Timing of preweanling maternal effects on development of hypertension in SHR rats

The effects of experimental gestational hypertension on maternal blood pressure and fluid intake and pre-weanling hypothalamic neuronal activity

To examine the fetal programming effects of maternal hypertension, natriophilia and hyperreninemia [experimentally induced in rats by partial inter-renal aortic ligature (PAL) prior to mating] fos immunoreactivity was studied in 6-day-old offspring of PAL and control mothers. The purposes of the present set of experiments were twofold. The first was to characterize the effects of PAL on the mother's arterial blood pressure and intake of salt (1.8% NaCl solution) and water over the course of gestation. Second, was to study the pattern of neuronal activation in key brain areas of 6-day-old offspring treated with the dipsogen isoproterenol that were from PAL and control mothers. Beta-adrenergic receptor agonist-treated pups allowed the determination whether there were neuroanatomical correlates within the neural substrates controlling thirst and the enhanced water intake evidenced by the isoproterenol treated pups of PAL mothers. Hydromineral ingestive behavior along with blood pressure and heart rate of PAL (M-PAL) and control (M-sPAL) dams throughout gestation was studied. Higher salt and water intakes along with blood pressures and heart rates were found during gestation and lactation in the M-PAL group. Maternal PAL evoked significantly increased isoproterenol-elicited Fos staining in brain regions (e.g. subfornical organ, organum vasculosum of the lamina terminalis, supraoptic nucleus, hypothalamic paraventricular nucleus and median preoptic nucleus) of 6-day-old pups, which is the age of animals shown enhanced thirst responses in PAL offspring. These results indicate that PAL is compatible with pregnancy, producing a sustained increase in blood pressure and heart rate, along with increased water and salt intake. The present study demonstrates that the neural substrates involved in cardiovascular homeostasis and fluid balance in adult rats are responsive in six-day-old rats, and can be altered by fetal programming.

Cross-fostering: Elucidating the effects of gene×environment interactions on phenotypic development

Cross-fostering of litters from soon after birth until weaning is a valuable tool to study the ways in which gene×environment interactions program the development of neural, physiological and behavioral characteristics of mammalian species. In laboratory mice and rats, the primary focus of this review, cross-fostering of litters between mothers of different strains or treatment groups (intraspecific) or between mothers of different species (interspecific) has been conducted over the past 9 decades. Areas of particular interest have included maternal effects on emotionality, social preferences, responses to stressful stimulation, nutrition and growth, blood pressure regulation, and epigenetic effects on brain development and behavior. Results from these areas of research highlight the critical role of the postnatal maternal environment in programming the development of offspring phenotypic characteristics. In addition, experimental paradigms that have included cross-fostering have permitted investigators to tease apart prenatal versus postnatal effects of various treatments on offspring development and behavior.

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.

Programming blood pressure in adult SHR by shifting perinatal balance of NO and reactive oxygen species toward NO: the inverted Barker phenomenon

The “programming hypothesis” proposes that an adverse perinatal milieu leads to adaptation that translates into cardiovascular disease in adulthood. The balance between nitric oxide (NO) and reactive oxygen species (ROS) is disturbed in cardiovascular diseases, including hypertension. Conceivably, this balance is also disturbed in pregnancy, altering the fetal environment; however, effects of perinatal manipulation of NO and ROS on adult blood pressure (BP) are unknown. In spontaneously hypertensive rats (SHR), NO availability is decreased and ROS are increased compared with normotensive Wistar-Kyoto rats, and, despite the genetic predisposition, the perinatal environment can modulate adult BP. Our hypothesis is that a disturbed NO-ROS balance in the SHR dam persistently affects BP in her offspring. Dietary supplements, which support NO formation and scavenge ROS, administered during pregnancy and lactation resulted in persistently lower BP for up to 48 wk in SHR offspring. The NO donor molsidomine and the superoxide dismutase mimic tempol-induced comparable effects. Specific inhibition of inducible nitric oxide synthase (NOS) reduces BP in adult SHR, suggesting that inducible NOS is predominantly a source of ROS in SHR. Indeed, inducible NOS inhibition in SHR dams persistently reduced BP in adult offspring. Persistent reductions in BP were accompanied by prevention of proteinuria in aged SHR. We propose that in SHR the known increase in ANG II type 1 receptor density during development leads to superoxide production, which enhances inducible NOS activity. The relative shortage of substrate and cofactors leads to uncoupling of inducible NOS, resulting in superoxide production, activating transcription factors that subsequently again increase inducible NOS expression. This vicious circle probably is perpetuated into adult life.

