Sodium metabolism and its requirement during reproduction in female rats

A distributed experiment demonstrates widespread sodium limitation in grassland food webs

Sodium (Na) has a unique role in food webs as a nutrient primarily limiting for plant consumers, but not other trophic levels. Environmental Na levels vary with proximity to coasts, local geomorphology, climate, and with anthropogenic inputs (e.g., road salt). We tested two key predictions across 54 grasslands in North America: Na shortfall commonly limits herbivore abundance, and the magnitude of this limitation varies inversely with environmental Na supplies. We tested them with a distributed pulse experiment and evaluated the relative importance of Na limitation to other classic drivers of climate, macronutrient levels, and plant productivity. Herbivore abundance increased by 45% with Na addition. Moreover, the magnitude of increase on Na addition plots decreased with increasing levels of plant Na, indicating Na satiation at sites with high Na concentrations in plant tissue. Our results demonstrate that invertebrate primary consumers are often Na limited and track local Na availability, with implications for the geography of invertebrate abundance and herbivory.

The effect of increasing the salt intake of pregnant dairy cows on the salt appetite and growth of their calves

An increased sodium appetite has been demonstrated in cattle following supplementation with sodium in the neonatal period, but it is unclear whether the sodium appetite of calves can be influenced in utero by the dam’s sodium intake during pregnancy. Twenty-two non-lactating, pregnant dairy cows received either a diet of silage and concentrates or the same diet with 70 g NaCl per day added to it for the last 2 months of pregnancy. The sodium supplement increased the birth weight of their calves but after 6 weeks there was no difference between treatments in calf weight. In addition to milk, calves in both treatments were offered a choice of concentrates with normal or high sodium concentrations (5·8 and 10·8 g/kg dry matter, respectively). Calves from the cows that had received supplementary sodium during pregnancy ate more of the high sodium concentrate than calves from cows without the sodium supplement, demonstrating that sodium appetite could be entrained by the sodium intake of the dam during late pregnancy.

Does salt have a permissive role in the induction of puberty?

Puberty is starting earlier than ever before and there are serious physiological and sociological implications as a result of this development. Current research has focused on the potential role of high caloric, and commensurate high adiposity, contributions to early puberty. However, girls with normal BMI also appear to be initiating puberty earlier. Westernized diets, in addition to being high in fat and sugar, are also high in salt. To date, no research has investigated a link between elevated salt and the reproductive axis. We hypothesize that a high salt diet can result in an earlier onset of puberty through three mechanisms that are not mutually exclusive. (1) High salt activates neurokinin B, a hormone that is involved in both the reproductive axis and salt regulation, and this induces kisspeptin release and ultimate activation of the reproductive axis. (2) Vasopressin released in response to high salt acts on vasopressin receptors expressed on kisspeptin neurons in the anteroventral periventricular nucleus, thereby stimulating gonadotropin releasing hormone and subsequently luteinizing hormone secretion. (3) Salt induces metabolic changes that affect the reproductive axis. Specifically, salt acts indirectly to modulate adiposity, ties in with the obesity epidemic, and further compounds the pathologic effects of obesity. Our overall hypothesis offers an additional cause behind the induction of puberty and provides testable postulates to determine the mechanism of potential salt-mediated affects on puberty.

Urine as an important source of sodium increases decomposition in an inland but not coastal tropical forest

Nutrient pulses can profoundly impact ecosystem processes and urine is a frequently deposited source of N and K, and Na. Na is unimportant to plants, but its addition can increase decomposition and change invertebrate community structure in Na-poor tropical forests. Here we used synthetic urine to separate the effects of Na from urine's other nutrients and contrasted their roles in promoting decomposition and detritivore recruitment in both a Na-poor inland Ecuadorian and Na-rich coastal Panamanian tropical forest. After 2 days, invertebrate communities did not vary among +Na, H2O, Urine+Na, and Urine-Na treatments. But after 2 weeks, Ecuador wood, but not cellulose, decomposition was twofold higher on Urine+Na and +Na plots compared to H2O and Urine-Na plots accompanied by >20-fold increases in termite abundance on these plots. Panama, in contrast, showed no effect of Na on decomposition. In both forests, plots fertilized with urine had nearly twofold decrease in detritivores after 2 weeks that was likely a shock effect from ammonification. Moreover, the non-Na nutrients in urine did not enhance decomposition at this time scale. On control plots, Panama had higher decomposition rates for both cellulose and wood than Ecuador, but the addition of Na in Ecuador alleviated these differences. These results support the hypothesis that in Na-poor tropical forests, urine can enhance wood decomposition and generate an important source of heterogeneity in the abundance and activity of brown food webs.

