Antidiuretic hormone reduces the high PGE2 synthesis in papillary collecting duct of DI rats

PGE₂ synthesis was measured along the nephron of Brattleboro (DI) rats, lacking ADH, and control LE rats, using an enzyme immunossay. Experiments were performed in vitro, in the absence of exogenous arachidonic acid, using microdissected tubular segments. The effect of a chronic treatment by dDAVP was tested on three ADH sensitive tubular segments, medullary thick ascending limb (MTAL), medullary collecting tubule (OMCD) and papillary collecting duct (IMCD).No difference in PGE₂ synthesis was present between LE and DI in glomerulus and tubular segments up to OMCD. In both strains, values were low in the proximal tubule and the loop of Henle, and gradually increased along the collecting tubule. In IMCD, PGE₂ synthesis was much higher in DI (12.8±2.0 pg per 30 min per mm tubular length) than in LE (3.8±0.5, LE vs. DIp<0.001).In MTAL and OMCD, dDAVP treatment did not affect PGE₂ synthesis. In IMCD, dDAVP reduced PGE₂ synthesis to values (5.3±0.8 pg per 30 min per mm tubular length), which were not significantly different from those of LE. Neither oxytocin, which has been shown to be elevated in DI rats, nor furosemide, that reduced papillary osmolarity to values comparable to those of DI rats, were able to increase PGE₂ synthesis in IMCD of LE rats. The mechanism of the increase in PGE₂ synthesis in IMCD of DI rats, and of the inhibitory effect of dDAVP is yet unknown; it may participate to compensate for the lack of ADH in the Brattleboro rat.     

Effects of osmolality and antidiuretic hormone on prostaglandin synthesis by renal papilla

Prostaglandin (PG) production by the kidney is known to be reduced both in vivo and in vitro in rats with hereditary diabetes insipidus (DI), totally lacking ADH. Exogenous ADH restores normal PG excretion in these rats. On the other hand, osmolality in vitro, and urine flow rate in vivo have been shown to influence PG synthesis rate. In order to determine whether the decreased PG synthesis of DI rats is due to the lack of antidiuretic hormone itself or to low tissue osmolality, we studied in vivo and in vitro PG production in DI rats in which urine osmolality had been raised either with ADG (infused by Alzet minipumps), or without ADH (by dehydratation) and in control DI rats.PGE₂ and PGF₂ were measured by radioimmunoassay in the urines and in supernatants of papillary homogenates incubated at 37°C for 15–120 min. ADH administration and dehydration led to similar urine osmolalities (900–1,000 mosmol/kg H₂O versus 150 in controls). However, only ADH administration but not dehydration increased PG urinary excretion (×5,P<0.001) and subsequent in vitro papillary synthesis (×1.6,P<0.01). These results show that antidiuretic hormone increases PG-synthesis of the renal papilla directly and not through its effects on papillary osmolality.     

The role of antidiuretic hormone in cold-induced diuresis in the anaesthetized rat

The aim of this study was to investigate whether the increased diuresis in consequence of hypothermia is due to a depression of the hypothalamic release of antidiuretic hormone (ADH). The plasma concentration of antidiuretic hormone and the effect of intravenous (i.v.) administration of 65 ng kg^?1 desmopressin (selective V₂-receptor agonist) were determined in the anaesthetized rat. In spite of a 50% (P < 0.001) decrease in glomerular filtration rate, urine flow increased sixfold (P < 0.01) and urine sodium excretion increased sevenfold (P < 0.05), whereas urine osmolality decreased (P < 0.001). At the same time plasma antidiuretic hormone decreased from 7.5 ± 1.1 to 3.8 ± 0.4 pg mL^?1 (P = 0.01). After injection of desmopressin urine flow was completely restored, whereas urine osmolality and sodium excretion were only partially normalized. Since tubular conservation of water and fractional water reabsorption decreased during hypothermia, the diuresis must have resulted from an augmented loss of water. This is further supported by the fact that osmolal excretion was not influenced either by hypothermia or by desmopressin. It is concluded that the diuresis in consequence to hypothermia is due both to a decrease in the release of ADH and to a reduction of renal medullary hypertonicity.     

