Renal effects of atrial natriuretic peptide infusion in young and adult rats

Lineage tracing aged mouse kidneys shows lower number of cells of renin lineage and reduced responsiveness to RAAS inhibition

Blocking the renin-angiotensin-aldosterone system (RAAS) remains a mainstay of therapy in hypertension and glomerular diseases. With the population aging, our understanding of renin-producing cells in kidneys with advanced age is more critical than ever. Accordingly, we administered tamoxifen to Ren1cCreERxRs-tdTomato-R mice to permanently fate map cells of renin lineage (CoRL). The number of Td-tomato-labeled CoRL decreased significantly in aged mice (24 mo of age) compared with young mice (3.5 mo of age), as did renin mRNA levels. To determine whether aged CoRL responded less to RAAS blockade, enalapril and losartan were administered over 25 days following uninephrectomy in young and aged mice. The number of CoRL increased in young mice in response to enalapril and losartan. However, this was significantly lower in aged mice compared with young mice due to limited proliferation, but not recruitment. Gene expression analysis of laser-captured CoRL showed a substantial increase in mRNA levels for proapoptotic and prosenescence genes, and an increase in a major prosenescence protein on immunostaining. These results show that CoRL are lower in aged mice and do not respond to RAAS inhibition to the same extent as young mice.

The moth and the aspen tree: sodium in early postnatal development

Over the past 25 years, our perception of the neonatal kidney has changed markedly from its being a "limited" organ compared with that of the adult to being extraordinarily well adapted in its role in maintaining homeostasis and making possible the rapid somatic growth necessary during this critical period of life. The present review focuses on the physiologic adaptations by the neonatal kidney in the maintenance of a positive sodium balance, which is necessary for normal growth not only in mammals but also in moths. There is a fine interplay between the developing brain, heart, thyroid, adrenals, and sympathetic nervous system, all converging on the kidney to conserve sodium, which is limited in the diet. The renin-angiotensin system plays a central role in this response and is balanced by developmental changes in the renal response to atrial natriuretic peptide, all of which contribute to sodium conservation. Over the next 25 years, advances in molecular genetics will doubtless elucidate many more facets of the mechanisms underlying neonatal sodium homeostasis. This will be particularly important as the survival of ever smaller preterm infants improves steadily.

Postnatal development and progression of renal dysplasia in cyclooxygenase-2 null mice

BACKGROUND

Genetic ablation of cyclooxygenase-2 (COX-2) resulted in cystic renal dysplasia and early death in adult mice. The ontologic development of the renal pathology and the biochemical and physiological abnormalities associated with the dysplasia are unknown.

METHODS

Mice homozygous for a targeted deletion of COX-2 (-/-) were compared with wild-type littermates (+/+). Somatic and kidney growth and renal histology were studied at the day of birth and at a number of postnatal ages. Systolic blood pressure, urinalysis, urine osmolality, serum and urine chemistries, and inulin clearance were evaluated in adult animals.

RESULTS

Beginning at postnatal day 10 (PN10), kidney growth was suppressed in -/- animals, while somatic growth and heart growth were unaffected. By PN10, -/- kidneys had thin nephrogenic cortexes and crowded, small, subcapsular glomeruli. The pathology increased with age with progressive outer cortical dysplasia, cystic subcapsular glomeruli, loss of proximal tubular mass, and tubular atrophy and cyst formation. Adult -/- kidneys had profound diffuse tubular cyst formation, outer cortical glomerular hypoplasia and periglomerular fibrosis, inner cortical nephron hypertrophy, and diffuse interstitial fibrosis. The glomerular filtration rate was reduced by more than 50% in -/- animals (6.82 +/- 0.65 mL/min/kg) compared with wild-type controls (14.7 +/- 1.01 mL/min/kg, P < 0. 001). Plasma blood urea nitrogen and creatinine were elevated in null animals compared with controls. Blood pressure, urinalysis, urine osmolality, and other plasma chemistries were unaffected by the deletion of COX-2.

