REGULATION OF PROXIMAL TUBULE FUNCTION BY ANGIOTENSIN

1. Independent of its effects on renal haemodynamics and glomerular filtration, angiotensin II (AII) has direct actions on the proximal tubule involving transepithelial Na+, H+, HCO3-, and water reabsorption, ammoniagenesis, gluconeogenesis and renal growth. 2. The effects of AII on water and electrolyte transport are biphasic and dose-dependent, such that low concentrations (10(-12)-10(-9) mol/L) stimulate reabsorption whereas high concentrations (10(-7)-10(-6) mol/L) inhibit reabsorption. Similar dose-response relations have been obtained for luminal and peritubular addition of AII. 3. The cellular responses to AII are mediated via an AT-1 receptor coupled via G-regulatory proteins to several parallel signal transduction pathways. Low doses inhibit the basolateral adenylate cyclase, lower intracellular cAMP and withdraw the inhibitory effect of protein kinase A on the luminal Na/H exchanger. Stimulation of this exchanger may also occur due to AII-receptor activation of phospholipase C to release diacyl glycerol, or by local transduction in the brush-border membrane involving phospholipase A2. 4. Inhibition of proximal fluid reabsorption is associated with increased intracellular Ca2+ released from intracellular stores, or entering via voltage-sensitive channels in response to the release of inositol-1,4,5-trisphosphate, or following Ca2+ channel opening induced by the arachidonic acid metabolite 5,6-epoxy-eicosatrienoic acid. 5. The stimulatory actions of peritubular AII on proximal transport are inhibited by physiological concentrations of atrial natriuretic factor (ANF) and by parathyroid hormone (PTH). 6. It is concluded that intrarenal AII acts to maintain optimal matching of fluid reabsorption and filtered load in response to changes in sodium balance, as well as to promote acidification of the urine during acidosis and perhaps to potentiate tubular growth following renal injury.     

FILTRATION FAILURE INDUCED BY pAMINOPHENOL IN RATS IS DUE TO RAISED INTRATUBULAR PRESSURE AND NOT CHANGES IN GLOMERULAR FUNCTION

1. The p-aminophenol (pAP) model of tubular necrosis displays elevated tubular pressures equivalent to 'stop-flow', with low glomerular filtration rate (GFR) but maintained blood flow and urine output. Renal function, micropuncture, and morphological studies were performed in anaesthetized rats to examine the causes of filtration failure. 2. At the height of pAP-induced renal failure proximal tubular fluid reabsorption (Jv(a] was markedly reduced while proximal and distal free-flow rates measured by tubular fluid collections during venting of the nephron were not significantly different from saline-injected controls. Renal blood flow was maintained over the 4 h observation period despite extensive and selective proximal tubular necrosis. There was no temporal relationship between increased tubular pressure and cast formation. 3. Maintained blood and tubular fluid flow rates indicate that activation of tubuloglomerular feedback plays little or no part in pAP-induced renal failure, which is apparently due to high fluid flow resistance in the region of the connecting tubule, late distal convolution or collecting ducts. Morphological appearances were consistent with compression of these segments.     

Proceedings of the Symposium ‘Angiotensin AT1 Receptors: From Molecular Physiology to Therapeutics’: REGULATION OF RENAL TUBULAR SODIUM TRANSPORT BY ANGIOTENSIN II AND ATRIAL NATRIURETIC FACTOR

• 1 The effects of angiotensin II (AngII) on water and electrolyte transport are biphasic and dose-dependent, such that low concentrations (10^?12 to 10^?9 mol/L) stimulate reabsorption and high concentrations (10^?7 to 10^?6 mol/L) inhibit reabsorption. Similar dose-response relationships have been obtained for luminal and peritubular addition of AngII.
• 2 The cellular responses to AngII are mediated via AT₁ receptors coupled via G-regulatory proteins to several possible signal transduction pathways. These include the inhibition of adenylyl cyclase, activation of phospholipases A₂, C or D and Ca²⁺ release in response to inositol-1,4,5,-triphosphate or following Ca²⁺ channel opening induced by the arachidonic acid metabolite 5,6,-epoxy-eicosatrienoic acid. In the brush border membrane, transduction of the AngII signal involves phospholipase A₂, but does not require second messengers.
• 3 Angiotensin II affects transepithelial sodium transport by modulation of Na⁺/H⁺ exchange at the luminal membrane and Na⁺/HCO₃ cotransport, Na⁺/K⁺-ATPase activity and K⁺ conductance at the basolateral membrane.
• 4 Atrial natriuretic factor (ANF) does not appear to affect proximal tubular sodium transport directly, but acts via specific receptors on the basolateral and brush border membranes to raise intracellular cGMP levels and inhibit AngII-stimulated transport.
• 5 It is concluded that there is a receptor-mediated action of ANF on proximal tubule reabsorption acting via elevation of cGMP to inhibit AngII-stimulated sodium transport. This effect is exerted by peptides delivered at both luminal and peritubular sides of the epithelium and provides a basis for the modulation by ANF of proximal glomerulotubular balance. The evidence reviewed supports the concept that in the proximal tubule, AngII and ANF act antagonistically in their roles as regulators of extracellular fluid volume.