Postnatal modulation of prenatally programmed hypertension by dietary Na and ACE inhibition

Adult hypertension in the rat can be programmed experimentally by changes in intrauterine environment. The offspring typically do not become hypertensive until 6 to 8 wk of age, and recent evidence suggests that renal dysfunction may participate in the pathogenesis. The present study was based on the hypothesis that the window for programming extends to the postnatal period in the rat. Adult hypertension was induced by maternal low-protein diet during the second half of gestation. After being weaned at 3 wk, the offspring were exposed to one of the following regimens for the subsequent 3 wk: 1) low-Na diet, 2) standard Na diet, 3) high-Na diet, and 4) standard Na diet with enalapril. The pups were followed for 10 wk after discontinuation of the treatments. The brief exposure to low-Na diet or enalapril totally prevented the development of hypertension and the effect lasted throughout the observation period. The development of hyperreninemia, present in the standard Na group at 16 wk of age, was abolished in the low-Na and enalapril groups. Conversely, 3-wk exposure to high-Na diet increased the severity of the later hypertension and did not prevent the hyperreninemia. The findings suggest that there is a period of susceptibility during which prenatally programmed hypertension can be modulated postnatally, possibly coinciding with a critical stage in renal maturation.

Effect of cross-fostering on blood pressure and renal function in the New Zealand genetically hypertensive rat

1. The severity of hypertension displayed by adult spontaneously hypertensive rats (SHR) and Dahl (SS/Jr) rats can be reduced by 20-30 mmHg if the hypertensive pup is cross-fostered to a normotensive mother within the first 2 weeks of birth. In the SHR, at least, this blood pressure-lowering effect arises through programming of the neonatal kidney to excrete sodium more effectively. Thus, cross-fostering may only be effective in lowering pressure in salt-sensitive hypertensive strains. Accordingly, the aim of the present study was to determine whether cross-fostering is effective in lowering adult blood pressure in the salt-resistant New Zealand genetically hypertensive (GH) rat. 2. Genetically hypertensive and control normotensive (N) rat pups were reared by either their natural mothers or a foster mother of the opposite strain (NX and GHX). Blood pressure was tracked from the age of 6-18 weeks, at which time renal function was assessed using standard clearance techniques in anaesthetized rats. Renal function was also assessed in a separate group of young rats at 5-6 weeks of age. 3. Cross-fostered GHX rats had lower blood pressure than GH rats, but this difference was only apparent until 9 weeks. The NX rats had higher blood pressures than N rats, but again pressure converged at 10 weeks. Basal renal function did not differ between GH and GHX rats or between N and NX rats at either age. However, young GH rats had lower renal blood flow, glomerular filtration rate, urine output and sodium excretion than N rats. 4. These data show that cross-fostering is effective in lowering blood pressure in GH rats, albeit transiently. The kidneys do not appear to play a role, because renal function did not differ under the current experimental conditions between GH and GHX rats. However, the kidney may play a greater role in the onset of hypertension in the GH rat than previously thought.

