The renal function of newborn infants

Fluid Homeostasis and Diuretic Therapy in the Neonate

Understanding physiologic water balance and homeostasis mechanisms in the neonate is critical for clinicians in the NICU as pathologic fluid accumulation increases the risk for morbidity and mortality. In addition, once this process occurs, treatment is limited. In this review, we will cover fluid homeostasis in the neonate, explain the implications of prematurity on this process, discuss the complexity of fluid accumulation and the development of fluid overload, identify mitigation strategies, and review treatment options.

Comparison Between Sodium Acetate and Sodium Chloride in Parenteral Nutrition for Very Preterm Infants on the Acid-Base Status and Neonatal Outcomes

PURPOSE

To compare between sodium acetate (SA) and sodium chloride (SC) in parenteral nutrition (PN) with associated metabolic acidosis and neonatal morbidities in preterm infants.

METHODS

Preterm infants below 33 weeks gestational age, and with a birth weight under 1,301 g were enrolled and further stratified into two groups: i) <1,000 g, or ii) ≥1,000 g in birth weight. The subjects were randomized to receive PN containing SA or SC within the first day of life. The results of routine blood investigations for the first 6 days of PN were collated, and the neonatal outcomes were recorded upon discharge or demise.

RESULTS

Fifty-two infants entered the study, with 26 in each group: 29 infants had extremely low birth weight (ELBW). There were no significant differences in birth weight, gestation, sex, exposure to chorioamnionitis and antenatal steroids, surfactant doses and duration of mechanical ventilation between groups. The SA group had significantly higher mean pH and base excess (BE) from days 4 to 6 than the SC (mean pH, 7.36 vs. 7.34; mean BE -1.6 vs. -3.5 [p<0.01]), with a two-fold increase in the mean BE among ELBW infants. Significantly fewer on SA required additional bicarbonate (n=4 vs. 13, p=0.01). The rate of bronchopulmonary dysplasia (BPD) was approximately four-fold lower in SA than SC (n=3 vs. 11, p<0.01). No significant differences were observed in necrotizing enterocolitis, patent ductus arteriosus, retinopathy of prematurity, cholestatic jaundice, and mortality between groups.

CONCLUSION

The use of SA in PN was associated with reduced metabolic acidosis and fewer BPD.

Developmental changes in renal function

PURPOSE OF REVIEW

Although nephrogenesis in a term infant is complete, there are a number of functional changes that occur in the kidney as the infant matures. Understanding these changes will aid in the evaluation of neonates to delineate what is normal development versus a pathophysiologic problem. In addition, as many drugs are either cleared by the kidneys or can affect renal function, dosing regimens are dramatically different in the neonate as compared with the adult. These differences are greatly exaggerated in the preterm infant, making it more difficult to determine if there is a pathophysiologic problem.

RECENT FINDINGS

While investigators in recent years have made great strides in understanding the early embryology of the kidney and the molecular signals involved in the formation of the kidney, there remains a paucity of functional studies. The most recent studies have re-examined the changes in the serum creatinine in the newborn and how this impacts the excretion of drugs. Developmental changes in the renal tubule transport systems and their regulation have also been more extensively studied.

SUMMARY

The kidney undergoes many developmental physiologic changes as the neonate adapts to extra-uterine life. Understanding these changes will help in the medical management of these infants.

Release of vasopressin and oxytocin from isolated pituitary glands of adult and new-born rats

1. Pituitary glands of adult rats of both sexes, of lactating female and of new-born rats, incubated in a Locke solution, release both oxytocin and vasopressin. The amount of hormones released, during a measured period of incubation, is related to the actual hormone content of the gland.2. Increasing the concentration of KCl in the incubation medium, with CaCl(2) present and in concentration of at least 2.2 mM, produces an enhanced release of both hormones from pituitary glands of adults, but does not affect the release of hormones from glands of new-born animals.3. Addition of ouabain to the incubation medium produces a marked increase of the release of the hormones from glands of both adult and new-born rats. This is accompanied by an extrusion of K ion and an influx of Na ion. The effect of ouabain on the hormone release and the shift of ions can be reversed by subsequent addition of adenosine triphosphate.4. The increased release of hormones produced by ouabain, in glands from new-born rats, is unaffected by the presence or absence of CaCl(2). In adults, however, the effect of ouabain, though present, is reduced in the absence of CaCl(2).5. It is suggested that in glands from adult animals, the hormones must be freed from their attachment on the protein-carrier, neurophysin and that this can be achieved by the entry of calcium ion into the cell. The subsequent secretion of the ;freed' hormones appears to be accompanied by a shift of ions across the cell membrane.6. In glands from neonates up to 3 weeks old, the absence of neurophysin, or its poor capacity for binding the hormones, explains the inability of calcium to operate in the same way as in the glands of adults. There is evidence suggesting that the secretion of the neurohypophysial hormones in the new-born animal consists mainly of their diffusion from the cells, without previous elution of the hormones as in adults.

