Birth weight has no influence on glomerular number and volume

Developmental origins of chronic renal disease: an integrative hypothesis

Cardiovascular diseases are one of the leading causes of mortality. Hypertension (HT) is one of the principal risk factors associated with death. Chronic kidney disease (CKD), which is probably underestimated, increases the risk and the severity of adverse cardiovascular events. It is now recognized that low birth weight is a risk factor for these diseases, and this relationship is amplified by a rapid catch-up growth or overfeeding during infancy or childhood. The pathophysiological and molecular mechanisms involved in the “early programming” of CKD are multiple and partially understood. It has been proposed that the developmental programming of arterial hypertension and chronic kidney disease is related to a reduced nephron endowment. However, this mechanism is still discussed. This review discusses the complex relationship between birth weight and nephron endowment and how early growth and nutrition influence long term HT and CKD. We hypothesize that fetal environment reduces moderately the nephron number which appears insufficient by itself to induce long term diseases. Reduced nephron number constitutes a “factor of vulnerability” when additional factors, in particular a rapid postnatal growth or overfeeding, promote the early onset of diseases through a complex combination of various pathophysiological pathways.

Birth weight and childhood blood pressure

A large body of literature suggests an inverse relationship between birth weight and blood pressure in children, adolescents and adults. The most persistent findings have been observed in children with a history of low birth weight or intrauterine growth restriction, while a large number of studies carried out in populations with normally distributed birth weight have shown conflicting results. A recently reported strong direct association between high birth weight and blood pressure, and the significant positive effect of postnatal growth on blood pressure suggests that the fetal origins of adult disease hypothesis should be expanded to include the role of excessive fetal and postnatal growth. In this paper, we review recent studies on the relationship between birth weight and blood pressure in childhood, with a focus on confounding variables that may explain the conflicting results of published work in this field.

Low nephron number and its clinical consequences

Epidemiologic studies now strongly support the hypothesis, proposed over two decades ago, that developmental programming of the kidney impacts an individual's risk for hypertension and renal disease in later life. Low birth weight is the strongest current clinical surrogate marker for an adverse intrauterine environment and, based on animal and human studies, is associated with a low nephron number. Other clinical correlates of low nephron number include female gender, short adult stature, small kidney size, and prematurity. Low nephron number in Caucasian and Australian Aboriginal subjects has been shown to be associated with higher blood pressures, and, conversely, hypertension is less prevalent in individuals with higher nephron numbers. In addition to nephron number, other programmed factors associated with the increased risk of hypertension include salt sensitivity, altered expression of renal sodium transporters, altered vascular reactivity, and sympathetic nervous system overactivity. Glomerular volume is universally found to vary inversely with nephron number, suggesting a degree of compensatory hypertrophy and hyperfunction in the setting of a low nephron number. This adaptation may become overwhelmed in the setting of superimposed renal insults, e.g. diabetes mellitus or rapid catch-up growth, leading to the vicious cycle of on-going hyperfiltration, proteinuria, nephron loss and progressive renal functional decline. Many millions of babies are born with low birth weight every year, and hypertension and renal disease prevalences are increasing around the globe. At present, little can be done clinically to augment nephron number; therefore adequate prenatal care and careful postnatal nutrition are crucial to optimize an individual's nephron number during development and potentially to stem the tide of the growing cardiovascular and renal disease epidemics worldwide.

The relationship between nephron number, kidney size and body weight in two inbred mouse strains

While some reports in humans have shown that nephron number is positively correlated with height, body weight or kidney weight, other studies have not reproduced these findings. To understand the impact of genetic and environmental variation on these relationships, we examined whether nephron number correlates with body weight, kidney planar surface area, or kidney weight in two inbred mouse strains with contrasting kidney sizes but no overt renal pathology: C3H/HeJ and C57BL/6J. C3H/HeJ mice had smaller kidneys at birth and larger kidneys by adulthood, however there was no significant difference in nephron number between the two strains. We did observe a correlation between kidney size and body weight at birth and at adulthood for both strains. However, there was no relationship between nephron number and body weight or between nephron number and kidney size. From other studies, it appears that a greater than two-fold variation is required in each of these parameters in order to demonstrate these relationships, suggesting they are highly dependent on scale. Our results are therefore not surprising since there was a less than two-fold variation in each of the parameters examined. In summary, the relationship between nephron number and body or kidney size is most likely to be demonstrated when there is greater phenotypic variation either from genetic and/or environmental factors.