Nutrients, growth, and the development of programmed metabolic function

For each individual, the genetic endowment at conception sets the limits on the capacity or metabolic function. The extent to which this capacity is achieved or constrained is determined by the environmental experience. The consequences of these experiences tend to be cumulative throughout life and express themselves phenotypically as achieved growth and body composition, hormonal status and the metabolic capacity for one or other function. At any time later in life the response to an environmental challenge, such as stress, infection or excess body weight is determined by an interaction amongst these factors. When the metabolic capacity to cope is exceeded, the limitation in function is exposed and expresses itself as overt disease. During early life and development the embryo, fetus and infant are relatively plastic in terms of metabolic function. The effect of any adverse environmental exposure is likely to be more marked than at later ages and the influence is more likely to exert a fundamental effect on the development of metabolic capacity. This has been characterised as "programming" and has come to be known as "the Barker hypothesis" or "the fetal origins hypothesis". Barker has shown that the size and shape of the infant at birth has considerable statistical power to predict the risk of chronic disease in later life. These relationships are graded and operate across a range of birth weight, which would generally be considered to be normal, and are not simply a feature of the extreme of growth retardation. The first evidence showed strong relations between birth weight and heart disease, the risk factors for heart disease, diabetes and hypertension, and the intermediary markers for heart disease, blood cholesterol and fibrinogen. Strong associations have also been found for bone disease, allergic disease and some aspects of brain function. In experimental studies in animals it is possible to reproduce all of the metabolic features predicted from this hypothesis by moderating the consumption of food, or its pattern during pregnancy, and determining metabolic behaviour in the offspring. It has been shown that aspects of maternal diet exert an influence on fetal growth, especially the dietary intake of carbohydrate, protein and some micronutrients. However, these relationships are less strong than might have been predicted, especially when compared with the associations which can be drawn with maternal shape, size and metabolic capacity. Maternal height, weight and body composition relate to the metabolic capacity of the mother and her ability to provide an environment in which the delivery of nutrients to the fetus is optimal. Current evidence suggests that the size of the mothers determines her ability to support protein synthesis, and that maternal protein synthesis, especially visceral protein synthesis, is very closely related to fetal growth and development. It is not clear the extent to which the effect of an adverse environment in utero can be reversed by improved conditions postnatally, but some care is needed in exploring this area, as the evidence suggests that "catch-up" growth imposes its own metabolic stress and may in itself exert a harmful effect.

Perinatal development and adult blood pressure

A growing body of evidence supports the concept of fetal programming in cardiovascular disease in man, which asserts that an insult experienced in utero exerts a long-term influence on cardiovascular function, leading to disease in adulthood. However, this hypothesis is not universally accepted, hence animal models may be of value in determining potential physiological mechanisms which could explain how fetal undernutrition results in cardiovascular disease in later life. This review describes two major animal models of cardiovascular programming, the in utero protein-restricted rat and the cross-fostered spontaneously hypertensive rat. In the former model, moderate maternal protein restriction during pregnancy induces an increase in offspring blood pressure of 20-30 mmHg. This hypertensive effect is mediated, in part, by fetal exposure to excess maternal glucocorticoids as a result of a deficiency in placental 11-ss hydroxysteroid dehydrogenase type 2. Furthermore, nephrogenesis is impaired in this model which, coupled with increased activity of the renin-angiotensin system, could also contribute to the greater blood pressure displayed by these animals. The second model discussed is the cross-fostered spontaneously hypertensive rat. Spontaneously hypertensive rats develop severe hypertension without external intervention; however, their adult blood pressure may be lowered by 20-30 mmHg by cross-fostering pups to a normotensive dam within the first two weeks of lactation. The mechanisms responsible for this antihypertensive effect are less clear, but may also involve altered renal function and down-regulation of the renin-angiotensin system. These two models clearly show that adult blood pressure is influenced by exposure to one of a number of stimuli during critical stages of perinatal development.

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.

