Recent results on the active site of amine oxidases

A preparation of bovine plasma amine oxidase, containing only one copper ion per molecule, was obtained by treatment with diethyldithiocarbamate. The enzyme preparation still retained full activity. Thus, only one of the two copper ions contained in the enzyme molecule appears to be involved in the catalytic process.

Reaction of beef plasma and lentil seedlings Cu-amine oxidases with phenylhydrazine

The reaction of copper amine oxidases from beef plasma and lentil seedlings with hydrazine derivatives has been studied. A 1:1 stoichiometry was always found for the irreversible binding to the dimeric proteins. The formation of the adduct does not require the presence of oxygen or copper. Substrates compete with hydrazine derivatives for the binding to the enzymes. The binding of hydrazines and of substrate has different effects on the EPR spectra of enzymic copper.

The acidosis of chronic renal failure

The acidosis of chronic renal failure is not due to bicarbonate wastage per se; rather, bicarbonate reabsorption per nephron is markedly enhanced. The ability to lower the urine pH is preserved. While overall ammonium production may be decreased in chronic renal failure, both ammonium production and excretion are markedly increased when expressed per remaining nephron. Titratable acid excretion in chronic renal failure is essentially maximal, owing to the effect of parathyroid hormone on phosphate excretion by the kidney. Thus, it appears that the acidosis of chronic renal failure is solely the consequence of the reduction in functional renal mass. Extrarenal buffering may contribute substantially to the maintenance of a near normal acid-base status in patients with marked reduction in glomerular filtration rate. That homeostasis is so well preserved until glomerular filtration rate falls to approximately 10 per cent of normal is remarkable; the price, however, may be considerable. Prolonged acidosis may magnify the tendency of renal failure to cause osteodystrophy. An obvious treatment for the acidosis of renal failure is exogenous alkali therapy. Most clinicians withhold alkali therapy until the bicarbonate concentration falls below 20 mEq per L. If the acidosis cannot be safely corrected with exogenous therapy, dialysis should be initiated.

[Substances inhibiting monoamine oxidase. II. Inhibition of bovine plasma amine oxidase by N-cyclopropyltryptamines]

In this paper, we report the inhibition constants obtained with N-cyclopropyl-5,6-dimethoxytryptamine and with N-cyclopropyl-6,7-dimethoxytryptamine on the activity of beef plasma amineoxidase. The inhibition constants are respectively: 0.3 x 10(-3) M and 0.65 x 10(-3) M. A dixon graph of the enzymic oxidation of benzylamine indicates a non-competitive inhibition of the enzyme by these dimethoxytryptamines.

Role of urinary concentrating ability in the generation of toxic papillary necrosis

We studied the pathogenesis of chemically induced papillary necrosis in six groups of rats. Papillary necrosis was produced by a single injection of 2-bromoethylamine hydrobromide (BEA), 50 mg, i.v.; the animals were followed for 7 to 10 days after the administration of the compound. Following BEA, heterozygous Brattleboro rats developed all the functional and morphologic lesions of papillary necrosis that we previously described in Sprague-Dawley rats. They were unable to maintain sodium balance when dietary sodium was withdrawn. Homozygous Brattleboro rats, on the other hand, developed none of the manifestations of papillary necrosis (that is, animals with central diabetes insipidus were protected completely from the nephrotoxic effects of BEA). They adapted normally to a zero sodium diet. Chronic administration of vasopressin to homozygous Brattleboro rats fully restored the toxic effects of BEA. Lowering urinary concentrating ability by inducing a water diuresis in Sprague-Dawley rats completely protected against BEA-induced papillary necrosis. Decreasing papillary solute concentration by furosemide or increasing urine flow after abrupt withdrawal of vasopressin to homozygous Brattleboro rats did not protect against BEA-induced papillary necrosis. We conclude that the combination, but not either alone, of increased urine flow and decreased papillary solute concentration protects against the development of BEA-induced papillary necrosis.

