Effect of amphotericin B on renal tubular acidification in the rat

Arterial Blood Gases and Acid-Base Regulation

Disorders of acid-base status are common in the critically ill and prompt recognition is central to clinical decision making. The bicarbonate/carbon dioxide buffer system plays a pivotal role in maintaining acid-base homeostasis, and measurements of pH, PCO2, and HCO3 - are routinely used in the estimation of metabolic and respiratory disturbance severity. Hypoventilation and hyperventilation cause primary respiratory acidosis and primary respiratory alkalosis, respectively. Metabolic acidosis and metabolic alkalosis have numerous origins, that include alterations in acid or base intake, body fluid losses, abnormalities of intermediary metabolism, and renal, hepatic, and gastrointestinal dysfunction. The concept of the anion gap is used to categorize metabolic acidoses, and urine chloride excretion helps define metabolic alkaloses. Both the lungs and kidneys employ compensatory mechanisms to minimize changes in pH caused by various physiologic and disease disturbances. Treatment of acid-base disorders should focus primarily on correcting the underlying cause and the hemodynamic and electrolyte derangements that ensue. Specific therapies under certain conditions include renal replacement therapy, mechanical ventilation, respiratory stimulants or depressants, and inhibition of specific enzymes in intermediary metabolism disorders.

The pathophysiology of distal renal tubular acidosis

The kidneys have a central role in the control of acid-base homeostasis owing to bicarbonate reabsorption and production of ammonia and ammonium in the proximal tubule and active acid secretion along the collecting duct. Impaired acid excretion by the collecting duct system causes distal renal tubular acidosis (dRTA), which is characterized by the failure to acidify urine below pH 5.5. This defect originates from reduced function of acid-secretory type A intercalated cells. Inherited forms of dRTA are caused by variants in SLC4A1, ATP6V1B1, ATP6V0A4, FOXI1, WDR72 and probably in other genes that are yet to be discovered. Inheritance of dRTA follows autosomal-dominant and -recessive patterns. Acquired forms of dRTA are caused by various types of autoimmune diseases or adverse effects of some drugs. Incomplete dRTA is frequently found in patients with and without kidney stone disease. These patients fail to appropriately acidify their urine when challenged, suggesting that incomplete dRTA may represent an intermediate state in the spectrum of the ability to excrete acids. Unrecognized or insufficiently treated dRTA can cause rickets and failure to thrive in children, osteomalacia in adults, nephrolithiasis and nephrocalcinosis. Electrolyte disorders are also often present and poorly controlled dRTA can increase the risk of developing chronic kidney disease.

Effects of cardiotonic steroids on isolated perfused kidney and NHE3 activity in renal proximal tubules

Cardiotonic steroids (CS) are known as modulators of sodium and water homeostasis. These compounds contribute to the excretion of sodium under overload conditions due to its natriuretic property related to the inhibition of the renal Na⁺/K⁺-ATPase (NKA) pump α1 isoform. NHE3, the main route for Na⁺ reabsorption in the proximal tubule, depends on the Na⁺ gradient generated by the NKA pump. In the present study we aimed to investigate the effects of marinobufagin (MBG) and telocinobufagin (TBG) on the renal function of isolated perfused rat kidney and on the inhibition of NKA activity. Furthermore, we investigated the mechanisms for the cardiotonic steroid-mediated natriuretic effect, by evaluating and comparing the effects of bufalin (BUF), ouabain (OUA), MBG and TBG on NHE3 activity in the renal proximal tubule in vivo. TBG significantly increased GFR, UF, natriuresis and kaliuresis in isolated perfused rat kidney, and inhibits the activity of NKA at a much higher rate than MBG. By stationary microperfusion technique, the perfusion with BUF, OUA, TBG or MBG promoted an inhibitory effect on NHE3 activity, whereas BUF was the most effective agent, and demonstrated a dose-dependent response, with maximal inhibition at 50nM. Furthermore, our data showed the role of NKA-Src kinase pathway in the inhibition of NHE3 by CS. Finally, a downstream step, MEK1/2-ERK1/2 was also investigated, and, similar to Src inhibition, the MEK1/2 inhibitor (U0126) suppressed the BUF effect. Our findings indicate the involvement of NKA-SRc-Kinase-Ras-Raf-ERK1/2 pathway in the downregulation of NHE3 by cardiotonic steroids in the renal proximal tubule, promoting a reduction of proximal sodium reabsorption and natriuresis.