No juvenile arterial hypertension in sheep multiples despite reduced nephron numbers

Low birth weight is associated with an increased risk of metabolic dysfunction and arterial hypertension in later life. Because of their reduced birth weight twins have been used repeatedly as a natural model to investigate prenatal programming of hypertension. To reveal an early impact of lower nephron endowment on blood pressure, we performed a longitudinal study on lambs from single, twin and triplet pregnancies. The lambs were studied from birth until adulthood, including regular blood analyses, measurements of body weight and blood pressure and post-mortem estimation of glomerular numbers. Relative weight differences between multiples and singletons at birth were -28% for twins and -44% for triplets, respectively. Some lambs showed rapid catch-up growth. Total nephron number of twins and triplets was reduced by 21 and 37% with respect to that of singletons (p < 0.01). However, multiples did not show increased blood pressure within the time frame of this study. No gender-specific effect was observed. Plasma concentrations of creatinine, urea, electrolytes or osmolality also did not differ. Our data indicate that the previously reported postnatal blood pressure differences between sheep multiples and singletons are a time-limited phenomenon. During infancy and adolescence, a reduced nephron number in sheep multiples is neither associated with increased blood pressure nor reflected by plasma parameters.

Assessment of renal circulation in small for gestational age and appropriate for gestational age term newborns: a prospective study

PURPOSE

To compare selected parameters of renal circulation between small for gestational age (SGA) and appropriate for gestational age (AGA) newborns.

METHODS

Fifty-two SGA and 100 AGA term newborns were examined. The size of the kidneys were measured, and renal blood flow in the central and intraparenchymal renal arteries were assessed via Doppler sonography. Peak systolic velocity (PSV), end diastolic velocity (EDV), mean blood flow velocity (V mean), resistance index (RI), and pulsatility index (PI) were determined and compared between the groups.

RESULTS

No statistically significant differences in the velocity parameters were found between SGA and AGA infants in central renal arteries. Slightly higher RIs and PIs were seen in AGA newborns (RI, 0.76 +/- 0.13 versus 0.78 +/- 0.06 [p < 0.05]; PI, 1.65 +/- 0.54 versus 1.84 +/- 0.46 [p < 0.05]). There were statistically significant differences between the groups in all measured parameters in intraparenchymal arteries (RI, 0.57 +/- 0.11 versus 0.63 +/- 0.05 [p < 0.001]; PI, 0.89 +/- 0.26 versus 1.09 +/- 0.16 [p < 0.001]) except PSV (7.11 +/- 1.55 versus 7.14 +/- 0.81 cm/s [p > 0.05]).

CONCLUSION

Based on our findings, we suggest that renal circulation is not negatively influenced by intrauterine growth restriction in SGA neonates compared with AGA newborns.

Consequences of Intrauterine Growth Restriction for the Kidney

Low birth weight due to intrauterine growth restriction is associated with various diseases in adulthood, such as hypertension, cardiovascular disease, insulin resistance and end-stage renal disease. The purpose of this review is to describe the effects of intrauterine growth restriction on the kidney. Nephrogenesis requires a fine balance of many factors that can be disturbed by intrauterine growth restriction, leading to a low nephron endowment. The compensatory hyperfiltration in the remaining nephrons results in glomerular and systemic hypertension. Hyperfiltration is attributed to several factors, including the renin-angiotensin system (RAS), insulin-like growth factor (IGF-I) and nitric oxide. Data from human and animal studies are presented, and suggest a faltering IGF-I and an inhibited RAS in intrauterine growth restriction. Hyperfiltration makes the kidney more vulnerable during additional kidney disease, and is associated with glomerular damage and kidney failure in the long run. Animal studies have provided a possible therapy with blockage of the RAS at an early stage in order to prevent the compensatory glomerular hyperfiltration, but this is far from being applicable to humans. Research is needed to further unravel the effect of intrauterine growth restriction on the kidney.