Furosemide, sodium appetite, and ingestive behavior

Sodium appetite is often produced experimentally by using the diuretic furosemide (Furo) to induce a rapid loss of urinary sodium. The present experiments were designed to investigate the dose-dependent relationship between renal and behavioral responses to Furo. We compared the effects of five different Furo doses (0.5, 1, 2, 6, and 10 mg) on 3% NaCl intake, water intake, Na(+)-free chow intake, urine quantity, electrolyte balance, and weight gain in rats. The Na(+) loss produced by Furo injection was dose dependent from 0.5 to 10 mg and did not change across repeated depletions. There was only a weak correspondence, however, between these dose-dependent changes in renal function and subsequent sodium appetite. This suggests that net Na(+) loss is not the only determinant of sodium intake. Moreover, at the two higher doses of Furo, both food intake and weight dropped significantly, but these did not change following the three lower ones. Given these substantial side effects, the preferred dose of Furo for inducing a salt appetite should not exceed 2.0 mg.

Potential mechanisms for functional changes in taste receptor cells following sodium deficiency in mammals

Sodium is an essential nutrient for life, and its level in the body is tightly regulated. When sodium deficient, some mammals alter their behavior towards salt by avidly consuming it, even at concentrations animals typically choose to avoid. This change in acceptance is accompanied by a reduction in the response of the gustatory chorda tympani nerve to sodium solutions. More specifically, the response rate of the sodium-specialist units to NaCl stimulation is reduced following sodium deficiency or adrenalectomy. The initial transduction of the chemical signal is mediated, in part, by Na+ influx through epithelial Na+ channels in the apical membrane of taste cells that synapse with the specialist neurons. Circulating hormones like angiotensin II and adrenocorticotropin hormone, which are released in response to sodium deficiency and adrenalectomy, could regulate the activity of Na+ channels through G-protein linked second-messenger systems. These putative pathways are of interest because they have been described in mammalian taste receptor cells. The present review will summarize evidence linking some hormones of fluid homeostasis with the apparent attenuation of input from sodium-specialist neurons.

NaCl preference increases during pregnancy and lactation: assessment using brief access tests

Pregnancy and lactation are characterized by increases in NaCl intake, as determined by long-term consumption tests, which cannot examine the relative contribution of taste and postingestive factors to this phenomenon. Consequently, in this study, changes in NaCl preference during pregnancy and lactation were studied in nulliparous Long-Evans rats using a brief access test (lickometer). In Experiment 1, rats were maintained on a Na(+)-adequate diet (0.03% Na(+)), habituated to lickometer testing, and subsequently assessed during pregnancy and lactation with three 30-s exposures to each of seven taste solutions: 0.075 M sucrose (base), 0.089 M NaCl in base, 0.158 M NaCl in base, 0.281 M NaCl in base, 0.5 M NaCl in base, 0.158 M NaCl and 0.281 M NaCl. Results indicated higher lick rates to the 0.5 M NaCl in base, 0.158 M NaCl and 0.281 M NaCl solutions during late pregnancy and late lactation (Day 13 and beyond). In Experiment 2, a comparison of two diets differing in sodium content (0.03% vs. 0.3% Na(+)) determined that these changes in NaCl preference during pregnancy and lactation were unrelated to dietary sodium. Thus, the apparent increase in NaCl preference during pregnancy and lactation, independent of dietary sodium, suggests that this change in preference is not in response to physiological sodium need.