Metabolomics as a tool to evaluate the toxicity of formulations containing amphotericin B, an antileishmanial drug

Amphotericin B (AmB) is a drug of choice against life-threatening systemic fungal infections and an alternative therapy for the treatment of all forms of leishmaniasis. It is known that AmB and its conventional formulation cause renal damage; however, the lipid formulations can reduce these effects. The aim of the present study was to identify metabolic changes in mice treated with two different AmB formulations, a nanoemulsion (NE) (lipid system carrier) loaded with AmB and the conventional formulation (C-AmB). For this purpose, metabolic fingerprinting represents a valuable strategy to monitor, in a non-targeted manner, the changes that are at the base of the toxicity mechanism of AmB. Plasma samples of BALB-c mice were collected after treatment with 3 alternate doses of AmB at 1 mg kg-1 administered intravenously and analysed with CE, LC and GC coupled to MS. Blood urea nitrogen (BUN) and plasma creatinine levels were also analysed. Kidney tissue specimens were collected and evaluated. It was not observed that there were any alterations in BUN and creatinine levels as well as in histopathological analysis. Approximately 30 metabolites were identified as potentially related to early C-AmB-induced nephrotoxicity. Disturbances in the arachidonic acid, glycerophospholipid, acylcarnitine and polyunsaturated fatty acid (PUFA) pathways were observed in C-AmB-treated mice. In the AmB-loaded NE group, it was observed that there were fewer metabolic changes, including changes in the plasma levels of cortisol and pyranose. The candidate biomarkers revealed in this study could be useful in the detection of the onset and severity of kidney injury induced by AmB formulations.

Proximal tubule NHE3 activity is inhibited by beta-arrestin-biased angiotensin II type 1 receptor signaling

Physiological concentrations of angiotensin II (ANG II) upregulate the activity of Na+/H+ exchanger isoform 3 (NHE3) in the renal proximal tubule through activation of the ANG II type I (AT1) receptor/G protein-coupled signaling. This effect is key for maintenance of extracellular fluid volume homeostasis and blood pressure. Recent findings have shown that selective activation of the beta-arrestin-biased AT1 receptor signaling pathway induces diuresis and natriuresis independent of G protein-mediated signaling. This study tested the hypothesis that activation of this AT1 receptor/beta-arrestin signaling inhibits NHE3 activity in proximal tubule. To this end, we determined the effects of the compound TRV120023, which binds to the AT1R, blocks G-protein coupling, and stimulates beta-arrestin signaling on NHE3 function in vivo and in vitro. NHE3 activity was measured in both native proximal tubules, by stationary microperfusion, and in opossum proximal tubule (OKP) cells, by Na+-dependent intracellular pH recovery. We found that 10−7 M TRV120023 remarkably inhibited proximal tubule NHE3 activity both in vivo and in vitro. Additionally, stimulation of NHE3 by ANG II was completely suppressed by TRV120023 both in vivo as well as in vitro. Inhibition of NHE3 activity by TRV120023 was associated with a decrease in NHE3 surface expression in OKP cells and with a redistribution from the body to the base of the microvilli in the rat proximal tubule. These findings indicate that biased signaling of the beta-arrestin pathway through the AT1 receptor inhibits NHE3 activity in the proximal tubule at least in part due to changes in NHE3 subcellular localization.

Uroguanylin inhibits H-ATPase activity and surface expression in renal distal tubules by a PKG-dependent pathway

