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  • 1
    ISSN: 1432-0568
    Keywords: Psammomys kidney ; Renal architecture ; Short-looped nephrons ; Long-looped nephrons ; Renal pelvis ; Urea recycling possibilities
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The architecture of the desert rodent Psammomys obesus has been studied by means of standard histologic procedures and by single nephron injections. As other rodent kidneys (rat, mouse), the Psammomys kidney consists of two types of nephrons, 66% short looped and 34% long looped nephrons. The cortex is composed of 4 to 5 layers of glomeruli, which lie closely put together, the glomeruli often touch each other. The superficial and the midcortical glomeruli give rise to short looped nephrons, the juxtamedullary to long looped nephrons. In the strongly developed medulla the inner stripe shows the most striking pattern. It consists of two distinct compartements, that of the giant vascular bundles and that of the interbundle regions. The giant vascular bundles consist of about 8 to 14% arterial vasa recta and 39 to 47% venous vasa recta; furthermore they include the thin descending limbs of the short loops of Henle which amount to 44 to 51% of the bundle structures. The tubules of the interbundle regions surround the bundles in a regular pattern. The inner zone is almost completely surrounded by the renal pelvis; the long broad papilla protrudes into the ureter. The thin descending limbs of short looped nephrons traverse the inner stripe inside the giant vascular bundles. Leaving the bundles they turn back within the inner stripe; their ascending limbs lie in the interbundle region. Both limbs of the long loops of Henle run in the interbundle region, together with the ascending limbs of the short loops and the collecting ducts. The long loops penetrate deeply the inner zone. Many bends are found near the tip of the papilla. The renal pelvis has a very specialized form. It penetrates the inner stripe with many complexely shaped extensions, which surround the giant vascular bundles. Large parts of the bundles with their thin walled structures are thus separated from the pelvic urine only by a single layer of cuboidal epithelium. The possible functional importance of the described specializations of the Psammomys kidney (giant vascular bundles, large inner zone, special shape of the renal pelvis) for the urine concentrating and urea recycling mechanisms is discussed.
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  • 2
    ISSN: 1432-1424
    Keywords: Key words: Medullary thick ascending limb — Ca2+ transport — Mg2+ transport — Electron microprobe analysis — Passive permeability — ADH
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. Recent studies from our laboratory have shown that in the mouse and rat nephron Ca2+ and Mg2+ are not reabsorbed in the medullary part of the thick ascending limb (mTAL) of Henle's loop. The aim of the present study was to investigate whether the absence of transepithelial Ca2+ and Mg2+ transport in the mouse mTAL is due to its relative low permeability to divalent cations. For this purpose, transepithelial ion net fluxes were measured by electron probe analysis in isolated perfused mouse mTAL segments, when the transepithelial potential difference (PDte.) was varied by chemical voltage clamp, during active NaCl transport inhibition by luminal furosemide. The results show that transepithelial Ca2+ and Mg2+ net fluxes in the mTAL are not driven by the transepithelial PDte. At zero voltage, a small but significant net secretion of Ca2+ into the tubular lumen was observed. With a high lumen-positive PDte generated by creating a transepithelial bath-to-lumen NaCl concentration gradient, no Ca2+ and Mg2+ reabsorption was noted; instead significant and sustained Ca2+ and Mg2+ net secretion occurred. When a lumen-positive PDte was generated in the absence of apical furosemide, but in the presence of a transepithelial bath-to-lumen NaCl concentration gradient, a huge Ca2+ net secretion and a lesser Mg2+ net secretion, not modified by ADH, were observed. Replacement of Na+ by K+ in the lumen perfusate induced, in the absence of PDte changes, important but reversible net secretions of Ca2+ and Mg2+. In conclusion, our results indicate that the passive permeability of the mouse mTAL to divalent cations is very low and not influenced by ADH. This nephron segment can secrete Ca2+ and Mg2+ into the luminal fluid under conditions which elicit large lumen-positive transepithelial potential differences. Given the impermeability of this epithelium to Ca2+ and Mg2+, the secretory processes would appear to be of cellular origin.