CONCLUSIONS

Deficiency of COX-2 results in progressive and specific renal architectural disruption and functional deterioration beginning in the final phases of nephrogenesis. Tissue-specific and time-dependent expression of COX-2 appears necessary for normal postnatal renal development and the maintenance of normal renal architecture and function.

Age-related changes in renal cyclic nucleotides and eicosanoids in response to sodium intake

The signaling molecules cGMP, cAMP, prostaglandin E(2) (PGE(2)), and prostaglandin F(2alpha) (PGF(2alpha)) play important roles in mediating the response of the kidney to changes in dietary sodium intake. We used a renal microdialysis technique in conscious rats to address the hypothesis that the renal ability to produce these mediators in response to dietary sodium intake is altered during maturation. Young (4-week-old) or adult (6-month-old) rats were studied after the consumption for 5 days of diets containing low (0. 04% NaCl), normal (0.28% NaCl), or high (4.0% NaCl) levels of sodium. Plasma renin activity was significantly increased by low-sodium diet and significantly decreased by high-sodium diet, with no significant difference between the responses of the 2 age groups. Renal interstitial fluid (RIF) levels of cGMP, cAMP, PGE(2), and PGF(2alpha) on normal-sodium diet were similar in the 2 age groups. Low-sodium diet caused a significant increase in RIF levels of all 4 mediators, with no significant differences between the responses of the 2 age groups. High-sodium diet also caused a significant increase in RIF levels of all 4 mediators. However, RIF production of cGMP, cAMP, and PGE(2) was significantly greater, and RIF PGF(2alpha) production was significantly lower, in young rats compared with adult rats. These data demonstrate that the kidneys of young and adult rats respond to dietary sodium restriction in a similar manner but that there are age-related changes in the renal response to sodium loading.

Regulation of the renal response to atrial natriuretic peptide by sodium intake in preweaned rats

The neonate conserves sodium avidly, and sodium intake is normally limited to that present in maternal milk. To evaluate the role of atrial natriuretic peptide (ANP) in this adaptation, preweaned rat pups were artificially reared and fed a formula with either normal sodium (25 mEq/L) or high sodium (145 mEq/L) for 7-8 d. To determine whether increased dietary sodium decreases ANP clearance receptor activity, animals were anesthetized, and the plasma ANP concentration (ANPp), urine flow (V), urinary sodium (UNa V), and cGMP excretion (UcGMP V) were measured before and after infusion of ANF(4-23), an ANP clearance receptor inhibitor (C-ANF), at 50 micrograms/kg/ min. Infusion of C-ANF increased ANPp 10-fold in both normal and high sodium groups, but V, UNa V, and UcGMP V increased only in animals receiving the high sodium diet (p < 0.05). Incubation of isolated glomeruli with 0.1 microM ANP increased extracellular cGMP more in high sodium than normal sodium groups (p < 0.05). We conclude that ANP clearance receptors in the neonate are highly activated regardless of sodium intake. Increased dietary sodium increases the renal diuretic and natriuretic response to circulating ANP through enhanced generation of cGMP.

Ureteral obstruction in the neonatal guinea pig: interaction of sympathetic nerves and angiotensin

The contribution of sympathetic nerves to the hemodynamic effects of unilateral ureteral obstruction (UUO) was investigated in the neonatal guinea pig. The left ureter was partially constricted (or sham-operated) at birth, and sympathetic innervation was inhibited by guanethidine and compared with saline vehicle-treated animals. At 15-20 days of age, blood pressure, cardiac output, total vascular resistance (TVR), renal blood flow, and renal vascular resistance (RVR) were determined before and after infusion of enalapril. UUO reduced cardiac output, increased TVR, and increased RVR of the ipsilateral kidney, whereas guanethidine treatment had no additional effects. Enalapril decreased RVR only in obstructed kidneys and not in intact opposite kidneys of animals with UUO. This was not affected by guanethidine administration. In contrast, enalapril decreased RVR only in guanethidine-treated (but not saline-treated) sham-operated guinea pigs. Therefore, UUO increases angiotensin-dependent vascular tone of the ipsilateral kidney independent of renal innervation. However, UUO decreases angiotensin-mediated vascular tone of the contralateral kidney, an effect unmasked by sympathectomy.