The influence of furosemide and Co2+ on electrolyte and water transport in newt distal tubule and frog skin

The technique of stationary microperfusion of the early distal tubule used in experiments on male newts (Triturus vulgaris) revealed the intratubular effect of CoCl₂ and furosemide on the ion transport. CoCl₂ in concentration 5·10^–4 M increased Na and Cl reabsorption, reduced Ca, Mg and K reabsorption, and increased (TF _exp/TF _o)_in from 1.14±0.04 to 1.68±0.27 (p<0.001). CoCl₂ in concentration 5·10^–5 M enhanced Na and Cl reabsorption, without affecting transport of other ions. 6·10^–4 M furosemide decreased reabsorption of water, Na, Cl, Ca and Mg. The combined effect of Co²⁺ and furosemide inhibited reabsorption of Mg and Ca to a greater extent, but stimulated Na reabsorption. Experiments on frog skin showed that being added to the apical membrane of cells, Co²⁺ stimulates short circuit current and potential differences, whereas furosemide reducing Cl absorption induces only hyperpolarization. Experimental findings allow to assume that Co²⁺ on the outer surface of the apical membrane of the distal tubule cells blocks the selective Ca and Mg channels and activates Na channels.     

Renal potassium bicarbonate release in humans exposed to an acute volume load

Summary Cells of the renal medulla regulate their volume by transmembrane ion movements when exposed to large changes in osmolality. Since renal cells in culture release KHCO₃ in response to hypotonic stress [11], we investigated the effect of an acute water load on urinary KHCO₃ excretion in 5 healthy individuals. Water diuresis was induced by the ingestion of 1.51 hypoosmolal fluid (22 mosm/kg H₂O) over 15 min. The rate of urinary volume excretion increased from an initial value of 1.4 ml/min to 9.3 ml/min after 75 min. Urinary osmolality dropped from an initial value of 940±32 mosm/kg H₂O to 74±4 mosm/kg H₂O (n = 5). The decrease of osmolality was accompanied by the transient release of potassium and bicarbonate. Peak values of KHCO₃ excretion were observed between 30 and 45 min after the onset of the experiment corresponding to the drop of urinary osmolality. The magnitude of renal potassium release correlated significantly (r=0.93; P < 0.05) with endogenous plasma aldosterone concentrations measured prior to the experiment in the 5 volunteers. We conclude that medullary epithelial cells release KHCO₃ when exposed to hypotonic stress. The volume regulatory response is upregulated by aldosterone.Abbreviations ADH antidiuretic hormone - MDCK Madin-Darby canine kidney     

Renal function in normal and potassium-depleted rats before and after preparation for micropuncture experimentation

Renal function was examined in unrestrained conscious rats maintained on either a control diet or a low-potassium diet, then re-examined in the same animals after thiobutabarbital (Inactin) anaesthesia and preparation for micropuncture studies. In conscious rats, inulin clearance (C_In) was not significantly different in the two groups (control 1012±43, low-K 904±58 l/min per 100g body wt; mean±SE), but lithium clearance (C _Li; used as an estimate of end-proximal fluid delivery) and fractional lithium excretion (FE_Li) were substantially reduced in the low-K group (C _Li: 246±11 vs 126±8 l/min per 100g body wt, P<0.001; FE_Li: 0.245±0.009 vs 0.143±0.008, P<0.001). Following anaesthesia and preparation for micropuncture, there were significant reductions in urine flow rate and sodium excretion in the control group, but not in the low-K rats. Potassium excretion increased in both groups, but values in the potassium-depleted animals remained extremely low. In neither group of rats was preparation for micropuncture associated with significant changes in C _In, C _Li or FE_Li. Thus, differences in tubular function between control and potassium-depleted rats were still apparent. The results suggest that preparation for micropuncture disturbs the function of the distal nephron, but that rates of glomerular filtration and proximal tubular reabsorption remain similar to values in conscious animals.     

Effect of somatostatin on kidney function and vasoactive hormone systems in healthy subjects