     

MODULATION OF PROXIMAL TUBULAR REABSORPTION BY ANGIOTENSIN II

In shrinking-drop micropuncture studies in anaesthetized rats proximal tubular fluid reabsorption (JVa) decreased by 36% following intravenous infusion of enalapril. In a separate group of rats enalapril reduced fractional lithium clearance indicating decreased proximal fluid reabsorption. Following enalapril, GFR rose by 46% but absolute proximal reabsorption rose by 22% indicating 48% effectiveness of proximal glomerulo-tubular balance (GTB). Since renal blood flow and glomerular filtration rate (GFR) increased in parallel and arterial pressure fell, fluid uptake and proximal GTB were unlikely to have been decreased by peritubular physical forces. In anaesthetized rats proximal fluid reabsorption and proximal GTB are modulated by endogenous AII through direct stimulation of proximal tubular transport.     

RENAL HANDLING OF ENDOGENOUS LITHIUM IN EXPERIMENTAL DIABETES MELLITUS IN THE RAT

1. The usefulness of determining the renal handling of endogenous lithium as a marker of proximal tubular sodium reabsorption was assessed in streptozotocin induced diabetes mellitus in the Sprague-Dawley rat. 2. The clearance and fractional excretion of lithium were determined before and following the development of diabetes mellitus, and compared with measurements of proximal tubular reabsorption made directly using micropuncture techniques. Endogenous lithium was measured in order to avoid the toxic tubular effects of exogenously administered lithium salts. 3. Although a trend existed for a reduction in the fractional excretion of lithium in diabetic animals (1.8 +/- 0.3 vs 2.4 +/- 0.5%; P greater than 0.20), this did not reach statistical significance and did not accurately reflect the change in directly measured tubular Na reabsorption. 4. The decrease in proximal tubular Na reabsorption demonstrated in diabetic animals treated with phlorizin was not significantly reflected in the fractional lithium excretion, although again a corresponding trend was evident (1.9 +/- 0.8 vs 0.6 +/- 0.2%; P greater than 0.10. 5. In summary, the significant alterations in tubular Na handling in diabetes mellitus, previously demonstrated directly using micropuncture techniques, are not reflected in the renal handling of endogenous lithium. This indirect method is inadequate to assess proximal tubular Na transport in experimental diabetes mellitus.     

Maleate induced fall of glomerular filtration rate

Summary Maleate causes an enhanced excretion of amino acids, glucose, phosphate and bicarbonate. In addition to this inhibition of fluid and electrolyte reabsorption malate decreases glomerular filtration rate (GFR). The present investigation was designed to study the mechanisms of this fall in GFR.In group I (Sprague-Dawley rats;N=8) maleate (2 mmol/kg body weight i.v.) increased the hydrostatic pressure in proximal tubule from 12.6±0.5 to 16.3±0.8 mm Hg (mean+SEM) and stop flow pressure in the first accessible loop of the proximal tubule was unchanged (33.6±0.4 vs 33.1±1.3 mm Hg; n.s.). Directly measured hydrostatic pressure in the glomerular capillaries in Munich-Wistar rats (N=7), however, was reduced by maleate from 47.6±1.6 to 42.4±1.9 mm Hg. In group II (N=8) we determined single nephron filtration rate (SNGFR) from distal and proximal collection sites in the same nephron in a paired fashion under control conditions and after maleate administration to assess the activity of the tubuloglomerular feedback. In the control periods SNGFR (16 nephrons) from distal collection sites was 26.3±1.6 nl/min whereas SNGFR from proximal collection sites was 31.8±2.4 nl/min. Following maleate distal SNGFR (17 nephrons) was 15.2±1.7 nl/min and proximal SNGFR was 24.3±2.2 nl/min. The ratio distal/proximal SNGFR was 1.23±0.07 under control conditions and increased to 1.76±0.1 following maleate indicating enhanced activity of tubuloglomerular feedback.Following maleate (group III;N=8) the chloride concentration in the late proximal tubular fluid fell from 135±4 mmol/l to 121±6 mmol/l (13 tubules), however, in the early distal tubule we found an increase in chloride concentration from 43±3 mmol/l to 62±6 mmol/l (12 tubules). Delivery of chloride to the loop of Henle was 2,107±221 pmol/min in the control periods and decreased to 1,475±200 pmol/min during maleate. Since early distal chloride delivery increased from 199±24 pmol/min to 364±47 pmol/min following maleate inhibition of chloride reabsorption in the loop of Henle has to be assumed. We conclude that maleate decreases glomerular filtration rate first by an elevation of hydrostatic pressure in the proximal tubule which is the result of inhibition of fluid and electrolyte reabsorption, and second by activation of the tubuloglomerular feedback.     