Atrial natriuretic peptide in renal development

Although discovered little more than a decade ago, atrial natriuretic peptide (ANP) has been shown to play a significant role in the maintenance of sodium homeostasis. Immediately after birth, plasma ANP concentration is very high concurrent with right atrial dilatation and a high urinary excretion of cyclic GMP (cGMP), the second messenger for ANP. Following postnatal diuresis and natriuresis, atrial volume, plasma ANP concentration, and urinary cGMP excretion decrease to baseline levels. In the ensuing suckling period, the diuretic and natriuretic response to acute saline volume expansion are attenuated, an effect which is offset by the lower hematocrit at this age. Increase in hematocrit by isovolemic exchange transfusion results in a greater rise of plasma ANP concentration following volume expansion, but a reduced excretion of cGMP. Intravenous infusion of ANP results in greater plasma ANP concentration, and greater urinary excretion of cGMP and sodium, in adult than in young rats. This increased metabolic clearance of ANP during early development is due at least in part to increased activity of clearance receptors. In addition, neutral endopeptidase contributes to removal of circulating ANP in maturing as well as adult rats. Infusion of ANP in neonatal or adult rats results in accumulation of cGMP in glomerular podocytes, with a higher threshold for activation in immature animals. Despite the similar response of intracellular generation of cGMP following exposure to ANP in neonatal and adult rats, egression of ANP out of glomeruli is low in neonates, an effect that is due to immaturity of an organic acid transporter.(ABSTRACT TRUNCATED AT 250 WORDS)

EDRF modulates renal hemodynamics during unilateral ureteral obstruction in the rat

Unilateral ureteral obstruction (UUO) results in vasoconstriction of the ipsilateral kidney, and vasodilatation of the intact opposite kidney. To investigate the role of endogenous nitric oxide, an endothelial-derived relaxing factor (EDRF), in the regulation of renal hemodynamics during UUO, Sprague-Dawley rats were anesthetized for study 24 hours after left UUO or sham-operation. Total vascular resistance (TVR) and renal vascular resistance (RVR) were measured using radioactive microspheres during control periods and following infusion of the nitric oxide synthase inhibitor, L-NAME (2.5 mg/kg). Blood pressure and RVR were increased by L-NAME, with a greater increment in the RVR/TVR ratio of the kidney with ipsilateral UUO than in the intact opposite kidney or sham-operated kidneys. Infusion of L-arginine (L-Arg), a substrate for nitric oxide synthase, did not alter the RVR/TVR ratio of either kidney of rats with UUO, but reduced the ratio in sham-operated animals. L-NAME tended to reduce urine flow and urinary sodium and cyclic GMP excretion, whereas L-Arg resulted in a marked diuresis, natriuresis, and increased excretion of cyclic GMP in both operative groups. We conclude that EDRF activity is increased in the kidney with ipsilateral UUO, which serves to counteract renal vasoconstriction. This response is not limited by availability of substrate (L-Arg). Vasodilatation of the intact opposite kidney appears to be mediated by factors other than EDRF.

Mechanisms regulating renal sodium excretion during development

The present review focuses on the ontogeny of mechanisms involved in renal sodium excretion during renal maturation. The effect of birth on renal excretion of sodium and the role played by the different tubular segments in the regulation of sodium excretion during maturation are discussed. The influence of circulating catecholamines and renal sympathetic innervation in regulating sodium excretion during renal development is reviewed. The effects of aldosterone, atrial natriuretic factor, and prostaglandins on sodium regulation during renal maturation are discussed. Special emphasis is given to the potential role of glucocorticoids in modulating sodium excretion early in life.