High dietary NaCl early in development enhances mean arterial pressure of adult rats

We investigated the long-term influence of early dietary NaCl on adult mean arterial pressure (MAP) and heart rate (HR), assessed continuously for 7 weeks when fed water and chow containing 1% NaCl (weeks 1, 4, 7), 0% NaCl (weeks 2-3), and 3% NaCl (weeks 5-6) while on a 12:12 light/dark cycle. Subjects were offspring of female Sprague-Dawley rats fed solid chow consisting of either 0.1% (basal), 1% (intermediate), or 3% (high) NaCl throughout pregnancy and lactation. After weaning on postnatal day (PD) 21, offspring were fed the same NaCl diet of their mother until PD 30, at which time all offspring were given Purina 5001 diet (1% NaCl) as their solid chow. On PD 60, 22 adult male offspring (eight basals, six intermediates, eight highs) were implanted with an aortic electronic sensor for transmitting blood-pressure signals by telemetry. MAP and HR varied significantly across the 7-week testing period in association with dietary NaCl levels. The three perinatal salt groups had similar HR levels on normal 1% NaCl chow. However, HR increased in all groups during 0% NaCl and decreased in all groups during 3% NaCl. In contrast, the average MAP levels were significantly greater in the highs compared to both the intermediate and basal groups during both dark and light periods. MAP increased in high and basal groups during both 0% and 3%; however, the increase was more pronounced in the highs. In conclusion, exposure to high levels of dietary NaCl early in development led to a persistent increase in MAP in adulthood.

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.

Induction of intrauterine growth restriction with a low-sodium diet fed to pregnant rats

OBJECTIVE

A low-sodium diet fed to female rats before mating through parturition leads to pups of lower weight. We characterized the effect of low dietary sodium during the last week of gestation (after fetal organogenesis) on fetal and maternal homeostasis.

STUDY DESIGN

Pregnant Sprague-Dawley rats were randomly assigned to a control group or to a group fed a low-sodium diet from gestational days 15 through 22. Systolic blood pressures were measured throughout pregnancy. On day 22 plasma volume was measured and blood samples were taken for electrolyte and hormonal measurements. Fetal and placental weights were also determined.

RESULTS

Plasma renin activity and aldosterone level were significantly higher in the experimental group than in the control group. Plasma volume was significantly lower in pregnant rats receiving a low-sodium than in those receiving a control diet. Rats receiving a low-sodium diet had pups of lower weight and length (4.45 +/- 0.22 g, 3.90 +/- 0.06 cm) than pups of the control group (5.21 +/- 0.12 g, 4.10 +/- 0.02 cm). Pups born to mothers with low-sodium diets recuperated from intrauterine growth restriction by 14 days after birth.

CONCLUSION

These data indicate that a low-sodium diet given to pregnant rats for the last 7 days of gestation leads to reduced plasma volume expansion and fetal growth restriction. This could prove to be a simple animal model for studying the relationship between maternal plasma volume and fetal growth.

Mechanism of volume adaptation in the awake early pregnant rat

The objective of the present study was to determine whether the increase in plasma volume (PV) during pregnancy is established by fluid retention or by a shift within the extracellular fluid volume (ECFV) from the interstitium toward the intravascular compartment. To this end, we simultaneously, measured total body water, (TBW), ECFV, and PV together with the hematocrit (Hct) and plasma osmolality 4, 8, and 12 days postsurgery in chronically instrumented pregnant (P) and nonpregnant (NP) rats. The P rats were instrumented with a catheter in the femoral artery on day 1 postconception. In the NP group, neither TBW nor ECFV and PV had changed consistently on days 8 and 12 postsurgery relative to day 4. In contrast, in the P animals, TBW, ECFV, and PV had increased by 16, 24, and 20%, respectively, by day 12 relative to day 4. To evaluate whether PV had increased in concert with an overall rise in TBW or as a result of a fluid shift at the cost of the interstitial fluid volume, we calculated the relative size of each fluid compartment on three consecutive measurement sessions. In the NP group, TBW, presented as percentage of maternal weight (%MW) as well as ECFV (%TBW) and PV (%ECFV) had not changed consistently throughout the measurement period. In the P animals, TBW (%MW) was slightly higher on day 12 compared with day 4, but ECFV (%TBW) and PV (%ECFV) had not changed significantly. Finally, in the NP group, Hct had not changed, whereas, in the P animals, Hct was 10% lower on days 8 and 12 compared with day 4. Plasma osmolality did not change consistently in either group during the course of the experimental period. The gradual synchronous increase in all fluid compartments, without consistent change in their relative distribution, suggests that, in normal rat pregnancy, PV expansion is primarily achieved by fluid retention rather than by a redistribution of the ECFV.