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  • 3
    ISSN: 1432-1424
    Keywords: Water fluxes ; Na fluxes ; proximal tubule microperfusion ; Li substitution ; rat kidney
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The relationship between water and sodium movements through the mammalian proximal convoluted tubule was investigated by substituting lithium for sodium. Proximal convoluted rat Kidney tubules were perfusedin vivo with a Ringer solution containing 107 meq/liter lithium and 42 meq/liter sodium. Several micropunctures were made along the same nephron, and [3H] inulin, [14C] glucose,22Na, osmolality, Na, Mg and Cl were determined on each sample. Measurements of22Na showed that sodium and lithium diffusion rates were practically identical throughout the entire epithelium. A one- for-one exchange of sodium for lithium induced a negative trans-epithelial net flux of Na from plasma to lumen. However, despite this negative flux, a positive net water movement was measured from lumen to plasma. This movement was proportional both to glucose reabsorption and to the rise in the chloride concentration, two mechanisms known to be dependent on the trans-cellular movement of sodium. It was therefore concluded that the net water flux was a function of the unidirectional transcellular net flux of Na. Rabbit proximal convoluted tubules were perfusedin vitro with a solution containing 75 meq/liter Li and 75 meq/liter Na on both the luminal and peritubular sides. Under these conditions, the water reabsorption rate dropped to half its control value. Water movement was therefore a function of the external sodium concentration, which in turn probably regulates the intracellular Na concentration.
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  • 4
    ISSN: 1432-2013
    Keywords: Renal Tubule ; Phosphate Transport ; Sodium Dependence ; Micropuncture ; Microperfusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The standing droplet method has been used in combination with the peritubular perfusion of blood capillaries to determine the build up of transtubular concentration differences of phosphate (P i ) in the renal proximal convoluted tubule of parathyroidectomized rats. Electron probe analysis was used to estimate P i . At zero time both the intraluminal and the contraluminal P i concentration was 2 mM. The time dependent decrease of the intraluminal P i concentration was approximately 4 times faster in the early than in the late proximal convoluted tubule. After 45 sec an intraluminal steady state concentration of 0.20 mM P i was achieved in the early part. In the late part the intraluminal P i concentration approached a steady state value of 0.54 mM at 120 sec. When sodium free solutions were used the intraluminal P i concentration increased to 2.22 mM in the earlier and to 2.76 mM in the late part. The data indicate that in the proximal convoluted tubule 1. The rate of phosphate reabsorption is greater in the early part than in the later part, and 2. phosphate reabsorption might occur as co-transport with Na+ ions.
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  • 5
    ISSN: 1432-2013
    Keywords: Concentrating mechanism ; Loop of Henle ; Vasa recta ; Counter-current system ; Desert rodent
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Micropunctures were performed at the tip of Henle's loops and vasa recta accessible at the extrarenal surface of the papilla in a desert rodent (Psammomys obesus) studied under mild NaCl (NaCl 4%, 0.0375 ml/min) and mild urea (urea 4%, 0.0375 ml/min) loading conditions. In NaCl loaded animals, it was confirmed that solute addition (mainly sodium) contributes in a large proportion to the concentrating process along the thin descending limb. Comparison of sodium and urea concentrations in the loops with those in vasa recta at the same level of the papilla demonstrated that 1. the transepithelial sodium gradient was compatible with a diffusion transport of this ion from the interstitium to the thin descending limb; 2. the sodium concentration higher in interstitium than in the loop fluid was not compatible with the existence of a purely passive concentrating process in inner medulla as was recently proposed [8], 3. the transepithelial urea gradient was very limited which indicates that this solute does not play an important part in the concentrating process. In urea loaded psammomys, solute addition (mainly urea) to the thin descending limb fluid was still present but water abstraction was enhanced as compared to salt loaded animals, probably on account to the higher interstitial urea concentration. It is, thus, brought to evidence that the relative contribution of water abstraction and solute addition to the concentrating process along the thin descending limb can vary in a given species as a function of the physiological state.