The evolution of evolutionary physiology

Studies on comparative and ontogenetic physiology appeared in the 19th and the beginning of the 20th centuries, and the view that these two methods are important for developing the bases of the evolution of functions was formulated. The term "evolutionary physiology" was proposed by Severtsov in 1914. At the beginning of the 1930s, the Laboratory for the Development of Problems in Evolutionary Physiology was established in the USSR and a review of these studies was published. The I. M. Sechenov Institute of Evolutionary Physiology was set up in Leningrad in 1956 under the direction of Orbeli, who formulated the main study areas and methods of evolutionary physiology. This field of physiology was actively developed over the following half century. Evolutionary physiology addresses problems of the evolution of functions and functional evolution, often recruiting methods from allied scientific fields, including biochemistry, morphology, and molecular biology.

Development of water transport in the collecting duct

The ability of the immature kidney to concentrate urine is lower than in adults. This can lead to severe water and electrolyte disorders, especially in premature babies. Resistance to AVP and lower tonicity of the medullary interstitium seem to be the major factors limiting urine concentration in newborns. AVP-stimulated cAMP generation is impaired. This is the result of inhibition of the production by PGE(2) acting through EP3 receptors and increased degradation by phosphodiesterase IV. The expression of aquaporin-2 (AQP2) in the immature kidney is low; however, under conditions of water deprivation and after stimulation with DDAVP, it rises to adult levels. The expression of AQP3 and AQP4 is intact at birth and does not seem to contribute to the hyporesponsiveness to AVP. Low sodium transport by thick ascending loops of Henle, immaturity of the medullary architecture, and adaptations in the transport of urea contribute to the lower tonicity of the medullary interstitium. This paper reviews the alterations in the AVP signal transduction pathway in the immature kidney.

Antidiuretic hormone resistance in the neonatal cortical collecting tubule is mediated in part by elevated phosphodiesterase activity

Neonates cannot concentrate their urine to the same degree as adults. One of the key factors in concentrating the urine is the renal collecting duct osmotic water permeability (Pf) response to antidiuretic hormone (ADH). Neonatal cortical collecting ducts have a blunted Pf response to ADH compared with adult tubules (Pf: 119.0 +/- 12.5 vs. 260.1 +/- 29.5 microm/s, P < 0.05). We found that the phosphodiesterase activity in the neonatal collecting ducts was higher than that in the adult collecting ducts (3,970 +/- 510 vs. 2,440 +/- 220 cpm.microg tubular protein-1.20 min-1, P < 0.05). After pretreatment of in vitro microperfused tubules with the nonspecific phosphodiesterase inhibitor IBMX (10-6 M in the bath), the Pf response to ADH in neonatal collecting ducts was 271.4 +/- 51.7 microm/s, which was identical to that of the adult collecting duct [315.3 +/- 31.3 microm/s, P = not significant (NS)]. Rolipram, a specific type IV phosphodiesterase inhibitor, lowered the elevated phosphodiesterase activity in the neonatal tubules to that in the adult tubules (2,460 +/- 210 vs. 2,160 +/- 230 cpm.microg tubular protein-1.20 min-1, P = NS). Neonatal tubules pretreated with rolipram (10-5 M) in the bath also had a Pf response to ADH that was comparable to that of the adult tubules (258.2 +/- 17.0 vs. 271.4 +/- 32.6 microm/s, P = NS). Thus the elevated phosphodiesterase activity in the neonatal tubules appears to be due to an increase in type IV phosphodiesterase activity. Hence, one of the key factors in the decreased ability of neonates to concentrate their urine is overactivity of phosphodiesterase in the cortical collecting duct that blunts the neonatal collecting duct Pf response to ADH.

Maturational changes in renal tubular transport

PURPOSE OF REVIEW

This review examines the maturational changes that occur in renal tubules during postnatal development.

RECENT FINDINGS

The ability to study transport in neonatal tubules and the use of molecular techniques have allowed studies that not only examine the mechanism of solute and water transport in neonates but also what causes the maturational changes in transport at a molecular and cellular level.

SUMMARY

This review demonstrates that there are significant quantitative and qualitative differences in transport during postnatal maturation in every nephron segment. In some segments the maturational changes involve simply a change in abundance of transporters, while in others the difference in transport is due to changes in transporter isoforms, changes in paracellular permeability or changes in intracellular signaling that regulate the transporter. This review focuses on these changes and what is known about what causes the maturational changes in transport.

