Physiological control of the urinary concentrating mechanism by peptide hormones

Effect of intravenous glucagon infusion on renal haemodynamics and renal tubular handling of sodium in healthy humans

The effects of a 2-h intravenous infusion of glucagon 5 ng kg-1 min-1 or placebo on glomerular filtration rate (GFR), renal plasma flow (RPF), tubular sodium handling as judged by the lithium clearance method, and plasma concentrations of angiotensin II (AngII), aldosterone (Aldo), and atrial natriuretic factor (ANF) were investigated in two groups of healthy human volunteers, glucagon group (n = 10), and placebo group (n = 10). Glucagon infusion resulted in a maximal increase in plasma concentrations of glucagon of 400%. GFR increased 5.9% (range 1.3-12.4, p < 0.001) through the whole infusion period, whereas RPF only increased transiently during the first hour of infusion 6.5% (range 2.6-15.3, p < 0.05). Whereas filtered load of sodium increased significantly in response to glucagon infusion (p < 0.001), urinary sodium excretion was unchanged. Neither of these variables were affected by placebo. As judged from assessments of tubular sodium handling derived from the renal clearance of lithium, the increased filtered load of sodium resulted in an increase in the output of sodium from the proximal tubules of a similar magnitude, and an increase in absolute reabsorption of sodium in the distal tubules totally counterbalancing this increased input to the distal tubules. These alterations in tubular sodium handling did not involve Ang II, Aldo, or ANF. We conclude that an increase in plasma concentration of glucagon within the physiological range is capable of inducing a small and sustained increase in GFR, whereas RPF increases only transiently.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma vasopressin and cortical nephron function in aging rats

The role of vasopressin and Henle's loop transport in age-related polyuria and decrease in urine osmolality was investigated in female WAG/Rij rats free of kidney disease. In these animals, urine osmolality dropped from 2000 mosmol/kg H2O to 1000-1200 mosmol/kg H2O between 10 and 30 months, and urinary volume increased in proportion. Vasopressin concentration measured in plasma withdrawn from conscious, unrestrained, chronically catheterized rats was not significantly different in 10, 20 and 30-month-old animals (mean values 2.5 +/- 0.7, 2.2 +/- 0.2 and 2.0 +/- 0.3 pg/ml (n = 8), respectively). This suggests an impaired responsiveness of old kidney to antidiuretic hormone. The possible involvement of Henle's loop in this defect was studied by micropuncture. Paired collections of tubular fluid were done in the early distal and late proximal convolutions of the same cortical nephrons. Single nephron filtration rates did not significantly differ with age. Tubular fluid osmolalities in the early distal convolution were 165 +/- 13, 178 +/- 9 and 160 +/- 11 (n = 14) mosmol/kg H2O in 10-, 20- and 30-month-old rats, indicating similar diluting capacity of the cortical thick ascending limb. The amount of sodium transported from lumen to peritubular space by Henle's loop was also unchanged with age as were water, calcium, magnesium and potassium reabsorptions. These data indicate that the age-related decrease in urine osmolality is not related to either a significant reduced vasopressin plasma concentration or an increased single glomerular filtration rate or a reduced transport capacity of Henle's loop of the cortical nephron. Rather they suggest an impaired response to vasopressin of other segments of the nephron that is, the medullary thick ascending limb of Henle's loop and/or the collecting duct.

Differential short-term desensitization to vasopressin, isoproterenol, glucagon, parathyroid hormone and calcitonin in the thick ascending limb of rat kidney

Short-term desensitization to hormone-induced cAMP accumulation was investigated in the medullary (MTAL) and the cortical (CTAL) thick ascending limbs of Henle's loop isolated by microdissection from the rat kidney. The following agonists were studied: vasopressin, glucagon and human calcitonin in the MTAL, and vasopressin, glucagon, human calcitonin, parathyroid hormone (PTH) and the beta-adrenergic agonist isoproterenol in the CTAL. Isolated tubules were preincubated in vitro for 60 min in the presence or absence of a maximal concentration of one of the five agonists (vasopressin 10 nM, glucagon 10 nM, calcitonin 100 nM, PTH 10 nM, isoproterenol 1 microM). Desensitization induced by each agent to its own action was then quantified by measuring the amount of cAMP accumulating in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine and the same agonist concentration as that used during preincubation. In the MTAL, as previously reported, preincubation with vasopressin led to a marked (80%-85%) desensitization to this hormone. A significant hormone self-induced desensitization of about 45% was also obtained with glucagon, but not with calcitonin. In the CTAL, the following order of potency to elicit desensitization was observed: vasopressin (80%) greater than isoproterenol (50%) greater than glucagon (30%) greater than PTH (20%, NS) greater than calcitonin (10%, NS). Thus, the magnitude of desensitization varied greatly from one hormone to another, but for a given hormone, was of roughly similar extent in both MTAL and CTAL.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular Mg2+ and magnesium depletion in isolated renal thick ascending limb cells