Cumulative evidence suggests that guanylin peptides play an important role on electrolyte homeostasis. We have previously reported that uroguanylin (UGN) inhibits bicarbonate reabsorption in a renal distal tubule. In the present study, we tested the hypothesis that the bicarbonaturic effect of UGN is at least in part attributable to inhibition of H(+)-ATPase-mediated hydrogen secretion in the distal nephron. By in vivo stationary microperfusion experiments, we were able to show that UGN inhibits H(+)-ATPase activity by a PKG-dependent pathway because KT5823 (PKG inhibitor) abolished the UGN effect on distal bicarbonate reabsorption and H89 (PKA inhibitor) was unable to prevent it. The in vivo results were confirmed by the in vitro experiments, where we used fluorescence microscopy to measure intracellular pH (pHi) recovery after an acid pulse with NH4Cl. By this technique, we observed that UGN and 8 bromoguanosine-cGMP (8Br-cGMP) inhibited H(+)-ATPase-dependent pHi recovery and that the UGN inhibitory effect was abolished in the presence of the PKG inhibitor. In addition, by using RT-PCR technique, we verified that Madin-Darby canine kidney (MDCK)-C11 cells express guanylate cyclase-C. Besides, UGN stimulated an increase of both cGMP content and PKG activity but was unable to increase the production of cellular cAMP content and PKA activity. Furthermore, we found that UGN reduced cell surface abundance of H+-ATPase B1 subunit in MDCK-C11 and that this effect was abolished by the PKG inhibitor. Taken together, our data suggest that UGN inhibits H(+)-ATPase activity and surface expression in renal distal cells by a cGMP/PKG-dependent pathway.

Changes in the activity and expression of protein phosphatase-1 accompany the differential regulation of NHE3 before and after the onset of hypertension in spontaneously hypertensive rats

AIM

The Na(+) /H(+) exchanger NHE3 activity decreases in the proximal tubule of spontaneously hypertensive rats (SHRs) as blood pressure increases, and this reduction is correlated with higher NHE3 phosphorylation levels at the PKA consensus site serine 552. This study tested the hypothesis that this lowered NHE3 activity is associated with an increase in PKA activity and expression, and/or a decrease in protein phosphatase-1 (PP1) activity and expression.

METHODS

Proximal tubule NHE3 activity was measured as the rate of bicarbonate reabsorption by stationary microperfusion. NHE3 phosphorylation and protein expression were determined by immunoblotting. PKA and PP1 activities were determined using specific substrates under optimal enzymatic conditions.

RESULTS

The PKA activator, 6-MB-cAMP, increased the phosphorylation levels of NHE3 at serine 552 in the renal cortex; this increase happens to a much greater extent in young pre-hypertensive SHRs (Y-SHRs) compared to adult SHRs with established hypertension (A-SHRs). Likewise, the inhibitory effect of 6-MB-cAMP on NHE3 transport activity was much more pronounced in the proximal tubules of Y-SHRs than in those of A-SHRs. Renal cortical activity of PKA was not significantly different between Y-SHRs and A-SHRs. On the other hand, Y-SHRs exhibited higher protein phosphatase 1 (PP1) activity, and their expression of the PP1 catalytic subunit PP1α in the renal cortex was also higher than in A-SHRs.

CONCLUSION

Collectively, these results support the idea that the lower NHE3 transport activity and higher phosphorylation occurring after the development of hypertension in SHRs are due, at least in part, to reduced PP1-mediated dephosphorylation of NHE3 at serine 552.

Dose-dependent effects of angiotensin-(1-7) on the NHE3 exchanger and [Ca(2+)](i) in in vivo proximal tubules

The acute direct action of angiotensin-(1-7) [ANG-(1-7)] on bicarbonate reabsorption (JHCO(3)(-)) was evaluated by stationary microperfusions on in vivo middle proximal tubules in rats using H ion-sensitive microelectrodes. The control JHCO(3)(-) is 2.82 ± 0.078 nmol·cm(-2)·s(-1) (50). ANG-(1-7) (10(-12) or 10(-9) M) in luminally perfused tubules decreases JHCO(3)(-) (36 or 60%, respectively), but ANG-(1-7) (10(-6) M) increases it (80%). A779 increases JHCO(3)(-) (30%) and prevents both the inhibitory and the stimulatory effects of ANG-(1-7) on it. S3226 decreases JHCO(3)(-) (45%) and changes the stimulatory effect of ANG-(1-7) to an inhibitory effect (30%) but does not affect the inhibitory effect of ANG-(1-7). Our results indicate that in the basal condition endogenous ANG-(1-7) inhibits JHCO(3)(-) and that the biphasic dose-dependent effect of ANG-(1-7) on JHCO(3)(-) is mediated by the Mas receptors via the Na(+)/H(+) exchanger 3 (NHE3). The control value of intracellular Ca(2+) concentration ([Ca(2+)](i)), as monitored using fura-2 AM, is 101 ± 2 nM (6), and ANG-(1-7) (10(-12), 10(-9), or 10(-6)M) transiently (3 min) increases it (by 151, 102, or 52%, respectively). A779 increases the [Ca(2+)](i) (25%) but impairs the stimulatory effect of all doses of ANG-(1-7) on it. The use of BAPTA or thapsigargin suggests a correlation between the ANG-(1-7) dose-dependent effects on [Ca(2+)](i) and JHCO(3)(-). Therefore, the interaction of the opposing dose-dependent effects of ANG II and ANG-(1-7) on [Ca(2+)](i) and JHCO(3)(-) may represent an physiological regulatory mechanism of extracellular volume and/or pH changes. However, whether [Ca(2+)](i) modification is an important direct mechanism for NHE3 activation by these peptides or is a side effect of other signaling pathways will require additional studies.

