunoreactive proximal tubule AT1R expression also was increased in 4 wk SHR and was reversed with captopril or L-DOPA treatment. Therefore, these results indicate that young, but not adult, SHR have increased expression of proximal tubule AT1R and that chronic L-DOPA or captopril treatment decreased the elevated AT1R expression to control levels. These results provide further support for an important role of the RAS in the development of hypertension in SHR.

Antisense inhibition of the brain kallikrein-kinin system

We used antisense oligodeoxynucleotide (ODN) strategy, based on interference of information flow from gene to protein, to determine the role of kininogen and bradykinin B2 receptor genes in the pathogenesis of genetic hypertension in rats. Mean blood pressure of 9-week-old spontaneously hypertensive rats (SHR) increased 4 hours after acute intracerebroventricular injection of synthetic 18-mer antisense ODNs targeting the translation initiation codon of kininogen mRNA (from 164 +/- 5 to 181 +/- 4 mm Hg, P < .01) or bradykinin B2 receptor mRNA (from 161 +/- 5 to 185 +/- 8 mm Hg, P < .01) and then returned to basal levels within 24 hours. Prolonged vasopressor effects were observed after repeated injections of antisense ODN targeting kininogen mRNA. Antisense ODNs to kininogen and B2 receptor mRNAs increased blood pressure of normotensive Wistar-Kyoto rats only slightly compared with SHR (from 116 +/- 3 to 124 +/- 1 and from 116 +/- 2 to 126 +/- 4 mm Hg, respectively; P < .05). Cardiovascular responses were confirmed by the use of antisense ODNs targeted to bind to different non-overlapping regions of kininogen or B2 receptor mRNA. Microinjection of antisense ODN to B2 receptor mRNA into the nucleus tractus solitarii increased mean blood pressure in SHR and prevented the vasodepressor effect induced by intranuclear microinjection of bradykinin. No significant change in mean blood pressure was induced in either strain by intravenous injection of antisense ODNs or by central injection of sense or scrambled ODNs. A strong fluorescent signal was detected at the level of the hippocampus, thalamus, hypothalamus periventricularis, midbrain, and cerebrum 1 hour after central injection of fluorescein isothiocyanate-conjugated antisense ODNs. Kininogen levels were significantly lower in the brain of rats given intracerebroventricular antisense kininogen ODN compared with controls. Our results indicate that the brain kallikrein-kinin system plays a role in the central regulation of blood pressure and suggest that this system may exert a protective action against further elevations of blood pressure levels in SHR.

Preweanling administration of terazosin decreases blood pressure of hypertensive rats in adulthood

To examine the contribution of the sympathetic nervous system to the development of hypertension, we injected spontaneously hypertensive rat (SHR) pups and normotensive Wistar-Kyoto rat (WKY) pups twice daily with saline (1.0 mL/kg SC) or terazosin (0.5 mg/kg SC), an alpha 1-adrenoceptor antagonist, from postnatal day 1 through 21. We determined the effectiveness and duration of action of this terazosin dose in pilot studies with adult SHR and WKY. Body weights of WKY pups were greater than body weights of SHR pups from postnatal day 1 through 21. In addition, body weights of terazosin-treated pups of both strains were comparable to body weights of saline-injected littermate controls. Indirectly measured systolic pressures of terazosin-treated SHR were reduced significantly at 60 and 90 days of age but not at 30 days of age compared with saline-injected littermate controls. Terazosin did not affect systolic pressures of WKY, measured at 30, 60, and 90 days of age. At 100 days of age, in chronically catheterized rats, mean arterial pressures of terazosin-treated SHR were reduced significantly compared with those of saline-injected littermate controls. In contrast, terazosin did not affect mean arterial pressures of WKY at 100 days of age. Finally, preweanling treatment with terazosin did not alter patterns of open field behavior of adult SHR or WKY. SHR were significantly more active and reared more frequently compared with WKY. These findings indicate that the time between birth and weaning at 21 days of age is critical for the full expression of the hypertensive phenotype in SHR. Chronic blockage of alpha 1-adrenoceptors during the preweanling period in SHR may reduce vascular hypertrophy, leading to long-term reductions in arterial pressure.