Neonatal Rat Kidney Research Articles

The AQP2 water channel: effect of vasopressin treatment, microtubule disruption, and distribution in neonatal rats.

Aquaporin 2 is a collecting duct water channel that is located in apical vesicles and in the apical plasma membrane of collecting duct principal cells. It shares 42% identity with the proximal tubule/thin descending limb water channel, CHIP28. The present study was aimed at addressing three questions concerning the location and behavior of the AQP2 protein under different conditions. First, does the AQP2 channel relocate to the apical membrane after vasopressin treatment? Our results show that AQP2 is diffusely distributed in cytoplasmic vesicles in collecting duct principal cells of homozygous Brattleboro rats that lack vasopressin. In rats injected with exogenous vasopressin, however, AQP2 became concentrated in the apical plasma membrane of principal cells, as determined by immunofluorescence and immunogold electron microscopy. This behavior is consistent with the idea that AQP2 is the vasopressin-sensitive water channel. Second, is the cellular location of AQP2 modified by microtubule disruption? In normal rats, AQP2 has a mainly apical and subapical location in principal cells, but in colchicine-treated rats, it is distributed on vesicles that are scattered throughout the entire cytoplasm. This is consistent with the dependence on microtubules of apical protein targeting in many cell types, and explains the inhibitory effect of microtubule disruption on the hydroosmotic response to vasopressin in sensitive epithelia, including the collecting duct. Third, is AQP2 present in neonatal rat kidneys? We show that AQP2 is abundant in principal cells from neonatal rats at all days after birth. The detection of AQP2 in early neonatal kidneys indicates that a lack of this protein is not responsible for the relatively weak urinary concentrating response to vasopressin seen in neonatal rats.

Cellular and ultrastructural location of angiotensinogen in rat and sheep kidney

Recent evidence suggests the involvement of a local renin-angiotensin system in some renal actions of angiotensin II (Ang II). In this study the renal distribution of the precursor to angiotensin formation, angiotensinogen, was investigated in rats and sheep using immunohistochemistry, immunoelectron microscopy and non-isotopic hybridization histochemistry. Immunostaining for angiotensinogen was seen in proximal tubules (PCT) of both rat and sheep kidneys. In the rat the strongest immunostaining was found in the kidneys of neonatal (1 day old) rats. Staining declined after birth. Non-isotopic hybridization histochemistry using oligodeoxynucleotide probes labeled with biotin confirmed the presence of angiotensinogen mRNA expression in PCT of the rat renal cortex. Electron microscopic immunohistochemistry using antibodies raised against rat angiotensinogen showed weak staining in the adult of granule-like structures close to the apical membrane of PCT cells. In the neonatal rat kidney, angiotensinogen immunostaining was found throughout the PCT cells and was markedly stronger than that seen in adult rat kidney. In sheep, angiotensinogen immunostaining with an antibody raised against purified ovine angiotensinogen showed staining of PCT in fetal, newborn and adult sheep kidney. The strongest immunostaining seen was in fetal sheep kidney with a decline seen after birth. Reverse transcription polymerase chain reaction (RT-PCR) showed that angiotensinogen mRNA was expressed in the sheep kidney at all ages studied. Angiotensinogen expression was higher in fetal sheep kidneys (77 day and 141 day gestation) than in adult sheep kidney. In conclusion, angiotensinogen mRNA expression was detected in both rat and sheep kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatially restricted expression of set mRNA in developing rat kidney.

A somatic translocation event fusing the novel gene set to the putative oncogene can has been implicated in the development of acute nonlymphocytic leukemia in humans. In this study, full-length cDNAs highly homologous with human set were cloned from a rat neonatal kidney library. The expression pattern of set mRNA was then examined in developing rat kidney. Two groups of set cDNAs (alpha and beta) with different translation initiation sites and open reading frames of 867 and 831 bp, respectively, were found. The predicted protein products are 33,385 and 32,085 Da in size and contain approximately 30% acidic residues, over half of them clustered at the COOH terminal, thus forming a long acidic tail. No signal peptide or membrane-spanning domains were identified, suggesting an intracellular protein product. By ribonuclease protection assay, both alpha and beta variants of set were expressed in kidney. On Northern blots of total kidney RNA, 3.0- and 2.2-kb mRNAs hybridized with the labeled set cDNA probe. Expression of both transcripts was four- to eightfold greater in neonatal compared with adult rat kidney. When neonatal rat kidneys were examined for set mRNA expression by in situ hybridization with 35S-labeled riboprobe, expression was densely localized in the cortical region of morphogenesis over primitive nephron structures, including S-shaped bodies. Thus mRNA for Set, a putative intracellular protein involved in leukemogenesis, is expressed in kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and characterization of angiotensin II AT2 receptors from neonatal rat kidney.