Mechanisms underlying the inhibitory effects of uroguanylin on NHE3 transport activity in renal proximal tubule

We previously demonstrated that uroguanylin (UGN) significantly inhibits Na(+)/H(+) exchanger (NHE)3-mediated bicarbonate reabsorption. In the present study, we aimed to elucidate the molecular mechanisms underlying the action of UGN on NHE3 in rat renal proximal tubules and in a proximal tubule cell line (LLC-PK(1)). The in vivo studies were performed by the stationary microperfusion technique, in which we measured H(+) secretion in rat renal proximal segments, through a H(+)-sensitive microelectrode. UGN (1 μM) significantly inhibited the net of proximal bicarbonate reabsorption. The inhibitory effect of UGN was completely abolished by either the protein kinase G (PKG) inhibitor KT5823 or by the protein kinase A (PKA) inhibitor H-89. The effects of UGN in vitro were found to be similar to those obtained by microperfusion. Indeed, we observed that incubation of LLC-PK(1) cells with UGN induced an increase in the intracellular levels of cAMP and cGMP, as well as activation of both PKA and PKG. Furthermore, we found that UGN can increase the levels of NHE3 phosphorylation at the PKA consensus sites 552 and 605 in LLC-PK(1) cells. Finally, treatment of LLC-PK(1) cells with UGN reduced the amount of NHE3 at the cell surface. Overall, our data suggest that the inhibitory effect of UGN on NHE3 transport activity in proximal tubule is mediated by activation of both cGMP/PKG and cAMP/PKA signaling pathways which in turn leads to NHE3 phosphorylation and reduced NHE3 surface expression. Moreover, this study sheds light on mechanisms by which guanylin peptides are intricately involved in the maintenance of salt and water homeostasis.

Pharmacologically-induced metabolic acidosis: a review

Metabolic acidosis may occasionally develop in the course of treatment with drugs used in everyday clinical practice, as well as with the exposure to certain chemicals. Drug-induced metabolic acidosis, although usually mild, may well be life-threatening, as in cases of lactic acidosis complicating antiretroviral therapy or treatment with biguanides. Therefore, a detailed medical history, with special attention to the recent use of culprit medications, is essential in patients with acid-base derangements. Effective clinical management can be handled through awareness of the adverse effect of certain pharmaceutical compounds on the acid-base status. In this review, we evaluate relevant literature with regard to metabolic acidosis associated with specific drug treatment, and discuss the clinical setting and underlying pathophysiological mechanisms. These mechanisms involve renal inability to excrete the dietary H+ load (including types I and IV renal tubular acidoses), metabolic acidosis owing to increased H+ load (including lactic acidosis, ketoacidosis, ingestion of various substances, administration of hyperalimentation solutions and massive rhabdomyolysis) and metabolic acidosis due to HCO3- loss (including gastrointestinal loss and type II renal tubular acidosis). Determinations of arterial blood gases, the serum anion gap and, in some circumstances, the serum osmolar gap are helpful in delineating the pathogenesis of the acid-base disorder. In all cases of drug-related metabolic acidosis, discontinuation of the culprit medications and avoidance of readministration is advised.