Effects of furosemide on the tubular reabsorption of nitrates in anesthetized dogs.

The present study was performed to determine the tubular sites of nitrite and nitrate (NO) reabsorption and the effects of furosemide on the renal handling of NOx in anesthetized dogs, using renal clearance and stop-flow methods. Furosemide (2 mg/kg, i.v.) increased the urinary excretion rates of Na+ (U(Na+)V) and NOx (U(NOx)V) with a reduction of tubular reabsorption rates of Na+ and NOx. During inhibition of renal nitric oxide (NO) synthesis by an intrarenal infusion of L-nitro arginine (30 microg/kg-min), furosemide also increased U(NOx)V and decreased tubular reabsorption rate of NOx from 96.5+/-0.8% to 86.6+/-1.7%. An intravenous infusion of 10% mannitol (0.5 ml/kg-min) also increased both U(Na+)V and U(NOx)V. In addition, after furosemide administration or mannitol infusion. U(NOx)V was correlated with U(Na+)V. In stop-flow experiments, the distal dip in NOx curve was observed and the site of the dip in NOx curve was identical to that of Na+ curve. Furosemide shifted upward the U/P(Na+)/U/P(Cr) and U/P(NOx)/U/P(Cr) at the distal dip, indicating inhibition of Na+ and NOx reabsorption at distal tubules. These results indicate that more than 96% of the filtered NOx is reabsorbed in the renal tubules, and that the tubular handling of NOx is very close to that of Na+. In addition, the stop-flow experiments demonstrate that furosemide inhibited the reabsorption of NOx as well as Na+ at the distal tubule.     

Regulation of renal tubular sodium transport by angiotensin II and atrial natriuretic factor

1. The effects of angiotensin II (AngII) on water and electrolyte transport are biphasic and dose-dependent, such that low concentrations (10(-12) to 10(-9) mol/L) stimulate reabsorption and high concentrations (10(-7) to 10(-6) mol/L) inhibit reabsorption. Similar dose-response relationships have been obtained for luminal and peritubular addition of AngII. 2. The cellular responses to AngII are mediated via AT(1) receptors coupled via G-regulatory proteins to several possible signal transduction pathways. These include the inhibition of adenylyl cyclase, activation of phospholipases A(2), C or D and Ca(2+) release in response to inositol-1,4,5,-triphosphate or following Ca(2+) channel opening induced by the arachidonic acid metabolite 5,6,-epoxy-eicosatrienoic acid. In the brush border membrane, transduction of the AngII signal involves phospholipase A(2), but does not require second messengers. 3. Angiotensin II affects transepithelial sodium transport by modulation of Na(+) /H(+) exchange at the luminal membrane and Na(+)/HCO(3) cotransport, Na(+)/K(+)-ATPase activity and K(+) conductance at the basolateral membrane. 4. Atrial natriuretic factor (ANF) does not appear to affect proximal tubular sodium transport directly, but acts via specific receptors on the basolateral and brush border membranes to raise intracellular cGMP levels and inhibit AngII-stimulated transport. 5. It is concluded that there is a receptor-mediated action of ANF on proximal tubule reabsorption acting via elevation of cGMP to inhibit AngII-stimulated sodium transport. This effect is exerted by peptides delivered at both luminal and peritubular sides of the epithelium and provides a basis for the modulation by ANF of proximal glomerulotubular balance. The evidence reviewed supports the concept that in the proximal tubule, AngII and ANF act antagonistically in their roles as regulators of extracellular fluid volume.     