Distal acidification in the rabbit: role of diet and blood pH

A distal acidification defect is said to exist in rabbits because this species does not achieve a normal urine minus blood (U-B) PCO2 gradient in response to sodium bicarbonate infusion. This observation contrasts with data derived from studies in isolated rabbit cortical collecting tubules that have shown an acidifying capacity when the tubules were obtained from acidotic animals. The present study was designed to examine the role of diet and blood pH on distal acidification in the rabbit. Maximal alkalinization of the urine by acute sodium bicarbonate infusion was associated with a low U-B PCO2 gradient (0.7 +/- 2.1 mmHg). Rabbits made acidotic by ammonium chloride administration for 1 wk achieved a substantial U-B PCO2 gradient (29 +/- 5 mmHg) in response to neutral sodium phosphate infusion. To further evaluate the role of blood pH on the ability to raise U-B PCO2 gradient, rabbits and rats made acidotic by chronic ammonium chloride administration were studied. Neutral sodium phosphate was then infused to stimulate distal acidification. At comparable levels of urinary phosphate concentration and blood pH, acidotic rabbits were able to achieve a U-B PCO2 (50 +/- 7 mmHg) comparable with that of acidotic rats (48 +/- 8.3 mmHg). These data show that the failure of rabbits to raise U-B PCO2 gradient can be partially corrected by prior exposure to acid in the diet and further corrected by maintaining the blood pH within the acidotic range.

Metabolic and hormonal factors influencing extrarenal buffering of an acute acid load

This study evaluates metabolic and hormonal factors influencing extrarenal buffering of an acute acid load. Phosphate deprivation of 2 weeks duration was associated with enhanced extrarenal acid buffering. The enhanced extrarenal buffering capacity of phosphate deprivation was not dependent on the presence of parathyroid glands. Parathyroid hormone administration to phosphate-deprived rats promoted a further enhancement of the buffering capacity of an acid load. Blood pH and HCO3 during acid loading were not significantly different between control and diphosphonate-treated rats and between phosphate-deprived rats and phosphate-deprived rats treated with diphosphonate. The mortality rate, however, was significantly higher in diphosphonate-treated rats than in rats not receiving the drug suggesting that diphosphonate blunts the buffering of an acid load in both control and phosphate-deprived rats. Chronic vitamin D administration and acute administration of arginine vasopressin in pharmacologic doses were associated with significant enhancement of buffering capacity as compared to control rats. Thyrocalcitonin administration to intact but not thyroparathyroidectomized rats was associated with diminished capacity to buffer an acid load. These data demonstrate that the buffering of an acute acid load is influenced by a number of dietary and hormonal factors probably acting at the level of the bone.

Serosal Na/Ca exchange and H+ and Na+ transport by the turtle and toad bladders

A Na/Ca exchange system has been described in the plasma membrane of several tissues and seems to regulate the concentration of calcium in cytosol. Replacement of extracellular Na by sucrose increases calcium uptake into and decreases calcium efflux from the cell, leading to an increase in cytosolic calcium. The effect of an increase in cytosolic calcium mediated by the Na/Ca exchange system on H+ and Na transport in the turtle and toad bladder was investigated by replacing serosal Na isosmotically by sucrose or choline. Replacement of serosal by sucrose was associated with a significant inhibition of H+ secretion or Na transport which was reversible by addition of NaCl. Replacement of mucosal Na by sucrose failed to alter H+ secretion. Removal of serosal Na was associated with a significant increase in 45Ca uptake which could be blocked by pretreatment with lanthanum chloride. Pretreatment with lanthanum chloride blunted the inhibitory effect of replacement of serosal Na by sucrose on H+ and Na transport, thus suggesting that the increase in calcium uptake and the inhibition of transport are causally related. Under anaerobic conditions the rate of H+ or Na transport are linked to the rate of lactate production. The inhibition of Na or H+ transport by removal of serosal Na was accompanied by a proportional decrease in lactate production, thus suggesting that an increase in cytosolic calcium does not inhibit transport by uncoupling glycolysis from transport. Replacement of serosal Na by sucrose did not alter the force of the H+ or Na pump but led to an increase in resistance of the active pathway of H+ and Na transport. The inhibition of Na transport by replacement of serosal Na with sucrose could be reversed by addition of amphotericin B, an agent which increases luminal permeability to Na, thus suggesting that decreased Na entry across the apical membrane is the mechanism responsible for the inhibition of Na transport. The results of the present studies strongly suggest that an increase in cytosolic calcium through the serosal Na/Ca exchange system inhibits H+ and Na transport in the turtle and toad bladder probably by increasing the resistance of the luminal membrane.