Atrial natriuretic peptide, cyclic guanosine monophosphate and sodium excretion during postnatal adaptation in male infants below 34 weeks gestation with severe respiratory distress syndrome

The role of atrial natriuretic peptide (ANP), in the perinatal period, is at present unclear. In adults urinary cyclic guanosine monophosphate (cGMP) is considered an index of the biological activity of plasma ANP. The aim of this study was to determine the relationship between plasma ANP, cGMP excretion (cGMPex) and sodium excretion (Naex) in preterm infants in the first days after birth. Sequential, 4 hourly, measurements of plasma ANP, cGMPex and Naex were made in 12 male neonates of median gestational age 27 weeks (range 25-33) and median birth weight 0.981 kg (range 0.635-2.029) over a median period of 5.2 days (range 2.3-10). The ratios of cGMPex to ANP and of Naex to cGMPex were each plotted against postnatal age. The ratio of cGMPex to ANP increased ten fold in the first 3-4 days after birth but then remained relatively constant; the ratio of Naex to cGMPex showed a steady increase from birth. We conclude that, in extremely immature infants, renal sodium loss in response to cGMP increases rapidly during the first 10 days after birth. In addition, after 3-4 days from birth, plasma ANP ia associated with a constant proportionate rate of cGMP excretion though, as the plateau ratio of cGMPex to ANP varied widely between babies, cGMPex cannot be used to predict plasma ANP in cross sectional studies. These changes may reflect postnatal adaptation and/or maturation of both ANP receptors and cGMP mediated cascades. In the immediate postnatal period, plasma ANP may also have a non-renal role.

Developmental pattern of cyclic guanosine monophosphate production stimulated by atrial natriuretic peptide in glomeruli microdissected from kidneys of young rats

Cyclic guanosine monophosphate (GMP) productions by alpha rat atrial natriuretic peptide 1-28 (alpha-rANP), carbamylcholine or sodium nitroprusside were assessed in isolated glomeruli microdissected from collagenase-treated kidneys of 2- to 34-day-old and adult rats. In both young and adult animals, alpha-rANP-stimulated cyclic GMP generation was proportional to the number of glomeruli and was enhanced in a dose-dependent and saturable fashion with increasing alpha-rANP concentrations. The apparent activation constant values were 6.4 nM for 5-day-old and 9.7 nM for adult rats. Maximal doses of either alpha-rANP or rANP 5-28 elicited similar responses in young and adult animals. Clear differences appeared between the developmental patterns of cyclic GMP productions stimulated by either alpha-rANP, carbamylcholine or sodium nitroprusside. The response to alpha-rANP was very large in the youngest rats tested, declined sharply during the suckling period and represented about 1.6 times the adult control level in 34-day-old rats. In contrast, the response to carbamylcholine was low after birth and rose progressively with age up to the adult level at the end of the weaning period, and the response to nitroprusside seemed to be independent of the animal's age.

Exaggerated natriuretic response to atrial natriuretic factor in rats developing spontaneous hypertension

The present study was designed to evaluate the renal response to atrial natriuretic factor (ANF) in young rats developing spontaneous hypertension (SHR) and compare this response to age-matched, normotensive controls (WKY) and adult animals. At 6 weeks of age, intravenous infusion of ANF (0.25 micrograms/kg min) in anesthetized, euvolemic rats produced a significantly larger natriuresis and diuresis in SHR compared with WKY rats; this strain difference was not observed in rats 11 weeks of age. SHR showed no age-related change in the natriuretic response to ANF, whereas adult WKY rats exhibited a greater response than young WKY rats. To determine the effect of renal perfusion pressure on the magnitude of the renal response to ANF, additional groups of 6- and 11-week-old SHR were studied while renal perfusion pressure was lowered acutely by aortic constriction (SHR-AC) to values similar to age-matched WKY rats. In young rats, the diuretic and natriuretic response to ANF was greatest in SHR, intermediate in SHR-AC, and lowest in WKY rats. In adult animals, the natriuretic and diuretic response was similar in SHR and WKY rats and tended to be less in SHR-AC. These results in both 6- and 11-week-old SHR are consistent with previous reports that the magnitude of the response to ANF is directly related to acute changes in renal perfusion pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Interrelationship of atrial natriuretic peptide, atrial volume, and renal function in premature infants

