Foetal fluid balance and hormone status following nephrectomy in the foetal sheep

Placental erythropoietin expression is upregulated in growth-restricted fetuses with abnormal umbilical artery Doppler findings: a case-control study of monochorionic twins

BACKGROUND

We previously reported that fetal plasma erythropoietin (EPO) concentrations are significantly increased in growth-restricted fetuses with abnormal umbilical artery (UA) Doppler. During hypoxia in an ovine model, the primary site of fetal EPO synthesis was switched from the kidneys to the placenta. Therefore, we designed this study to evaluate human placental EPO gene expression and the correlation to fetal serum EPO concentration in growth-restricted fetuses in a monochorionic (MC) twin model.

METHODS

In MC twin pairs, selective intrauterine growth restriction (sIUGR) was defined as the presence of (i) birth weight discordance of > 20% and (ii) a smaller twin with a birth weight less than the 10th percentile. Fetal UA and middle cerebral artery (MCA) Doppler were checked within 1 week before delivery. An abnormal UA Doppler was defined as persistently absent or reverse end-diastolic flow. Cerebroplacental ratio (CPR) was defined as MCA-pulsatility index (PI)/UA-PI. Fetal plasma EPO concentrations were measured in cord blood, and EPO gene expression was assayed in each twin's placental territory. The intertwin plasma EPO ratio was calculated as the cord plasma EPO level of the smaller (or sIUGR) twin divided by the EPO concentration of the larger (or appropriate-for-gestational-age (AGA)) twin, and the intertwin placental EPO gene expression ratio was calculated similarly.

RESULTS

Twenty-six MC twins were analyzed, including normal twins (Group 1, n = 9), twins with sIUGR without UA Doppler abnormalities (Group 2, n = 9), and twins with sIUGR and UA Doppler abnormalities (Group 3, n = 8). The CPRs of smaller (sIUGR) fetuses were significantly decreased in Group 3 MC twins (p < 0.001), but not significantly different between Group 1 and Group 2. The highest fetal plasma EPO ratio and placental EPO gene expression ratio were identified in Group 3 MC twins (p < 0.001). The placental EPO gene expression ratios were significantly correlated with the fetal plasma EPO ratios (Pearson's correlation test, p = 0.004).

CONCLUSION

This study provides evidence of increased placental EPO expression in MC twin fetuses with sIUGR and abnormal UA Doppler. Future studies are needed to confirm the similar role of placental EPO in severe IUGR singletons.

Statistical analysis with dilatation for development process of human fetuses

This paper is concerned with the development process of human fetuses. Though the development process of human fetuses still includes many unknown issues, it is known that a certain harmonious relationship between the organs can be observed. This knowledge is based on our intuition, but we have no theory which clarifies these harmonized developments. The paper aims to give a mathematical understanding of the notion of harmonized development through the use of dilatation, which is a measure of the departure from conformal mapping. The asymptotics for dilatation have been developed using certain efficient models of quasiconformal mapping. The proposed method of dilatation is effectively applied to the human fetus data.

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Renal dysfunction is associated with a reduced contribution of nitric oxide and enhanced vasoconstriction after a congenital renal mass reduction in sheep

BACKGROUND

Children born with reduced congenital renal mass have an increased risk of hypertension and chronic kidney disease in adulthood, although the mechanisms are poorly understood. Similar sequelae occur after fetal uninephrectomy (uni-x) in sheep, leading to a 30% nephron deficit. We hypothesized that renal dysfunction is underpinned by a reduced contribution of nitric oxide (NO) and vascular dysfunction in uni-x sheep.