Increased salt preference in adult offspring raised by mother rats consuming excessive amounts of salt and water

This study was designed to evaluate the effects of an early high-salt environment on the maternal and the young offspring physiology and on the adult offspring sodium appetite. Twenty-five-adult female Wistar rats were pseudorandomly divided into two groups. Twelve animals underwent a partial ligature of their abdominal aorta (PAL). Once polydipsia and sodium appetite (tested by measuring water and a 2.7% NaCl intakes) developed, they were mated. The other 13 rats (SHAM) were sham-operated and also mated. Throughout pregnancy and lactation, water and salt intake of PAL rats was consistently and significantly higher than that of the Sham. On gestation day 20, amniotic fluid and maternal plasma sodium concentration of PAL and Sham rats did not differ. Sodium concentration in the milk of the lactating PAL group was elevated (P < 0.05) on day 20 after delivery. At 0, 10 and 21 days of age, plasma sodium concentration of PAL offspring (PAL-O) and Sham offspring (Sh-O) were not significantly different. At 90 days of age, the salt preference of PAL-O rats was greater than that of Sh-O rats after 7 days of sodium deprivation (P < 0.01).

Altered gustatory development in Na(+)-restricted rats is not explained by low Na+ levels in mothers' milk

Placing pregnant rats on a sodium-deficient diet (0.03% NaCl) very early in gestation and then weaning the offspring (sodium-restricted rats) to the same diet precludes development of amiloride-sensitive sodium taste transduction pathways in the offspring. However, normal amiloride-sensitive sodium taste responses can be restored by permitting sodium ingestion by sodium-restricted rats. The present study tested the hypothesis that the concentration of sodium in sodium-restricted mothers' milk must be abnormally low in order to preserve altered gustatory function in the offspring. Other milk electrolyte and total protein concentrations were determined as well. Milk sodium was similar between sodium-restricted and control rat mothers at 10-13 and 16-20 days postpartum, as were levels of potassium and chloride. At 10-13 days postpartum, total protein was higher in milk from sodium-restricted mothers. Sodium-restricted mothers' milk calcium concentrations were higher versus controls at 16-20 days postpartum. These results indicate that the lack of gustatory amiloride sensitivity in sodium-restricted rats cannot be attributed to deficient dietary sodium levels during the suckling period.

Sex difference in blood pressure of spontaneously hypertensive rats influenced by perinatal NaCl exposure

Our prior study showed that the basal blood pressure level and pressor response to peripheral angiotensin II of adult Sprague-Dawley rats were enhanced by perinatal exposure to a high NaCl diet. The purpose of the present study was to assess further the relationship between NaCl-sensitivity of blood pressure and perinatal NaCl exposure. We tested the hypothesis that the basal blood pressure level and pressor responses to angiotensin II could be increased by perinatal exposure to high NaCl in NaCl-resistant spontaneously hypertensive rats (SHR-R). Adult female SHR-R were maintained on a diet containing either basal 1% or high 8% NaCl throughout pregnancy and lactation. The offspring were continued on these same diets to 30 days postpartum. Thereafter, all offspring were maintained on a diet containing 1% NaCl. After being adapted to restraint, systolic blood pressure was measured indirectly by the tail-cuff procedure when the rats were 30, 44, and 58 days of age. Subsequently, baseline mean arterial pressure (MAP) and pressor responses to intravenous administration of angiotensin II (20, 40, 80, and 120 ng/kg body weight) were obtained from the catheterized femoral artery in conscious unrestrained rats. The MAP levels of adult female SHR-R exposed perinatally to 8% NaCl were significantly greater than those of females exposed to 1% NaCl. This elevated blood pressure was accompanied by an elevation in plasma osmolality. Perinatal exposure to 8% NaCl did not raise the blood pressure and plasma osmolality levels of adult male SHR-R, but did enhance pressor responses to angiotensin II.(ABSTRACT TRUNCATED AT 250 WORDS)

