Intrarenal blood flow distribution in canine puppies

Changes in Renal Resistive Index Values in Healthy Puppies during the First Months of Life

The purpose of this study is to establish renal resistive index (RRI) of normal kidneys in puppies aged from newborn to 20 weeks of age and to determine the age at which RRI reaches adult dog values. Six healthy adult intact beagles and six puppies from 1 day after birth to 20 weeks of age were used. In the adult dogs, the ultrasonographic scans were performed once, and in the puppies, the ultrasonographic studies were performed on the first day after birth and at 1, 2, 3, 4, 6, 8, 12, 16, and 20 weeks of age. RRI was obtained at the interlobular and arcuate arteries in each kidney. There were no statistical differences between the RRI values obtained between the right and left kidney nor between intrarenal arteries (interlobar and arcuate). The RRI was the highest during the first weeks of life, after which it declined gradually with increasing age reaching adult dog values at 12 weeks of age. In conclusion, the normal mean RRI is age dependent in dogs. Twelve weeks can be regarded as the age at which adult mean RRI criteria can be applied to puppies.

Postnatal development of the renal medulla; role of the renin-angiotensin system

Adverse events during foetal development can predispose the individual for cardiovascular disease later in life, a correlation known as foetal programming of adult hypertension. The 'programming' events have been associated with the kidneys due to the significant role in extracellular volume control and long-term blood pressure regulation. Previously, nephron endowment and functional consequences of a low nephron number have been extensively investigated without achieving a full explanation of the underlying pathophysiological mechanisms. In this review, we will focus on mechanisms of postnatal development in the renal medulla with regard to the programming effects. The renin-angiotensin system is critically involved in mammalian kidney development and impaired signalling gives rise to developmental renal lesions that have been associated with hypertension later in life. A consistent finding in both experimental animal models and in human case reports is atrophy of the renal medulla with developmental lesions to both medullary nephron segments and vascular development with concomitant functional disturbances reaching into adulthood. A review of current knowledge of the role of the renin-angiotensin system for renal medullary development will be given.

Neuronal nitric oxide synthase, nNOS, regulates renal hemodynamics in the postnatal developing piglet

INTRODUCTION

Nitric oxide (NO) vasodilation critically modulates renal hemodynamics in the neonate compared with the adult. Based on the postnatal expression pattern of renal neuronal nitric oxide synthase (nNOS), the hypothesis was that nNOS is the major NOS isoform regulating renal hemodynamics in the immature, but not mature, kidney.

RESULTS

NOS inhibitors did not alter mean arterial pressure (MAP) in either group. Intrarenal S-methyl-L-thiocitrulline (L-SMTC) in newborns significantly reduced renal blood flow (RBF) 38 ± 4%, glomerular filtration rate (GFR) 42 ± 6%, and increased renal vascular resistance (RVR) 37 ± 7%, whereas intrarenal L-nitro-arginine methyl ester (L-NAME) affected RBF, GFR, and RVR equivalent to L-SMTC treatment. When L-NAME was administered after L-SMTC treatment, newborn renal hemodynamic changes were not further altered from what was observed when L-SMTC was administered alone. In contrast, in the adult, only intrarenal L-NAME, and not L-SMTC, affected renal hemodynamic responses.

DISCUSSION

In conclusion, these studies demonstrate that nNOS is an important regulator of renal hemodynamics in the newborn kidney, but not in the adult.

METHODS

Experiments compared renal hemodynamic responses with intrarenal infusion of L-NAME, an inhibitor of all NOS isoforms, with the selective nNOS inhibitor L-SMTC in the newborn piglet and the adult pig.

Angiotensin II regulates NOS expression in afferent arterioles of the developing porcine kidney

NO protection is crucial against angiotensin II (ANG II) mediated vasoconstriction in postnatal preglomerular resistance vessels. Although whole kidney NOS is developmentally regulated, NOS regulation in developing renal resistance vessels is unknown. The hypothesis was NOS expression and function in developing afferent arterioles are regulated by ANG II through AT1 and AT2 receptors. Afferent arterioles from porcine kidneys, ages newborn, 7, 21 d, and adult, were dissected using a polybead perfusion technique. Dissected afferent arterioles were treated with ANG II and with either the AT1 receptor inhibitor candesartan or the AT2 receptor inhibitor PD 123319 and evaluated for NOS isoform expression and NOS enzymatic activity. Although NOS activity and neuronal NOS (nNOS) expression were greater in the newborn than in the adult, endothelial NOS (eNOS) expression was greater in the adult. ANG II increased NOS activity and eNOS expression at all ages, but nNOS expression only in developing afferents. AT1 and AT2 receptor blockade significantly attenuated NOS activity and eNOS expression at all ages, but nNOS expression only in developing afferents. ANG II regulates nNOS and eNOS expression and NOS activity in afferent arterioles of the developing kidney via AT1 and AT2 receptors.