Effects of dextran sulphate on renal dysfunctions induced by gentamicin as determined by the kidney perfusion technique in rats.

The effect of dextran sulphate on accumulation of gentamicin within the renal cortex and on renal functional changes induced by gentamicin has been investigated using the rat renal perfusion technique. Gentamicin accumulation decreased by the coadministration of dextran sulphate, the degree being proportional to dextran sulphate concentration. The filtered load of gentamicin was unaffected by dextran sulphate. The perfusion of kidney with gentamicin brought about a decrease in the reabsorption of Na+ and an increase in the rate of urine flow with no effect on that of glomerular filtration. The addition of dextran sulphate in the perfusate resulted in the resumption of these levels to those of the control. Dextran sulphate thus apparently interacts with gentamicin in the lumen of proximal tubules to inhibit the reabsorption of gentamicin by proximal tubular cells, and prevents the renal dysfunctions induced by gentamicin.     

Sodium reabsorption in thick ascending limb of Henle's loop: effect of potassium channel blockade in vivo

1. Based on previous in vitro studies, inhibition of K(+) recycling in thick ascending limb (TAL) is expected to lower Na(+) reabsorption through (i) reducing the luminal availability of K(+) to reload the Na(+)-2Cl(-)-K(+) cotransporter and (ii) diminishing the lumen positive transepithelial potential difference which drives paracellular cation transport. 2. This issue was investigated in anaesthetized rats employing microperfusion of Henle's loop downstream from late proximal tubular site with K(+)-free artificial tubular fluid in nephrons with superficial glomeruli. 3. The unselective K(+) channel blocker Cs(+) (5 - 40 mM) dose-dependently increased early distal tubular delivery of fluid and Na(+) with a maximum increase of approximately 20 and 185%, respectively, indicating predominant effects on water-impermeable TAL. 4. The modest inhibition of Na(+) reabsorption in response to the 15 mM of Cs(+) but not the enhanced inhibition by 20 mM Cs(+) was prevented by luminal K(+) supplementation. Furthermore, pretreatment with 20 mM Cs(+) did not attenuate the inhibitory effect of furosemide (100 microM) on Na(+)-2Cl(-)-K(+) cotransport. 5. Neither inhibitors of large (charybdotoxin 1 microM) nor low (glibenclamide 250 microM; U37883A 100 microM) conductance K(+) channels altered loop of Henle fluid or Na(+) reabsorption. 6. The intermediate conductance K(+) channel blockers verapamil and quinine (100 microM) modestly increased early distal tubular Na(+) but not fluid delivery, indicating a role for this K(+) channel in Na(+) reabsorption in TAL. As observed for equieffective concentrations of Cs(+) (15 mM), Na(+) reabsorption was preserved by K(+) supplementation. 7. The results indicate that modest inhibition of K(+) channels lowers the luminal availability of K(+) and thus transcellular Na(+) reabsorption in TAL. More complete inhibition lowers paracellular Na(+) transport probably by reducing or even abolishing the lumen positive transepithelial potential difference. Under the latter conditions, transcellular Na(+) transport may be restored by paracellular K(+) backleak.     

Sodium reabsorption during intrarenal diazoxide infusion in the dog

Infusion of diazoxide, a potent benzothiazide antihypertensive, into the renal artery results in diuresis and natriuresis. The site within the nephron of decreased reabsorption has been controversial. Thus free flow recollection micropuncture studies of the superficial proximal tubule of the dog were undertaken to determine if diazoxide decreased sodium reabsorption from this part of the nephron. Renal blood flow, monitored by an electromagnetic flow meter, was increased by about 15% with the diazoxide infusion. Systemic blood pressure and hematocrit remained unchanged. Glomerular filtration rate increased significantly from 26 +/- 2 to 34 +/- 3 ml/min, urine flow and sodium excretion also increased (0.13 +/- 0.01 to 0.33 +/- 0.06 ml/min and 5.5 +/- 0.90 to 35.5 +/- 11.0 microEq/min, respectively). Decreased sodium reabsorption from the proximal tubule was demonstrated by a decrease in the tubular fluid to plasma inulin ratio (1.62 +/- 0.1 1.47 +/- 0.1) thus giving a reduction in fractional sodium reabsorption to the site of micropuncture (36.1 +/- 4.3 to 29.5 +/- 5.1%, p less than 0.05). To examine peritubular effects of diazoxide infusion, capillary protein concentration and pressure were measured; the former increasing significantly (9.17 +/- 0.32 to 9.80 +/- 0.35, p less than 0.05) and the latter did not change (13.1 +/- 1.0 vs. 15.2 +/- 1.4 mm Hg). Thus intrarenal diazoxide causes whole kidney vasodilatation with diuresis, natriuresis and decreased sodium reabsorption from the superficial proximal tubule. Additional studies provided no data to indicate that changes in peritubular physical factors account for the changes in sodium handling.(ABSTRACT TRUNCATED AT 250 WORDS)     