Angiotensin II (Ang II) AT2 receptors were purified 40,000-fold to a nearly homogeneous state after solubilization from neonatal rat kidney membranes with 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propane-sulfonic acid. Comparable IC50 values for the soluble extract (0.32 nM) and membranes (0.31 nM) were obtained by competition curves with 125I-labeled CGP42112, a selective AT2 ligand. Binding to AT2 receptors in the soluble extract was not sensitive to dithiothreitol. AT2 receptors were further purified by gel filtration and a CGP42112 Sepharose affinity column. Ang II AT2 receptors were selectively eluted with 5 microM CGP42112 at 4 degrees C, and a single band with an apparent molecular mass of 71 kDa was obtained after SDS/PAGE. Two-dimensional electrophoresis confirmed the purity of the protein and an isoelectric point of 5.3-5.5 was obtained. A highly selective elution of the AT2 receptors from the affinity column was performed with 5 nM 125I-labeled CGP42112 at room temperature after the column was treated with 1 microM losartan in the presence of high salt. After cross-linking, a major labeled protein with similar molecular mass and isoelectric point was obtained. Dissociation of the radiolabeled protein was insensitive to losartan but was enhanced by CGP42112, PD123177, Ang II, and [Sar1]Ang II. In summary, Ang II AT2 receptors were purified by CGP42112 affinity chromatography and selective elution and retain the pharmacological specificity of particulate receptors.

Collagen studies in newborn rat kidneys with incomplete ureteric obstruction

Collagen studies in newborn rats with incomplete ureteric obstruction were performed to describe and quantify changes in collagen deposition resulting from urinary tract obstruction at an early developmental age. Incomplete ureteric obstruction was created in three-day-old rats by placing the left ureter in a tunnel formed by the psoas muscle, and sham-operated controls underwent a laparotomy. The rats were sacrificed at 10, 17, 24 or 31 days. Collagen types I, III, IV, and V were localized by indirect immunofluorescence microscopy, the total collagen content of the kidney was quantitated using hydroxyproline analysis, and collagen types I and III were quantitated using cyanogen bromide (CNBr) peptide analysis. Increased immunofluorescent staining for all of the collagens was found in the diffusely widened medullary interstitium of the obstructed kidney, and more focally in the cortical interstitium. Collagen types I, III and V, but not collagen type IV, were also found in bands in the interstitium at the junction of the cortex with the medulla. Increased staining for collagen type IV was found in thickened and tortuous tubular basement membranes (TBM) of the obstructed kidneys. The total collagen content of the obstructed kidney was significantly increased compared to the amounts in both the contralateral kidneys and in the kidneys from sham-operated controls at 24 and 31 days of age (P < 0.01 in each case, Wilcoxon matched pairs rank sum test and Mann Whitney U-test, respectively). The amount of collagen in the kidneys correlated with the degree of hydronephrosis (Spearman correlation test, r = 0.78, P < 0.02). CNBr peptide analysis demonstrated that over 50% of the collagen in the normal neonatal rat kidney was collagen type I and approximately 25% was collagen type III. In the obstructed kidneys most of the collagen was also collagen type I and collagen type III, although the proportion of total collagen comprised by these collagen types was decreased compared with the controls. The amount of collagen type III in the contralateral kidneys was reduced compared to that in the controls. Thus, the neonatal renal response to obstruction resulted in increased amounts of a range of collagens in the interstitium and TBM, and the extent of this response was partially related to the degree of hydronephrosis.