Nitric oxide synthase and renin-angiotensin gene expression and NOS function in the postnatal renal resistance vasculature

Nitric oxide (NO), produced by nitric oxide synthase (NOS), critically counteracts angiotensin-II-enhanced vascular resistance in the immature kidney, perhaps due to the developmental regulation of NOS expression and function in the postnatal preglomerular resistance vessels (PRV). Our experiments measured the messenger ribonucleic acid (mRNA) gene expression of neuronal NOS (nNOS), endothelial NOS (eNOS), and components of the renin-angiotensin system (renin, AT1 and AT2 receptors), by real-time RT-PCR, as well as NOS enzymatic activity by citrulline assay in PRVs (afferent, interlobular, and arcuate arterioles) obtained from swine ages newborn, 7 and 21 days, and adult. NOS enzymatic activity was upregulated in PRVs immediately after birth but decreased to adult levels with maturation. Neuronal NOS, renin, and AT2 receptor expression in PRVs were upregulated in the newborn and decreased with age to lowest levels in the adult. In contrast, eNOS and AT1 receptor expression were downregulated at birth but increased to the highest levels in the adult. Upregulated NOS enzymatic activity in newborn PRVs supports the critical neonatal role for NO renal vascular vasodilation. Upregulated nNOS gene expression, concomitant with downregulated eNOS gene expression in neonatal PRVs, suggests that the nNOS isoform may be responsible for counteracting angiotensin II increased vascular resistance in immature porcine PRVs.

Glomerular eNOS gene expression during postnatal maturation and AT1 receptor inhibition

Glomerular maturation increases from immature superficial to advanced juxtamedullary nephrons, while nephrogenesis continues postnatally in porcine kidneys. Endothelial NOS, eNOS, shows significant postnatal renal developmental regulation, perhaps mediated by Angiotensin II (AII). The objective was to compare eNOS mRNA gene expression between superficial and juxtamedullary glomeruli obtained from piglets and adult pigs utilizing laser capture microdissection during basal conditions and, to determine the role of the AII AT1 receptor, AT1, after chronic AT1 inhibition (AT1X) with candesartan. Superficial glomerular eNOS expression was lowest in newborns (NB) and at 7 days, and was highest in 14, 21 day old piglets and adults. Juxtamedullary glomerular eNOS, while similar in NB, 14, 21 day and adult, dipped to the lowest level at 7 days. Juxtamedullary glomerular eNOS expression in the NB was 7 fold greater than in superficial glomeruli. AT1X did not change eNOS expression in adult glomeruli. AT1X significantly reduced NB eNOS expression in both superficial, 90+/-10%, and juxtamedullary glomeruli, 89+/-5% respectively. In conclusion, eNOS gene expression demonstrates significant differences between NB superficial and juxtamedullary glomeruli, significant postnatal developmental regulation of both glomerular locations, and this expression may be mediated in the NB by AII via the AT1 receptor.

Quantitative evaluation of renal function in healthy Beagle puppies and mature dogs

Daily urinary collection and assessment of glomerular filtration rate (GFR) and effective renal plasma flow were performed in ten 2-month-old Beagle puppies and ten 6-9 year-old Beagle dogs to identify age-associated differences in renal function. The most striking differences in puppies compared to mature dogs were a higher daily urinary volume (+65%), GFR (+87%), free water reabsorption (+159%), a lower daily protein excretion (-88%), and fractional excretion of phosphorus (-35%). Renal function in Beagle puppies, but not mature dogs, was also quite different compared to data published in younger adult dogs.

