Angiotensinogen and angiotensin-converting enzyme gene copy number and angiotensin and bradykinin peptide levels in mice

OBJECTIVE

To test the hypothesis that changes in gene expression that may accompany angiotensinogen (AGT) and angiotensin-converting enzyme (ACE) gene polymorphism cause alteration in angiotensin and bradykinin peptide levels.

DESIGN

Mice with one or two genes for AGT and ACE allow assessment of the effects of modest alteration in AGT and ACE gene expression on angiotensin and bradykinin peptide levels.

METHODS

Angiotensin and bradykinin peptides were measured in the blood, kidney, heart, lung, adrenal, brain, and aorta of mice that were either wild-type (+/+), heterozygous (+/-) or null (-/-) for either the AGT or ACE gene.

RESULTS

Angiotensin I and angiotensin II were not detectable in blood or tissues of AGT -/- mice, which had increased bradykinin levels in kidney and lung. ACE -/- mice had markedly reduced angiotensin II levels and increased bradykinin levels in blood and tissues. However, despite reduced AGT and ACE gene expression, angiotensin and bradykinin peptide levels in AGT and ACE +/- mice were no different from the levels in wild-type mice.

CONCLUSION

Although the AGT and ACE genes are fundamental determinants of angiotensin and bradykinin peptide levels, compensatory mechanisms attenuate the effect of modest change in AGT and ACE gene expression on the levels of these peptides. Identification of these compensatory mechanisms may provide new candidate genes for investigation in humans.

Osmoregulatory fluid intake but not hypovolemic thirst is intact in mice lacking angiotensin

Water intakes in response to hypertonic, hypovolemic, and dehydrational stimuli were investigated in mice lacking angiotensin II as a result of deletion of the angiotensinogen gene (Agt−/− mice), and in C57BL6 wild-type (WT) mice. Baseline daily water intake in Agt−/− mice was approximately threefold that of WT mice because of a renal developmental disorder of the urinary concentrating mechanisms in Agt−/− mice. Intraperitoneal injection of hypertonic saline (0.4 and 0.8 mol/l NaCl) caused a similar dose-dependent increase in water intake in both Agt−/− and WT mice during the hour following injection. As well, Agt−/− mice drank appropriate volumes of water following water deprivation for 7 h. However, Agt−/− mice did not increase water or 0.3 mol/l NaCl intake in the 8 h following administration of a hypovolemic stimulus (30% polyethylene glycol sc), whereas WT mice increased intakes of both solutions during this time. Osmoregulatory regions of the brain [hypothalamic paraventricular and supraoptic nuclei, median preoptic nucleus, organum vasculosum of the lamina terminalis (OVLT), and subfornical organ] showed an increased number of neurons exhibiting Fos-immunoreactivity in response to intraperitoneal hypertonic NaCl in both Agt−/− mice and WT mice. Polyethylene glycol treatment increased Fos-immunoreactivity in the subfornical organ, OVLT, and supraoptic nuclei in WT mice but only increased Fos-immunoreactivity in the supraoptic nucleus in Agt−/− mice. These data show that brain angiotensin is not essential for the adequate functioning of neural pathways mediating osmoregulatory thirst. However, angiotensin II of either peripheral or central origin is probably necessary for thirst and salt appetite that results from hypovolemia.

Genetic relationship between placental and fetal weights and markers of the metabolic syndrome in rat recombinant inbred strains