Summary The acute effects of i.v. somatostatin (250 mcg bolus followed by 250 mcg/h continuous infusion for two hours) on renal hemodynamics, renal electrolyte and water handling, and urinary excretion of catecholamines and prostaglandins, as well as on plasma concentrations of arginine vasopressin, atrial natriuretic factor, norepinephrine, epinephrine, dopamine, glucagon, and plasma renin activity were studied in seven normal subjects. Somatostatin decreased effective renal plasma flow and glomerular filtration rate, osmotic and free water clearances, urine volume, and sodium and potassium excretion, while urinary osmolality, fractional excretion of sodium, and phosphate excretion increased significantly. Plasma concentrations of arginine vasopressin, atrial natriuretic factor, norepinephrine, epinephrine, and dopamine remained unchanged, while plasma renin activity (3.0±0.25 vs 2.4±0.2 ng AngI/ml/h;p}<0.01) and glucagon levels (40±11 vs 20±16 pg/ml;p}<0.01) decreased. Urinary excretion of norepinephrine, epinephrine, dopamine, PGE₂, and PGF_2alpha was suppressed under somatostatin. A significant positive correlation was found between urinary dopamine and sodium excretion (r=0.7;p}<0.001) and urinary postaglandin E₂ and glomerular filtration (r=0.52;p}<0.01). Without accompanying changes in plasma osmolality and vasopressin concentration significant antidiuresis occurred, suggesting a direct tubular effect of somatostatin. However, the hormone-induced changes are due mainly to the decrease in renal plasma flow. The results demonstrate that somatostatin at supraphysiological doses exerts significant effects on the kidney.Abbreviations PAH paraaminohippuric acid - ANF atrial natriuretic factor - AVP arginine vasopressin - PRA plasma renin activity - ERPF effective renal plasma flow - GFR glomerular filtration rate - TRP tubular reabsorption of phosphate - NE norepinephrine - E epinephrine - DA dopamine - GH growth hormone     

Effect of saline infusion on net reabsorption of endogenous sulfate relative to that of phosphate in intact and thyroparathyroidectomized rats

Clearance experiments have been performed to study the effects of saline infusion on the reabsorption of inorganic sulfate (SO₄) at endogenous levels. Adult female Sprague-Dawley rats on a standard diet were used. Both intact and thyroparathyroidectomized (TPTX) animals were infused with a 130 mmol/l sodium chloride solution at a low (0.15 ml/min) and a high (0.375 ml/min) rate. This increase of the infusion rate decreased the reabsorption of SO₄ in both groups of animals significantly. The fractional excretion of SO₄ in theintact rats increased from 9.9±5.6 to 18.4±3.6% (mean values±SD,p<0.001) and in theTPTX rats from 5.3±2.5 to 22.4±6.3% (p<0.001). It is concluded that endogenous parathyroid hormone has no major effect on the saline-induced inhibition of reabsorption of SO₄.This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Fr 239/9-1)     

Effect of indomethacin in vivo and PGE2 in vitro on MTAL Na-K-ATPase of the rat kidney

We have previously shown that prostaglandin synthesis inhibition in rats which reduces urinary excretion of PGE₂ and sodium, is associated with increased Na-K-ATPase activity in renal medulla. To further characterize this interaction studies were performed in isolated segments of medullary thick ascending limb of Henle's loop (MTAL) in rats. The effect of pretreatment with indomethacin in vivo and incubation with PGE₂ in vitro on MTAL Na-K-ATPase activity was studied. Pretreatment of rats with indomethacin increased Na-K-ATPase of the MTAL from 37.2±2.0×10⁻¹¹ mol/mm/min in controls to 62.7±2.2 (p<0.001) while Mg-ATPase was only slightly decreased. Incubation of MTAL Na-K-ATPase from indomethacin pretreated rats with increasing concentration of PGE₂ in vitro dose dependently inhibited MTAL Na-K-ATPase activity with no effect on Mg-ATPase. Baseline Na-K-ATPase was 62.7±2.2 in MTAL from indomethacin pretreated rats and decreased to 36.9±1.4 (p<0.001) with 1 uM of PGE₂, to 26.5±2.3 (p<0.001) with 10 uM PGS₂ and to 22.0±1.0 (p<0.001) with 100 uM PGE₂. 100 uM PGE₂ in the incubation medium inhibited MTAL Na-K-ATPase of intact rats from 37.2±2 to 21.3±1.2 (p<0.001) and completely abolished the indomethacin induced increase in MTAL Na-K-ATPase. The results of this study show stimulation of MTAL Na-K-ATPase by pretreatment with indomethacin in vivo and, direct inhibition of MTAL Na-K-ATPase by PGE₂ in vitro. These findings suggest that the increase in MTAL Na-K-ATPase induced by pretreatment with indomethacin may stem directly from PGE₂ depletion. These observations are consistent with direct inhibitory effect of PGE₂ on Na reabsorption in the MTAL.     