Pharmacogenomics and renal drug disposition in the newborn

Genetic polymorphisms in the genes coding for drug metabolizing enzymes, drug transporters, and drug receptors are major determinants of an individual's response to drugs. The potential interactions of pharmacogenomics of renal drug transporters and drug receptors with renal drug disposition and the immature kidneys are briefly reviewed. Examples of gene polymorphisms seen in the RAAS (renin angiotensin system), beta-adrenergic receptors, dopamine receptors and cytochrome P450 and their potential clinical impact are discussed. The human newborn has deficient hepatic and renal drug metabolism and disposition. This immaturity in drug-handling capacity may potentially be superimposed to genetic polymorphisms determining drug metabolism and transport thereby substantially increasing interpatient variability in drug dose requirements and in drug responses in the newborn. Pharmacogenomics is a tool that can be used to individualize drug therapy in newborns to minimize adverse drug effects and to optimize efficacy.

Glomerular filtration rate, effective renal plasma flow, blood pressure and pulse rate in the equine neonate during the first 10 days post partum

The aim of this study was to determine glomerular filtration rate (GFR), effective renal plasma flow (RPF) and filtration fraction (FF) values in Pony foals during the first 10 days post partum in order to assess any age-related changes and the reliability of the single injection inulin/p-aminohippurate (PAH) method. Arterial blood pressure and pulse rate were determined in Pony and Thoroughbred (TB) foals during the first 10 days post partum to establish age-related changes, differences between breeds and possible interrelationships with GFR and RPF. Inulin and PAH were injected intravenously (i.v.) in healthy Pony foals (n = 6) on Days 1, 2, 4, 7 and 10 post partum. Blood samples were collected via the contralateral jugular catheter over a 4 h period. Plasma clearances of inulin and PAH showed that GFR and RPF, respectively, remained relatively constant throughout the post natal period, and were similar to values reported for mature horses. The GFR measurements obtained using the inulin method gave clearance values that were significantly higher (P < 0.01) than those of creatinine clearance performed in a separate group of Pony foals (n = 7). Blood pressure and pulse rates were measured using an electronic sphygmomanometer in TB (n = 6) and Pony foals (n = 7) on Days 1, 2, 3, 4, 7 and 10 postpartum. There were no significant differences in these measurements between the breeds, and both cardiovascular parameters increased during the first 3 days post partum. Values remained unchanged for the rest of the study period. Although measurements of GFR, RPF, blood pressure and pulse rates were not determined in the same animals, blood pressure and pulse rates did not appear to correlate with either GFR or RPF in the Pony foals during the post natal period.

Developmental renal hemodynamics

Renal blood flow, which is lower in the immature than in the mature animal, achieves adult values in human subjects by 1-2 years of age. The age-related increase in renal blood flow cannot be completely explained by increases in kidney size, since nephrogenesis is complete by 36 weeks' gestation in humans. Thus, other factors, especially changes in renal hemodynamics, are likely to be responsible for the increase in renal blood flow. The increase in renal blood flow appears to be directly related to the decrease in renal vascular resistance during the postnatal period. Decreases in the effect of renal vasoconstrictors, increases in the effect of renal vasodilators, or a combination of the two, may be responsible. Many mediators of vasoconstriction have been studied, including adenosine, catecholamines, endothelin, endogenous digitalis-like peptide, and the renin-angiotensin system. Mediators of vasodilation include endothelium-derived relaxing factor (e.g., nitric oxide), prostaglandins, atrial natriuretic peptide, dopamine, and kinins. However, the decrease in renal vascular resistance with age is most likely related to decreases in activity of the renin-angiotensin system and responsiveness to catecholamines; these effects are modulated by nitric oxide. Other mediators may also be important in determining the age-related decrease in renal vascular resistance, but their exact roles remain to be defined.

Renal cortical and renal medullary necrosis in the first 3 months of life

Renal cortical necrosis, renal medullary necrosis, and combined renal cortical-medullary necrosis result from renal ischemia without vascular occlusion. Renal hypoperfusion and ischemic injury in infants have been ascribed to massive blood loss, hemolytic disease, septicemia, and severe hypoxemia. In a postmortem study we identified 82 cases among 1,638 autopsies during the 20 years between 1970 and 1989 in infants 3 months old or less at the time of death. The frequency of renal necrosis in autopsy cases increased significantly during the last 6 years of the study. The distribution of the renal lesion was cortical in 28, medullary in 23, and combined in 31. Forty infants carried diagnoses of congenital heart disease, 17 of asphyxial shock, 9 of sepsis, 3 of infectious myocarditis, 9 of major malformations, 4 of anemic shock, 1 of vascular malformation, and 1 of gastroenteritis and dehydration. A significantly higher proportion of babies with congenital heart disease had cortical involvement. Comparison of clinical characteristics revealed a significantly higher frequency of prematurity, respiratory distress syndrome, bleeding diathesis, and possibly sepsis in the children with congenital heart disease, suggesting that these factors are important in the pathogenesis of the renal lesion. Fourteen infants underwent cardiac catheterization; there was no demonstrable association between the renal lesions and the use of radiographic contrast medium. We conclude that severe congenital heart disease itself is a risk factor for life-threatening renal cortical and medullary necrosis.