METHODS AND RESULTS

In 5-year-old female uni-x and sham sheep, mean arterial pressure, glomerular filtration rate, and renal blood flow were measured before and during NO inhibition (N(ω)-nitro-l-arginine methyl ester [L-NAME]). Reactivity was assessed in resistance arteries, including renal lobar and arcuate arteries. Basal mean arterial pressure was 15 mm Hg higher and glomerular filtration rate and renal blood flow were ≈30% lower (P<0.001) in uni-x animals. L-NAME increased mean arterial pressure by ≈17 mm Hg in both groups, whereas glomerular filtration rate and renal blood flow were decreased less in uni-x sheep (PInteraction<0.01). Endothelial NO synthase and Ser-1177-phosphorylated endothelial NO synthase protein levels were upregulated in renal cortex of uni-x sheep (P<0.05). Lobar arteries of uni-x sheep had enhanced responsiveness to phenylephrine and nitrotyrosine staining and reduced sensitivity to endothelial stimulation. Vasodilator prostanoid contribution to endothelium-dependent relaxation was reduced in lobar arteries of uni-x sheep, accompanied by reduced cyclooxygenase-1 and -2 gene expression (P<0.05). Neurovascular constriction was enhanced ≈1.5-fold in renal arteries of uni-x sheep (P<0.05).

CONCLUSIONS

Renal dysfunction after congenital renal mass reduction is associated with impaired regulation of renal hemodynamics by NO. Reductions in renal blood flow and glomerular filtration rate are underpinned by impaired basal NO contribution, endothelial dysfunction, and enhanced vascular responsiveness to sympathetic nerve stimulation.

Impaired ability to modulate glomerular filtration rate in aged female sheep following fetal uninephrectomy

Fetal uninephrectomy (uni-x) results in hypertension at a later age in female than male sheep. We hypothesized that dysregulation of tubular sodium handling contributes to the reduced ability to regulate extracellular fluid (ECF) homeostasis in older females born with a congenital nephron deficit. Following renal excretory balance studies, the response to inhibition of the Na(+)K(+)2Cl(-) cotransporter with furosemide (0.5 mg/kg bolus + 1 mg/kg per hour, i.v) or vehicle treatment was examined in conscious 5-year-old female uni-x (n = 7) and sham (n = 7) sheep. Balance studies in meal-fed sheep demonstrated that while average 24 h sodium excretion over 6 days was not different between the groups, the daily variation in sodium excretion was significantly greater in uni-x compared to sham sheep (31 ± 4% vs. 12 ± 2%; P < 0.001). Basal plasma renin activity (PRA) and renal cortical cyclooxygenase-2 (COX-2) gene expression were lower in uni-x sheep (both, P < 0.01). The increases in glomerular filtration rate (GFR) and renal blood flow observed in sham sheep in response to furosemide were significantly attenuated in uni-x sheep (both P GROUP×TREAT < 0.05). However, fractional sodium excretion increased by a greater extent in the uni-x (4.4 ± 1.0%) as compared to the sham sheep (2.0 ± 0.4%; P GROUP×TIME < 0.05) in response to furosemide. In conclusion, fetal uni-x was associated with altered renal sodium handling and hypertension in aged females. The impaired ability to modulate PRA and GFR in the adults with a congenital nephron deficit may reduce the capacity of the kidney to respond to gains or losses in ECF to maintain a stable internal environment.

Blunted sodium excretion in response to a saline load in 5 year old female sheep following fetal uninephrectomy

Previously, we have shown that fetal uninephrectomy (uni-x) causes hypertension in female sheep by 2 years of age. Whilst the hypertension was not exacerbated by 5 years of age, these uni-x sheep had greater reductions in renal blood flow (RBF). To further explore these early indications of a decline in renal function, we investigated the renal response to a saline load (25 ml/kg/40 min) in 5-year old female uni-x and sham sheep. Basal mean arterial pressure was ∼15 mmHg greater (P(Group)<0.001), and sodium excretion (∼50%), glomerular filtration rate (∼30%, GFR) and RBF (∼40%) were all significantly lower (P(Group)<0.01) in uni-x compared to sham animals. In response to saline loading, sodium excretion increased significantly in both groups (P(Time)<0.001), however this response was blunted in uni-x sheep (P(GroupxTime)<0.01). This was accompanied with an attenuated increase in GFR and fractional sodium excretion (both P(GroupxTime)<0.05), and reduced activation of the renin-angiotensin system (both P<0.05), as compared to the sham group. The reduction in sodium excretion was associated with up-regulations in the renal gene expression of NHE3 and Na(+)/K(+) ATPase α and β subunits in the kidney cortex of the uni-x compared to the sham animals (P<0.05). Notably, neither group completely excreted the saline load within the recovery period, but the uni-x retained a higher percentage of the total volume (uni-x: 48±7%; sham: 22±9%, P<0.05). In conclusion, a reduced ability to efficiently regulate extracellular fluid homeostasis is evident in female sheep at 5 years of age, which was exacerbated in animals born with a congenital nephron deficit. Whilst there was no overt exacerbation of hypertension and renal insufficiency with age in the uni-x sheep, these animals may be more vulnerable to secondary renal insults.