Expression of rat fibroblast growth factor receptor 1 as three splicing variants during kidney development

Fibroblast growth factors (FGF) are known to participate in the processes of embryogenesis and angiogenesis. This study was undertaken to examine the transcriptional and posttranscriptional regulation of the FGF receptor 1 (FGFR-1) subclass in the embryonic rat kidney. Two full-length FGF receptor cDNAs were cloned using low-stringency screening of a neonatal rat kidney library with a chicken FGFR-1 cDNA probe. Sequencing revealed these cloned cDNAs to be rat homologues of the FGFR-1 subtype, with the two clones representing splicing variants beta and gamma of the FGFR-1. Evidence for renal expression of a third splicing variant (alpha) was obtained by use of the polymerase chain reaction. Splicing variants alpha and beta of FGFR-1 are predicted to produce cell-surface FGF receptors with three and two immunoglobulin-like domains, respectively, whereas the gamma-isoform may represent an intracellular form of the receptor. Although all three splicing variants were expressed in the developing kidney at days 14, 17, and 20 of gestation, at neonatal days 1 and 7 and in mature rats the beta-isoform was present in vastly larger abundance than alpha- and gamma-isoforms at all stages studied. Northern blot analysis revealed enhanced expression of FGFR-1 in the neonatal compared with the mature kidney. It is concluded that FGFR-1 is expressed in the kidney predominantly as the beta-isoform splicing variant and that expression of this receptor is enhanced during kidney development.

Immunohistochemical Study on Perinatal Development of Rat Superoxide Dismutases in Lungs and Kidneys

It has been reported that levels of antioxidant enzymes are low in fetal rat lungs and kidneys, and that they increase rapidly during late gestation. Among the antioxidant enzymes, both copper-zinc and manganese superoxide dismutases (CuZnSOD and MnSOD) are assumed to play a key role in protection against oxidative tissue injury. To determine the nature of the rapid perinatal increase in CuZnSOD and MnSOD, immunoenzyme staining was performed in the lungs and kidneys of fetal (d 18 and 20 of gestation) and neonatal (d 22) rats. The CuZnSOD and MnSOD in the homogenates were assayed by RIA, and they were found to be higher in the neonatal organs than in the respective fetal organs. The neonatal bronchiolar epithelium was stained for both CuZnSOD and MnSOD more intensely than the fetal one. The CuZnSOD staining in the neonatal alveolar wall was more intense than that in the fetal one. There was a significant reactivity for MnSOD in the neonatal, but not in the fetal, alveolar walls. In the kidneys, the reactivities for CuZnSOD and MnSOD were confined to the undifferentiated tubules. Although the tubules were increased in numbers in the neonatal kidneys, the intensity of the staining for both CuZnSOD and MnSOD was unchanged. The histochemical study disclosed that CuZnSOD and MnSOD increased in the kidneys in a manner different from that in the lungs. The low concentration of both CuZnSOD and MnSOD in the fetal lung tissues may contribute to the vulnerability to oxygen toxicity. Such changes in the concentrations in specific tissues were not delineated in the kidneys.

Renal synthesis of atriopeptin-like protein in physiology and pathophysiology

Atriopeptin is synthesized in mammalian atria as a 126-amino acid (14 kDa) prohormone, but it is secreted and circulates as a 28-amino acid (2.5 kDa) peptide. We have demonstrated the synthesis and secretion of an atriopeptin-like peptide in neonatal and adult rat kidney cell cultures. In this study, we evaluated the site of renal synthesis of this protein and its expression in normal rats and rats made nephrotic with puromycin aminonucleoside. The major form of atriopeptin in normal kidneys comigrated with an apparent molecular mass of 2.5 kDa assessed by gel filtration chromatography. However, the major form of this atriopeptin-like protein in nephrotic kidneys was determined to have an apparent molecular mass similar to the heart prohormone. No atriopeptin prohormone was detected in the plasma of nephrotic rats. Localization of this renal atriopeptin-like protein was accomplished by immunocytochemistry of rat kidney frozen sections. Using an antibody generated against either the COOH-terminal or NH3-terminal region of the cardiac atriopeptin prohormone, we detected specific immunostaining in the distal cortical nephron of the nephrotic kidney. This is the first report of the anatomic localization of a renal atriopeptin-like protein and its upregulation in nephrosis.