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  • 6
    ISSN: 1432-2013
    Keywords: ADH ; Transepithelial ion net fluxes ; Na+, Cl−, K+, Ca2+ and Mg2+ transport ; Electron microprobe ; Mouse kidney ; Cortical and medullary thick ascending limb of Henle's loop ; In vitro microperfusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The effect of antidiuretic hormone (arginine vasopressin, AVP) on transepithelial Na+, Cl−, K+, Ca2+ and Mg2+ net transports was investigated in medullary (mTAL) and cortical (cTAL) segments of the thick ascending limb (TAL) of mouse nephron, perfused in vitro. Transepithelial net fluxes (J Na +,J Cl −,J K +,J Ca 2+,J Mg 2+) were determined by electron probe analysis of the collected tubular fluid. Transepithelial potential difference (PDte) and transepithelial resistance (Rte) were measured simultaneously. cTAL segments were bathed and perfused with isoosmolal, HCO 3 − containing Ringer solutions, mTAL segments were bathed and perfused with isoosmolal HCO 3 − free Ringer solutions. In cTAL segments, AVP (10−10 mol·l−1) significantly increasedJ Mg 2+ andJ Ca 2+ from 0.39±0.08 to 0.58±0.10 and from 0.86±0.13 to 1.19±0.15 pmol·min−1 mm−1 respectively. NeitherJ Na + norJ Cl −, (J Na +: 213±30 versus 221±28 pmol·min−1 mm−1,J Cl −: 206±30 versus 220±23 pmol·min−1 mm−1) nor PDte (13.4±1.3 mV versus 14.1±1.9 mV) or Rte (24.6±6.5Ω cm2 versus 22.6±6.4Ω cm2) were significantly changed by AVP. No significant effect of AVP on net K+ transport was observed. In mTAL segments, Mg2+ and Ca2+ net transports were close to zero and AVP (10−10 mol·l−1) elicited no effect. However NaCl net reabsorption was significantly stimulated by the hormone,J Na + increased from 107±33 to 148±30 andJ Cl − from 121±33 to 165±32 pmol·min−1 mm−1. The rise inJ NaCl was accompanied by an increase in PDte from 9.0±0.7 to 13.5±0.9 mV and a decrease in Rte from 14.4±2.0 to 11.2±1.7 Ω cm2. No K+ net transport was detected, either under control conditions or in the presence of AVP. To test for a possible effect of HCO 3 − on transepithelial ion fluxes, mTAL segments were bathed and perfused with HCO 3 − containing Ringer solutions. With the exception ofJ Ca 2+ which was significantly different from zero (J Ca 2+: 0.26±0.06 pmol·min−1 mm−1), net transepithelial fluxes of Na+, Cl−, K+ and Mg2+ were unaffected by HCO 3 − . In the presence of AVP,J Mg 2+ andJ Ca 2+ were unaltered whereasJ NaCl was stimulated to the same extent as observed in the absence of HCO 3 − . In conclusion our results indicate heterogeneity of response to AVP in cortical and medullary segments of the TAL segment, since AVP stimulates Ca2+ and Mg2+ reabsorption in the cortical part and Na+ and Cl− reabsorption in the medullary part of this nephron segment.
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  • 7
    ISSN: 1432-2013
    Keywords: Isolated thick ascending limb ; Microperfusion ; Divalent cation and potassium transport ; Microprobe analysis ; Transepithelial voltage
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Isolated segments of rat cortical (cTAL) and medullary (mTAL) thick ascending limbs were microperfused and the transepithelial net fluxes (Jx) were determined by measuring the composition of the collected fluid with an electron microprobe. When perfused with symmetrical solutions both segments showed similar JNa and jCl and lumen-positive transepithelial voltage (V te=7–8 mV). JMg, JCa and JK were not significantly different from zero. When perfused with asymmetrical solutions (lumen 50 mM, bath 150 mM NaCl), the mean Vte were 23 mV and 17 mV in the cTAL and mTAL respectively; this rise was accompanied by significant increases in JMg and JCa in the cTAL, but not in the mTAL, and a marked increase in JK in both segments. It is concluded that, in the rat, divalent cations can be reabsorbed in the cTAL, and K+ can be reabsorbed in the cTAL and mTAL. The transport is voltage-dependent. The mTAL can reabsorb neither Mg2+ nor Ca2+, whatever Vte.
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  • 8
    ISSN: 1432-2013
    Keywords: Key words Cortical thick ascending limb of Henle’s loop ; Microperfusion ; Calcium ; Magnesium ; Cell maturation ; Gender ; Transepithelial ion net fluxes ; Electron microprobe
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Previous studies from our laboratory have shown that Ca2+ and Mg2+ absorption in the mouse cortical thick ascending limb of Henle’s loop (cTAL) is a passive, paracellular process driven by the transepithelial voltage. The passive permeability of the epithelium is enhanced by peptide hormones. The present study investigated whether divalent cation absorption in the cTAL is influenced by cell maturation and/or gender. For this purpose, mouse cTAL segments were microdissected from kidneys of female and male animals aged 4 and 8 weeks. The microdissected tubules were perfused in vitro at a luminal flow rate of 1.5 to 2.5 nl/min. Transepithelial Na+, Cl–, Ca2+ and Mg2+ net fluxes (J X , pmol·min–1·mm–1) were measured using electron microprobe analysis, and the transepithelial potential difference (PDte) was measured continuously. No differences were found in the PDte, J Na and J Cl of the various animal groups but the transepithelial Ca2+ and Mg2+ transport capacity of the cTAL was higher in adults (8 weeks) than in young animals (4 weeks). Furthermore, irrespective of age, transepithelial Ca2+ net absorption was greater in male than in female animals. In contrast, the NaCl transport was maximal at 4 weeks in both genders. We conclude therefore that transepithelial divalent cation absorption in the mouse cTAL is an inductive process influenced by cell maturation and gender. The molecular basis of these inductions remains to be elucidated.
