Influence of an enriched dietary sodium chloride regime during gestation and suckling and post-natally on the ontogeny of hypertension in the rat

Decreased sympathetic nerve activity in young hypertensive rats reared by normotensive mothers

AIMS

Early postnatal development can be significantly compromised by changes in factors provided by the mother, leading to increased vulnerability to hypertension in her offspring. TGR(mRen-2)27 (TGR) mothers, characterised by an overactivated renin-angiotensin system, exhibit altered ion composition in their breast milk. Therefore, we aimed to analyse the impact of cross-fostering on cardiovascular parameters in hypertensive TGR and normotensive Hannover Sprague-Dawley (HanSD) offspring.

MATERIALS AND METHODS

We measured cardiovascular parameters in 5- to 10-week-old male offspring by telemetry. The expression of proteins related to vascular function was assessed by western blotting in the aortic samples obtained from 6- to 12-week-old male offspring. Plasma renin activity and plasma angiotensin II (Ang II) levels were evaluated by radioimmunoassay (RIA).

KEY FINDINGS

The development of hypertension was in TGR accompanied by increased low-to-high frequency ratio (LF/HF; a marker of sympathovagal balance; 0.51 ± 0.16 in week 10). Furthermore, TGR exhibited increased aortic expression of mineralocorticoid receptor (MR; p < 0.05) and transforming growth factor beta type 1 (TGF-β1; p = 0.002) compared to HanSD offspring. Fostering significantly decreased sympathovagal balance (0.23 ± 0.10 in week 10) and, transiently, plasma Ang II levels and MR expression in TGR offspring reared by HanSD mothers.

SIGNIFICANCE

These findings highlight the importance of understanding the complex interplay between early life experiences, maternal factors, and later cardiovascular function. Understanding the mechanisms behind the observed effects may help to identify potential interventions to prevent the development of hypertension later in life.

Excessive maternal salt intake gives rise to vasopressin-dependent salt sensitivity of blood pressure in male offspring

Salt sensitivity of blood pressure (SSBP) is a trait carrying strong prognostic implications for various cardiovascular diseases. To test the hypothesis that excessive maternal salt intake causes SSBP in offspring through a mechanism dependent upon arginine-vasopressin (AVP), we performed a series of experiments using offspring of the rat dams salt-loaded during pregnancy and lactation with 1.5% saline drink ("experimental offspring") and those with normal perinatal salt exposure ("control offspring"). Salt challenge, given at 7-8 weeks of age with either 2% saline drink (3 days) or 8% NaCl-containing chow (4 weeks), had little or no effect on systolic blood pressure (SBP) in female offspring, whereas the salt challenge significantly raised SBP in male offspring, with the magnitude of increase being greater in experimental, than control, rats. Furthermore, the salt challenge not only raised plasma AVP level more and caused greater depressor responses to V1a and V2 AVP receptor antagonists to occur in experimental, than control, males, but it also made GABA excitatory in a significant proportion of magnocellular AVP neurons of experimental males by depolarizing GABA equilibrium potential. The effect of the maternal salt loading on the salt challenge-elicited SBP response in male offspring was precluded by maternal conivaptan treatment (non-selective AVP receptor antagonist) during the salt-loading period, whereas it was mimicked by neonatal AVP treatment. These results suggest that the excessive maternal salt intake brings about SSBP in male offspring, both the programming and the expression of which depend on increased AVP secretion that may partly result from excitatory GABAergic action.

Vascular Response to Graded Angiotensin II Infusion in Offspring Subjected to High-Salt Drinking Water during Pregnancy: The Effect of Blood Pressure, Heart Rate, Urine Output, Endothelial Permeability, and Gender

Introduction. Rennin-angiotensin system and salt diet play important roles in blood pressure control. We hypothesized that the high-salt intake during pregnancy influences the degree of angiotensin-dependent control of the blood pressure in adult offspring. Methods. Female Wistar rats in two groups (A and B) were subjected to drink tap and salt water, respectively, during pregnancy. The offspring were divided into four groups as male and female offspring from group A (groups 1 and 2) and from group B (groups 3 and 4). In anesthetized matured offspring mean arterial pressure (MAP), heart rate and urine output were measured in response to angiotensin II (AngII) (0-1000 ng/kg/min, iv) infusion. Results. An increase in MAP was detected in mothers with salt drinking water (P < 0.05). The body weight increased and kidney weight decreased significantly in male offspring from group 3 in comparison to group 1 (P < 0.05). MAP and urine volume in response to AngII infusion increased in group 3 (P < 0.05). These findings were not observed in female rats. Conclusion. Salt overloading during pregnancy had long-term effects on kidney weight and increased sex-dependent response to AngII infusion in offspring (adult) that may reveal the important role of diet during pregnancy in AngII receptors.