Perinatal exposure to a high NaCl diet increases the NaCl intake of adult rats

To determine the effect of differences in perinatal NaCl exposure on NaCl intake, adult Sprague-Dawley female rats were maintained on diets containing either 0.12, 1.0, or 3% NaCl throughout pregnancy and lactation. The offspring were continued on the these same diets to 30 days postpartum. Thereafter, all offspring were fed the same basal diet containing 1% NaCl. At 90 days of age, the adult offspring were placed in metabolism cages for 7 days and fed 1% NaCl chow for days 1-2, and 0% NaCl chow for days 3-7. On days 6-7, the animals were free to consume both water and 0.3 M NaCl. When dietary NaCl was available, adult rats exposed perinatally to the high NaCl diet excreted significantly more sodium on days 1-2 and 6-7 than did the rats exposed to either the mid or low NaCl diets. There were no differences in sodium excretion during sodium deprivation on days 3-5. The 0.3 M NaCl intake of the high NaCl-exposed rats was also significantly greater than the intake of the mid and low NaCl-exposed rats. In another group of adult rats, exposed perinatally to either a low or high NaCl diet, the spontaneous 24-hr intake of water and 0.3 M NaCl was measured after repeated episodes of acute sodium depletion. Sodium depletion was induced by 48 hr of dietary sodium deprivation combined with a single subcutaneous injection of 5 mg furosemide. Acute sodium depletion was found to augment existing differences in NaCl intake between low and high NaCl-exposed rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular correlates of predisposition to hypertension in pups of one kidney: one clip renal hypertensive dams

We reported that the pups of one kidney:one-clip Goldblatt hypertensive (HT) rats have a predisposition to the induction of experimental hypertension. To investigate the nature of the predisposition, pups of HT dams were challenged with vasopressor agents at 28-days-of-age or treated with DOCA and 6.5% salt diet between 35 and 81 days-of-age. Pups of HT dams had a significantly greater blood pressure between 28 and 81 days-of-age and also showed a greater change in blood pressure when provided a 6.5% salt diet than did pups of normotensive dams. Pups of HT dams demonstrated a decreased bradycardic response after injection of angiotensin II (p less than .05) and a lesser change in mean arterial blood pressure after injection of angiotensin II or norepinephrine than did the pups from normotensive dams. These results suggest an effect of maternal hypertension on the development of cardiovascular control mechanisms in her offspring. The decreased pressor response suggests the predisposition is not due to an increased vascular sensitivity to vasopressors.

Volume homeostasis during pregnancy in the rat

This article reviews alterations in volume and sodium handling during rat pregnancy, noting similarities and contrasts to events in human gestation. Gravid rodents undergo extracellular and plasma volume increases of 50% to 70%, and these changes accompany a marked cumulative sodium retention shared by both dam and fetuses. Pregnancy alters several factors, with opposing effects on renal salt handling; however, mechanisms by which gestational sodium accumulation and volume expansion are achieved remain obscure. Furthermore, despite substantial increases in absolute blood volume, considerable uncertainty exists as to how this volume is sensed, particularly during the final gestational week when a rapid increase in volume is associated with decreases in peripheral resistance and BP. Attempts to assess "effective" intravascular volume by measuring responses to intravenous (IV) or oral sodium loading or to chronic mineralocorticoid administration indicate that pregnant and nonpregnant rats respond similarly, suggesting that such animals sense their volume as normal. In contrast, when salt-restricted, gravid rats fail to expand their plasma volume normally; this relative hypovolemia activates mechanisms leading to free water retention and pathologic hyponatremia, responses not observed in virgin animals.

Experimental hypertension in young and adult animals

The susceptibility of immature and adult animals to various environmental factors often differs because the response of the young organism can only involve those regulatory mechanisms that are available at the particular stage of development. Increased sensitivity to certain (e.g., hypertensive) stimuli may be limited to a relatively short age period that is usually characterized by the maturation of some important physiological functions. High salt intake seems to influence the animals especially during the weaning period and prepuberty, in the course of which profound developmental changes of circulation, electrolyte metabolism, and neurohumoral regulation have been demonstrated. Indeed, salt-dependent forms of experimental hypertension are more severe when they are induced in immature animals. Moreover, substantial differences in hemodynamics, distribution of body fluids, and involvement of pressor and natriuretic agents indicate that the mechanisms of salt hypertension need not be the same in immature and adult animals. For this reason, increased attention should be paid to developmental factors in the study of induced forms of experimental hypertension.