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  • 9
    ISSN: 1432-2013
    Keywords: Parathyroid hormone ; Human calcitonin ; Transepithelial ion net fluxes ; Na+, Cl−, K+, Mg2+, Ca2+ transport ; Electron microprobe analysis-Mouse kidney ; In vitro microperfusion ; Cortical and medullary thick ascending limb of Henle's loop
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The effect of parathyroid hormone (PTH) on transepithelial Na+, Cl−, K+, Ca2+ and Mg2+ transport was investigated in isolated perfused cortical thick ascending limbs (cTAL) and that of human calcitonin (hCT) was tested in both cortical and medullary thick ascending limbs (mTAL) of the mouse nephron. The transepithelial ion net fluxes (J x) were determined by electron probe analysis of the perfused and collected fluids. Simultaneously, the transepithelial voltage (PDte) and resistance (R te) were recorded. In cTAL segments, PTH and hCT significantly stimulated the reabsorption of Na+, Cl−, Ca2+ and Mg2+. hCT generated a net K+ secretion towards the lumen and PTH tended to exert the same effect. Neither PDte nor R te were significantly altered by either PTH or hCT. However, in the post-experimental period a significant decrease in PDte was noted. Time control experiments carried out under similar conditions revealed a significant decrease in PDte with time, which could have masked the hormonal response. In mTAL segments, Mg2+ and Ca2+ transport was close to zero. hCT did not exert any detectable effect on either PDte or J Cl −, J Na + J K +, J Mg 2+ and J Ca 2+ in these segments. In conclusion, our data demonstrate that PTH and hCT stimulate NaCl reabsorption as well as Mg2+ and Ca2+ reabsorption in the cTAL segment of the mouse. These data are in agreement with and extend data obtained in vivo in the rat.
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  • 10
    ISSN: 1432-2013
    Keywords: Ca2+ transport ; Mg2+ transport ; Electron microprobe analysis ; Cortical thick ascending limb ; Furosemide ; Parathyroid hormone ; Paracellular shunt pathway permeability ; Tight junctions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Recent studies from our laboratory have shown that in the cortical thick ascending limb of Henle's loop of the mouse (cTAL) Ca2+ and Mg2+ are reabsorbed passively, via the paracellular shunt pathway. In the present study, cellular mechanisms responsible for the hormone-stimulated Ca2+ and Mg2+ transport were investigated. Transepithelial voltages (PDte) and transepithelial ion net fluxes (J Na, J Cl, J K, J Ca, J Mg) were measured in isolated perfused mouse cTAL segments. Whether parathyroid hormone (PTH) is able to stimulate Ca2+ and Mg2+ reabsorption when active NaCl reabsorption, and thus PDte, is abolished by luminal furosemide was first tested. With symmetrical lumen and bath Ringer's solutions, no Ca2+ and Mg2+ net transport was detectable, either in the absence or in the presence of PTH. In the presence of luminal furosemide and a chemically imposed lumen-to-bath directed NaCl gradient, which generates a lumen-negative PDte, PTH slightly but significantly increased Ca2+ and Mg2+ net secretion. In the presence of luminal furosemide and a chemically imposed bath-to-lumen-directed NaCl gradient, which generates a lumen-positive PDte, PTH slightly but significantly increased Ca2+ and Mg2+ net reabsorption. In view of the observed small effect of PTH on passive Ca2+ and Mg2+ movement, a possible interference of furosemide with the hormonal response was considered. To investigate this possibility, Ca2+ and Mg2+ transport was first stimulated with PTH in tubules under control conditions. Then active NaCl reabsorption was abolished by furosemide and the effect of PTH on J Ca and J Mg measured. In the absence of PDte and under symmetrical conditions, no Ca2+ and Mg2+ transport was detectable, either in the presence or absence of PTH. In the presence of a bath-to-lumen-directed NaCl gradient, Ca2+ and Mg2+ reabsorption was significantly higher in the presence than in the absence of PTH. Finally, when active NaCl transport was not inhibited by furosemide, but reduced by a bath-to-lumen-directed NaCl gradient, PTH strongly increased J Ca and J Mg, whereas only a small increase in PDte was noted. In conclusion, these data suggest that PTH exerts a dual action on Ca2+ and Mg2+ transport in the mouse cTAL by increasing the transepithelial driving force for Ca2+ and Mg2+ reabsorption through hormone-mediated PDte alterations and by modifying the passive permeability for Ca2+ and Mg2+ of the epithelium, very probably at the level of the paracellular shunt pathway.
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