Pharmacological blockade of blood pressure and heart rate increases following milk ingestion in 15-day-old SHR and WKY rat pups

Role of the orexin 2 receptor in palatable-food consumption-associated cardiovascular reactivity in spontaneously hypertensive rats

Hypertensive subjects often exhibit exaggerated cardiovascular reactivity. An overactive orexin system underlies the pathophysiology of hypertension. We examined orexin's roles in eating-associated cardiovascular reactivity in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. Results showed eating regular chow or palatable food (sucrose agar) was accompanied by elevated arterial pressure and heart rate. In both SHRs and WKY rats, the cardiovascular responses associated with sucrose-agar consumption were greater than that with regular-chow consumption. Additionally, SHRs exhibited greater cardiovascular responses than WKY rats did to regular-chow and palatable food consumption. Central orexin 2 receptor (OX2R) blockade attenuated sucrose-agar consumption-associated cardiovascular response only in SHRs. In both SHRs and WKY rats, OX2R blockade did not affect regular-chow consumption-associated cardiovascular responses. Greater numbers of c-Fos-positive cells in the rostral ventrolateral medulla (RVLM) and of c-Fos-positive orexin neurons in the dorsomedial hypothalamus (DMH) were detected in sucrose agar-treated SHRs, compared to regular chow-treated SHRs and to sucrose agar-treated WKY rats. Central OX2R blockade reduced the number of c-Fos-positive cells in the RVLM only in sucrose agar-treated SHRs. We concluded that in SHRs, orexin neurons in the DMH might be overactive during eating palatable food and may further elicit exaggerated cardiovascular responses via an OX2R-RVLM pathway.

Blood pressure responses to feeding in infancy: spin-offs of serendipity

This article summarizes results from a program of research that has focused on understanding the characteristics of, and factors that shape, acute cardiovascular responses to feeding in infants. The work developed from behavioral observations in rats suggesting a linkage between variations in maternal behavior and adult blood pressure. However, the hypothesis that specific types of interactions, in particular those associated with feeding, might alter cardiovascular function was greatly influenced by a serendipitous observation made by Myron Hofer several years earlier. Myron discovered that nutrient availability is a key regulator of cardiovascular function in newborn rats. Our spin-off studies established that, in newborn rats and human infants, ingestion of nutrient is associated with robust increases in heart rate and blood pressure.

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.

Cardiovascular responses to feeding in the neonate during the first four days of life

Prior work has demonstrated acute cardiovascular responses associated with breast and bottle feeding of newborn infants that consist of increases in both blood pressure and heart rate. This current study sought to determine if the amplitude of these responses is related to the age of the infant and/or the amount of nutrient ingested during the study period. Results show that the magnitude of the feeding responses does not change over the first four days of life. In a second study it was found that changes in systolic blood pressure during feeding are positively correlated with volume of nutrient intake during the initial phase of feeding (r = +0.75, p < 0.01). These results suggest that some of the individual variability in the increase in blood pressure during feeding is related to the amount of nutrient ingested. It is not known whether these differences are due to differences in sucking effort or other aspects of feeding efficiency.

Aspects of autonomic and neuroendocrine function

This article provides a brief review of aspects of autonomic and neuroendocrine function studied initially in collaboration with the late Marian Silver. The importance of the sympathetic innervation to the liver in the control of glycogenolysis was established in anaesthetised animals of various species. Otherwise the work has been carried out mainly in conscious animals under strictly physiological conditions and below behavioural threshold. Investigations of the role of the autonomic innervation to the endocrine pancreas in controlling the release of pancreatic hormones, led to the realisation that the parasympathetic innervation mediates responses to glycaemic stimuli while the sympathetic innervation mediates responses to any form of stress. Studies of adrenal medullary function have confirmed that its threshold for many forms of stress is much higher than that of other components of the sympathetic system and revealed the importance of the pattern of electrical stimulation in determining the rates of release of catecholamines, enkephalins, corticotrophin-releasing factor (CRF) and adrendocorticotrophin (ACTH). The splanchnic sympathetic innervation to the adrenal cortex also plays an important role in determining glucocorticoid output by sensitising the cells to ACTH, probably mainly by the release of vasoactive intestinal peptide (VIP) from cortical nerve terminals. Finally studies of feeding in milk-fed calves have shown that suckling is associated with a remarkable hypertension and tachycardia. These cardiovascular effects are due to a selective sympathetic discharge, which does not involve the adrenal medullae, or the release of neuropeptide Y (NPY) and, at least in the calf, can be attributed to activation of adrenoceptors.

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.

Parasympathetic control of heart period during early postnatal development in the rat

Basal autonomic control of the heart period (HP) changes considerably during the early postnatal period in the rat. Although studies in the developing animal have examined the ability of the sympathetic branch to decrease HP during physiological challenge, few studies have examined the emerging capabilities of the parasympathetic branch to alter HP during early development. To determine the extent of parasympathetic control of HP in the young rat, we used a modified dive reflex procedure and electrical stimulation of the vagal nerve to examine the range of parasympathetic effects on HP in postnatal day 3-24 rats. Modified dive reflex manipulations produced maximal parasympathetically-mediated HPs that were longer just after birth and at weaning than at intervening ages. Direct vagal nerve stimulation studies revealed significant decreases with age in the HP at maximal vagal activation and in the intrinsic HP. The dynamic range, or difference between minimal and maximal parasympathetic effects on HP was similar across ages when assessed from the results of vagal stimulation. Nerve stimulations also revealed age-independent and relatively linear transfer functions relating parasympathetic stimulation frequency and HP during early life. Therefore, several parameters characterizing parasympathetic control of HP, including the dynamic range and transfer function, remain reasonably stable throughout the early postnatal period in the rat. These data provide a framework delineating the autonomic limits within which cardiac responses operate in the young rat. Knowledge of changes in these limits across time affords a firmer physiological basis for cross-age comparisons of autonomically-mediated cardiac changes.