Excess maternal salt intake produces sex-specific hypertension in offspring: putative roles for kidney and gastrointestinal sodium handling

Hypertension is common and contributes, via cardiovascular disease, towards a large proportion of adult deaths in the Western World. High salt intake leads to high blood pressure, even when occurring prior to birth – a mechanism purported to reside in altered kidney development and later function. Using a combination of in vitro and in vivo approaches we tested whether increased maternal salt intake influences fetal kidney development to render the adult individual more susceptible to salt retention and hypertension. We found that salt-loaded pregnant rat dams were hypernatraemic at day 20 gestation (147±5 vs. 128±5 mmoles/L). Increased extracellular salt impeded murine kidney development in vitro, but had little effect in vivo. Kidneys of the adult offspring had few structural or functional abnormalities, but male and female offspring were hypernatraemic (166±4 vs. 149±2 mmoles/L), with a marked increase in plasma corticosterone (e.g. male offspring; 11.9 [9.3–14.8] vs. 2.8 [2.0–8.3] nmol/L median [IQR]). Furthermore, adult male, but not female, offspring had higher mean arterial blood pressure (effect size, +16 [9–21] mm Hg; mean [95% C.I.]. With no clear indication that the kidneys of salt-exposed offspring retained more sodium per se, we conducted a preliminary investigation of their gastrointestinal electrolyte handling and found increased expression of proximal colon solute carrier family 9 (sodium/hydrogen exchanger), member 3 (SLC9A3) together with altered faecal characteristics and electrolyte handling, relative to control offspring. On the basis of these data we suggest that excess salt exposure, via maternal diet, at a vulnerable period of brain and gut development in the rat neonate lays the foundation for sustained increases in blood pressure later in life. Hence, our evidence further supports the argument that excess dietary salt should be avoided per se, particularly in the range of foods consumed by physiologically immature young.

Renal Development and Blood Pressure in Offspring from Dams Submitted to High-Sodium Intake during Pregnancy and Lactation

Exposure to an adverse environment in utero appears to programme physiology and metabolism permanently, with long-term consequences for health of the fetus or offspring. It was observed that the offspring from dams submitted to high-sodium intake during pregnancy present disturbances in renal development and in blood pressure. These alterations were associated with lower plasma levels of angiotensin II (AII) and changes in renal AII receptor I (AT₁) and mitogen-activated protein kinase (MAPK) expressions during post natal kidney development. Clinical and experimental evidence show that the renin-angiotensin system (RAS) participates in renal development. Many effects of AII are mediated through MAPK pathways. Extracellular signal-regulated protein kinases (ERKs) play a pivotal role in cellular proliferation and differentiation. In conclusion, high-sodium intake during pregnancy and lactation can provoke disturbances in renal development in offspring leading to functional and structural alterations that persist in adult life. These changes can be related at least in part with the decrease in RAS activity considering that this system has an important role in renal development.

Renal structure and function evaluation of rats from dams that received increased sodium intake during pregnancy and lactation submitted or not to 5/6 nephrectomy

Adult rats submitted to perinatal salt overload presented renin-angiotensin system (RAS) functional disturbances. The RAS contributes to the renal development and renal damage in a 5/6 nephrectomy model. The aim of the present study was to analyze the renal structure and function of offspring from dams that received a high-salt intake during pregnancy and lactation. We also evaluated the influence of the prenatal high-salt intake on the evolution of 5/6 nephrectomy in adult rats. A total of 111 sixty-day-old rat pups from dams that received saline or water during pregnancy and lactation were submitted to 5/6 nephrectomy (nephrectomized) or to a sham operation (sham). The animals were killed 120 days after surgery, and the kidneys were removed for immunohistochemical and histological analysis. Systolic blood pressure (SBP), albuminuria, and glomerular filtration rate (GFR) were evaluated. Increased SBP, albuminuria, and decreased GFR were observed in the rats from dams submitted to high-sodium intake before surgery. However, there was no difference in these parameters between the groups after the 5/6 nephrectomy. The scores for tubulointerstitial lesions and glomerulosclerosis were higher in the rats from the sham saline group compared to the same age control rats, but there was no difference in the histological findings between the groups of nephrectomized rats. In conclusion, our data showed that the high-salt intake during pregnancy and lactation in rats leads to structural changes in the kidney of adult offspring. However, the progression of the renal lesions after 5/6 nephrectomy was similar in both groups.