Infants experience dramatic changes in fluid balance during the first few days of life, which provides an opportunity to observe the interrelationships of changing atrial size, atrial natriuretic peptide (ANP) secretion, and renal function during a relatively short period. To study these relationships, we examined nine infant boys (mean birth weight 1180 gm and gestational age 30 weeks) at 20 to 28 hours of age and then at four 24-hour intervals. Measurements included plasma ANP concentration, two-dimensional echocardiographic estimations of left and right atrial volumes, Doppler determination of ductus arteriosus patency, creatinine clearance, urine flow rate, urinary sodium excretion, and cyclic guanosine monophosphate (cGMP) excretion. Plasma ANP concentration was found to decrease with age and to correlate with decreasing size of the right atrium, closure of the ductus arteriosus, urinary cGMP excretion, and sodium excretion. We speculate that elevated plasma ANP values in a preterm neonate reflect an expanded volume state. As volume contraction, reflected by decreasing atrial volume and body weight occurs, ANP levels decrease, which may diminish diuresis. These findings are compatible with a significant role for ANP in volume homeostasis of newborn infants.

Special needs of the newborn infant in fluid therapy

The preterm baby, although less able to tolerate high fluid and solute loads, has a remarkable ability to adapt to a rather broad range of environmental challenges, and thus renal function in the preterm infant should no longer be considered impaired. Lower rather than higher fluid administration is the current trend in neonatal care because it is safer and more physiologic. Although changes in body weight over time provide a good indicator of water balance, determination of plasma and urine electrolytes and tonicity are also essential in the monitoring of fluid therapy in preterm infants.

Tubular response to hormones is blunted in weanling rats

The renal responses to atrial natriuretic peptide (ANP), parathyroid hormone (PTH) and arginine vasopressin (AVP) were studied in anaesthetized weanling and adult rats with clearance methods. In rats receiving PTH, thyroparathyroidectomy (TPTX) was performed prior to the study. The results showed that GRF remained unchanged in both age groups during infusion with ANP, AVP and PTH after TPTX as well as with TPTX alone. During ANP infusion, the urine flow rate increased from 0.01 +/- 0.002 to 0.029 +/- 0.004 ml (100 g body wt)-1 min-1 (P less than 0.001) and the fractional sodium excretion increased from 0.48 +/- 0.1 to 3.0 +/- 0.4% (P less than 0.001) in the adult rats; no significant changes were observed in the weanling rats. Phosphate excretion was not influenced by ANP in either age group. After TPTX, both net and fractional phosphate excretion decreased in both age groups. During PTH infusion in TPTX rats, both net and fractional phosphate excretions increased significantly in the adult rats but were unchanged in the weanling rats. In rats of the same age, the renal concentrating capacity and urinary excretion of prostaglandin E2 (PGE2) were determined after dehydration. The renal concentrating capacity was lower, and the renal excretion of PGE2 was higher, in the weanling than in the adult rats. Furthermore, the concentrating mechanism seems not to be influenced by indomethacin treatment in either age group. It is concluded that the tubular responses to ANP and PTH are blunted in the immature kidney and that the renal excretion of PGE2 is not an important factor in the regulation of the concentrating capacity in the weanling rat.