Glomerular Number and Function Are Influenced by Spontaneous and Induced Low Birth Weight in Rats

A link exists between low birth weight and diseases in adulthood, such as hypertension, cardiovascular disease, and insulin resistance. Intrauterine growth restriction (IUGR) has been used to explain this association and has been shown to lead to a nephron endowment in humans. A reduction in glomerular number has been described in animal models with induced low birth weight as well but not in animals with spontaneous low birth weight. It therefore is debatable whether the models are suitable. The effect on glomerular number and size was studied in rats with naturally occurring IUGR and experimental IUGR, induced by bilateral uterine artery ligation. Design-based stereologic methods were used. Urinary protein excretion was determined as a measure of renal damage. Results showed a decrease of approximately 20% in glomerular number in both groups of IUGR (control 35,400, naturally occurring IUGR 30,900, and experimental IUGR 28,000 glomeruli per kidney). Mean glomerular volume was increased in both IUGR groups, which was associated with an increased proteinuria. It is concluded that IUGR leads to a nephron endowment with a compensatory glomerular enlargement. This compensation is associated with more proteinuria in the long run. Uterine artery ligation in the pregnant rat is a suitable model to study the effects of IUGR on the kidney.

Impaired kidney growth in low-birth-weight children: Distinct effects of maturity and weight for gestational age

BACKGROUND

Chronic kidney disease (CKD) alters the regulation of calcium and phosphate homeostasis, leading to secondary hyperparathyroidism, metabolic bone disease, soft tissue calcifications, and other metabolic derangements that have a significant impact on morbidity and mortality. The parathyroid gland is the central organ responsible for regulating these adaptive responses. Suppression of parathyroid hormone (PTH) secretion, hypertrophy, and hyperplasia are a major goal of treatment of CKD.

METHODS

Current literature was reviewed and combined with the author's experience to address a number of issues regarding the optimal treatment of secondary hyperparathyroidism in hemodialysis patients.

RESULTS

The calcium sensing receptor (CASR) is the most important factor regulating parathyroid gland function, and allosteric modulators of CASR, called calcimimetics, provide a novel drug therapy to suppress PTH secretion. The current use of active vitamin D analogues to suppress PTH is often limited by hypercalcemia and hyperphosphatemia. Clinical trials of cinacalcet HCl, the first calcimimetic to be approved for treatment of secondary hyperparathyroidism in CKD, have demonstrated suppression of circulating PTH levels without increments in the calcium-phosphorus (Ca x P) product, making it easier to achieve the stringent management guidelines proposed for subjects with CKD by the National Kidney Foundation Kidney Disease Outcomes Quality Initiative (K/DOQI).

CONCLUSION

The management of disordered calcium and phosphate homeostasis in CKD patients is evolving based on our knowledge of the major importance of the calcium sensing receptor (CASR) in controlling parathyroid gland function and the potent actions of calcimimetics to target CASR. The purpose of this presentation is to provide an overview of the role of the CASR in regulation of parathyroid gland function, to examine the mechanisms whereby calcimimetics target the CASR, and to review the clinical trials that support the use of cinacalcet HCl for the treatment of secondary hyperparathyroidism in stage 5 chronic kidney disease (CKD).

Long-term consequences of low birth weight

There is accumulating evidence of the impact of low birth weight in adult age. Thus, the Barker theory and Brenner hypothesis gain more power. This article reviews and analyzes the evidence that supports the intrauterine origin of chronic noncommunicable diseases in adult age, particularly systemic arterial hypertension and chronic renal insufficiency. These are possibly related to lower nephron numbers, acquired in utero or later in life, which can increase susceptibility to kidney damage from diseases such as hypertension and diabetes mellitus, or cause arterial hypertension and secondary renal damage.