Magnesium reabsorption and regulation within the kidney occur principally within the cortical thick ascending limb (cTAL) cells of the loop of Henle. Fluorometry with the dye, mag-fura-2, was used to characterize intracellular Mg2+ concentration ([Mg2+]i) in single cTAL cells. Primary cell cultures were prepared from porcine kidneys using a double antibody technique (goat anti-human Tamm-Horsfall and rabbit anti-goat IgG antibodies). Basal [Mg2+]i was 0.52 +/- 0.02 mM, which was approximately 2% of the total cellular Mg. Cells cultured (16 h) in high magnesium media (5 mM) maintained basal [Mg2+]i, 0.48 +/- 0.02, in the normal range. However, cells cultured in nominally magnesium-free media possessed [Mg2+]i, 0.27 +/- 0.01 mM, which was associated with a significant increase in net Mg transport, (control, 0.19 +/- 0.03 and low Mg, 0.35 +/- 0.01 nmol.mg-1 protein.min-1) as assessed by 28Mg uptake. Mg(2+)-depleted cells were subsequently placed in high Mg solution (5 mM) and the Mg2+ refill rate was assessed by fluorescence. [Mg2+]i returned to normal basal levels, 0.53 +/- 0.03 mM, with a refill rate of 257 +/- 37 nM/s. Mg2+ entry was not changed by 5.0 mM Ca2+ or 2 mM Sr2+, Cd2+, Co2+, nor Ba2+ but was inhibited by Mn2+ approximately La3+ approximately Gd3+ approximately Zn2+ approximately Be2+ at 2 mM. Intracellular Ca2+ and 45Ca uptake was not altered by Mg depletion or Mg2+ refill, indicating that the entry is relatively specific to Mg2+. Mg2+ uptake was inhibited by nifedipine (117 +/- 20 nM/s), verapamil (165 +/- 34 nM/s), and diltiazem (194 +/- 19 nM/s) but enhanced by the dihydropyridine analogue, Bay K 8644 (366 +/- 71 nM/s). These antagonists and agonists were reversible with removal and [Mg2+]i subsequently returned to normal basal levels. Mg2+ entry rate was concentration and voltage dependent and maximally stimulated after 4 h in magnesium-free media. Cellular magnesium depletion results in increases in a Mg2+ refill rate which is dependent, in part, on de novo protein synthesis. These data provide evidence for novel Mg2+ entry pathways in cTAL cells which are specific for Mg2+ and highly regulated. These entry pathways are likely involved with renal Mg2+ homeostasis.

Vasopressin stimulation of NaCl transport in the medullary thick ascending limb of Henle's loop is decreased in aging mice

The maximal urinary osmolality that can be reached by the kidney is reduced with age. This may be due to impaired NaCl transport by the medullary thick ascending limb of Henle's loop, which is part of the renal concentrating mechanism and is modulated by antidiuretic hormone (ADH). We therefore tested in vitro a possible age-related change in the transport capacity and in the response of this nephron segment to ADH in young (1-2 months) and old (20-24 months) mice. The transepithelial potential difference (Vte) was significantly higher in young mice (+8.5 +/- 0.4 mV, n = 13) than in old ones (+6.6 +/- 0.5 mV, n = 17). Addition of 0.1 nmol.1-1 ADH to the bath solution significantly increased Vte by 5.2 +/- 0.5 mV in the young and by 3.1 +/- 0.6 mV in the old animals. Application of dibutyryl-cAMP (0.1 mmol.1-1) did not further increase the hormonal response in both groups. The ADH-mediated increase in the corresponding equivalent short-circuit current (ISC = Vte/Rte) was twice as great in young mice as in old, indicating that the stimulation of NaCl transport by ADH across the medullary thick ascending limb is significantly reduced with age. These results suggest that the previously reported age-related defect in the urinary concentrating ability of the kidney is partly due to a decreased response of the medullary thick ascending limb to ADH.