Intrauterine nutrition: its importance during critical periods for cardiovascular and endocrine development

Experimental investigations in animals have highlighted the role of early reduced calorie and protein nutrition on fetal cardiovascular development, and the occurrence of a transition from a low fetal arterial blood pressure in late gestation to a high arterial blood pressure postnatally. These observations may explain the correlation between health, including appropriate nutrition, in pregnant women and the outcome of their pregnancies. Emphasis has been placed on low birth weight infants who have an increased risk of developing cardiovascular diseases, including hypertension, coronary heart disease and stroke in adulthood. Vascular pathology in adults is not always associated with low birth weight and animal experiments indicate that substantial changes in cardiovascular and endocrine function can result from maternal or fetal undernutrition without impairing fetal growth. Experimental investigation on organogenesis shows the pivotal role of adequate protein availability as well as total caloric intake. Amino acid metabolism in the feto-maternal unit appears to have a key influence on the development of organs involved in chronic degenerative disease in the adult. Experimental investigation has also highlighted the role of carbohydrate metabolism and its effect on the fetus in this respect. Either restriction of protein intake or diabetes in pregnant rats has intergenerational effects at least on the endocrine pancreas and the brain. Further investigation is needed to clarify the mechanisms involved and lead to a new understanding of the importance of nutrition during pregnancy. This will provide an important approach to the primary prevention of diabetes and chronic degenerative diseases.

Effect of cross-fostering on neonatal sodium balance and adult blood pressure in the spontaneously hypertensive rat

1. The aim of the present study was to compare electrolyte handling in naturally reared neonatal spontaneously hypertensive rats (SHR) with those reared by a Wistar-Kyoto (WKY) rat foster mother (denoted SHRX), as cross-fostering SHR pups to a WKY rat dam lowers adult blood pressure in the SHR. 2. The electrolyte content of WKY rat and SHR dams' milk was determined and electrolyte intake and urinary excretion rates were calculated in both naturally reared and cross-fostered WKY rat and SHR pups. 3. The milk sodium concentration fell in both strains (WKY rat: 31.8 +/- 2.0 to 15.2 +/- 1.2 mmol/L; SHR 31.9 +/- 2.5 to 18.2 +/- 1.6 mmol/L; P < 0.001), as did potassium (P < 0.001), over lactation, but there were no differences between strains. Calcium and magnesium concentrations increased (P < 0.001), although SHR dam's milk contained less calcium (P < 0.001) than that of WKY rat dams during the third week of lactation. 4. Spontaneously hypertensive rat pups ingested less milk (P < 0.05) than WKY rat pups; therefore, their cumulative sodium intake over postnatal days 4-15 was significantly lower than that of WKY rat pups (WKY rat vs SHR: 84.4 +/- 3.6 vs 59.7 +/- 2.6 mumol/g bodyweight, respectively; P < 0.05) and fostered SHRX pups (77.7 +/- 7.0 mumol/g bodyweight; P < 0.05). Potassium and magnesium intakes were comparable between SHR, WKY rat and SHRX pups, but SHR pups ingested significantly less calcium than either WKY rat pups (136.1 +/- 6.4 vs 200.1 +/- 9.5 mumol/g bodyweight, respectively; P < 0.05) or SHRX pups (200.0 +/- 18.0 mumol/g bodyweight; P < 0.05). 5. These data show that the neonatal SHR experiences a period of sodium deficiency during the developmental stage when cross-fostering is effective in lowering blood pressure. This is consistent with the reported up-regulation of the renin-angiotensin system observed in SHR at this time and may have a long-term influence on blood pressure.