Growth patterns of the heart and kidney suggest inter-organ collaboration in facultative fetal growth

Maternal smoking during pregnancy has been associated with a number of negative sequelae among offspring, including elevated postnatal blood pressure. While animal studies have described organ level alterations with smoke exposure, human data have been more limited. Thirty-four healthy maternal/fetal pairs (24 nonsmokers, 10 smokers) participated in a longitudinal growth study from the thirteenth week of pregnancy to document fetal kidney and heart growth trajectories and morphology. Curve fitting followed by a mixed model for repeated measures identified significantly different growth patterns in kidney width, thickness, length, and volume growth with exposure: the smoke-exposed fetal kidney was wide and thick compared to the unexposed kidney during the second and early third trimester, declining to proportionately thin kidneys for length and width subsequently. Cardiac growth in width and volume followed a reverse pattern: a surge in cardiac volume occurred after 30 weeks with acceleration in cardiac width, resulting in a heart that was wide for length and for fetal weight. Smoke exposure altered fetal growth in size and timing of the heart and kidneys during midgestation, with changes in organ morphology suggesting compensatory growth. These are the first data providing anatomical evidence of altered renal/cardiac volume relationships that may provide a mechanism to previously reported sequelae of in utero smoke exposure. They suggest that cell-level adaptive responses to hypoxia and/or chemical insults are operative and illustrate the importance of longitudinal ultrasound to directly assess the organ-level growth response of the human fetus to a prenatal stress, in lieu of relying on birth outcome measures.

Impact of Graft Mass on the Clinical Outcome of Kidney Transplants

The effect of nephronic mass reduction of kidney transplants has not been analyzed specifically in a large cohort. Transplant injuries in cadaver kidney graft may have led to an underestimation of the magnitude of this factor. The aim of this study was to analyze the consequences of kidney mass reduction on transplantation outcome. The weights of 1142 kidney grafts were collected prospectively immediately before grafting. Donors and recipients <15 yr of age, simultaneous kidney/pancreas grafts, and technical failures before day 7 were excluded from the analysis. The analysis was performed on Cockroft-calculated creatinine clearance and proteinuria in 964 patients for whom all of the necessary information was available. This study reports that the smallest kidneys transplanted into the largest recipients (donor kidney weight/recipient body weight [DKW/RBW] <2 g/kg, n = 88) increased their clearance by 2.38 ml/min every month for 6 mo (P < 0.0001) and by 0.27 ml/min thereafter (P < 0.0001). Conversely, creatinine clearance did not change for the largest kidneys transplanted into the smallest recipients (DKW/RBW ratios >/=4 g/kg). Next, using a Cox model analysis, it was shown that the risk of having a proteinuria >0.5 g/kg was significantly increased for the low DKW/RBW ratios <2 g/kg with 50% of patients having a proteinuria, compared with DKW/RBW ratios >/=4 g/kg (P < 0.001). In cadaver transplant recipients, graft mass has a rapid impact on graft filtration rate and proteinuria. Avoiding major kidney/recipient inadequacy should have a significant influence on long-term transplant function.

Naturally occurring intrauterine growth retardation and adult blood pressure in rats

In humans, infants who are born small have been reported to have higher blood pressure in adulthood than do larger infants. This suggests that factors in the intrauterine environment that affect fetal growth can program the individual for hypertension later in life. The present study determined whether there is a similar, naturally occurring relationship between birth weight and adult blood pressure in rats. Female Sprague-Dawley rats bred in our colony were fed a normal diet during pregnancy. On the day of delivery, any pups that weighed <90% of the mean pup weight for the litter were identified as runts. For each runt, a sex-matched littermate of normal weight was also identified and assigned to this study. These pairs were chronically instrumented at approximately 20 wk of age. Mean arterial pressure was significantly higher in runt male and female offspring compared with their normal birth weight littermates (males: 149 +/- 7, runts versus 129 +/- 4 mmHg, controls; females: 128 +/- 1, runts versus 119 +/- 2 mmHg, controls). Although the runts had smaller body weights at study than did their littermate controls, the kidney-to-body weight ratio and renal function normalized to kidney or body weight were not different. These studies indicate that adult blood pressure is related to birth weight in rats, as it is in humans. The relative hypertension in runt animals is not due to gross differences in renal function but may be related to more subtle renal structural and/or functional differences.