Renal blood flow and flow velocity in children and adolescents: duplex Doppler evaluation

The renal arteries of 62 children and adolescents aged 1-16 years without renal or renovascular disease were examined by computer Doppler duplex sonography (DDS) to measure absolute renal blood flow velocities. Maximum systolic velocity (Vmax) and time average velocity (TAV) were not age-dependent. In addition, absolute values of renal artery and renal blood flow were measured. Renal blood flow was 4.1 +/- 1.2 ml/min per gram kidney (two standard deviations), independent of age and comparable to commonly accepted physiological values. The coefficient of variation of blood flow calculations was 6%-15% depending on vessel diameter.

Renal function following acute renal failure in childhood: a long term follow-up study

We measured glomerular and tubular function in 10 children, aged 7 to 19 years, 7 to 12 years after apparent recovery from their episodes of acute renal failure. Although glomerular filtration rate was normal in all, filtration fraction (inulin clearance/PAH clearance) was elevated in six of eight patients. Segmental tubular sodium transport was assessed using clearance techniques during hypotonic saline diuresis; both proximal and distal sodium reabsorption were normal when compared with age-appropriate normal standards. Tubular reabsorption of phosphate, glucose and amino acids were also normal, as were urinary concentrating and diluting capacity and distal tubular hydrogen ion secretion. We conclude that, despite normal glomerular filtration rate, glomerular function is abnormal in a significant number of children who have apparently recovered completely from ARF, probably due to destruction of a proportion of the total nephron population, predominantly those located in the superficial layers of the cortex.

Maturation of circulatory system in three mammalian models of human development

The review surveys the literature on maturation of vasoconstrictor and vasodilator functions in cerebral, renal and intestinal circulations of three non-primate models of human development. An ovine model has been refined for use at both fetal and neonatal stages of development. Important variables controlling regional circulations in the lamb fetus at term include arterial O2 content and pCO2 (brain), angiotensin-II (kidney) and norepinephrine (small intestine). Blood flow autoregulation to decreasing perfusion pressure has been inferred for the renal circulation of the neonate. A canine model has been employed in the postnatal period, usually later than the first week after birth. Important variables controlling regional circulations in the young puppy include arterial pO2 and pCO2 (brain) and epinephrine and angiotensin-II (kidney). Blood flow autoregulation to decreasing pressure has been demonstrated in the cerebral circulation at birth and in the renal circulation at one week thereafter. The intestinal circulation has not been studied with respect to blood flow control. A porcine model has been examined from birth through at least two months of postnatal life. Important variables controlling regional circulations in swine at birth include adrenergic nerve stimulation, arterial pCO2 (brain), angiotensin-II (kidney) and norepinephrine (kidney and small intestine). Blood flow autoregulation to decreasing perfusion pressure has been demonstrated in the brain by the fourth day, in the kidney by the end of the second week and in the small intestine by the end of the first month after birth. The advantage of each model for further investigation of functional maturation of regional circulatory control is summarized.

Renal perfusion in experimental coarctation of the aorta during antihypertensive therapy

Renal perfusion and the effectiveness of antihypertensive therapy were evaluated in experimental coarctation. A sham operation was performed at the age of two months on one group of dogs (n = 5); coarctation of the thoracic aorta was produced at the same age in a second group (n = 7) which was not treated with antihypertensive drugs; and coarctation was similarly produced in a third group (n = 6) which was treated with antihypertensive drugs; the drug treatment was begun one month after the coarctation operation. After operation, all dogs were observed for seven months under similar circumstances. With high doses of antihypertensive agents (hydrochlorothiazide 25 mg/day, propranolol 80 mg/day, and prazosin 20 mg/day), the blood pressure decreased significantly in the coarctation with treatment group compared with the coarctation without treatment group. Renal perfusion, when measured by xenon-133 washout at the age of five months, was similar in the coarctation without drug treatment and control groups but significantly lower (23% decrease, p less than 0.01) in the coarctation with drug treatment group; renal perfusion was similar in all three groups when measured at the ages of eight and nine months. The observed temporary decrease suggests a need to evaluate renal perfusion during antihypertensive treatment in coarctation in man.