The effects of chronic papillary necrosis on acid excretion

Complete papillary necrosis in rats can be induced within 1 month following a single injection of 2-bromoethylamine hydrobromide (BEA) (50 mg, i.v.). Utilizing a combination of clearance and balance techniques the effects of complete absence of the papilla was examined as regards urinary acidification, whole kidney glomerular filtration rate (GFR), single nephron GFR, and morphology. Whole kidney GFR was not different from control, however, the percent filtering juxtamedullary nephrons was markedly diminished (87.2 +/- 2.1 vs. 31.5 +/- 3.6% filtering, control vs. BEA, respectively, P less than 0.001) and significantly reduced in the superficial nephrons (80.6 +/- 3.6 vs. 62.2 +/- 6.1% filtering, control vs. BEA, respectively, P less than 0.05). There was a significant decrease in juxtamedullary single nephron GFR and an increase in the superficial single nephron GFR as assessed by the quantitative Hanssen's technique in the animals with chronic papillary necrosis. Complete papillary necrosis was associated with normal arterial bicarbonate concentration, pH, and plasma electrolyte concentrations. At the same degree of acidemia (induced by NH4Cl administration) minimal urinary pH, ammonium excretion, and titratable acid excretion were not different than seen in age matched controls. The response to Na2SO4 infusion and phosphate infusion was the same in both groups of animals. The urine-blood (U-B)pCO2, and index of urinary acidification, was identical in BEA and control animals. Scanning electron microscopy showed scarring of the juxtamedullary glomeruli one month after BEA. The papilla was sloughed and lying free in the renal pelvis in every experimental animal. These data demonstrate that complete papillary necrosis is not associated with acidosis nor a defect in urinary acidification.

Factors influencing vascular hyporesponsiveness to angiotensin II

Bartter's syndrome is characterized, in part, by hyporesponsiveness to the pressor effect of exogenous angiotensin II (AII). This has been attributed to volume contraction, hypokalemia, and/or increased prostaglandin (PG) levels. In order to investigate factors responsible for a diminished response to the pressor effect of AII, rats were made hypokalemic or volume contracted and hypokalemic (VCHK) by dietary restriction. AII sensitivity was examined by determining the dose of AII required to raise the mean arterial pressure 20 mm Hg. When compared with control rats. VCHK and hypokalemic rats were significantly less sensitive to AII. VCHK rats were significantly less sensitive to AII than hypokalemic rats. Both experimental groups were similarly hypokalemic, but plasma renin activity (PRA) of VCHK only was greater than control values. In VCHK rats, acute K+ restoration partially corrected AII hyporesponsiveness, although plasma K+ increased to normal. In VCHK rats, acute volume expansion with normal saline similarly achieved only partial correction of AII hyporesponsiveness although PRA values fell to the control range. Simultaneous K+ restoration and volume expansion to VCHK rats successfully restored AII sensitivity to the control range. Dietary sodium, chloride, and potassium restriction did not increase urinary excretion to PGE2. Indomethacin (5 mg/kg, iv) given acutely to VCHK rats did not significantly after baseline hyporesponsiveness to AII. Norepinephrine vascular sensitivity was not affected by either volume contraction or hypokalemia. These data demonstrate that volume contraction and hypokalemia individually depress exogenous AII sensitivity in the rat and do so by separate and additive mechanisms. Furthermore, these mechanisms appear to be independent of PG.