Amphotericin B-induced nephrotoxicity: characterization of blood and urinary biochemistry and renal morphology in mice

This study was conducted to characterize blood and urinary biochemistry, and renal morphology, after single or 1-week repeated dosing of mice with the polyene macrolide antibiotic, amphotericin B (AMB). AMB was intravenously administered to mice at 2 or 4 mg/kg for the single-dose experiment or once daily at 1 or 2 mg/kg for 1 week for the repeated-dose experiment. The most prominent histopathological findings included necrosis of the tubular epithelial cells in the thick ascending limb of Henle's loop in the renal outer medulla at a single dose of 2 or 4 mg/kg, and the severity of the lesion was dose-dependent. Blood chemistry and urinalysis revealed several changes suggestive of renal dysfunction such as reduction of plasma filtration ability (increases in plasma creatinine and blood urea nitrogen, a decrease in creatinine clearance) and polyuria accompanied with dehydration (decrease in renal water reabsorption, increases in plasma total protein and albumin) at a dose of 4 mg/kg in the single-dose experiment. Among the parameters analyzed, urinary lactate dehydrogenase was the most sensitive and reliable parameter for the prediction of AMB-induced nephrotoxicity in mice. These data provided comprehensive information on the nephrotoxicity of AMB and indicate useful markers for the sensitive detection of AMB-induced renal injury in mice.

Effect of renoguanylin on hydrogen/bicarbonate ion transport in rat renal tubules

Renoguanylin (REN) is a recently described member of the guanylin family, which was first isolated from eels and is expressed in intestinal and specially kidney tissues. In the present work we evaluate the effects of REN on the mechanisms of hydrogen transport in rat renal tubules by the stationary microperfusion method. We evaluated the effect of 1 muM and 10 muM of renoguanylin (REN) on the reabsorption of bicarbonate in proximal and distal segments and found that there was a significant reduction in bicarbonate reabsorption. In proximal segments, REN promoted a significant effect at both 1 and 10 muM concentrations. Comparing control and REN concentration of 1 muM, JHCO(3)(-), nmol cm(-2) s(-1)-1,76+/-0,11(control)x1,29+/-0,08(REN 10 muM); P<0.05, was obtained. In distal segments the effect of both concentrations of REN was also effective, being significant e.g. at a concentration of 1 muM (JHCO(3)(-), nmol cm(-2) s(-)1-0.80+/-0.07(control)x0.60+/-0.06(REN 1 muM); P<0.05), although at a lower level than in the proximal tubule. Our results suggest that the action of REN on hydrogen transport involves the inhibition of Na(+)/H(+)exchanger and H(+)-ATPase in the luminal membrane of the perfused tubules by a PKG dependent pathway.

Direct action of aldosterone on bicarbonate reabsorption in in vivo cortical proximal tubule

The direct action of aldosterone (10−12 M) on net bicarbonate reabsorption ( JHCO3−) was evaluated by stationary microperfusion of an in vivo middle proximal tubule (S2) of rat kidney, using H ion-sensitive microelectrodes. Aldosterone in luminally perfused tubules caused a significant increase in JHCO3− from a mean control value of 2.84 ± 0.08 [49/19 ( n° of measurements/ n° of tubules)] to 4.20 ± 0.15 nmol·cm−2·s−1 (58/10). Aldosterone perfused into peritubular capillaries also increased JHCO3−, compared with basal levels during intact capillary perfusion with blood. In addition, in isolated perfused tubules aldosterone causes a transient increase of cytosolic free calcium ([Ca2+]i), monitored fluorometrically. In the presence of ethanol (in similar concentration used to prepare the hormonal solution), spironolactone (10−6 M, a mineralocorticoid receptor antagonist), actinomycin D (10−6 M, an inhibitor of gene transcription), or cycloheximide (40 mM, an inhibitor of protein synthesis), the JHCO3− and the [Ca2+]i were not different from the control value; these drugs also did not prevent the stimulatory effect of aldosterone on JHCO3− and on [Ca2+]i. However, in the presence of RU 486 alone [10−6 M, a classic glucocorticoid receptor (GR) antagonist], a significant decrease on JHCO3− and on [Ca2+]i was observed; this antagonist also inhibited the stimulatory effect of aldosterone on JHCO3− and on [Ca2+]i. These studies indicate that luminal or peritubular aldosterone (10−12 M) has a direct nongenomic stimulatory effect on JHCO3− and on [Ca2+]i in proximal tubule and that probably GR participates in this process. The data also indicate that endogenous aldosterone stimulates JHCO3− in middle proximal tubule.