Urinary prostaglandins, vasopressin, and kallikrein excretion in healthy children from birth to adolescence

The urinary excretion of PGE2, PGF2 alpha, arginine-vasopressin, and kallikrein was measured in 58 healthy subjects aged 2 days to 13 years. In contrast to the activity of the renin-angiotensin-aldosterone system, which is known to decrease from birth, urinary excretion of PGE2, PGF2 alpha, vasopressin, and kallikrein significantly increased with age. These changes were not correlated to urine volume, natruresis, or glomerular filtration rate. Correction of the values by body surface area obliterated the differences for PGs and vasopressin, but not for kallikrein.

The role of the kidney in sodium homeostasis during maturation

Evidence is presented that the retention of sodium observed during development is consequent primarily to enhanced tubular reabsorption rather than to low rates of glomerular filtration. The enhanced transport of sodium occurs in nephron segments located beyond the proximal tubule, apparently under the stimulation of the high plasma concentration of aldosterone. This adaptive mechanism may account for the fact that the infant thrives on a rather low intake of sodium, as prevails during the period of breast-feeding. The renin-angiotensin-aldosterone system cannot be fully inhibited even by intravascular volume expansion and this may account for the blunted natriuretic response of the developing animal and human to the acute infusion of saline or albumin solutions. Conversely, the renal sodium loss and the hyponatremia often encountered in premature babies appear to be due to an insufficient rise in aldosterone secretion or to a limited responsiveness of the distal tubule to aldosterone stimulation.

Renal blood flow in high and low positions of aortal catheter in rabbits

The effect of aortal catheterization on renal blood flow (RBF) was evaluated in seven rabbits weighing 2500 to 3500 g. RBF was measured 21 times from 13 kidneys during aortal catheterization with an isotope (133 Xe) technique. The catheter was positioned in the aorta of each rabbit both above and below the origins of the renal arteries. The results of these two measurements were compared. Two phases of RBF, a fast first phase and a slow second phase, were determined. The fast phase of RBF was 5.3 ml/g/min, when it was in the high position. The results of the slow phase were identical, 0.16 ml/g/min, in both positions. No significant statistical difference could be found according to the phase of the catheter (paired t-test). These results in rabbits suggest that the aortal catheter does not cause any immediate decrease in RBF. This finding has clinical importance in aortal catheterization in neonates to obtain arterial blood samples. When considering the position of the aortal catheter in neonates factors other than RBF should be taken into account.

Maturation of the proximal tubule in the puppy kidney: a comparison to the adult

Two- to four-day-old beagle puppy kidneys were preapred for transmission and scanning electron microscopy and compared to similarly prepared adult tissues. Proximal tubules of puppy kidneys which contained nephrons in various stages of differentiation were examined and maturational changes were described. Lateral surface contours of proximal tubular cells were initially smooth and relatively unfolded, but progressively acquired complex processes that may be recognized as lateral ridges and lateral-basal processes. Basal projections began as short, stubby processes and gradually took on either a narrow, plate-like or finger-like appearance. Mitochondria lysosomes and apical vacuoles increased in number as the tubules matured. Mitochondria lacked orientation in outer cortical tubules, but exhibited some vertical arrangement within basal processes in inner cortical tubules. Despite features indicating advanced maturation of tubules in the inner cortex, puppy kidneys lacked the lipid droplets characteristic of the adult. Thus, differentiation of this portion of the developing nephron into S1, S2 and S3 segments was not possible at day 4. Morphometric analyses of the lateral and basal membrane surface concentration of proximal convulted tubules from the puppy revealed all cells to have a significantly smaller membrane area than that of the adult. However, the inner cortical cells of the puppy had a greater surface concentration than those of the outer cortex. The reduced transport capacity of the puppy proximal tubule may be realted to the lack of segmentation and/or reduced lateral-basal surface area.