Nephron endowment and blood pressure: what do we really know?

It has been hypothesized that a reduced number of nephrons at birth contributes to the development of essential hypertension. Nephron number in normal human kidneys has been shown to vary up to eightfold. Therefore, a significant proportion of the population appears to be at risk for developing hypertension. Furthermore, nephron deficits might explain why some racial groups have a higher incidence of hypertension and end-stage renal disease than others. Animal studies have demonstrated that maternal limitations in nutrient supply, both gross and nutrient-specific; exposure to elevated levels of hormones or toxins; and genetic factors can lead to permanent deficits in nephron number and, when examined, elevated blood pressure. In this review, maternal and genetic factors influencing nephron endowment and the implications of nephron deficit for hypertension and renal disease in humans are discussed.

Exploring the pathways leading from disadvantage to end-stage renal disease for Indigenous Australians

Indigenous Australians are disadvantaged, relative to other Australians, over a range of socio-economic and health measures. The age- and sex-adjusted incidence of end-stage renal disease (ESRD)--the irreversible preterminal phase of chronic renal failure--is almost nine times higher amongst Indigenous than it is amongst non-indigenous Australians. A striking gradient exists from urban to remote regions, where the standardised ESRD incidence is from 20 to more than 30 times the national incidence. We discuss the profound impact of renal disease on Indigenous Australians and their communities. We explore the linkages between disadvantage, often accompanied by geographic isolation, and both the initiation of renal disease, and its progression to ESRD. Purported explanations for the excess burden of renal disease in indigenous populations can be categorised as: primary renal disease explanations;genetic explanations;early development explanations; and socio-economic explanations. We discuss the strengths and weaknesses of these explanations and suggest a new hypothesis which integrates the existing evidence. We use this hypothesis to illuminate the pathways between disadvantage and the human biological processes which culminate in ESRD, and to propose prevention strategies across the life-course of Indigenous Australians to reduce their ESRD risk. Our hypothesis is likely to be relevant to an understanding of patterns of renal disease in other high-risk populations, particularly indigenous people in the developed world and people in developing countries. Furthermore, analogous pathways might be relevant to other chronic diseases, such as diabetes and cardiovascular disease. If we are able to confirm the various pathways from disadvantage to human biology, we will be better placed to advocate evidence-based interventions, both within and beyond the scope of the health-care system, to address the excess burden of renal and other chronic diseases among affected populations.

Effect of intrauterine growth retardation on the progression of nephrotic syndrome

BACKGROUND/AIM

Neonates with intrauterine growth retardation (IUGR) experience higher morbidity and mortality rates than appropriate-for-gestational-age (AGA) neonates. The purpose of our study was to clarify whether IUGR has any influences on the progression of nephrotic syndrome in children.

METHODS

We performed a retrospective review of 56 children with nephrotic syndrome. IUGR was defined as a birth weight less than the tenth percentile for gestational age. Among 56 patients having nephrotic syndrome, 8 had IUGR, and 48 were AGA.

RESULTS

The 24-hour urinary protein level in children with IUGR was significantly higher than that in children who were AGA (7.61 +/- 6.75 vs. 3.92 +/- 3.70 g/day, p < 0.05). There was a statistically significant difference in the incidence of steroid resistance (62.5 vs. 10.4%, p < 0.05) and in the time to remission (median 60 vs. 13 days, p < 0.05) between the children with IUGR and those being AGA. Also, there was a significant difference in the incidences of treatment with cytotoxic agents (75 vs. 29.2%, p < 0.05) and complications such as hypertension.

CONCLUSIONS

Our report indicates that IUGR predicts an unfavorable progression of nephrotic syndrome. So, it is important for nephrologists to pay attention to the clinical course of nephrotic syndrome in neonates with IUGR.

Birth weight and renal function

Low birth weight due to premature birth or to intrauterine growth retardation adversely affects normal renal development. In the period immediately after birth, the short-term renal-related consequences of low birth weight are an increased risk of acute renal failure as well as transient imbalance of fluid and electrolyte homeostasis. Epidemiological studies show that low birth weight adults are at risk of developing chronic renal disease. There are thought to be several causes for this phenomenon. Adult low birth weight subjects have an increased incidence of hypertension and type 2 diabetes, both of which are well-established risk factors for chronic renal injury. Moreover, interference with normal kidney development, as seen in low birth weight babies, results in a lower than normal number of nephrons, which might also play a role in the renal disturbances.