Animal experiments on the question of the renal toleration of the horse chestnut saponin aescin.

The possibility that aescin might have a nephrotoxic side effect has been investigated by clearance studies in kidneys of healthy rats and by toleration studies in rats with damaged kidneys. The effect of aescin, both free and albumin-bound, on renal tubular transport processes was studied in the model of the isolated, artificially perfused frog kidney. The rates at which different concentrations of aescin were bound to rat plasma proteins were determined in vitro. The clearance of i.v. aescin was 13% of creatinine clearance and 7% of p-aminohippurate (PAH) clearance; this rules out the tubular secretion of aescin. No deaths occurred among aminonucleoside-damaged rats given i.v. sodium aescinate 2.2 mg/kg, but rats damaged with mercuric chloride or uranyl nitrate had exactly the same mortality rate as those given 2.2 mg/kg i.v. of sodium aescinate alone. The rats received four injections in all of aescin 0.35 mg/kg i.v., given at intervals of two days. Aescin had no effect on renal damage caused by aminonucleoside, mercuric chloride or uranyl nitrate. Aescin concentrations of 0.2 mg/l and 2.0 mg/l in the perfusion fluid increased the excretion of Na+ and glucose by the frog kidney and reduced the reabsorption of both these substances. With a sodium aescinate concentration of 5 mg/l the production of urine ceased. When 1% (w/v) of albumin was added to the perfusion fluid, even sodium aescinate 5 mg/l had no effect on the tubular transport of Na+, glucose and water. The fact that about 50% of aescin was not bound to plasma protein in vitro suggests that some of the small amount of aescin in the glomerular filtrate is reabsorbed in the tubules.     

AMYLIN: PHYSIOLOGICAL ROLES IN THE KIDNEY AND A HYPOTHESIS FOR ITS ROLE IN HYPERTENSION

1. There are high-affinity binding sites for amylin in the renal cortex associated with proximal tubules. These appear to represent seven transmembrane (heptatopic) receptors that are known to form ternary complexes with G-proteins and activate second messenger systems. 2.Amylin stimulates sodium/water reabsorption from the basolateral side of the proximal tubules and plays a role in sodium homeostasis. 3. The transient expression of amylin-like mRNA has been detected perinatally, using in situ hybridization, in the sub-nephrogenic zone of the metanephros and is associated with proximal tubules of the developing nephron. There it is thought to play a role as a growth factor for brush border epithelial cells in the developing kidney and in renal regrowth in the adult kidney. 4. In two models of hypertension, the spontaneously hypertensive rat (SHR) and one created surgically by subtotal nephrectomy, renal amylin receptors are activated. In the SHR, activation precedes the rise in blood pressure and suggests that activation of the amylin system may be an important event in the development of hypertension.     

Sodium-dependence of the potency of inhibitors of the neuronal noradrenaline carrier in the rat vas deferens

Vasa deferentia obtained from reserpine-pretreated rats were incubated (monoamine oxidase and catechol-O-methyltransferase inhibited) in media containing various concentrations of 3H-(-)noradrenaline and Na+ and initial rates of the neuronal uptake of 3H-noradrenaline measured both in the absence and presence of uptake inhibitors after 1 min of incubation. When rates of uptake were determined at various 3H-noradrenaline (1.0-12.2 mumol/l) and two fixed Na+ concentrations (25 and 140 mmol/l), the inhibition of uptake produced by (+)amphetamine, (-)metaraminol, desipramine, nomifensine and cocaine was competitive with respect to 3H-noradrenaline at both Na+ concentrations. While the Ki for (+)amphetamine, (-)metaraminol desipramine and nomifensine increased when the Na+ concentration was lowered, that for cocaine decreased. When the Na+ concentration was varied (10-140 mmol/l) and the 3H-noradrenaline concentration held constant (1.2 mumol/l), (+)amphetamine, (-)metaraminol, nomifensine and desipramine acted as mixed-type inhibitors with respect to Na+, and the inhibition of uptake produced by these drugs was the more pronounced, the higher the Na+ concentration. On the other hand, cocaine was competitive with Na+ and the inhibition produced by this drug was the more pronounced, the lower the Na+ concentration. It is concluded that the inhibitors of neuronal uptake tested here act in dependence on the external Na+ concentration. Desipramine and nomifensine resemble alternative amine substrates in being more potent at high than at low Na+ concentrations. On the other hand, cocaine is more potent at low than at high Na+ concentrations.     