Postnatal oxytocin injections cause sustained weight gain and increased nociceptive thresholds in male and female rats

The aim of the present study was to investigate possible long-term effects of postnatally administered oxytocin on weight gain, gastrointestinal hormone levels, and nociceptive thresholds in rats. For this purpose, s.c. daily injections of oxytocin (1 mg/kg) or saline (NaCl, 0.9%) were given to male and female rat pups on d 10-14 after birth. The animals were killed at the age of 60 or 94 d. Treatment with oxytocin resulted in higher body weight in males, 60 d after birth, and in females from d 60 and throughout the rest of the experiment, compared with controls. The higher body weight was due to an increased weight gain in oxytocin-treated rats, compared with controls, which was most pronounced between 40 and 60 d after birth. Oxytocin-treated male rats had increased circulating levels of cholecystokinin, a tendency to increased plasma levels of insulin (p = 0.066), and relatively more adipose tissue in the thigh and interscapular region, compared with controls. At the age of 60 d, oxytocin-treated female and male rats had a prolonged withdrawal latency when measured in the tail-flick test, compared with controls. This study shows that oxytocin can induce long-lasting changes in weight gain, hormone levels, and nociceptive thresholds, when administered postnatally, in female and male rats.

Intrauterine programming of cardiovascular disease by maternal nutritional status

The origins of cardiovascular disease are related to genetic factors, postnatal environmental and behavioral influences, and also the environment experienced in utero. Patterns of disproportionate fetal growth consistent with maternal undernutrition appear to be predictive of later hypertension and coronary heart disease. These findings from epidemiologic studies are strongly supported by animal studies. Experimental models are suggestive of a role for glucocorticoid hormones in the intrauterine programming of cardiovascular function. New understanding of the relationships between maternal diet and the development and maturation of fetal tissues may enable prevention of cardiovascular disease by intervention in early life.

Intrauterine programming of hypertension by glucocorticoids

An ever increasing body of evidence indicates that cardiovascular disease is determined by prenatal experience of undernutrition. Animal experiments suggest that in addition to maternal dietary interventions, exposure of the fetus to glucocorticoid hormones may programme fetal physiology and metabolism, such that cardiovascular functions are permanently altered. Through nutritional regulation of maternal-placental-fetal hormone interactions long term risk of hypertension may be established long before exposure to adult lifestyle factors more usually associated with cardiovascular mortality.

Enduring effects of infant feeding experiences on adult blood pressure

OBJECTIVE

Vulnerability to psychosomatic diseases is influenced by events early in life. The objective of this article is to discuss animal research that demonstrates relationships between feeding experiences and growth in infancy and risk of hypertension in adulthood.

METHOD

Subjects were spontaneously hypertensive rats (SHR) and their normotensive Wistar Kyoto progenitors. Initial experiments involved observations of the behaviors of rat mothers and their infants and follow-up measurements of blood pressures. Further studies focused on measurements of infant blood pressure during feeding, and recent investigations manipulated weight gain and sex hormones early in life.

RESULTS

Infant rats whose mothers were seen nursing more often had increased blood pressure as adults. Each time rat mothers delivered milk to their young, the nursing pups' blood pressures rose dramatically. These feeding-induced increases in blood pressure have been observed in the young of many species including humans. They are mediated by autonomic nervous system activity and are larger in SHR pups. Finally, animals that gain weight rapidly as infants as a consequence of being reared in small litters had higher adult blood pressure; but, this effect is seen only in intact males.

CONCLUSIONS

Adult physiologic traits can be influenced by the joint actions of genetic predisposition and essential psychosocial interactions during early development. Animal models can stimulate new ideas, provide important confirmations and elaborations of hypotheses from human investigations, and afford experimental approaches for identifying mechanisms underlying the transduction of behavioral experience to disease susceptibility.

Intrauterine programming of hypertension: nutrient-hormone interactions

Geographic, retrospective, and prospective epidemiologic studies have revealed evidence that the environment in utero is a major determinant of later degenerative disease. Observations from Britain, Sweden, Jamaica, Australia, India, and China support the hypothesis that disproportionate retardation of fetal growth results in increased blood pressure and increased risk of cardiovascular mortality. On the basis of what has been a limited pool of observations linking maternal undernutrition and fetal growth retardation, it has been proposed that hypertension and coronary heart disease are "programmed" by nutrition status.

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.