Sodium restriction versus daily maintenance replacement in very low birth weight premature neonates: a randomized, blind therapeutic trial

To test the hypothesis that restriction of sodium intake during the first 3 to 5 days of life will prevent the occurrence of hypernatremia and the need for administration of large fluid volumes, we prospectively and randomly assigned 17 babies (mean +/- SD: 850 +/- 120 gm; 27 +/- 1 weeks of gestation) to receive in blind fashion either daily maintenance sodium or salt restriction with physician-prescribed parenteral fluid intake. Maintenance-group infants received 3 to 4 mEq of sodium per kilogram per day; restricted infants received no sodium supplement other than with such treatments as transfusion. Sodium balance studies conducted for 5 days demonstrated that maintenance salt intake resulted in a daily sodium balance near zero, whereas sodium-restricted infants continued to excrete urinary sodium at a high rate, which promoted a more negative balance (average daily sodium balance -0.30 +/- 1.78 SD in maintenance group vs -3.71 +/- 1.47 mEq/kg per day in restriction group; p less than 0.001). Care givers tended to prescribe daily increases in parenteral fluids for the salt-supplemented infants, perhaps because serum sodium concentrations were elevated in these infants after the first day of the study (p less than 0.001). Hypernatremia developed in two sodium-supplemented infants (greater than 150 mEq/L), and hyponatremia developed in two sodium-restricted infants (less than 130 mEq/L); however, the restricted infants were more likely to have normal serum osmolality (p less than 0.05). Both groups of infants produced urine that was neither concentrated nor dilute, with a high fractional excretion of sodium; renal failure was not observed. The mortality rate was not affected, but the incidence of bronchopulmonary dysplasia was significantly less in the sodium-restricted babies (p less than 0.02). We conclude that in tiny premature infants, a fluid regimen that restricts sodium may simplify parenteral fluid therapy targeted to prevent hypernatremia and excessive administration of parenteral fluids.

Modern fluid and electrolyte management of the critically ill premature infant

In this article, the authors introduce the concept of a transitional physiology which governs fluid and electrolyte balance in the immediate postnatal period. The important impact of the extrauterine environment on fluid balance is also discussed. Finally, the pathophysiology of diuresis in RDS, and fluid shifts in the VLBW infant with therapeutic recommendations are presented.

Urinary cyclic AMP and renal concentrating capacity in infants

In 15 infants between 1 and 31 weeks the effect of antidiuretic hormone (ADH) on the renal concentrating capacity and urinary cyclic AMP (cAMP) was tested. A significant decrease of urine flow and a significant increase of osmolality, urea and cAMP was observed indicating that the distal nephron of the infant kidney is responsive to exogenous ADH and that its effect is mediated by cAMP. The results of a second series with 52 normally hydrated infants demonstrate that the nonlinear age-related increase of osmolality and urea in urine is accompanied by a similar pattern of cAMP excretion, pointing out that the maturation of the concentrating capacity seems to be related to an increasing responsiveness of the cAMP system to ADH. Furthermore the results raise the possibility that increasing concentrations of urea and solutes in the medulla and papilla of the infant kidney may have--in the presence of very low ADH secretion--an additional stimulating effect on cAMP formation.

ostnatal development of renal function: micropuncture and clearance studies in the dog

Postnatal renal development was studied in dogs between 2 and 77 days. Single, superficial nephrons were evaluated by micropuncture, concurrently with measurements of total renal function and morphometric analyses in the same animals. Glomerular filtration rate for the entire kidney increased linearly from 0.13 ml/min per g kidney weight at 2 days to 0.91 at 77 days. Extraction of p-aminohippurate increased from about 20 to 80%, and renal plasma flow per g kidney weight, measured as Cpah/Epah, increased threefold during the same period. Filtration fraction increased to the mature value during the first half of the postnatal period studied. The clearance of urea per unit of renal mass increased with age, whereas the fraction of filtered urea reabsorbed declined during the early part of the postnatal period. The pattern of fractional urea reabsorption may be due mainly to increased medullary recycling of urea and to a rise in the reabsorption of water from the medullary collecting duct. Urine osmolality was higher than plasma from birth onward and rose with age. Osmolal equality of collecting duct fluid and medullary interstitium reflected mature vasopressin (ADH)-induced water permeability. The rise in urinary concentration was predominantly due to increasing medullary sequestration of urea. Glomerular filtration rate of the superficial nephron increased from 3.2 nl/min at 21 days, when subcapsular nephrons were uniformly patent, to 23.1 at 77 days. Despite this rise in filtered load, fractional reabsorption of sodium and water in superficial proximal tubules was constant and at the mature level from the onset of intratubular perfusion. Changes in arterial plasma protein concentration, in filtration fraction, and in the hydrostatic pressure gradient between proximal tubule and peritubular capillary may interact to maintain glomerulotubular balance. The data, together with results of an accompanying morphological study, demonstrate a sequence of coordinated changes during postnatal renal maturation.

Surgical aspects of renal damage in childhood: assessment, salvage and aftermath

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