Infants thinner at birth exhibit smaller kidneys for their size in late gestation in a sample of fetuses with appropriate growth

Fetal ultrasound measurements were employed to investigate the relationship between weight and ponderal index at birth and kidney size during the second (23 weeks) and third (32 weeks) trimesters of pregnancy in a sample of 25 normally growing fetuses. Kidney volume and kidney volume / fetal weight ratio at 32 weeks are significantly and positively related to both weight and ponderal index at birth, controlling for sex, gestational age at birth, and day of ultrasound measurement. A second-degree polynomial relationship approximates the predictability of kidney volume fetal weight ratio at 23 weeks to that at 32 weeks, demonstrating shifting growth rates in fetal organ and body growth relationships during midgestation. Sex and parental size are suggested as contributing to these patterns. Females have a surge in renal growth between 23 and 32 weeks to catch up to earlier growing males, and maternal weight significantly predicts incremental growth in kidney volume and the kidney volume / fetal weight ratio at 32 weeks of gestation. The observation that fetuses relatively thin at birth have relatively smaller kidneys for their size in late gestation suggests that the influence of maternal weight on birth outcome may act through organ growth.

Protein restriction in pregnancy is associated with increased apoptosis of mesenchymal cells at the start of rat metanephrogenesis

BACKGROUND

In rats, offspring born to mothers supplied low protein diets during pregnancy have fewer glomeruli than normal. We hypothesized that such nephron deficits are associated with altered cell turnover in the metanephros, the embryonic precursor of the adult kidney.

METHODS

Wistar rats were supplied with one of three isocaloric diets from day 0 of pregnancy: control (18% protein) or low protein (9% or 6%) diets. All had a normal chow after birth. Groups were compared by multilevel statistical modeling.

RESULTS

At two weeks postnatally, when nephrogenesis has finished, controls had 16.8 x 103 +/- 0.7 x 10(3) (mean +/- SEM) glomeruli/kidney, whereas offspring exposed to 9% diet had 5.1 x 10(3) +/- 1.2 x 10(3) fewer and those exposed to 6% diet had 6.9 x 10(3) +/- 1.7 x 10(3) fewer glomeruli/kidney (P < 0.001, both diets). At embryonic day 13 (E13), when the metanephros has just formed, control metanephroi contained 2.35 x 10(4) +/- 0.15 x 10(4) cells, with no significant differences in low protein groups. At E15, when mesenchyme begins forming primitive nephrons but glomeruli are still absent, controls had 2.00 x 10(6) +/- 0.13 x 10(6) cells. E15 embryos exposed to 9% protein had 1.09 x 10(6) +/- 0.36 x 10(6) fewer cells/metanephros than controls, while those exposed to 6% diet had 1.45 x 10(6) +/- 0.37 x 10(6) fewer (P < 0.01, both diets). Apoptotic cells were detected by molecular (in-situ end-labeling) and morphological (propidium iodide staining) techniques. In all diets, apoptosis was noted in condensing mesenchyme (nephron precursors) and loose mesenchyme (interstitial precursors). Control E13 metanephroi had 63 +/- 7 apoptotic cells/mm2, whereas those exposed to 9% diet had an increase of 77 +/- 26 cells/mm2 (P < 0.01) and those exposed to 6% diet had an increase of 55 +/- 26 cells/mm2 (P < 0.05). By E15, apoptosis was similar in all groups but metanephric mitosis was significantly increased in the 6% protein diet group. No change was found in the level of apoptosis in E13 mesonephroi.

CONCLUSIONS

Maternal low protein diets reduce final numbers of glomeruli in association with enhanced deletion of mesenchymal cells at the start of kidney development. Whether aberrant nephrogenesis is a direct effect from deletion of nephron precursors, or an indirect effect from loss of supportive interstitial precursors, requires further investigation.