Development of cardiovascular disease due to renal insufficiency in male sheep following fetal unilateral nephrectomy

BACKGROUND

Renal insufficiency is associated with the development of cardiovascular disease.

OBJECTIVES

This study investigated whether reduced fetal renal mass resulted in renal insufficiency, hypertension, cardiac dysfunction and whether these changes progressed with age.

METHODS AND RESULTS

Fetal uninephrectomy was performed at 100-day gestation (term, 150 days) and studies performed in male sheep from 6 weeks to 24 months of age. Renal function declined with age in sham animals as demonstrated by increasing plasma creatinine levels and urinary excretion of albumin. The age-related decline in renal function was exacerbated in animals that had undergone fetal uninephrectomy. Evidence of renal insufficiency was indicated from as early as 6 weeks of age with elevations in plasma creatinine (Ptreatment < 0.001), urea (Ptreatment < 0.001) and sodium (Ptreatment < 0.05) levels in uninephrectomized lambs as compared with sham animals. At 6 months, urinary albumin excretion (P < 0.001) was increased and urinary sodium excretion (P < 0.001) decreased in the uninephrectomized animals. By 24 months, renal function had deteriorated further with significant progression of albuminuria (P(treatment x age) < 0.001). Elevation of mean arterial pressure (approximately 15 mmHg) was associated with significantly increased cardiac output, stroke volume and plasma volume at 6 months; arterial pressure (approximately 27 mmHg) had increased further in uninephrectomized animals at 24 months and was driven by increased total peripheral resistance. Cardiac functional reserve (dobutamine challenge) was reduced in uninephrectomized animals at 6 and 24 months of age (Ptreatment < 0.001), and this was associated with left ventricular enlargement (P < 0.001) and reduced fractional shortening (P < 0.01).

CONCLUSION

Fetal uninephrectomy causing a reduction in nephron endowment results in an accelerated age-related decline in renal function. This is associated with an early onset of elevated blood pressure and impairments in cardiac structure and function.

Renal and placental secretion of erythropoietin during anemia or hypoxia in the ovine fetus

OBJECTIVE

The source of the erythropoietin (EPO) that circulates in the fetus is unknown although it is known that EPO does not cross the placenta and that fetal kidneys, liver, and placenta express the EPO gene. This study tested to what extent in vivo EPO secretion by the fetal kidneys and placenta can be demonstrated under normoxic and hypoxic conditions.

STUDY DESIGN

Renal arterial and venous EPO concentrations were determined in eight late-gestation chronically catheterized fetal sheep made progressively anemic by exchange transfusion with saline solution over 5 to 8 days. In a separate additional series of experiments, umbilical arterial and venous EPO concentrations were determined in nine normoxic fetuses and in nine fetuses subjected to 12 hours of hypoxia induced by lowering maternal-inspired oxygen content. Organ secretion rates were calculated as the product of plasma flow rate and the arteriovenous concentration differences.

RESULTS

Renal vein plasma EPO concentration was higher than the arterial concentration in 36 of 40 paired samples (P<.0001) by 16.3%+/-2.7% (mean+/-SE). This difference was concentration independent over a range of 12 to 4100 mU/mL. Renal EPO secretion rates were variable and averaged 155+/-105 mU/min when hematocrit was 31.3%+/-1.6% (n=5) and 1124+/-300 mU/min post-exchange transfusion when hematocrit was 15.6%+/-0.8% (n=12). In contrast, umbilical venous and arterial EPO concentrations (range 9-35 mU/mL), although highly correlated (r=0.94), were not different during normoxia (Po(2)=21.6+/-0.5 mm Hg, n=9). Under hypoxic conditions (Po(2)=15.6+/-0.4 mm Hg, n=9), umbilical vein EPO concentration (range 151-1245 mU/mL) was higher than arterial concentration (range 140-951 mU/mL) in eight of nine paired samples by 13.6%+/-3.3% (P<.01). Under these conditions, estimated umbilical EPO secretion rate was 27,900+/-11,500 mU/min.