Development of mechanisms for drug excretion

Renal excretion of many drugs is less in the newborn than what would be predicted in terms of body weight. This is due, in part, to low renal blood flow and glomerular filtration rate during the immediate newborn period. In the first weeks to months of postnatal life, renal vascular resistance decreases and blood flow increases. Subsequently, the glomerular filtration rate increases. During this time, drug elimination may also be affected by qualitative and quantitative differences in binding of drugs to plasma proteins. In addition, drug elimination may be prolonged due to an immature ability in the newborn to transport or metabolize drugs in the kidney. Recent evidence demonstrated that drugs administered pre- or postnatally may alter the rate of renal excretion of drugs in the newborn. Such effects are of great concern to the clinician and underscore the importance of monitoring drug blood levels in these patients. From another perspective, the ability to stimulate drug excretion has been used as a tool to study basic renal physiology. Quantitative patterns of development coupled with the use of substrate stimulation provided evidence for at least three transport systems for organic anions in the proximal tubule. Similar technics provided preliminary data linking drug metabolism and organic anion transport in the proximal tubule, whereas an obligatory, rate-limiting role for ligandin in anion transport could not be established.

Renal hemodynamics in the perinatal period. A study in lambs

The perinatal changes in renal hemodynamics have been studied in lambs. Eleven of the lambs were exteriorized, but maintained on placental circulation (fetal lambs). Eight of the lambs were delivered with cesarean section and studied immediately after clamping of the cord (newborn lambs). Nine lambs were delivered spontaneously and studied during the first 9 days of life. Renal blood flow (RBF) was determined by the microsphere method using a sample drawn from the iliac artery as the reference flow. The filtering capacity of the nephrons was evaluated after ferrocyanide injection and dissection of the nephrons. Total RBF did not seem to change much at birth but increased in relation to kidney weight during the first postnatal week. Clamping of the cord did, however, result in a change in intrarenal blood flow distribution, so that in the newborn lambs relatively more of the blood flow was perfusing the outer cortical region. During the first postnatal week there was a slight, but insignificant further relative increase in outer cortical blood flow. The relative increase in outer cortical blood flow at birth was accompanied by an increased frequency of filtering superficial nephrons from 22 to 77%. Practically all juxtamedullary nephrons were filtering before birth. Three days postnatally 98% of the superficial and 100% of the juxtamedullary nephrons were filtering.

Mechanisms of fluid absorption during proximal tubule development

Passive permeation characteristics and paracellular pathway ultrastructure were studied in vitro by perfusion of rabbit isolated proximal convoluted tubules during postnatal ontogenesis. The influence upon net volume flow (Jv) of transepithelial hydrostatic and protein osmotic pressure was significantly higher during early postnatal maturation than in the mature tubule. Hydraulic hydrostatic conductance (X10(-3) ml - cm-2 - min-1 - cm H2O-1) was 0.0367 +/- 0.0048 during early postnatal maturation (N = 99) and 0.0052 +/- 0.002 at maturity (N = 78). Hyperoncotic serum (12.7 +/- 0.4 g/100 ml) in the bath increased Jv by 67.5 +/- 21.1% from 0.31 +/- 0.06 to 0.52 +/- 0.08 nl-mm-1-min-1 in the neonatal proximal tubule, whereas an increase of only 25.7 +/- 20.4% from 1.08 +/- 0.15 to 1.32 +/- 0.18 nl-mm-1-min-1 was noted in the mature proximal tubule during this elevated bath protein-osmotic pressure. Electron microscopic observations showed that microperoxidase passed from tubule lumen through the basement membrane via intercellular spaces in immature tubules perfused at an increased transtubular pressure gradient. This suggests that a transepithelial shunt pathway may participate in changes of conductance during ontogenesis, although length and ultrastructural configuration of tight junctions did not vary with these variables.

CONCLUSIONS

1) Hydrostatic and oncotic water conductance of the rabbit proximal tubule changes with postnatal development. 2) Ultrastructural tracer studies suggest that the change in conductance is due to alteration of the paracellular pathway. 3) Isotonic absorption of the neonatal proximal tubule may depend more on transepithelial pressure gradients than in the mature tubule.