Behavioral variability in SHR and WKY rats as a function of rearing environment and reinforcement contingency

The spontaneously hypertensive rat (SHR) may model aspects of human attention deficit hyperactivity disorder (ADHD). For example, just as responses by children with ADHD tend to be variable, so too SHRs often respond more variably than do Wistar-Kyoto (WKY) control rats. The present study asked whether behavioral variability in the SHR strain is influenced by rearing environment, a question related to hypotheses concerning the etiology of human ADHD. Some rats from each strain were reared in an enriched environment (housed socially), and others were reared in an impoverished environment (housed in isolation). Four groups--enriched SHR, impoverished SHR, enriched WKY, and impoverished WKY--were studied under two reinforcement contingencies, one in which reinforcement was independent of response variability and the other in which reinforcement depended upon high variability. The main finding was that rearing environment did not influence response variability (enriched and impoverished subjects responded similarly throughout). However, rearing environment affected body weight (enriched subjects weighted more than impoverished subjects) and response rate (impoverished subjects generally responded faster than enriched subjects). In addition, SHRs tended to respond variably throughout the experiment, whereas WKYs were more sensitive to the variability contingencies. Thus, behavioral variability was affected by genetic strain and by reinforcement contingency but not by the environment in which the subjects were reared.

Maternal influences on adult blood pressure of SHRs: a single pup cross-fostering study

To examine the effects of the preweanling maternal environment on the development of adult blood pressure, individual spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive pups were reciprocally cross-fostered between mothers of the same or opposite strain. With this design, each SHR and WKY foster mother reared 7 of her own pups and 1 SHR or WKY foster pup. All litters were weaned at 21 days of age and body weights were obtained. At 110 days of age, SHR and WKY males were weighed and surgically prepared with chronic tail artery catheters. One day later, basal measures of mean arterial pressure (MAP, mmHg) and heart rate (HR, beats per min) were obtained while animals were resting and undisturbed in their home cages. Our findings indicate that adult SHRs reared by WKY foster dams displayed significant reductions in MAP compared to SHRs that were reared by their natural mothers or by SHR foster mothers. In contrast, MAPs of WKYs did not differ across rearing conditions. Weanling body weights of cross-fostered SHRs were significantly greater than control-reared SHRs while weanling body weights of cross-fostered WKYs were significantly lower than control-reared WKYs. However, by 110 days of age, there were no treatment-related effects on body weights of males of either strain. These findings underscore the importance of genotype x maternal environment interactions during the preweanling period for the development of cardiovascular phenotype in SHRs.

Nutritional influences in early life upon obesity and body proportions

Close relationships exist between patterns of intra-uterine growth and the risk of ischaemic heart disease, hypertension, diabetes, insulin-resistance syndrome, obesity and some cancers later in life. Earlier studies placed emphasis on low birth weight and reduced growth, but it is now clear that disproportions in early growth are of great importance. Disproportion may be identified as disproportions of fetal and placental growth (and the risk of high blood pressure), or in head circumference, length and weight. It is hypothesized that the availability of nutrients at different times during gestation, by interacting with the maternal and fetal hormonal profile, predisposes to different patterns of growth. The same interaction programmes critical metabolic functions and determines the metabolic capacity at all later ages. People who were exposed to severe undernutrition during the Dutch hunger winter showed increased adiposity if the exposure was during early pregnancy, but decreased adiposity if the exposure was during late pregnancy. In men born in the UK, those with evidence of retarded fetal growth had significantly greater waist/hip circumference ratios for any given body mass index (the ratio fell with increasing weight at one year of age). In Mexican-Americans and non-Hispanic Caucasian Americans, people in the lowest third of birth weight had more truncal fat than those in the highest third. Offspring of rats exposed to marginally reduced protein intakes during pregnancy manifest a similar pattern of growth and metabolic change to that seen in humans, with perturbations of appetite and body fat patterning. Studies in rats suggest that programming of the hypothalamus, especially the hypothalamic-pituitary-adrenal axis might be the mechanism through which these changes are brought about.