In vitro desensitization of isolated nephron segments to vasopressin

Recent studies have demonstrated that in vivo administration of 1-deamino-8-D-arginine-vasopressin, an analog of arginine-8-vasopressin, induces homologous desensitization to vasopressin in the thick ascending limb of the loop of Henle. Desensitization has been documented by a decreased physiological response to vasopressin in vivo and by a reduced cAMP accumulation in the cortical thick ascending limb (CTAL). By measuring cAMP content in single isolated medullary thick ascending limbs (MTALs), we now report that desensitization can occur all along the thick ascending limb and, more importantly, that it can also be induced in vitro. In a first series of experiments, we observed that 1 hr after in vivo injection of 1-deamino-8-D-arginine-vasopressin, MTALs were desensitized by 80% to vasopressin, whereas the effects of the other hormones acting on the same cyclase pool (glucagon, calcitonin) were fully maintained. In a second set of experiments, desensitization was induced in vitro by vasopressin, the natural hormone. A 60-min preincubation of MTALs with vasopressin caused a marked (up to 86%) and highly reproducible desensitization. The process was dose and time dependent. The apparent Ka for desensitization was 0.2 nM, and the half-maximal effect was obtained within 20 min. The desensitization induced in vitro by vasopressin was again essentially homologous in nature, with 80% of the maximal stimulation of cAMP accumulation being obtained in the presence of glucagon. Desensitization to vasopressin was observed in the presence and absence of indomethacin, indicating that it is independent of prostaglandin synthesis. It is concluded that (i) vasopressin and its analog 1-deamino-8-D-arginine-vasopressin cause marked desensitization in the CTAL and MTAL and (ii) the low vasopressin concentrations required to induce desensitization and the rapid onset of the process suggest that it has a physiological significance.

Inhibition of bicarbonate absorption by peptide hormones and cyclic adenosine monophosphate in rat medullary thick ascending limb

In vitro microperfusion experiments were performed to examine the effects of peptide hormones on bicarbonate and ammonium transport by the medullary thick ascending limb (MTAL) of the rat. Arginine vasopressin (AVP; 2.8 X 10(-10) M in the bath) reduced bicarbonate absorption by 50% (from 7.8 to 3.7 pmol/min per mm). AVP caused a similar reduction in bicarbonate absorption in tubules perfused with 10(-4) M furosemide to inhibit net NaCl absorption. Glucagon (2 X 10(-9) M in the bath) also reduced bicarbonate absorption (from 11.7 to 7.6 pmol/min per mm). The inhibition of bicarbonate absorption could be reproduced with either exogenous 8-bromo-cAMP or forskolin. With 8-bromo-cAMP (10(-3) M) in the bath, addition of vasopressin to the bath did not significantly affect bicarbonate absorption. PTH significantly inhibited bicarbonate absorption, but the extent of inhibition was less than that observed with either AVP or glucagon. Vasopressin had no effect on net ammonium absorption in MTAL perfused and bathed with 4 mM NH4Cl. These findings indicate that: (a) vasopressin, glucagon, and PTH directly inhibit bicarbonate absorption in the MTAL of the rat; (b) this inhibition occurs independent of effects on net NaCl absorption and appears to be mediated in part by cAMP; and (c) HCO3- and NH4+ absorption can be regulated independently in the MTAL.

Regulation of acid-base transport in the rat thick ascending limb

The thick ascending limb of the rat influences urinary net acid excretion by reabsorbing both bicarbonate and ammonium. The bicarbonate absorption is mediated predominantly by apical membrane Na+-H+ exchange and occurs at rates that are comparable to or greater than rates measured in cortical and medullary collecting ducts. The ammonium absorption is mediated predominantly by apical membrane Na+-NH4+-2Cl- cotransport and enhances urinary ammonium excretion by promoting countercurrent multiplication of ammonium, which facilitates ammonium secretion into medullary collecting ducts. Studies with medullary thick ascending limbs (MTAL) in vitro have shown that the regulation of these transport processes involves both acute responses to changes in the luminal and peritubular environment and adaptive changes in tubule transport capacity in response to chronic systemic acid-base perturbations. In particular, an increase in potassium concentration inhibits ammonium absorption with no effect on net bicarbonate absorption whereas vasopressin inhibits bicarbonate absorption with no effect on net ammonium absorption. Chronic metabolic acidosis causes an adaptive increase in the ability of the MTAL to reabsorb both bicarbonate and ammonium. These results demonstrate that the MTAL is a site of regulation of renal acid-base transport and that ammonium and bicarbonate transport rates can vary independently in this nephron segment.