Functional and morphologic maturation of superficial and juxtamedullary nephrons in the rat

The development of single nephron glomerular filtration rate (SNGFER) was studied in both superficial and juxtamedullary nephrons in rats in relation to concomitant morphologic maturation. These experiments were carried out in rats between 23 and 91 days of age (between 36 nad 275 g body weight) with the [14C]ferrocyanide infusion technique. 2. SNGFR of the superficial and juxtamedullary nephrons increased with body weight, glomerular volume and proximal tubular length. 3. The ratio SNGFR of the superficial (S) nephrons/SNGFR of the juxtamedullary (JM) nephrons rose from 0-60 in the 40-60 g rats to 0-84 in the adult rats, demonstrating the centrifugal functional maturation of the nephrons. 4. The S/JM ratio for both glomerular volume and tubular length was constant and averaged 0-72+/--0-12 and 0-81+/-0-05, respectively, indicating that while the increase in SNGFR was greater for S than for JM nephrons, this was not accompanied by concomitant disproportionate increases of glomerular volume and/or proximal tubular length between these nephron categories during development in the rat.

Fluid and electrolyte metabolism in the newborn infant

This chapter has sought to gather together the background information on systems controlling homoeostasis of salt and water and their clinical derangement. The temptation to adopt an all-embracing approach to the management of these problems is strong but such an approach is difficult to achieve and indeed dangerous. The circumstances of each sick infant are unique and plans for treatment must be individually tailored and flexible, dependent upon clinical and biochemical progress. Future developments in this field are likely to involve further understanding of renal and hormonal control of fluid and electrolyte and it might be expected that as new methods of management emerge they will be accompanied by their own peculiar risks of inducing secondary homoeostatic disturbances. With regard to infant feeding each advance appears to underline the desirability of breast feeding and support current moves toward provision of low solute feeds for those who are artificially fed. Paediatricians should be vocal in their advocation of breast feeding and ensure that the major principles of salt and water handling are understood by all who work with small infants. An extra scoop of powdered milk can be more of a poison than a food.

Maturational changes in the fetal glomerular filtration rate

Few data are available documenting the evolution of glomerular filtration rate (GFR) during gestation. In order to investigate this aspect, 33 fetal sheep have been studied, with the use of an intrauterine fetal preparation. It appears that the actual GFR (AGFR) in milliliters per minute increased during the last third of gestation without a significant increase in GFR expressed as milliliters per minute per kilogram of fetal weight (GFR-FW) and as milliliters per minute per gram of kidney weight (GFR-KW). It is suggested that a parallel and constant increase in AGFR and in total fetal weight or kidney weight might explain the absence of variation if GFR-FW or GFR-KW during the last third of gestation.

Functional and morphologic maturation of the superficial nephrons. Relationship to total kidney function

The functional and morphologic pattern of superficial nephron development was studied in guinea pigs ranging in age between 2 h and 38 days. Concomitent measurements of total kidney function and glomerular counts were also performed. Superficial nephron glomerular filtration rate was found to increase from 0.92 to 19.32 nl/min. The filtration rate of the entire kidney rose from 0.19 to 1.31 ml/min. During the first 15 days of life the average rate of increase in glomerular filtration rate per nephron (0.48 nl/min.day), obtained by dividing the increase in total kidney glomerular filtration rate by glomerular number, was more than twice the observed rate of increase in the superficial nephrons (0.21 nl/min.day). During the remainder of the first month, the increase in superficial nephron glomerular filtration rate (0.97 nl/min.day) was greater than the average increase for all nephrons (0.71 nl/min.day). Thus, the initial increase in total kidney glomerular filtration rate was primarily a consequence of the activity of the deep nephrons, whereas during the ensuing period the superficial nephrons appeared to be the sole contributors to the change in total kidney glomerular filtration rate.The increase in superficial nephron glomerular filtration rate was found to correlate closely with the increase in proximal tubular length. Functional glomerulotubular balance was maintained throughout the entire period of renal maturation.