Epidemiological studies have shown a clear link between fetal growth retardation and an increased propensity for later cardiovascular disease in adults. It has been hypothesized that such early fetal deprivation “programs” individuals toward a life-long metabolical “thrifty phenotype” that predisposes adults to such diseases. Here we test this hypothesis, and its possible genetic basis, in rat recombinant inbred (RI) strains that uniquely allow the longitudinal studies necessary for its testing. Placental and fetal weights were determined on day 20 of pregnancy in (at least) 6 litters from each of 25 available BXH/HXB RI strains and from their SHR and BN-Lx progenitors and were correlated with metabolic traits determined in adult rats from the same inbred lines. Quantitative trait loci (QTLs) associated with placental and fetal weights were identified by total genome scanning of RI strains using the Map Manager QTX program. Heritabilities of placental and fetal weights were 56% and 62%, respectively, and total genome scanning of RI strains revealed QTLs near the D1Rat266 marker on chromosome 1 and near the D15Rat101 marker on chromosome 15 that were significantly associated with fetal and placental weights respectively. Placental weights correlated with fetal weights ( r = 0.60, P = 0.001), while reduced fetal weights correlated with increased insulin concentrations during glucose tolerance test ( r = −0.71, P = 0.0001) and with increased serum triglycerides ( r = −0.54, P = 0.006) in adult rats. Our results suggest that predisposition toward a thrifty phenotype associated with decreased placental weight and restricted fetal growth is in part genetically determined.

TRANSITORY REDUCTION IN ANGIOTENSIN AT2 RECEPTOR EXPRESSION LEVELS IN POSTINFARCT REMODELLING IN RAT MYOCARDIUM

1. Myocardial infarction (MI) poses a significant risk for sudden cardiac death. The effectiveness of angiotensin-converting enzyme (ACE) inhibitors and AT1 receptor blockade in attenuating unfavourable post-MI outcomes indicates an important role for angiotensin (Ang) II signalling in the post-MI remodelling process. 2. AT1 and AT2 receptor expression is known to be altered during the early postinjury period and at the later failure stage in the infarcted heart. The aim of the present investigation was to characterize AngII receptor expression shifts in the intermediate, adaptive phases of post-MI hypertrophic remodelling. 3. The present study investigated relative cardiac AT1 and AT2 receptor expression levels using semiquantitative reverse transcription-polymerase chain reaction (GAPDH normalized) in rats at 4 and 20 weeks after ligation of the left anterior descending coronary artery. 4. Heart weight and normalized heart weight were significantly higher in the MI group than in the sham group 4 weeks post-MI, with significant hypertrophy of the left ventricle, left atrium and right ventricle in MI rats. At 20 weeks post-MI, left ventricular hypertrophy remained significant, whereas the mass of the other cardiac tissues was not different to that of sham controls. 5. AT2 receptor expression was markedly reduced in both the non-infarct and infarcted areas of the left ventricular wall in the MI group compared with the sham-operated group 4 weeks after surgery. Expression levels were reduced to 8 and 13% of sham values in the viable and scar tissue regions, respectively. By 20 weeks post-MI, there was no evidence of AT2 receptor expression suppression in the left ventricle. No significant relative changes in AT1 receptor mRNA levels were observed at either 4 or 20 weeks post-MI. 6. The present study demonstrates, for the first time, a selective downregulation of left ventricular AT2 receptor expression in the intermediate phase of post-MI ventricular remodelling in the rat. This downregulation may provide an enhanced AT1 receptor-mediated compensatory progrowth signal in the early adaptive post-MI growth phase. A more detailed understanding of the time-course of differential AT1 and AT2 receptor expression regulation post-MI may potentially identify an optimal window for targeted pharmacological intervention in the treatment of MI.

Relative contribution of the prenatal versus postnatal period on development of hypertension and growth rate of the spontaneously hypertensive rat

1. To determine the relative roles of the prenatal and postnatal (preweaning) environment on the development of blood pressure and growth rate in the spontaneously hypertensive rat (SHR) of the Okamoto strain, we used combined embryo transfer and cross-fostering techniques between SHR and normotensive Wistar-Kyoto (WKY) rats to produce offspring whose development was examined during the first 20 weeks of life. 2. We measured litter sizes, bodyweights and tail-cuff blood pressures in offspring at 4, 8, 12 and 20 weeks of age. We also recorded heart, kidney and adrenal weights at 20 weeks of age, when the study concluded. 3. We found that both the in utero and postnatal environments provided by the SHR mother could significantly affect WKY rat offspring growth rates, but blood pressure was unaffected in this strain. In SHR offspring, the SHR maternal in utero and suckling period both contributed to the rate of blood pressure development in the SHR, but not the final blood pressure of offspring at 20 weeks of age. This effect was greater for male than female offspring. Organ weights were largely unaffected by the perinatal environment in either strain. 4. We conclude that although the SHR maternal in utero and immediate postnatal environment both contribute to the rate of blood pressure development in the SHR, they do not appear to contribute to the final blood pressure of offspring at maturity. The SHR maternal environment also alters growth rate that may, in turn, underlie these effects on SHR blood pressure development, particularly in males.