The role of the kidney in the maintenance of water balance

This chapter shows how the mammalian kidney is able to regulate the excretion of water independently from that of solutes. For this function, which derives from several evolutionary steps among vertebrates, it takes advantage of the diluting ability of the thick ascending limb to produce osmotic energy which is then used to concentrate solutes in the urine. This concentration is permitted by a highly sophisticated architecture of nephrons and vessels in the renal medulla, combined with special permeability characteristics of the different nephron segments and specific hormonal regulation. Two different types of loops of Henle and several well-insulated vascular compartments contribute to this process. The major nitrogenous waste product, urea, is concentrated by an indirect process involving a transfer of osmotic energy from the outer to the inner medulla. As known for several decades, concentrating function is primarily regulated by the effect of antidiuretic hormone (ADH) on water permeability of the collecting duct. However, as discovered more recently, it is also largely dependent upon the effect of the same hormone on urea permeability in the terminal collecting duct. In addition, recent investigations have revealed a much more complex hormonal regulation of the concentrating process than previously thought. ADH itself acts on many other structures in the kidney, and many other hormones and mediators, the secretion of which is not thought to be influenced by the water status, do affect urine concentration either directly or by their interaction with ADH. Rodents display a wide spectrum of morphological and functional renal adaptations improving water conservation. Their study has brought a better understanding of the significant steps and anatomical structures that contribute to the concentrating process. Finally, it is also apparent that the capacity to concentrate urine is influenced in individual animals of a given species by the availability of water, by specific feeding patterns, and by the protein content of the diet.

Effect of arginine vasopressin on urine formation and plasma atrial natriuretic peptide level in conscious dogs

We investigated the relationship between plasma arginine vasopressin (AVP) and atrial natriuretic peptide (ANP) levels and urine formation in conscious dogs. The rate of urine production and urinary electrolyte excretion were determined following infusion of high doses of AVP to dogs in different volume states. Water deprivation for 24 h induced an antidiuresis while the plasma osmolality, plasma AVP and plasma ANP remained at physiological levels. Subsequent i.v. administration of AVP did not alter the production of urine. Oral water loading (20 ml/kg body weight) induced low plasma AVP levels (1.3 +/- 0.5 pg/ml, mean +/- S.E.M.). Following AVP administration to the water-loaded group, the urine production rate decreased significantly from baseline while the osmolality of the urine increased significantly. Plasma ANP levels did not differ significantly between the two experimental groups, and did not change following i.v. administration of AVP. The results show that, in conscious dogs, (1) the production of a small volume of highly concentrated urine can occur with plasma AVP levels of below 5 pg/ml, (2) 24 h of dehydration induces an antidiuresis while plasma peptides, as well as the different biochemical variables, remain within normal limits, (3) increased plasma AVP levels do not induce a change in plasma ANP levels under these experimental conditions, (4) infusion of AVP induces a significantly increased K+ excretion but only in overhydrated animals.

Short and long loop nephrons

The explanation for the necessity to have both short and long loop nephrons for urinary concentration is unknown but may represent nature's resolution of conflicting ideal conditions for maximum urinary concentration. Ideally, one would like the thick ascending limb to extend throughout the entire medulla to the papillary tip and be supplied by a blood flow vigorous enough to provide oxygen and remove waste products as rapidly as needed. One would also like to have a progressively smaller volume of tissue to be concentrated toward the papillary tip to lessen the osmotic work required and a highly efficient vascular exchange system to sequester the medullary interstitial solute effectively. But the same efficiency of countercurrent exchange of oxygen causes the inner medulla to have a relatively low oxygen content. The presence of the thin loops of Henle in the inner medulla may represent a compromise between these conflicting ideals. The papilla tapers to a low mass, which allows a mechanism requiring only a modest energy supply to increase the tonicity of the interstitium enormously. The reduced work requirement obivates the need for thick ascending limbs to extend into the papilla where they would be highly vulnerable to anoxia. The outer medulla with its larger mass and thick ascending limbs supplied by a high blood flow can initiate the operation to reduce the volume of fluid and solute to be concentrated, and at the same time carry out other functions required of the filtration-reabsorption kidney.(ABSTRACT TRUNCATED AT 250 WORDS)