Effect of uroguanylin on potassium and bicarbonate transport in rat renal tubules

The effect of uroguanylin (UGN) on K+and H+secretion in the renal tubules of the rat kidney was studied using in vivo stationary microperfusion. For the study of K+secretion, a tubule was punctured to inject a column of FDC-green-colored Ringer's solution with 0.5 mmol KCl/L ± 10−6mol UGN/L, and oil was used to block fluid flow. K+activity and transepithelial potential differences (PD) were measured with double microelectrodes (K+ion-selective resin vs. reference) in the distal tubules of the same nephron. During perfusion, K+activity rose exponentially, from 0.5 mmol/L to stationary concentration, allowing for the calculation of K+secretion (JK). JKincreased from 0.63 ± 0.06 nmol·cm–2·s–1in the control group to 0.85 ± 0.06 in the UGN group (p < 0.01). PD was –51.0 ± 5.3 mV in the control group and –50.3 ± 4.98 mV in the UGN group. In the presence of 10−7mol iberiotoxin/L, the UGN effect was abolished: JKwas 0.37 ± 0.038 nmol·cm–2·s–1in the absence of, and 0.38 ± 0.025 in the presence of, UGN, indicating its action on maxi-K channels. In another series of experiments, renal tubule acidification was studied, using a similar method: proximal and distal tubules were perfused with solutions containing 25 mmol NaHCO3/L. Acidification half-time was increased both in proximal and distal segments and, as a consequence, bicarbonate reabsorption decreased in the presence of UGN (in proximal tubules, from 2.40 ± 0.26 to 1.56 ± 0.21 nmol·cm–2·s–1). When the Na+/H+exchanger was inhibited by 10−4mol hexamethylene amiloride (HMA)/L, the control and UGN groups were not significantly different. In the late distal tubule, after HMA, UGN significantly reduced JHCO3–, indicating an effect of UGN on H+-ATPase. These data show that UGN stimulated JK+by acting on maxi-K channels, and decreased JHCO3–by acting on NHE3 in proximal and H+-ATPase in distal tubules.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Peritubular AVP regulates bicarbonate reabsorption in cortical distal tubule via V(1) and V(2) receptors

Peritubular arginine vasopressin (AVP) regulates bicarbonate reabsorption in the cortical distal tubule via V(1) and V(2) receptors. The dose-dependent effects of peritubular AVP on net bicarbonate reabsorption (J(HCO)) were evaluated by stationary microperfusion of in vivo early (ED; distal convoluted tubule) and late distal (LD; connecting tubule and initial collecting duct) segments of rat kidney, using double-barreled H(+)-sensitive, ion-exchange resin/reference (1 M KCl) microelectrodes. AVP (10(-11) M) perfused into peritubular capillaries increased J(HCO), compared with basal levels during intact capillary perfusion with blood, in ED and LD segments. AVP (10(-9) M) also increased J(HCO) in both segments, but the effect of AVP (10(-11) M) was significantly higher. A specificV(1)-receptor antagonist alone or with AVP (10(-11) or 10(-9) M) reduced J(HCO) below basal levels. A specific V(2)-receptor antagonist alone or plus AVP (10(-11) M) did not affect J(HCO) but increased AVP (10(-9) M)-mediated stimulation. 8-Bromoadenosine 3',5'-cyclic monophosphate alone reduced J(HCO) below basal levels and also reduced AVP (10(-11) M)-mediated stimulation. (Deamino-Cys(1), D-Arg(8)) vasopressin (a V(2)-selective agonist) also reduced J(HCO) below basal levels. These results show that peritubular AVP stimulates J(HCO) in ED and LD segments via basolateral V(1) receptors and that basolateral V(2) receptors have a dose-dependent inhibitory effect mediated by cAMP. The data also indicate that endogenous AVP stimulates distal J(HCO) via basolateral V(1) receptors.