Renal function and concentrating ability in a desert rodent: The gundi(Ctenodactylus vali)

Clearance and cortical micropuncture experiments were carried out on non diuretic gundis. In this species, the kidney has a long and well developed papilla but, unlike other desert rodents, the vascular organization of the outer medulla is very simple. After withdrawal of water supply for either 24 h or 3 days before the experiments, the urine osmolality was only 1,361±57,n=9, before and 1,136±89 mosmol ·kg^–1 during anesthesia. The GFR per 100 g B. W. (0.450 ml ·min^–1) is lower than in the rat studied under similar conditions. With regard to electrolytes the tubular handling of Na, Ca, K and Mg is similar to that observed for another desert rodent, psammomys obesus. For P, massive reabsorption (more than 30% of the filtered load) takes place along the distal convoluted tubule. The relatively poor concentrating ability of the gundi's kidney is not due to a lack of medullary recycling of urea since a net addition of urea to short loops of Henle is observed in this species. Physiological and morphological observations concerning the gundi and other desert rodent species suggest that the vascular bundle development in the outer medulla might affect the renal response to water deprivation.     

Valine oxidation in the rat medullary thick ascending limb

In the kidney, a branched-chain amino acid transferase (BCAAT) activity has been localized mainly in the medullary thick ascending limb (MTAL) of the rat nephron. BCAAT is the first enzyme involved in the metabolic pathway of the three branched-chain amino acids (BCAA): leucine, isoleucine and valine. The present work has been designed to study valine catabolism. Valine and leucine oxidation in MTAL were compared by measuring the rate of ¹⁴CO₂ produced when these substrates were incubated as sole substrates at a final concentration of 1 mM. Since glucose is also metabolized in MTAL, valine and leucine oxidation were quantified also in the presence of glucose (1 mM). The results show that: (1) valine oxidation was greater than that of leucine (63.0±4.7 vs 39.7±5.2 pmol · h^–1 · (g^–1 protein, respectively; P<0.001). As previously shown, leucine oxidation was found to be increased in the presence of glucose whereas glucose oxidation decreased. In contrast, the presence of glucose strongly diminished valine oxidation (19.2±1.9 vs 63.1±4.7 pmol · h^–1 · (g^–1 protein; P<0.001) whereas glucose oxidation was increased in the presence of valine (268.2±14.9 vs 229.6±16.2 pmol · h^–1 · g^–1 protein; P<0.05). We conclude that in rat MTAL, under near physiological conditions (in the presence of glucose, as in vivo), leucine is a preferential respiratory fuel as compared to valine. However, valine supports energetic salt transport and facilitates glucose oxidation.     

Dissociation between antidiuretic response and renal medullary cyclic AMP levels in the rat

Renal tubular bicarbonate reabsorption and acidification were evaluated in phosphate depleted rats (PD) and controls. After 33 days of phosphate depletion, urine pH of PD rats (N=5, 6.36±0.15) was significantly higher than control (N=5, 5.64±0.09,P<0.005) following an NH₄Cl load. Urinary titratable acid of PD rats (9.6±1.8) was significantly reduced compared to control (117.2±19.7 Eq/3 h,P<0.001), whereas NH ₄ ⁺ excretion was not different. The plasma HCO ₃ ^– thresholds at which bicarbonaturia occurred (approximately 25 mEq/l) were identical in controls and phosphate depleted rats during isotonic bicarbonate infusion. The higher urine pH of phosphate depleted rats following NH₄Cl administration was not due to low urinary phosphate as 3-day phosphate depleted rats could normally acidify urine after NH₄Cl (pH=5.86±0.09,N=6 vs. control 5.87±0.08,N=6,P=N.S.) despite urinary phosphate excretion as low as in 33-day PD rats. These data indicate the presence of impaired distal tubular acidification in chronically phosphate depleted rats.Former trainee of the Cardiovascular Research Program and currently a third year medical student at The University of Michigan Medical School.     

Proximal tubular reabsorption of inorganic phosphate in adrenalectomized rats

Micropuncture studies have been performed in adrenalectomized (Adr.X) and sham-adrenalectomized (Sham-Adr.X) rats in order to examine the effects of acute adrenalectomy on the inorganic phosphate (Pi) transport in the convoluted proximal tubule of superficial nephrons under free flow conditions. Adrenalectomy diminished single nephron glomerular filtration rate from 27.0±12.4 (Sham-Adr.X) to 10.9±7.1 nl/min (mean ± S.D.,P<0.001), increased the inulin ratio (tubular fluid-to-plasma) at the end of the proximal convolution from 2.14±0.60 (Sham-Adr.X) to 3.62±1.67 (P<0.001) and decreased both the (TF/P)_Pi ratio from 0.87±0.20 (Sham-Adr.X) to 0.46±0.28 (P<0.001) and the fractional delivery of inorganic phosphate into the loop of Henle from 44±18 (Sham-Adr.X) to 15±10% (P<0.001). Thus the fractional reabsorption of inorganic phosphate in the convoluted proximal tubule in acutely Adr.X rats was higher than in Sham-Adr.X animals. It is concluded that glucocorticoid hormones play an important role in the regulation of the fractional reabsorption of Pi in the proximal tubule. The mechanism of this regulation is discussed.Supported by the Deutsche Forschungsgemeinschaft (Fr239/7). Parts of this work [6] have been presented at the 52th Meeting of the German Society of Physiology, Kiel. FRG, 1979, and at the XIIIth Symposion of the German Society of Nephrology, Berlin, 1979     