CONCLUSION

Under nonanemic, normoxic basal conditions, the kidneys secreted EPO into the fetal circulation, whereas secretion by the placenta was not demonstrated. In the phlebotomy-induced fetal anemia experiments, the kidney demonstrated marked, progressive increases in the rate of EPO production. Similarly, in the fetal hypoxemia experiments, the placenta demonstrated progressive increases--albeit an order of magnitude greater than the kidneys--in EPO production rate. As an extension of these findings, we speculate that the hypoproliferative neonatal anemia that invariably occurs in the early weeks after birth is in part the result of loss of EPO production by the placenta.

Fetal uninephrectomy leads to postnatal hypertension and compromised renal function

It has been proposed that the number of nephrons an individual has may be inversely related to his or her blood pressure. In this study using female ovine fetuses, nephron number was reduced by performing a fetal uninephrectomy during the period of active nephrogenesis (100 days of gestation, term=150 days). Lambs were born at term and grew at a similar rate. At 5 months of age, ovaries were removed and the carotid artery exteriorized into a fold of skin. Blood pressure and renal function were studied at 6 and 12 months of age. At 6 months of age, uninephrectomized lambs had significantly higher mean arterial blood pressure than sham-operated lambs (89+/-2 versus 82+/-2 mm Hg, P<0.05) when measured over a 3-day period. Heart rate was not different between the groups. Urine flow rate was similar, but glomerular filtration rate was significantly lower in uninephrectomized animals (P<0.05). Urinary concentrations and excretion rates of sodium tended to be higher in uninephrectomized animals but were similar for chloride and potassium. There was no evidence of proteinuria in the uninephrectomized lambs. Similar differences were observed in blood pressure and renal function at 12 months of age. Plasma renin concentrations at this age were lower in the uninephrectomized lambs (P<0.05). An oral salt load for 10 days did not increase blood pressure significantly in either group at 12 months of age, nor were there differences in the responsiveness to graded doses of angiotensin II. These results suggest that formation of a low nephron number in utero, may result in elevated blood pressure and compromised renal function in later life.

Compensatory renal growth after unilateral nephrectomy in the ovine fetus

Unilateral nephrectomy of the adult animal results in compensatory renal growth but does not involve formation of new nephrons. It is not clear whether compensatory growth can occur during the period of active nephrogenesis in utero and if so, whether more nephrons can be formed. Male ovine fetuses (n = 20) underwent unilateral nephrectomy (n = 10) or sham nephrectomy (n = 10) at 100 d of gestation (term, 150 d). After 27 to 34 d, ewes and fetuses were killed and the right kidney of each fetus was removed and weighed. The wet weight of the right kidney was greater in the unilaterally nephrectomized fetuses (16.3 +/- 1.3 g compared with 12.2 +/- 0.7 g; mean +/- SEM, P < 0.05) as was the kidney to body weight ratio (5.2 +/- 0.3 g/kg compared with 3.8 +/- 0.2 g/kg; P < 0.001). Nephron number in the right kidney was estimated by an unbiased stereologic technique. There was a 45% increase in the number of nephrons in the kidneys from unilaterally nephrectomized animals compared with the kidneys from sham-operated animals (530,763 +/- 37,136 nephrons in the unilaterally nephrectomized group compared with 365,672 +/- 36,016 nephrons in the sham-operated group; P < 0.01). Mean glomerular volume was lower in the unilaterally nephrectomized group; however, total glomerular volume per kidney was not different between groups. This study demonstrates that there is a significant amount of compensatory growth and nephron endowment in a remaining kidney after unilateral nephrectomy during the period of active nephrogenesis in the sheep. This is the first time such events have been shown to occur in utero.