V(1) receptors in luminal action of vasopressin on distal K(+) secretion

Luminal perfusion with collected proximal fluid increases distal K(+) secretion compared with artificial solutions. Arginine vasopressin (AVP), present in luminal fluid, might be responsible for this observation. K(+) secretion rate (J(K)) was measured by K(+)-sensitive microelectrodes during paired luminal stationary microperfusion with control and AVP-containing 0.5 mM K(+) solutions. J(K) was 1.34 +/- 0.35 (n = 24 tubules) nmol x cm(-2) x s(-1) during perfusion with 10(-9) M AVP, against 0.90+/-0.12 nmol x cm(-2) x s(-1) (n = 21) in control (P<0.02). With 10(-9) M AVP+10(-6) M beta-mercapto-beta-beta-cyclopenta-methylenepropionyl(1), O-Me-Tyr(2)-Arg(8) vasopressin (MCMV), a specific peptide V(1)-receptor antagonist, J(K) was 0.36+/-0.067 against 0.77+/-0.10 (control; n = 9) nmol x cm(-2) x s(-1) (P<0.01). With 10(-6) M MCMV alone, J(K) was 0.37+/-0.04 against a control of 0.62+/-0.06 (n = 19) nmol. cm(-2). s(-1) (P<0.01). A peptide V(2) antagonist had no such effect. In Brattleboro rats, which do not produce endogenous AVP, MCMV had no effect when given alone, although AVP still stimulated J(K). In conclusion, luminal AVP stimulates distal J(K) significantly. The V(1) antagonist MCMV inhibits the effect of AVP but also reduces J(K) when given alone. This suggests that AVP acts luminally via V(1) receptors but also that there appears to be a background effect of endogenous AVP blocked by the antagonist.

Luminal arginine vasopressin stimulates Na(+)-H+ exchange and H(+)-ATPase in cortical distal tubule via V1 receptor

Bicarbonate reabsorption was evaluated by stationary microperfusion of in vivo early distal (ED) and late distal (LD) segments of rat kidney. Intratubular pH was recorded by double-barreled H ion-exchange resin/reference (1 M KCl) microelectrodes for the determination of HCO3- reabsorption. In the presence of luminal arginine vasopressin (AVP, 10(-9) M), a significant increase in HCO3- reabsorption was observed both in ED (from 0.931 +/- 0.061 to 2.12 +/- 0.171 nmol.cm-2.s-1] and LD segments [from 0.542 +/- 0.086 to 1.67 +/- 0.111 nmol.cm-2.s-1]. The addition of the V1-receptor antagonist [(d (CH2)5, Tyr (Et)2) arginine vasopressin] (10(-5) M) to luminal perfusion blocked luminal AVP mediated stimulation in ED and LD segments. 5-(N, N-hexamethylene) amiloride (10(-4) M) added to luminal perfusion inhibited luminal AVP-mediated stimulation in ED (by 63.7%) and LD (by 34.1%) segments. The addition of Bafilomycin A1 (2 x 10(-7) M) to the luminal perfusion did not affect luminal AVP-mediated stimulation in ED segments, but reduced it (by 31.7%) in LD segments. Our results indicate that luminal AVP acts to stimulate the Na(+)-H+ exchange in ED and LD segments via activation of V1 receptors, as well as the vacuolar H(+)-ATPase in LD segments.

Regulation of nephron acidification by corticosteroids

The present paper reviews work from our laboratories evaluating the importance of adrenal cortical hormones in acidification by proximal and cortical distal tubules. Proximal acidification was determined by stationary microperfusion, and measurement of bicarbonate reabsorption using luminal pH determination was performed with H(+)-ionsensitive microelectrodes. Rats were adrenalectomized (ADX) 48 h before the experiments, and corticosteroids (aldosterone (A), corticosterone (B), and 18-OH corticosterone (18-OH-B)) were injected intramuscularly 100 and 40 min before the experiments. In ADX rats stationary pH increased significantly to 7.03 as compared to sham-operated rats (6.78). Bicarbonate reabsorption decreased from 2.65 +/- 0.18 in sham-operated rats to 0.50 +/- 0.07 nmol cm-2 s-1 after ADX. The administration of the three hormones stimulated proximal tubule acidification, reaching, however, only 47.2% of the sham values in aldosterone-treated rats. Distal nephron acidification was studied by measuring urine minus blood pCO2 differences (U-B pCO2) in bicarbonate-loaded rats treated as above. This pCO2 difference is used as a measure of the distal nephron ability to secrete H+ ions into an alkaline urine. U-B pCO2 decreased significantly from 39.9 +/- 1.26 to 11.9 +/- 1.99 mmHg in ADX rats. When corticosteroids were given to ADX rats before the experiment, U-B pCO2 increased significantly, but reached control levels only when aldosterone (two 3-microgram doses per rat) plus corticosterone (220 micrograms) were given together. In order to control for the effect of aldosterone on distal transepithelial potential difference one group of rats was treated with amiloride, which blocks distal sodium channels. Amiloride-treated rats still showed a significant reduction in U-B pCO2 after ADX. Only corticosterone and 18-OH-B but not aldosterone increased U-B pCO2 back to the levels of sham-operated rats. These results show that corticosteroids stimulate renal tubule acidification both in proximal and distal nephrons and provide some clues about the mechanism of action of these steroids.