Developmental patterns of renal atrial natriuretic peptide receptors: [125I]alpha-rat atrial natriuretic peptide binding in glomeruli and inner medullary collecting tubules microdissected from kidneys of young rats

The ontogenic developmental patterns of atrial natriuretic peptide (ANP) receptors of glomeruli and inner medullary collecting tubules (IMCT) were studied by measuring the specific binding of [125I]alpha-rat ANP 1-28 ([125I]alpha-RANP) to isolated glomeruli and IMCT microdissected from collagenase-treated kidneys of young rats aged from 2 to 35 days post-partum. For glomeruli and IMCT from young and adult animals, total and non-specific binding increased linearly with glomerulus number or tubular length. ANP receptors detected in glomeruli and IMCT from young rats showed the same stereospecifities as those from adult rats for recognition of ANP analogues (alpha-RANP 1-28, ANP 3-28, atriopeptin III and atriopeptin II). The numbers of ANP receptors in glomeruli and IMCT (expressed in terms of 10(-18) mol labelled ANP bound per glomerulus or per mm IMCT length, respectively) exhibited marked variations during postnatal ontogenesis; they were low after birth and rose progressively with age up to the corresponding adult levels (20 +/- 2 X 10(-18) mol.glom-1 and 4.4 +/- 0.8 X 10(-18) mol.mm-1) at the end of the 5th week of postnatal life.

Age differences in renal response to atrial natriuretic peptide in rabbits

Plasma levels of atrial natriuretic peptide (ANP) and the effect of exogenous ANP on renal function have been studied in newborn and adult rabbits. In order to investigate an age difference in responsiveness to ANP, we studied the renal effects of alpha-human ANP (1-28) administered at the same dose per kg body weight in adult and neonatal rabbits. Plasma basal ANP levels were similar in 18 newborn (4- to 11-day-old) compared to 7 adult rabbits (150 +/- 16 and 151 +/- 28 pg/ml, resp.). Eleven newborn and 11 adult rabbits were anesthetized and mechanically ventilated. After a control period, each animal received an hANP loading dose (3 micrograms/kg i.v.), followed by an infusion of 0.3 micrograms/kg/min. Blood gases remained stable throughout the experiment in both groups. Mean blood pressure decreased in newborn (28.5 +/- 0.8 to 26.2 +/- 1.0 mmHg) and adult (92 +/- 3 to 84 +/- 3 mmHg) animals. Percent hANP-induced changes in renal functions in newborn and adult rabbits were, respectively: urine flow rate: -21 +/- 4% and +57 +/- 8%; urinary sodium excretion: +4 +/- 7% and +81 +/- 11%; glomerular filtration rate (GFR): -19 +/- 4% and -4 +/- 6%; renal blood flow (RBF): -22 +/- 4% and -11 +/- 5%. As expected, diuresis and natriuresis increased in adult rabbits. Failure of hANP to increase natriuresis and diuresis in newborn rabbits could be related to the marked decrease in GFR, receptor immaturity and/or interactions with other hormonal systems.

Endocrine control of electrolyte balance during development

The endocrine control of electrolyte balance during development is reviewed. It is suggested that the high urinary sodium excretion observed in premature infants may be secondary to the immaturity of the adrenal gland to adequately increase the secretion of aldosterone (Sulyok et al, 1979b), and to the inability of the distal tubule to respond appropriately to a rise in circulating aldosterone levels (Sulyok et al, 1979a). On the other hand, the elevated plasma aldosterone levels observed in term newborn infants may play an important role in the blunted response of the newborn kidney to saline loading (Sulyok et al, 1979a; Spitzer, 1982). The ability of ANP to induce a natriuresis and to contribute to fluid and electrolyte homeostasis during development has been investigated. It has been found that the immature kidney is less responsive to ANP than later in life (Chevalier et al, 1988; Robillard et al, 1988). On the other hand, it has been suggested that a rise in plasma ANP during the first five days of life may contribute to the physiological weight loss associated with the extracellular volume contraction occurring shortly after birth (Tulassay et al, 1987). The role of glucocorticoids, prostaglandins and the kallikrein-kinin system in regulating electrolyte balance during development is also reviewed.