Developmental windows and environment as important factors in the expression of genetic information: a cardiovascular physiologist's view

Genetic studies in humans and rodent models should help to identify altered genes important in the development of cardiovascular diseases, such as hypertension. Despite the considerable research effort, it is still difficult to identify all of the genes involved in altered blood pressure regulation thereby leading to essential hypertension. We should keep in mind that genetic hypertension and other cardiovascular diseases might develop as a consequence of early errors in well-co-ordinated systems regulating cardiovascular homoeostasis. If these early abnormalities in the ontogenetic cascade of expression of genetic information occur in critical periods of development (developmental windows), they can adversely modify subsequent development of the cardiovascular system. The consideration that hypertension and/or other cardiovascular diseases are late consequences of abnormal ontogeny of the cardiovascular system could explain why so many complex interactions among genes and environmental factors play such a significant role in the pathogenesis of these diseases. The detailed description and precise time resolution of major developmental events occurring during particular stages of ontogeny in healthy individuals (including advanced knowledge of gene expression) could facilitate the detection of abnormalities crucial for the development of cardiovascular alterations characteristic of the respective diseases. Transient gene switch-on or switch-off in specific developmental windows might be a useful approach for in vivo modelling of pathological processes. This should help to elucidate the mechanisms underlying cardiovascular diseases (including hypertension) and to develop strategies to prevent the development of such diseases.

Postnatal renal development of rats from mothers that received increased sodium intake

The newborn rat kidney is not fully developed until approximately 12 days after birth. Several lines of evidence suggest that angiotensin II (AII) participates in the postnatal development of the kidney. The aim of the present study was to analyze proliferating cell nuclear antigen (PCNA), fibronectin, alpha-smooth muscle-actin (alpha-SM-actin), and AII expression in renal cortex during development in rats born to mothers that received a normal (control) or increased (experimental) sodium intake during pregnancy. Ninety Wistar rats aged 1, 7, 15, and 30 days from the control and experimental groups were killed and the kidneys removed for histological and immunohistochemical studies. The results showed higher fibronectin, alpha-SM-actin, PCNA, and AII expression in the glomerular and tubulointerstitial areas of the renal cortex of 1- and 7-day-old animals, which decreased with renal development. The animals from the experimental group showed at 1 day of age a decrease in alpha-SM-actin, fibronectin, PCNA, and AII expression compared with controls of the same age ( P<0.05). In conclusion, our data show that increased sodium intake during pregnancy induces a reduction of alpha-SM-actin, fibronectin, and PCNA expression in the renal cortex tubulointerstitium and glomeruli of neonatal rats. These alterations may be related to the decrease of AII expression also observed in the renal cortex from these animals.

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.

Developmental sensitivity to high dietary sodium chloride in borderline hypertensive rats

The present study compared the postweaning blood pressures and body weights of borderline hypertensive rats exposed to a high (8%) sodium chloride maternal diet either from conception to weaning or only during the weaning period with borderline hypertensive rats consistently exposed to a normal (1%) sodium chloride maternal diet. Because the effects of early sodium chloride exposure may be most evident during a subsequent challenge, rats from each group were assigned to receive either an 8% sodium chloride or a 1% sodium chloride diet from 8 to 17 weeks of age. Exposure to an 8% sodium chloride diet from conception through weaning increased the adult blood pressure of borderline hypertensive rats compared with that of controls exposed to a 1% sodium chloride diet; exposure to an 8% sodium chloride diet only during weaning did not increase blood pressure. An 8% sodium chloride diet beginning at 8 weeks of age increased systolic blood pressure. The effects of perinatal and adult exposure to high dietary sodium chloride were additive. Behavioral observations and urinary electrolyte measures confirmed that pups exposed to an 8% sodium chloride diet during weaning ingested the high-sodium chloride diet. The blood pressure and heart rate response to autonomic nervous system ganglionic blockade were assessed at 17 weeks of age. Borderline hypertensive rats exposed to an 8% sodium chloride diet from conception through weaning showed an increased bradycardic response, but no difference in depressor response, to ganglionic blockade. These data suggest that the window of developmental sensitivity for modulation of blood pressure regulation by high dietary sodium chloride occurs during prenatal and early postnatal development.