Thiazide-sensitive Na-Cl cotransport mediates NaCl absorption in amphibian distal tubule

To find out the mechanism(s) underlying NaCl absorption in the distal tubule of Necturus, we devised a variant of the split-drop technique. Following injection an oil column, subsequently split by a NaCl solution isotonic to plasma, a double-barrelled microelectrode (conventional/selective to Na+ or to Cl- ions) recorded Na+ (alpha Na) or Cl- (alpha Cl) activity and transepithelial potential (Vte). Paired control/low-Na+ solutions yielded reabsorptive half-times (t1/2) of 0.68 +/- 0.11 min and 7.6 +/- 1.8 min respectively; corresponding Vte values were -22.2 +/- 4.0 mV and -7.6 +/- 1.9 mV. t1/2 values of control versus low-Cl- solutions were 0.77 +/- 0.32 min and 6.5 +/- 1.7 min respectively, whereas respective Vte values were not different from one another: -23.8 +/- 4.3 mV versus -18.8 +/- 5.5 mV. Nominally K(+)-free solutions or bumetanide, 10 mumol/l, did not alter t1/2 or Vte, with regard to the paired control. Amiloride, 5 mumol/l or 2 mmol/l, failed to decrease t1/2 or to lower Vte; apparently, the role of a Na+/H+ antiport does not contribute significantly to NaCl absorption. Furosemide, 0.1 mmol/l, reduced t1/2 by 54% with regard to the control state. Determination of t1/2 as a function of increasing hydrochlorothiazide concentrations revealed apical high- and low-affinity sites, estimated at 0.56 mumol/l and 0.115 mmol/l respectively. Taken together these observations indicate that NaCl absorption is predominantly carried out by an electroneutral Na(+)-Cl- cotransport.

Comparison of nephrotoxicities of different polyoxyethyleneglycol formulations of amphotericin B in rats

The aim of the present study was to assess whether amphotericin B (AmB)-Myrj 59, AmB-polyoxyethyleneglycol 24 cholesterol (PC), and AmB-Synperonic A50 (SA50) were less nephrotoxic than AmB-deoxycholate (DC). Rats were treated with the different AmB formulations (10 mg/kg of body weight) intraperitoneally or with the surfactants alone. A group of control rats receiving the vehicle was also examined. After 6 days of daily intraperitoneal injections of AmB-DC, decreased body weight and glomerular filtration rate as well as increased degree of diuresis, uremia, microalbuminuria, and N-acetyl-beta-D-glucosaminidase excretion in urine were noted. Urinary excretion of potassium and sodium was also decreased in AmB-DC-treated rats. Most of these effects were more pronounced with AmB-PC and AmB-SA50. In contrast, AmB-Myrj 59 was less nephrotoxic than AmB-DC. Indeed, after 6 days of treatment with AmB-Myrj 59, the natriuria, kaliuria, albuminuria, and glomerular filtration rates were unchanged compared with those of controls. Moreover, the body weight loss and uremia increase of the rats treated by AmB-Myrj 59 were less than those of the rats treated with the commercial preparation. Among the surfactants, only PC was toxic for the rats. The intrinsic toxicity of PC and the higher systemic exposure to AmB could contribute to increased toxicities of AmB-PC and AmB-SA50, respectively. AmB-Myrj 59 was less nephrotoxic than AmB-DC at equivalent areas under the plasma concentration-time curves. These preliminary results suggest that this formulation could be a good alternative to the commercial product.