Fetal dexamethasone exposure impairs cellular development in neonatal rat heart and kidney: Effects on DNA and protein in whole tissues

Fetal glucocorticoid exposure causes postnatal growth retardation. To examine the mechanisms underlying effects on specific organ systems, we administered 0.2 or 0.8 mg/kg of dexamethasone to pregnant rats on gestational days 17, 18, and 19 and assessed three biochemical markers of cell development in heart and kidney of the offspring: DNA content per organ as an index of total cell numbers, DNA per g tissue as an index of cell packing density, and protein/DNA ratio as an index of relative cell size. In both tissues, DNA content became markedly subnormal during the first postnatal week, the ontogenetic period of rapid cell division. Partial recovery occurred by the end of the first postnatal month. In the heart, cell packing density was subnormal initially and the cells were significantly enlarged. In contrast, packing density was slightly elevated in the kidney; protein/DNA was increased by the low dose of dexamethasone, but markedly decreased by the high dose. These results suggest that tissue growth impairment caused by prenatal dexamethasone treatment reflects primary deficits in cell proliferation that extend to a variety of different cell types; however, consequent effects on cell packing density and cell size are dose-specific, possibly reflecting actions of glucocorticoids selective for certain cell types or phases of cell development.

Immunohistochemical localization of atrial natriuretic peptide in the developing and adult mammalian kidney

The discovery, within the last decade, of atrial natriuretic peptide (ANP), a family of peptides with natriuretic/diuretic and vasorelaxant properties, has prompted much research into the mechanisms and sites of action of ANP within the kidney. In the present study, ANP was localized in the kidneys of several mammalian species by immunohistochemical techniques 1) to identify possible sites of synthesis; 2) to compare the localization of ANP to known physiological effects; 3) to determine species differences, if any, in ANP localization; and 4) to study the development of ANP immunoreactivity in the fetal and neonatal rat kidney. Using an antibody against rat ANP, IV, ANP was localized exclusively on the proximal convoluted tubule (PCT) brush border and within intercalated cells of the outer medullary and cortical collecting tubules and ducts of adult mouse, rat, pig, monkey, and human kidneys. The development of ANP immunoreactivity paralleled the differentiation and maturation of collecting duct epithelium in rat fetal kidney. Atrial natriuretic peptide found within intercalated cells of the cortical and outer medullary collecting ducts may be the result of endogenous synthesis and, following secretion, may be available to receptors in the inner medullary collecting ducts.

Synthesis and secretion of an atriopeptin-like protein in rat kidney cell culture.

The synthesis and secretion of an atriopeptin(AP)-like prohormone (AP126ir) has been demonstrated in rat neonatal renal cell cultures. AP126ir could be detected in the cellular extract and the medium from cultured kidney cells of neonatal and adult rats using an enzyme immunoassay specific for cardiac AP prohormone. On reverse-phase high-performance liquid chromatography, the AP obtained from the extract and the medium comigrated with cardiac AP prohormone. Incubation of the renal AP in the medium with thrombin resulted in the generation of a single low molecular mass peak which migrated with the cardiac carboxy-terminal 28-amino acid AP. Neonatal kidney cells pulsed with [35S]methionine secreted radiolabeled AP126ir, which was detected by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis chromatography. Incubation of neonatal kidney cell cultures with the protein synthesis inhibitor cycloheximide resulted in a significant decrease in both the cellular and media AP. No decrease in cellular and media AP was detected when neonatal atrial cultures were treated with cycloheximide. These data demonstrate the de novo synthesis of an AP prohormone-like protein in neonatal rat kidney cultures. Furthermore, unlike the atria, kidney cells appear to secrete AP solely by constitutive means. In primary adult rat kidney cultures, most of AP126ir was detected in the cortical tubule fraction demonstrating that these cells secrete AP126ir in the adult rat kidney. We hypothesize that the renal AP may be important as an autocrine or paracrine regulator of renal function.

Thyroid hormones and growth of the neonatal rat liver and kidney : J. Easton a, S.K. Winterburn a, J. Holt a, J.P. Canavan b and D.F. Goldspink b, aThe Cardiac Research Unit, Killingbeck Hospital, Leeds and bDepartment of Cardiovascular Studies, University of Leeds, U.K.

Thyroid hormones and growth of the neonatal rat liver and kidney : J. Easton a, S.K. Winterburn a, J. Holt a, J.P. Canavan b and D.F. Goldspink b, aThe Cardiac Research Unit, Killingbeck Hospital, Leeds and bDepartment of Cardiovascular Studies, University of Leeds, U.K.

Immunohistochemical study of .GAMMA.-glutamyl transpeptidase during the development and maturation of rat kidney.