Effects of antidiuretic hormone on electrolyte reabsorption and secretion in distal tubules of rat kidney

The effects of (1-desamino-8-d-arginine) vasopressin (dDAVP) on water and electrolyte transport in the distal tubule were investigated by micropuncture. Since, in addition to antidiuretic hormone, parathyroid hormone, calcitonin and glucagon stimulate the adenylate-cyclase system in this nephron segment, experiments were performed on hormone-deprived rats, i.e. homozygous DI Brattleboro rats with reduced levels of endogenous parathyroid hormone, calcitonin and glucagon. Along the distal tubule, dDAVP enhanced water, Cl, Na and Ca reabsorption and sharply increased net K secretion. Phosphate transport was left unchanged and Mg reabsorption was not significantly altered by dDAVP between the early and late distal tubule. Antidiuretic hormone also slightly increased water filtration rate in the superficial nephron, which rose in proportion to whole kidney glomerular filtration rate. It is concluded that, in rats: 1) antidiuretic hormone stimulates water, NaCl and Ca absorption and enhances K secretion along the distal tubule and 2) the tubular effects of dDAVP on electrolyte transport in the loop and distal tubule are responsible for decreasing Mg and Ca urinary excretion.     

The effect of streptozotocin-induced diabetes mellitus on urinary excretion of sodium and renal Na+−K+-ATPase activity

Renal sodium handling and microsomal Na⁺–K⁺-ATPase activity in kidney cortex, medulla and papilla of rats with streptozotocin-induced diabetes mellitus (DM) was studied.During 7 days following the administration of streptozotocin GFR, urinary excretion, filtered load and tubular reabsorption of Na⁺ averaged (mean±SE) 1.18±0.016 ml/min, 1.74±0.14, 177.3±8.9 and 175.6±8.9 mEq/min respectively in experimental rats as compared to corresponding rates of 0.85±0.04 (P<0.001), 0.85±0.03 (P<0.001), 129.8±5.8 (P<0.001) and 129±5.8 (P<0.001) respectively in the control rats.The activity of microsomal Na–K-ATPase in the kidney cortex, medulla and papilla of the control group was (mean±SE) 44.7±1.7, 150±7.5 and 37.4±3.6 (moles Pi/mg prot/h) respectively. 24 h after DM induction Na–K-ATPase activity in the cortex rose to 59.3±2.4 (P<0.001) and remained high after 3 and 7 days. Medullary Na–K-ATPase activity was unchanged 24 h after streptozotocin administration but was markedly increased to 260±9 (P<0.001) after 3 days and remained high after 7 days.These findings show that stretozotocin-induced DM in rats causes a substantial increase in GFR which is associated with a net increase in filtered and reabsorbed load of Na⁺ and natriuresis. These alterations are accompanied by a marked increase in Na–K-ATPase activity in renal medulla and in the cortex.This study was supported by the Morton S. Kaufman Hemodialysis Foundation and by the Joint Research Fund of the Hebrew University and Hadassah     

Regulation of the renal transport of inorganic sulfate: Effects of metabolic changes in arterial blood pH

The present experiments were designed to examine the effects of mild metabolic changes of blood pH on the reabsorption of inorganic sulfate (SO₄) in the rat. Female adult Sprague Dawley rats (which were kept on a standard diet) were used. In order to produce either a metabolic alkalosis (Alk) or a metabolic acidosis (Ac), 130 mmol/l NaHCO₃ or a mixture of 130 mmol/l NH₄Cl and 130 mmol/l NaCl was infused. The blood pH values were 7.49±0.02 and 7.29±0.02, respectively. The fractional excretion (FE) of SO₄ was 28.6±5.7 (Alk) and 9.5±2.7 (Ac), respectively. These changes were significant (P<0.001). It is concluded, that physiological metabolic changes of blood pH may be important in the renal regulation of the transport of SO₄.This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Fr 239/9-2)     