Postnatal changes in renal glomerular blood flow distribution in puppies

The intrarenal distribution of radionuclide microspheres injected into the thoracic aorta was used to examine glomerular blood flow distribution (GBFD) in 26 healthy, unanesthetized puppies, ranging in age from 5 h to 42 days, and in 5 adult dogs. For analysis, the cortex was divided into four equally thick zones designated zone I (subcapsular) to zone IV (juxtamedullary). During the first 36 h of life, the highest flow rate was in zone II, which received 35.5+/-2.0%/g, compared with 26.8+/-1.4% to zone I, 23.7+/-1.4% to zone III, and 13.4+/-1.4% to zone IV. At age 6 wk, zone I had the highest rate of perfusion (48.6+/-2.1%, compared with 28.8+/-1.4% in zone II, 15.8+/-0.8%, in zone III, and 6.8+/-0.6% in zone IV). The 6-wk old animals resembled the adult animals, except for relatively greater perfusion per gram of zone I in the former group. Changes in relative GBFD did not correlate with those in arterial pressure or peripheral hematocrit. The distribution of glomeruli among the four zones of the cortex followed its own pattern of development. At birth and at 6 wk, the greatest density of glomeruli was in zone I (50.6+/-5.4 and 42.7+/-3.9%/g respectively, as compared with 24.1+/-2.9% in adults); in adults zone II contained the greatest density (39.1+/-1.6%). At birth the relative perfusion of glomeruli in zone I was only one-fifth that of glomeruli in zone IV, with intermediate values in zones II and III. By 6 wk of age, increased perfusion of the outer cortical glomeruli resulted in rates of flow in the four zones that did not differ significantly from each other. Relative perfusion in zone I continued to increase, so that in the adult animals perfusion in that zone was significantly greater than in the three deeper zones. These data demonstrate the marked hemodynamic changes that take place within the kidney during the first few weeks of life. The relatively greater blood flow of the most deeply situated nephrons in the early postnatal period suggests ascendancy of this population of nephrons and may have important functional implications.

Maturation of glomerular blood flow distribution in the new-born dog

1. Glomerular blood flow distribution was studied in seventy-eight new-born mongrel dogs aged 1-40 days by measuring the distribution of radioactive labelled microspheres within the kidney.2. The microsphere technique was found to be a valid indicator of glomerular blood flow distribution for the new-born dog since (a) the spheres were completely extracted by the kidney, (b) more than 95% of the spheres were trapped in glomerular capillaries, (c) the spheres were evenly distributed within any specific region of the kidney and, (d) the spheres did not interfere with renal haemodynamics.3. Plasma flow per gram tissue to inner cortical glomeruli, relative to that to outer cortical glomeruli, the IC/OC flow ratio, was high at birth, decreased over the first 2 weeks of life and remained relatively constant thereafter. Plasma flow per gram tissue to the outer cortex increased over the whole 40 day period while that to the inner cortex decreased slightly and then increased after 2 weeks.4. The IC/OC flow ratio decreased in a curvilinear fashion as blood pressure rose with maturation. Acute increases or decreases in blood pressure in any animal produced decreases or increases, respectively, in the IC/OC flow ratio.5. There was no correlation between the IC/OC flow ratio and renal extraction of p-amino-hippurate (E(PAH)).6. There was histological evidence that glomerular differentiation persists for 2 weeks during the post-natal period in the dog. This continuing post-natal glomerulogenesis takes place only in the outer cortex.7. These results are consistent with the hypothesis that an important factor in renal maturation is a redistribution of blood flow within the kidney. As the animal matures a greater fraction of the total glomerular blood flow goes to outer cortical glomeruli. This is due partly to the continuing glomerular differentiation taking place in the outer cortical region and partly to the increasing arterial blood pressure occurring with maturation.

Factors affecting the maturation of renal PAH extraction in the new-born dog

1. The maturation of renal para-amino-hippurate extraction (E(PAH)) was studied in thirty-nine mongrel new-born dogs aged 2-29 days. E(PAH) was also measured in five adult dogs for comparison.2. There was no apparent change of PAH extraction during the first month of life but E(PAH) was lower in the puppy than in the adult. Under mannitol diuresis E(PAH) averaged 0.49 for the puppies and 0.83 for the adult dogs (P < 0.001). E(PAH) averaged 0.48 for the puppies under nondiuretic conditions, a value not significantly different from that observed during mannitol diuresis. There was no correlation of E(PAH) with mean arterial blood pressure.3. Inulin extraction (E(In)) remained constant during the first month of life and was the same in the new-born and adult dog suggesting that preglomerular shunting of plasma away from functioning nephrons was the same for both the puppy and adult dog.4. E(PAH) did not change in the puppy when plasma concentrations varied from trace (using [(3)H]PAH) to 0.25 mM indicating that excessive PAH load is not a factor in the low E(PAH) of the puppy.5. Diffusion of PAH from the red blood cell was excluded as an explanation for the low E(PAH) since diffusion was the same for both the adult and new-born dog.