mTOR inhibitor everolimus ameliorates progressive tubular dysfunction in chronic renal failure rats

Responsible factors in progressive tubular dysfunction in chronic renal failure have not been fully identified. In the present study, we hypothesized that the mammalian target of rapamycin, mTOR, was a key molecule in the degenerative and progressive tubular damage in chronic renal failure. Everolimus, an mTOR inhibitor, was administered for 14 days in 5/6 nephrectomized (Nx) rats at 2 and 8 weeks after renal ablation. Marked activation of the mTOR pathway was found at glomeruli and proximal tubules in remnant kidneys of Nx rats. The reduced expression levels of the phosphorylated S6 indicated the satisfactory pharmacological effects of treatment with everolimus for 14 days. Everolimus suppressed the accumulation of smooth muscle alpha actin, infiltration of macrophages and expression of kidney injury molecule-1 in the proximal tubules. In addition, everolimus-treatment restored the tubular reabsorption of albumin, and had a restorative effect on the expression levels of membrane transporters in the polarized proximal tubular epithelium, when its administration was started at 8 weeks after Nx. These results indicate that the constitutively activated mTOR pathway in proximal tubules has an important role in the progressive tubular dysfunction, and that mTOR inhibitors have renoprotective effects to improve the proximal tubular functions in end-stage renal disease.     

Effect of azosemide on the in vitro perfused thick ascending limb of Henle's loop from the mouse

Direct effect of azosemide on the mouse thick ascending limb of Henle's loop was examined by the isolated tubules perfusion technique, and the effect was compared with that of furosemide. Azosemide added to the tubular lumen reversibly suppressed the lumen positive potential (PDt) and decreased the net chloride reabsorption. Dose-response studies of these two drugs suggested that azosemide was about 5 times as potent as furosemide. The more potent effect of azosemide compared with furosemide was confirmed by a paired study in the same tubules. At a concentration of 10(-5) mol/l azosemide suppressed PDt by 91.4%, furosemide by 65.6%.     

THE EFFECT OF CAPTOPRIL OR ENALAPRILIC ACID ON THE Na APPETITE OF Na-DEPLETE RATS

1. The converting enzyme inhibitors, captopril (SQ14 225, Squibb) or enalaprilic acid (MK 422, Merck Sharp and Dohme), were used to evaluate the role of angiotensin II in sodium (Na) appetite of Na-deplete rats given a choice of water and 0.5 mol/l NaCl to drink. Also, the effect of these drugs on taste was evaluated in water-deprived Na-replete rats and in Na-deplete rats given a choice of NaCl, sucrose and water. 2. Intraperitoneal (i.p.) injection of furosemide (20 mg/kg) increased urinary Na loss by 1.3-1.4 mmol and subsequent Na intake by 1.5-1.8 mmol. This increase in Na intake was abolished by a high dose of captopril (50 mg/kg, i.p.) or enalaprilic acid (80 mg/kg, i.p.), but was enhanced by a low dose of captopril (50 micrograms/kg, i.p.). 3. There was no clear evidence that either captopril or enalapril affected taste for NaCl, sucrose or water in water-deprived Na-replete rats. However, the high dose of captopril or enalapril decreased the intake of NaCl and sucrose in Na-deplete rats. 4. Thus, while the results are consistent with the possibility that angiotensin II is involved in Na appetite induced by Na depletion and that the difference between the high and low doses of converting enzyme inhibitors on Na appetite may be related to their relative effectiveness in blocking angiotensin I conversion at central sites, the observation that sucrose intake is decreased by high dose converting enzyme inhibitor in Na-deplete rats suggests that other actions of the converting enzyme inhibitors may be involved.     