Papillary plasma flow in rats

Papillary plasma flow (PPF) was measured by the albumin accumulation technique in rats of the Brattleboro strain with or without diabetes insipidus (DI and HZ respectively) and in Wistar rats. Measurements were also performed in DI rats receiving antidiuretic hormone for 30 min or 5 days and in dehydrated Wistar rats. PPF in HZ control and Wistar control rats was similar to previously published measurements. In contrast PPF was significantly higher in DI rats (461±26l/min·g versus 263±28 in HZ) and decreased significantly after acute ADH administration. It returned to control values after prolonged ADH administration (262±40). Plasma flow entering the papilla was inversely correlated with urine osmolality up to 1000 mosmol/kg H₂O. Further increases in urine concentration (dehydration of Wistar rats) did not modify further PPF (255±28 versus 270±16 in non dehydrated Wistar). PPF might be influenced indirectly by ADH or prostaglandins and seems to depend on the osmotic environment of the papilla up to a certain limit. The factors which maintain PPF at a given minimum level with further increases in urine concentration are not known.     

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 (V _te) 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.l^–1 ADH to the bath solution significantly increased V _te 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 (I _SC = V _te/R_te) 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.     

Modulation of the effect of arginine-vasopressin on water and ion transport in the newt early distal tubule and frog urinary bladder by V1-antagonists

In the early distal tubule of the newtTriturus vulgaris L., 1 nM arginine-vasopressin (AVP) increased water reabsorption; the fractional reabsorption of Na⁺ was elevated from 46.2±6.9% to 67.8±3.9% (P<0.001), of Cl^– from 52.7±6.7% to 73.1±3.5% (P<0.001), of Mg²⁺ from 48.0±7.7% to 71.7±6.3% (P<0.001). When V₁-receptors were blocked by 1 nM peptide V₁-antagonist [1-(-mercapto-,-cyclopentamethylene propionic acid), 2-(O-methyl) Tyr]-[Arg⁸]vasopressin, 1 nM AVP increased the fractional reabsorption of fluid by 8.9% of Na⁺ by 10.7% and of Cl^– by 11.2%, as compared with the effect of AVP alone. The fractional reabsorption of Ca²⁺ after addition of AVP did not differ from control; when V₁-receptors were blocked in the presence of AVP, the fractional reabsorption of Ca²⁺ was increased by AVP. The V₁-receptor block in the presence of AVP did not change the fractional reabsorption of Mg²⁺. Experiments on the urinary bladder of the frogRana temporaria L. showed that 1 nM SR 49059, a non-peptide antagonist of V_1a-receptors, like the peptide V₁antagonist, enhanced the AVP effect by 29%. Inhibition of protein kinase C activity by calphostin C (1 nM) mimicked the effect of V₁-antagonists; the AVP hydroosmotic effect was increased by 60%. The results obtained indicate that V₁-receptors modulate the effects of V₂-receptor activation: their block is accompanied by an enhancement of the AVP hydroosmotic effect in the frog urinary bladder and by an increase of Na⁺ and Cl^– reabsorption in the newt early distal tubule. The enhancement of the AVP effect owing to the V₁-receptor activation seems to be mediated by a decrease in protein kinase C activity.     

Simultaneous Mg,Ca, P,K, Na and Cl analysis in rat tubular fluid

Summary The micropuncture recollection technique was used for further analysis of Mg, Ca, P, Na, Cl and K tubular handling during and after acute magnesium plasma loading in the rat. The concentrations of these different ion species in late proximal and early distal fluid samples, ureteral urine and plasma ultrafiltrates were measured using electron probe microanalysis.³H-inulin was used as glomerular indicator.The results indicate that 1, late proximal (TF/P_f)Mg/In values remained proportional to the filtered load (about 80%); 2. part of the filtered Mg was reabsorbed in the loop of Henle by an active and saturable transport mechanism; 3. early distal (TF/P_f)Mg/In values during and after Mg loading were below the corresponding (U/P_f)Mg/In values; this observation might either indicate net Mg tubular excretion by the terminal nephron segments or result from the building up of a medullary Mg ion pool during the loading period; 4. acute hypermagnesemia induced transient calciuria, shown to result from decreased Ca reabsorption in the terminal segments; 5. it enhanced P reabsorption in the terminal segments; the drop in P excretion might have been mediated by decreased PTH release; 6. tubular handling of the other ion species was relatively unaffected by increasing tubular Mg loading.