Prenatal high salt diet increases blood pressure and salt retention in the spontaneously hypertensive rat

1. An examination was made of the effect of prenatal, high salt (5% w/w) and low salt (0.1% w/w) diet on the blood pressure and ability to excrete a salt load of mature spontaneously hypertensive rats (SHR) of the Okamoto strain maintained on normal salt (0.8% w/w) diet after weaning. 2. Prenatal high salt diet resulted in a significant exacerbation of the hypertension of 4 month old SHR when compared with animals given prenatal low salt diet. 3. Three month old SHR given prenatal, high salt diet exhibited a significantly reduced Na+ excretion following a single, oral salt load (150 mmol/l, 1% bodyweight) when compared with the low salt group. 4. Thus, prenatal, high salt diet may influence body fluid homeostasis in genetically susceptible individuals later in life.

Why does the spontaneously hypertensive rat have an exaggerated preference for sweet and salty solutions? An hypothesis

OBJECTIVE

We hypothesised that the elevated saline preference of the spontaneously hypertensive rat (SHR) might actually represent an 'appetite' for volume expanding osmolyte rather than an exaggerated hedonic drive. Further, we propose that this drive is due to an inappropriately perceived hypovolemia in the SHR. In order to test this hypothesis, we examined the preference of the spontaneously hypertensive rat for other isotonic solutions, namely, glucose and urea. We predicted that glucose, by exerting an osmotic force, would be over-ingested, while urea, being freely permeable across cell membranes, would not.

DESIGN

We used two-bottle preference tests, whereby individually housed SHR and Brown Norway (BN) control rats were given a choice of water and either isotonic saline, glucose or urea to drink over a period of 6-7 days. During this period, the total fluid intake (water volume plus solute solution volume) and saline preference (proportion of total fluid intake taken as solute solution) were calculated daily.

RESULTS

We found that while the spontaneously hypertensive rat did in fact exhibit an exaggerated preference for isotonic glucose solutions when compared with normotensive Brown Norway rats, this was not the case for urea solutions possessing the same particle density.

CONCLUSIONS

These findings support the notion that the spontaneously hypertensive rat might actually be seeking out solutions that provide a volume stimulus to the circulation. Thus, these findings, in combination with the other known disturbances in the spontaneously hypertensive rat (sympathetic overdrive, altered activity of the hypothalamo-pituitary-adrenal axis), leads us to propose that the spontaneously hypertensive rat might over-ingest 'tonic' osmolytes as a result of some disturbance in central volume sensing or regulation.

Influence of brain angiotensin on thermoregulation and hydromineral balance during pregnancy in rats

During mammalian pregnancy, body temperature decreases and there are changes in fluid and electrolyte balance. Angiotensin signaling mechanisms in the brain have been shown to influence thermoregulation and body fluid balance in the nonpregnant state. We hypothesized that brain angiotensin is also implicated in adjusting these physiological systems in the pregnant rat. We compared core temperature and fluid regulation in three groups of pregnant rats: untreated rats, rats receiving continuous infusion of an AT1 antagonist candesartan (5 μg·kg−1·day−1) into a lateral cerebral ventricle to block brain AT1 receptors, and rats receiving vehicle [artificial cerebrospinal fluid (aCSF)] vehicle. Untreated and aCSF-treated rats showed a decrease in colonic temperature (−0.5 and −0.8°C respectively) by day 20 of gestation. However, rats treated with candesartan had increased colonic temperature compared with baseline (+0.9°C), and their temperature was significantly higher on days 7 ( P < 0.05), 17 ( P < 0.05), and 20 ( P < 0.001) compared with the other groups (aCSF and untreated). Daily food and water intakes and body weight were not different between the three groups. Similarly, litter sizes and pup weights were equal in all groups. Finally, the expected decreases in plasma Na+ and osmolality during pregnancy were equivalent in all groups. This study suggests that brain angiotensin mediates the progressive decrease in body temperature that occurs during pregnancy. However, the changes in fluid balance associated with pregnancy are not dependent on brain angiotensin.