Dietary restriction sensitizes the rat distal colon to aldosterone

1. The effects of starvation and undernutrition were assessed on rat colonic electrogenic Na+ absorption in fed controls, 72 h starved and acute undernourished (fed one-third of the control group's daily food intake for up to 9 days). The basal short-circuit currents (Isc) of three segments of rat colon (proximal, mid- and distal), stripped of their external muscle layers were monitored before and during addition of 0.1 mM-mucosal amiloride. The decrease in Isc was used as the measure of the electrogenic Na+ absorption. 2. Acute undernutrition and to a lesser extent 72 h starvation elevated the basal Isc only in the distal colon. The increase was inhibited by amiloride (0.1 mM, mucosal) indicating that it was due to electrogenic Na+ transport. 3. Allowing the 9 days acute undernourished rats to drink 0.9% NaCl failed to prevent the increase in the basal Isc in the distal colon but it was reduced by administration of spironolactone. 4. Adrenalectomy completely abolished the increased basal Isc in the distal colon induced by the 9 day undernutrition. However, the plasma aldosterone levels in the fed and 9 day undernutrition groups were not significantly different. 5. Injection of aldosterone into adrenalectomized rats drinking 0.9% NaCl and which were undernourished for 9 days induced a large increase in their distal colonic Isc which was inhibited by mucosal amiloride. Similar treatment of sham-operated rats on 0.9% NaCl or adrenalectomized control fed rats on 0.9% NaCl had no effect on the distal colonic Isc. 6. The results indicate that acute undernutrition for 9 days makes the distal colonic epithelium more sensitive to the prevailing plasma aldosterone level allowing an enhanced electrogenic Na+ absorptive capacity to be induced.

Evidence for two binding sites on membrane-bound angiotensin-converting enzymes (ACE) for exogenous inhibitors except in testis

It was possible to assess that ACE inhibitors bound to two different sites on the enzymes which consist in two homologous domains. Accordingly, it was also shown that testis ACE which consists in a single domain exhibits only one site for inhibitors. The common binding site of both enzymes contains Zn2+, bears the enzymatic activity, is less sensitive to added Zn2+ and Cl- and has a lower affinity for inhibitors than the first one, found only in duplicated domain enzymes and highly sensitive to the effect of ions.

Circulatory and ionoregulatory effects of atrial natriuretic peptide on rainbow trout (Salmo gairdneri Richardson) fed normal or high levels of dietary salt

Rainbow trout fed a normal salt diet (1.3% NaCl) or a high salt diet (12% NaCl for at least 6 months) were chronically cannulated in the dorsal aorta and received 10 μg kg(-1) ANP (1-28 human, UBC-Bioproducts) infused over a 10 min period. This had an insignificant influence on sodium balance, blood electrolytes and branchial sodium fluxes. In fish given a normal diet, the blood pressure and heart rate were uninfluenced by ANP, but pulse pressure was reduced by on average 60% and in some cases was not evident at all. Blood pressure in the fish fed a high salt diet was significantly higher than in the control fish; this together with heart rate and pulse pressure was not affected by ANP administration.

Dissociation of autonomic controls of heart rate in weaning-aged borderline hypertensive rats by perinatal NaCl

The ontogeny of functional sympathetic neural, adrenal medullary, and extra-adrenal components of adrenergic control of heart rate was investigated in borderline hypertensive rats exposed to either high or low sodium chloride (NaCl) from conception through weaning. Borderline hypertensive rats were produced by mating spontaneously hypertensive females with normotensive Wistar-Kyoto males. Females were maintained on diets containing either low (0.12% NaCl) or high (3% NaCl) dietary NaCl throughout pregnancy and lactation. At 28 days of age, baseline heart rates recorded from awake and unrestrained pups did not differ between low and high NaCl-exposed pups. Overall sympathetic tone, inferred from heart rate change after beta 1-adrenergic blockade with atenolol, did not differ between high and low NaCl-exposed pups. Early NaCl exposure did not alter the neural component of sympathetic control of heart rate as inferred from heart rate decrease after bretylium tosylate. Parasympathetic nervous system control, as reflected by tachycardic response to muscarinic receptor blockade with atropine methyl nitrate was also unchanged by early NaCl exposure. The adrenal catecholamine component of sympathetic control of heart rate was inferred from bradycardia following administration of the ganglion blocking agent, chlorisondamine, to pups pretreated with bretylium and atropine methyl nitrate. Pups exposed to low NaCl showed increased adrenal control of heart rate compared to high NaCl-exposed pups. The influence of residual catecholamines on heart rate was inferred from bradycardia following administration of the beta 1-adrenergic receptor blocking agent, atenolol, in pups pretreated with bretylium, atropine methyl nitrate, and chlorisondamine. Residual catecholamine influence was greater in rats exposed to high NaCl.(ABSTRACT TRUNCATED AT 250 WORDS)