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.

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.

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

Cross-fostering of litters was used to determine the timing of preweanling maternal influences on the development of high blood pressure in spontaneously hypertensive (SHR) rats. The SHR litters were either reared by their natural mothers or reciprocally cross-fostered to normotensive Wistar-Kyoto (WKY) mothers for postnatal days 1-7, 1-14, 1-21, 8-21, or 15-21. All SHR litters were weaned at 21 days of age and males were housed in groups of two to three per cage until physiological measures were obtained at 100 days of age. At 100 days of age, all rats were surgically prepared with tail artery catheters and, on the day after surgery, direct measures of mean arterial pressure (MAP, mmHg) and heart rate (HR, bpm) were obtained while rats were resting and undisturbed in their individual home cages. Our findings indicate that cross-fostering SHR pups to WKY foster mothers was attended by significant effects on body weights at weaning and on adult MAPs. Compared to control SHRs, cross-fostered SHRs, with the exception of the 15-21-day group, were significantly heavier at weaning. By 100 days of age, body weights of SHRs were similar across treatment groups. Basal MAPs of SHRs cross-fostered for days 1-7, 1-14, 1-21, and 8-21, but not days 15-21, were reduced significantly compared to SHR controls reared by their natural mothers. In contrast, basal HRs were not affected in any of the cross-fostered SHR groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent control of pressor responses to feeding in young rats

A series of experiments investigated the eliciting stimuli and physiological systems involved in triggering the blood pressure (BP) response to feeding in the young rat. The studies included 1) tongue cannula (TC) infusions of milk to nipple-attached and unattached pups; 2) TC infusions of saline, sucrose, and water to nipple-attached pups; 3) maternal milk ejections to 3-week-old pups that rarely have stretch responses; 4) gastric cannula infusions of milk to nipple-attached pups; and 5) maternal milk ejections and TC infusions to pups whose mouths were topically anesthetized. The results implicate the contact of fluid with oral and/or lingual surfaces as the critical stimulus primarily responsible for the BP rise. Although nipple attachment appears to be an important precondition for the typical pressor response, the co-occurrence of a behavioral stretch response is not essential.

Sucrose drinking reduces dorsal hypothalamic beta-endorphin levels in spontaneously hypertensive rats but not in Wistar-Kyoto rats

The present study was performed to test whether drinking of a 10% sucrose solution would preferentially alter tissue content of beta-endorphin in dorsal and ventral hypothalamic fragments from the spontaneously hypertensive rat (SHR), as opposed to the Wistar-Kyoto rat (WKY). Changes were correlated with cardiovascular function and circulating catecholamine levels to assess the role of hypothalamic beta-endorphin, a putative mediator of sucrose-induced changes in cardiovascular sympathetic nervous tone. Male rats were trained to consume their total 24-h water intake in a single period between 0900-1100 h. Catheters were chronically implanted to sample blood and to record arterial blood pressure and heart rate. The experimental protocol consisted of a recording session, which included a 10-min drinking period wherein rats consumed 8 ml of either sucrose solution or deionized water. Rats were sacrificed and hypothalami removed for analysis of beta-endorphin content. Comparable increases in blood pressure were noted in SHR and WKY during drinking of either sucrose or water. Drinking-induced tachycardia was blunted in SHR drinking sucrose. Plasma norepinephrine was increased only in sucrose-drinking SHR. Plasma glucose levels were elevated in both SHR and WKY following drinking of sucrose, but not water. beta-Endorphin levels were reduced (p < 0.05) in dorsal, but not ventral, hypothalamic fragments only in SHR drinking sucrose. The sucrose-induced changes in beta-endorphin did not correlate with blood pressure responses. The results indicate an exaggerated stimulation of beta-endorphin release in the dorsal hypothalamus following sucrose ingestion in SHR.

Blood pressure increases in response to feeding in the term neonate

We studied blood pressure and heart rate of 18 medically well newborn babies before, during, and after feeding. Seven were breast-fed and 11 bottle-fed and all were studied between 24 and 92 hr of age. There was an increase in heart rate associated with sucking on a pacifier but no additional increase with feeding. By 30-60 min after feeding, heart rate had returned to baseline value. Blood pressure, however, did increase in response to feeding and showed a greater increase in the breast-fed group than in the bottle-fed group. Systolic blood pressure rose from 72 mmHg in the basal state to 85 mmHg during the first 3 min of feeding in the bottle-fed babies and from 73 to 99 mmHg during comparable periods in the breast-fed babies. The results for diastolic blood pressure were similar except that changes within the bottle-fed group were not statistically significant. Bottle-fed babies rose only from 42 to 47 mmHg while breast-fed babies rose from 42 to 61 mmHg from basal conditions to feeding. Blood pressure responses to feeding may have implications for susceptibility to necrotizing enterocolitis, for future blood pressure liability in adulthood and for later hypertensive disease.

Feeding in infancy: short- and long-term effects on cardiovascular function

Cardiovascular responses of adult organisms to feeding are well characterized and, in general, are understood as acute adaptations required for processing and distributing nutrients. Research over the past several years has shown that infants also have important cardiovascular responses to nutrient intake and that these are regulated by changes in autonomic activity to the heart and vasculature. Recent studies have provided results that suggest these responses in infancy may make an important contribution to the long-term development of cardiovascular function, in particular, adult blood pressure (BP). The purpose of this presentation will be to review the evidence that has led to this conclusion, offer ideas about how this potential early-life shaping of subsequent cardiovascular function may come about, and suggest further studies that will be required in order to characterize the mechanisms responsible for these effects.