Salt preference of congenic strains derived from the spontaneously hypertensive rat

Previous studies using reciprocal crosses between the spontaneously hypertensive rat (SHR) and the normotensive Wistar-Kyoto (WKY) strain suggested a role for the Y chromosome in the SHR's exaggerated preference for saline solutions. We have reexamined the role of the Y chromosome in the salt preference of the SHR using a consomic strain derived from SHR and Brown Norway (BN-Lx) progenitors. We also studied congenic lines in which regions of BN-Lx chromosomes 8 and 20 had been introgressed into the SHR genome. Animals were given a choice of water and 0.9% saline to drink over a period of 7 days and their total fluid intake (TFI; water plus saline) and saline preference (proportion of the TFI taken as saline) calculated. SHR bearing the BN-Lx Y chromosome had a significantly reduced saline preference when compared to progenitor SHR. Evidence was also found for the existence of a region on chromosome 8, which influences fluid intake in the SHR. The causative genes involved in these effects however remain to be determined.

Lack of evidence for a role for either the in utero or suckling periods in the exaggerated salt preference of the spontaneously hypertensive rat

When offered as a choice with drinking water in two-bottle preference tests, the spontaneously hypertensive rats (SHR) of the Okamoto strain exhibit a marked preference for saline solutions. While this behaviour is thought to be in part genetically determined, the role of environmental influences-in particular, perinatal ones-are poorly understood. In this study, we have used combined embryo transfer and cross-fostering techniques between SHR and normotensive Wistar Kyoto (WKY) rats to delineate the relative roles of the prenatal and postnatal, suckling environment on the exaggerated saline preference of male SHR and WKY offspring at 20 weeks of age. We found, using two-bottle preference tests using water and 140 mmol/l sodium chloride solution, that neither the in utero period nor the postnatal, suckling period played a role in the development of the much larger total fluid intake (water plus saline) or saline preference (proportion of the total fluid intake taken as saline) of the SHR. We thus conclude that maternal and perinatal environmental factors do not play a major role in this behaviour and that the exaggerated saline preference of the SHR is probably largely genetically determined.

Sequence analysis of the fibroblast growth factor 2 gene from the spontaneously hypertensive and hypertrophic heart rats

We have previously reported a quantitative trait locus associated with pressure-independent cardiac hypertrophy in the spontaneously hypertensive rat (SHR) of the Okamoto strain. This locus (Lvm1; left ventricular mass locus 1) contains the gene Fgf2 that codes for the potent cardiac growth factor, Fibroblast Growth Factor 2 (FGF2). Given that FGF2 appears essential for the induction of certain forms of cardiac hypertrophy in the rat, we proposed this gene as a candidate for the cardiac enlargement seen in the SHR. Previous reports of elevated FGF2 mRNA levels in the SHR, led us to hypothesise that nucleotide sequence variations occurring in the coding regions or in putative transcriptional factor binding sites within the Fgf2 promoter might play a role in cardiac hypertrophy in this strain. Given that we have also recently derived from the SHR a rat strain that develops spontaneous cardiac hypertrophy in the absence of hypertension (the Hypertrophic Heart Rat; HHR), we also took the opportunity to examine the sequence of its Fgf2 promoter and coding region. However, extensive sequence analysis of the promoter and coding regions of the SHR and HHR Fgf2 genes failed to reveal any nucleotide variations between strains. Thus, we conclude that variations in the nucleotide sequence of the promoter and coding region of the SHR Fgf2 gene do not play a role in the cardiac hypertrophy of the SHR and HHR strains.