Gamma-glutamyl transpeptidase (γ-GTP) was localized in prenatal and neonatal rat kidneys using an immunohistochemical method with monoclonal antibodies to rat kidney γ-GTP. γ-GTP was demonstrated on the brush border and basolateral membrane of proximal convoluted tubules from the 18th fetal day. The basolateral membrane of proximal tubules revealed strong reaction at 1 and 3 days after birth. The reaction of basolateral membrane decreased with the maturation of nephrons, however, moderate reaction was still observed at 30 days of age in the S3 segment. Throughout the postnatal development of the kidney, the S3 exhibited the strongest immunostainingand S1 showed the weakest reaction, and proximal tubules in the deep layer of the cortex always provided a stronger reaction than those in the superficial layer of the cortex. Maturating cuboidal and squamous epithelial cells of descending Henle's loop showed a positive reaction on the luminal and lateral cell membranes. On immunoelectron microscopy, γ-GTP was found on the brush border, basolateral membrane, membranes of pinocytotic vesicles and apical invaginations in proximal tubular cells at 1-7 days after birth. However, tight junctions were devoid of the reaction. The development of basal infolding and localization of γ-GTP there seemed to relate closely to the development of surrounding capillaries. γ-GTP was localized on the cell membrane and occasionally in the cytoplasm of interstitial cells of renal interstitium.

Effects of birth on energy metabolism in the rat kidney.

The oxygen-consumption rates and the activities of fumarase and beta-hydroxyacyl-CoA dehydrogenase were compared in mitochondria isolated from fetal- and neonatal-rat kidney. Whole-organ ATP, phosphocreatine and creatine contents were determined in parallel. Kidney mitochondrial respiratory rates in the presence of succinate, glutamate/malate and palmitoyl-L-carnitine increased between 21 days post coitum and 1 day post partum, together with activities of oxidative enzymes. However, this postnatal maturation of oxidative metabolism was not yet initiated in mitochondria isolated from kidney 1 h post partum. An increase in ATP and phosphocreatine was observed immediately after delivery; newborn-rat kidney ATP content then remained high, whereas phosphocreatine reserves decreased considerably between 6 h and 1 day post partum. It is concluded that the increase in high-energy phosphate compounds observed at birth is not initially related to an activation of oxidative phosphorylation, and probably involves a transient stimulation of anaerobic glycolysis, while a progressive mitochondrial maturation takes place in the rat kidney during the first day of newborn life.

Cell death by apoptosis following X-irradiation of the foetal and neonatal rat kidney.

A light and electron microscopic study was undertaken to determine the type of cell death induced by X-irradiation in the developing kidney. Five-day-old Sprague-Dawley rats were exposed to a whole-body dose of either 2 or 5 Gy, and foetuses in the eighteenth day of development were exposed to a dose of 4 Gy. The kidneys were examined at 4, 8 and 24 h, and at 1 and 2 weeks post-irradiation. The dying cells from both control and treated kidneys showed the morphological features of apoptosis, a distinct form of cell death that has been identified in mammalian tissues under physiological as well as pathological conditions. Necrosis was not detected. Apoptosis was infrequent in control kidneys and insignificant in extent when compared with the proliferative activity of the cells of the superficial nephrons. There was a pronounced increase in apoptosis during the first day after irradiation. The findings are in agreement with recent ultrastructural studies which report the presence of apoptosis following irradiation of rapidly proliferating adult cell populations, and irradiation of other immature mammalian tissues. There is evidence that apoptosis involves active cellular self-destruction, and it has been suggested that activation of apoptosis might bring about selective elimination of cells with critical DNA damage in irradiated tissues, thus minimizing propagation of genetic abnormalities.

Ultracytochemical study of the ouabain-sensitive, K-dependent p-nitrophenylphosphatase (Na-K-ATPase) activity in the developing rat kidney.

The ultracytochemical localization of ouabain-sensitive, K-dependent p-nitrophenylphosphatase (K-NPPase) activity of Na-K-ATPase was investigated in the fetal and neonatal rat kidneys with the one-step lead citrate method (17) in order to study the developmental changes in the site of the Na-pump.In fetal rat kidneys, the K-NPPase positive renal tubules first appeared in the inner cortex at 18 gestation days and increased with the maturation of the fetus. Electron microscopically, the reaction products were found on the basolateral plasma membranes of distal convoluted tubule cells at various levels of differentiation. In the kidney of 19 and more gestation days, K-NPPase positive tubules were found also in the ascending thick tubules of the loop of Henle in the medulla. In newborn rats, some of the ascending thick tubules with intense K-NPPase activity reached to the tip of the papilla. These tubules in the papilla gradually lost K-NPPase activity and redifferentiated into ascending thin tubules during the period of lactation. The clear separation of K-NPPase positive outer medulla and K-NPPase negative inner medulla was completed around 3 weeks after birth. The above results indicate that suckling rats have Na-pumps even in the inner medulla and thus have different urine concentration mechanisms from those in the adults.