Stereospecificity of the effects of ozolinone on renal hemodynamics and on segmental tubular sodium reabsorption in conscious rats

The study was performed to elucidate the effects of the two stereoisomers of ozolinone (d,l) on renal hemodynamics and proximal tubular Na reabsorption. Clearance experiments were performed in conscious water-loaded female Wistar rats. The clearances of [3H]inulin, [14C]tetraethylammonium and lithium were used as estimates for glomerular filtration rate, renal plasma flow and delivery of fluid from the proximal tubules, respectively. When the baseline parameters had stabilized, d- or l-ozolinone was injected i.v. in doses of 4, 20 and 100 mg/kg. 1-Ozolinone caused a transient and dose-dependent diuretic-natriuretic response with no evidence of a ceiling. At peak natriuresis, 2.5-5 min after 100 mg/kg of 1-ozolinone, the fractional Na excretion was increased from 0.5 to 25%; this was associated with an increased fractional excretion of lithium from 27 to 60%, and small transient decreases of renal hemodynamic parameters. d-Ozolinone had no significant effects except for a small natriuresis after 100 mg/kg. It is concluded that in water-loaded conscious rats 1-ozolinone is a powerful diuretic which, in contrast to d-ozolinone, increases the delivery of fluid from the proximal tubule as judged from changes in lithium clearance.     

Transport of cimetidine across the basolateral membrane of rabbit kidney proximal tubules: interaction with organic anions.

Since in whole animal studies and in man the renal clearance of cimetidine was prolonged by the coadministration of probenecid, the aim of the present study was to examine the interaction of the organic base cimetidine with the organic anion transport system at the basolateral membrane of isolated non-perfused rabbit proximal tubules. S2 segments of proximal tubules were incubated at 37.5 degrees C with 3H-cimetidine (2 x 10(-7) mol/l) or 3H-PAH (4 x 10(-6) mol/l, as a marker for the organic anion transport system) and 14C-inulin (marker for the extracellular space) for 25 min to achieve a steady state. Afterwards, a nonradiolabelled substance was added to the bath, and the change of the cellularly stored radioactivity was measured at 5-min intervals. Probenecid (5 x 10(-5) mol/l) decreased the cellular amount of 3H-cimetidine to 26% of the control value. At this concentration, furosemide and Na2SO4 had no effect. At a concentration of 10(-3) mol/l, these substances reduced the cellular 3H-cimetidine uptake to 33% (furosemide) and 57% (Na2SO4) of the control value. 10(-4) and 10(-3) mol/l succinate diminished the steady-state uptake of cimetidine to 77% and 53% of the control value, respectively. On the other hand, cimetidine (10(-3) mol/l) decreased the cellular uptake of 3H-PAH to 52% of the control value. N1-methylnicotinamide (5 x 10(-5) mol/l and 10(-3)mol/l) had no effect on the steady-state uptake of 3H-PAH. These results indicate that the organic base cimetidine, besides its high affinity for the cation transporter, also interacts with the organic anion transport system at the basolateral membrane of rabbit proximal tubules.     

A micropuncture study of the effect of isoprenaline on renal tubular fluid and electrolyte transport in the rat

Summary To explain the mechanism of the isoprenaline induced antidiuresis the effect of this substance on fluid and electrolyte reabsorption in various nephron segments of the rat kidney has been studied using micropuncture techniques. Isoprenaline (1.5×10^–9 mol/kg·min i.v.) decreased GFR of superficial nephrons and increased fractional proximal fluid, sodium and potassium reabsorption. However, the fractions of filtered fluid and electrolytes, which had been reabsorbed up to the early distal tubule were unchanged after isoprenaline. This indicates that the increased fractional proximal reabsorption has been completely compensated by decreased reabsorption from Henle's loops. In the distal convoluted tubules an increase in the fractional fluid and sodium reabsorption could be established using a recollection technique. Given directly into the distal tubular lumen isoprenaline stimulated the isotonic fluid reabsorption, which was measured by the Gertz split oil droplet method.From these results it is concluded that the decrease in superficial GFR and the increased distal tubular fluid and sodium reabsorption lead to the decrease in urine volume and in urinary electrolyte excretion after isoprenaline.with the technical assistance of U. Greven and S. HildebrandSupported by Deutsche ForschungsgemeinschaftPortions of this work were presented at the Meeting of the Deutsche Pharmakologische Gesellschaft in Graz, 1974 (Greven, 1974b)