Effects of neonatal growth on adult blood pressures of borderline hypertensive rats

We conducted this study to test the hypothesis that there are long-term effects of litter-size manipulations during the preweaning period on growth and adult blood pressure of rats. Litter size of genetically homogeneous borderline hypertensive rats, which were produced by cross-mating male Wistar-Kyoto rats with female spontaneously hypertensive rats, was manipulated from 10 to 16 days of age. In addition, a subset of males and females was castrated within the first 30 hours of life. Body weights were measured at several preweaning and postweaning ages, and tail-cuff blood pressures were recorded at 90 days of age. Intact and castrated pups of both sexes that were reared in small (n = 4) litters from 10 to 16 days of age gained nearly twice the weight of animals reared in large (n = 9 to 12) litters during this period. Intact males from small litters had significantly higher adult blood pressures than those from large litters. These long-term effects remained even in groups matched for adult weight and length. Neonatal castration of males completely blocked the consequences of litter-size manipulation on adult blood pressure, suggesting either an organizational or activational role for androgens. Neither intact nor neonatally castrated females exhibited differences in adult blood pressure as a function of litter-size manipulation.

Development of hypertension in spontaneously hypertensive rats: role of milk electrolytes

1. Milk samples were collected from lactating spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) normotensive females at 8, 14 or 18 days postpartum. 2. Milk samples were later analysed for content of calcium (Ca++), sodium (Na+), potassium (K+), chloride (Cl-) and total protein. 3. Milk samples from SHR females had significantly higher concentrations of Na+ (at 8 and 14 days) and Cl- and significantly lower concentrations of Ca++, K+ (at 8 and 14 days) and total protein compared to milk samples from WKY females. 4. Preweanling dietary content of several electrolytes implicated in the development of hypertension differed dramatically between SHR and WKY strains. This altered diet early in life may serve as an environmental trigger for progressive age-related increases in arterial pressure in SHR.

Milk electrolyte content of Dahl hypertensive and normotensive rats

Milk samples were collected from lightly anesthetized lactating female rats of the Dahl hypertension-sensitive (SS/Jr) and Dahl hypertension-resistent (SR/Jr) inbred strains on postnatal days 8, 14 and 18. These milk samples were stored at 4 degrees C until analyzed for content of calcium (Ca++), magnesium (Mg++), chloride (Cl-), potassium (K+), sodium (Na+), and total protein. Our findings revealed that milk samples from females of the two strains were remarkably similar in content of electrolytes and total protein at each of the three sampling times. The only significant difference from among a total of 18 strain comparisons across the three sample times was that milk levels of Ca++ were significantly higher in SS/Jrs compared to SR/Jrs on postnatal day 14. These findings clearly indicate that these milk constituents do not differ between mothers of the two Dahl strains. Thus, milk concentrations of electrolytes and protein do not appear to serve as an environmental stimulus during the preweanling period for the dramatic age-related increases in arterial pressure characteristic of the SS/Jr strain.

Maternal influences on milk intake in SHR and WKY pups

Mother to pup milk transfer was examined in the spontaneously hypertensive rat (SHR) and its normotensive progenitor, the Wistar-Kyoto (WKY). Litters of SHR and WKY pups were either reared by their natural mothers or cross-fostered to mothers of the opposite strain shortly after birth. At postpartum days 7 and 14, pup body weights were monitored throughout a 2-h maternal separation period followed by a 3-h refeeding period during which pups remained with their mothers. Weight increases of pups over the refeeding period served as an indirect measure of milk ingestion. At postpartum day 7, both SHR and WKY pups nursed by SHR mothers received lower quantities of milk compared to their counterparts nursed by WKY mothers. Maternal strain differences in milk transfer were largely attenuated at postpartum day 14. The different nutritional environments provided by SHR and WKY mothers may mediate the alterations in offspring cardiovascular physiology and physical development that occur with reciprocal cross-fostering.