Salt level in weaning diet affects saline preference and fluid intake in Dahl rats

Weanling Dahl salt-sensitive (DS) and salt-resistant (DR) rats were used to compare effects of feeding high or low NaCl diets on taste preference for, and intake of, a wide range of saline concentrations. The DS and DR were fed either 8.0 or 0.4% dietary NaCl for 4 weeks. Then, with all animals fed the 0.4% NaCl diet, their taste preferences for 0.0001 to 0.56 M saline were assessed using three 24-hour two-bottle preference tests of each solution versus distilled deionized water. Saline preference and intake were influenced by concentration and its interaction with genotype, with DS exhibiting higher preferences than DR for hypotonic saline. The DS preexposed to 8.0% dietary NaCl showed elevated consumption levels of water and total fluid (saline + water) that persisted throughout the 5-week test period, despite transfer to the 0.4% NaCl diet before the initiation of preference testing. Findings indicate that genotype, dietary NaCl levels in weaning diet, and saline concentration of preference test solutions interact to influence saline preference and saline and water intake in Dahl rats.

Dietary salt affects fluid intake and output patterns of pregnant rats

This study was done to determine how differences in dietary NaCl influence water and electrolyte balance during gestation. Eighteen adult female rats were fed diets containing either 0.12 (low), 1.0 (mid, control), or 3.0% (high) NaCl throughout gestation. On gestation days 1-19, body weight, water and food intakes, urine volume and electrolyte levels were measured. On gestation day 20, amniotic fluid electrolyte levels were determined; litter sizes were assessed 24-hr after birth. Because food intakes were similar throughout gestation, the dams of the three groups ingested different amounts of NaCl. Differences in dietary NaCl were accompanied by large differences in the daily patterns of urine sodium, urine sodium/potassium ratios, water intake, and urine volume. Despite these differences in intake and output, water and electrolyte balance (intake minus output) was the same for the three groups. There were no differences in the dams' body weights, amniotic fluid electrolyte levels, or litter sizes. The long-term consequences of perinatal dietary NaCl for the developing pups are discussed.

Maternal dietary sodium chloride levels affect the sex ratio in rat litters

Eighty-eight adult female rats were fed diets containing either 0.08, 0.12, 1, 3, or 4% NaCl for at least one week prior to breeding and throughout gestation. Within 24 hours of birth, all pups were sexed based on anogenital distance, and the number of males and females were recorded. The amount of sodium chloride in mother rats' diets was inversely related to the proportion of phenotypic males in the litter. As dietary salt increased, the proportion of males decreased. These alterations in dietary NaCl affected the sex ratio without disrupting litter size or the general health of the offspring. Dietary mineral content may affect the phenotypic sex ratio through changes in the genotypic sex ratio, or alternatively, via changes in the environment in which the genes are expressed. These findings are important for basic research concerning maternal nutrition and development.

Effect of variations in dietary sodium intake on sodium excretion in mature rats

Sprague-Dawley rats weighing 400 g or more were studied to determine whether their continued weight gain affects renal sodium handling. Rats maintained on a wide range of sodium intakes gained 3.9 +/- 0.4 g/day. The intercept of a linear regression of intake against urinary excretion provided an estimate of the minimum daily requirement for sodium intake of 247 +/- 33 microEq/day. When more than this required amount was ingested, the animals excreted the excess quantitatively in the urine. When less was ingested they continued to gain weight at a slower rate, 1.6 +/- 0.6 g/day, and remained in positive sodium balance. Nonetheless, they developed a sodium deficit manifested as retention of a sodium challenge. Thus, on an adequate dietary intake the normal physiological state of Sprague-Dawley rats of this size is one of chronic sodium retention rather than neutral sodium balance. In contrast, when inadequate sodium is ingested a deficit develops in the absence of external losses. These observations have important implications for the interpretation of studies of renal sodium handling in these animals.