Maturation of diluting capacity in loop of Henle of rat superficial nephrons

An ontogenic study of adrenomedullin gene expression in the rat lung, adrenal, kidney, and heart

In this study, the gene expression of adrenomedullin (Adm) in the peripheral tissues which include lung, adrenal, kidney, and heart during development was investigated in the rat. The preproadrenomedullin (preproAdm) mRNA and mRNAs of its related receptor components, calcitonin receptor-like receptor (Crlr), and receptor activity-modifying proteins (Ramp1, 2 and 3) of the lung, adrenal, kidney, and heart were measured by real-time RT-PCR and the ADM peptide measured by radioimmunoassay in 1-, 7-, 21-day-old rats and the adult rats. From day 1 to 21, preproAdm mRNA levels increased with age in the lung, the kidney, and the heart but decreased with age in the adrenal. ADM levels, however, increased with age in the lung but decreased with age in the kidney, the adrenal, and the heart. The preproAdm levels in the lung, in the kidney, and in the adrenal all increased in the adult rat. ADM peptide levels, however, decreased in the adult rat. Crlr and Ramp2 gene expression increased with age in the lung, in the kidney, and in the heart but decreased with age in the adrenal in the prepubertal rats. The results indicate that the levels of preproAdm mRNA, ADM peptide and its receptor component mRNAs in different tissues followed different patterns of changes during development.

Tubuloglomerular feedback response in the prenatal and postnatal ovine kidney

The tubuloglomerular feedback mechanism (TGF) plays an important role in regulating single-nephron glomerular filtration rate (GFR) by coupling distal tubular flow to arteriolar tone. It is not known whether TGF is active in the developing kidney or whether it can regulate renal vascular tone and thus GFR during intrauterine life. TGF characteristics were examined in late-gestation ovine fetuses and lambs under normovolemic and volume-expanded (VE) conditions. Lambs and pregnant ewes were anesthetized and the fetuses were delivered via a caesarean incision into a heated water bath, with the umbilical cord intact. Under normovolemic conditions, mean arterial pressure of the fetuses was lower than lambs (51 ± 1 vs. 64 ± 3 mmHg). The maximum TGF response (ΔPSFmax) was found to be lower in fetuses than lambs when tubular perfusion was increased from 0 to 40 nl/min (5.4 ± 0.7 vs. 10.6 ± 0.4 mmHg). Furthermore, the flow rate eliciting half-maximal response [turning point (TP)] was 15.7 ± 0.9 nl/min in fetuses compared with 19.3 ± 1.0 nl/min in lambs, indicating a greater TGF sensitivity of the prenatal kidney. VE decreased ΔPSFmax (4.2 ± 0.4 mmHg) and increased TP to 23.7 ± 1.3 nl/min in lambs. In fetuses, VE increased stop-flow pressure from 26.6 ± 1.5 to 30.3 ± 0.8 mmHg, and reset TGF sensitivity so that TP increased to 21.3 ± 0.7 nl/min, but it had no effect on ΔPSFmax. This study provides direct evidence that the TGF mechanism is active during fetal life and responds to physiological stimuli. Moreover, reductions in TGF sensitivity may contribute to the increase in GFR at birth.

Potassium transport in the maturing kidney

The distal nephron and colon are the primary sites of regulation of potassium (K(+)) homeostasis, responsible for maintaining a zero balance in adults and net positive balance in growing infants and children. Distal nephron segments can either secrete or reabsorb K(+) depending on the metabolic needs of the organism. In the healthy adult kidney, K(+) secretion predominates over K(+) absorption. Baseline K(+) secretion occurs via the apical low-conductance secretory K(+) (SK) channel, whereas the maxi-K channel mediates flow-stimulated net urinary K(+) secretion. The K(+) retention characteristic of the neonatal kidney appears to be due not only to the absence of apical secretory K(+) channels in the distal nephron but also to a predominance of apical H-K-adenosine triphosphatase (ATPase), which presumably mediates K(+) absorption. Both luminal and peritubular factors regulate the balance between K(+) secretion and absorption. Perturbation in any of these factors can lead to K(+) imbalance. In turn, these factors may serve as effective targets for the treatment of both hyper-and hypokalemia. The purpose of this review is to present an overview of recent advances in our understanding of mechanisms of K(+) transport in the maturing kidney.

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.

Developmental regulation of expression of renal potassium secretory channels

PURPOSE OF REVIEW

Somatic growth is associated with an increase in total body K content. K homeostasis is regulated, in large part, by urinary K excretion. Within the adult kidney and specifically the cortical collecting duct, K secretion is accomplished by the passive diffusion of cell K into the urinary fluid down a favorable electrochemical gradient through K selective channels. The purpose of this review is to summarize the results of recent studies that provide insight into how the cortical collecting duct is uniquely adapted for K retention early in life.

RECENT FINDINGS

Electrophysiological analyses have identified two types of apical K channels in the mammalian cortical collecting duct. The prevalence of the secretory K channel and its high open probability at the resting membrane potential in the adult has led to the belief that this channel mediates baseline K secretion. The Ca and stretch-activated maxi-K channel has been proposed to mediate flow-stimulated K secretion. In contrast to the high rates of K secretion observed in adult cortical collecting ducts microperfused in vitro, segments isolated from neonatal animals show no significant net K transport until after the third week of postnatal life. The temporal delay between expression of conducting secretory K channels (baseline K secretion) and maxi-K channels (flow-stimulated K secretion) in the maturing cortical collecting duct reflect unique developmental programs regulating the transcription and/or translation of ROMK (rat outer medullary K channel) and slo, the molecular correlates of the secretory K and maxi-K channels, respectively.

SUMMARY

The K retention characteristic of the neonatal kidney is due, in part, to a paucity of distinct K channels mediating baseline and flow-stimulated K secretion in the collecting duct. The signals directing the developmental regulation of channel expression are as yet unknown.

Maturation of renal potassium transport

The studies outlined in this review suggest that the immaturity of distal nephron segments may hinder urinary excretion of potassium early in life. Among the factors that may limit potassium secretion by principal cells in the neonatal cortical collecting duct are an unfavorable electrochemical gradient (reduced Ki, Na(+)-K(+)-ATPase activity and/or Vte), limited membrane permeability to potassium and sodium, low tubular fluid flow rate, reduced luminal sodium concentration, or increased paracellular backleak. Alternatively, enhanced potassium absorption by other relatively well-differentiated distal nephron segments may contribute in part to a reduced net potassium excretory rate in the newborn. It should be kept in mind, however, that the limited potassium secretory capacity of the immature kidney becomes clinically relevant only under conditions of potassium excess. Under normal circumstances, the tendency of the newborn to retain potassium is an appropriate and necessary condition for growth.(ABSTRACT TRUNCATED AT 250 WORDS)