The development of carnitine synthesis from gamma-butyrobetaine in the rat.

Abstract The ability of the rat liver to synthesize camitine from γ-butyrobetaine increases from low values in the fetus to adult values on the 8th day after birth. The rate of synthesis of camitine is greater when determined in the high-speed supernatant than in the low-speed supernatant of the liver. No synthesis could be shown to occur in neonatal rat kidney or neonatal brown adipose tissue.

Choline pathways during normal and stimulated renal growth in rats.

Cellular membrane synthesis occurs during normal and stimulated renal growth. Choline in the kidney is utilized as a precursor for membrane synthesis via the choline kinase reaction. We investigated choline phosphorylation during normal and stimulated renal growth. Rapidly growing neonatal rat kidneys contained relatively high levels of choline kinase activity (61 pmol phosphorylcholine/min per mg protein). Choline kinase activity and phosphorylcholine production then fell gradually over the 1st mo of life; by 1 mo phosphorylcholine production was 34 pmol phosphorylcholine/min per mg protein. Choline kinase activity increased by 27% (P less than 0.001) in 28-day-old rats when renal growth was stimulated by contralateral nephrectomy; the increase occurred within 2 h after surgery. Thus, changes in the activity of this important enzyme in the initiation of membrane synthesis is associated both with normal renal development and with adaptation to nephron loss. The findings further suggest that the cell membrane may be involved in the initiation of compensatory renal growth.

Adenosine deaminase and adenosine kinase in rat hepatomas and kidney tumours

Adenosine deaminase and adenosine kinase have been measured in rat liver, 12 transplantable hepatomas, regenerating, foetal and neonatal liver, adult and neonatal rat kidney and 2 transplantable kidney tumours. Adenosine, deaminase activity, relative to the normal liver value, was elevated 2-4 fold in hepatomas of rapid growth rate, was in the normal range in more slowly growing hepatomas and in regernerating liver, and was low in foetal and neonatal liver. Adenosine kinase activity was decreased, relative to rat liver values, in all the hepatomas; activity of this enzyme gave a negative correlation with tumour growth rate. Kinetic properties of the two enzymes were examined in partially purified preparations. Adenosine deaminases from both liver and rapidly growing hepatoma 3924A were subject to weak product inhibition by inosine. Adenosine kinase from liver and hepatoma 3924A was inhibited by the reaction products ADP and AMP, and the enzyme was also subject to excess substrate inhibition by concentrations of ATP in excess of 1 mM. In rat hepatoma cell lines growing in culture, the toxicity of adenosine correlated inversely with the ratio of adenosine deaminase activity to adenosine kinase activity. Chromatographic measurements showed that hepatoma cells incorporated less extracellular adenosine into their adenine nucleotide pools than did isolated liver cells. These results indicate that increased adenosine deaminase activity and decreased adenosine kinase activity may confer a selective advantage upon the cancer cell.

Vasopressin responsiveness of renal adenylate cyclase in newborn rats and rabbits.

Newborns show an inability to concentrate maximally their urine. The vasopressin responsiveness of adenylate cyclase was, therefore, examined in membranes obtained from kidneys of neonatal and adult rats and from renal medulla and isolated collecting tubules of newborn and adult rabbits. In spite of higher basal and NaF-stimulated activity, vasopressin failed to stimulate adenylate cyclase from neonatal rat kidneys. In neonatal and adult rabbits, basal and NaF-stimulated adenylate cyclase activities of renal medullary membranes were comparable but vasopressin stimulation was significantly lower in the newborns. No change in hormonal activation constant was observed. This hyporesponsiveness of neonatal adenylate cyclase to vasopressin was confirmed with single isolated rabbit collecting tubules for adenylate cyclase determination, a highly sensitive preparation. It is intriguing to speculate that the low vasopressin stimulation of the medullary adenylate cyclase in the developing kidney may contribute to the known limitations of the urinary concentrating mechanism in the newborn period.