Drug-induced papillary necrosis: electrolyte excretion and nephron heterogeneity

The functional expression of papillary necrosis was investigated in rats following administration of 2-bromoethylamine hydrobromide (BEA). The percentage of filtering superficial and juxtamedullary nephrons was assessed using the qualitative Hanssen technique. There was a marked decrease in the percentage of filtering juxtamedullary nephrons 24 h after the induction of papillary necrosis (as compared with control rats), which was blunted in salt-loaded animals. The percentage of filtering superficial nephrons was the same in all three groups. BEA administration to normal rats was associated with a significant increase in sodium, chloride, phosphate, and calcium excretion. BEA administration to rats on a chloride-restricted diet was associated with chloride wastage. These animals also failed to adapt to a low chloride diet as late as 7 days after the administration of BEA. The phosphaturia observed in intact rats treated with BEA was abolished by parathyroidectomy. BEA-treated rats were able to adapt to a low phosphate diet and to a low magnesium diet. These data demonstrate that papillary necrosis is associated with profound impairment of juxtamedullary nephron filtration and with chloride wastage. In the absence of the papillary structures adaptation to phosphorus or magnesium deprivation is still possible.

Parathyroid hormone and extrarenal acid buffering

The role of parathyroid hormone (PTH) on the extrarenal buffering of an acid load was examined during HCl infusion (5 meq x kg-1 x h-1) to bilaterally nephrectomized rats. Thyroparathyroidectomized (TPTX) rats replaced with PTH had significantly higher blood pH and HCO3 values than TPTX rats not infused with PTH. Administration of EDTA, in a dose shown to release PTH, was associated with a significant increase in buffering capacity in intact but not in TPTX rats. Colchicine, given in a dose capable of stimulating PTH release, was also associated with enhanced buffering capacity in intact but not in TPTX rats. In TPTX rats infused with acetazolamide and PTH, the hormone failed to enhance extrarenal buffering of an acid load. Animals with chronic renal failure, induced by infarction of the kidney, also had an enhanced capacity to buffer an acid load. This enhanced buffering capacity in chronic renal failure was abolished by TPTX. Acute renal failure induced by bilateral ureteral ligation was also associated with increased buffering only in the presence of parathyroid glands. These data demonstrated that PTH, from either an exogenous or endogenous source, enhances extrarenal buffering capacity of an acid load. Chronic and acute renal failure are associated with increased buffering capacity, which is dependent on the presence of parathyroid glands. The data suggest that this effect is mediated through carbonic anhydrase.

Inhibition of H+ secretion in the turtle bladder by colchicine and vinblastine

In order to investigate a possible role of the microtubules in urinary acidification we measured H+ secretion by the turtle bladder in vitro before and after addition of either colchicine or vinblastine to the serosal phase. Both colchicine and vinblastine inhibited H+ secretion in a dose-dependent fashion; in the control hemibladders H+ secretion remained unchanged. The half-maximal inhibition of H+ secretion occurred at 5.2 x 10(-5) M for colchicine and 7.2 x 10(-6)M for vinblastine. The inhibitory effect of colchicine and vinblastine on H+ secretion was maximal at 10(-4)M and 5 x 10(-4)M, respectively. At these concentrations these compounds failed to alter SCC and resistance. The inhibition of H+ secretion by colchicine or vinblastine increased with time of exposure to the drugs. The effect of colchicine or vinblastine on H+ secretion was not reversible with removal of the drug or with addition of 5% CO2 in the serosal phase. Lumicolchicine, an isomer of colchicine devoid of capacity to interact with microtubules, failed to alter the rate of H+ secretion, suggesting that the observed effect of colchicine on H+ secretion was the result of disruption of microtubule function and not the consequence of nonspecific effect of the drug. These data provide evidence for a role of microtubules in urinary acidification by the turtle bladder in vitro, but an effect of these drugs on membrane-bound tubulin cannot be excluded.

[Monoamine oxidase inhibitors. I. Synthesis of N-cyclopropyltryptamines]

The synthesis of two new N-cyclopropyltryptamines is described. By treating 5,6-dimethoxyindole with oxalyl chloride and N-benzylcyclopropylamine, N-benzyl-N-cyclopropyl-5,6-dimethoxyindole-3-glyoxalamide is obtained. The reduction of this compound by LiAlH4, gives N-benzyl-N-cyclopropyl-5,6-dimethoxytryptamine, which is hydrogenated to N-cyclopropyl-5,6-dimethoxytryptamine. Similarly N-cyclopropyl-6,7-dimethoxytryptamine is prepared. Preliminary results indicate a different specificity of the inhibitors used on mitochondrial and bovine plasma enzyme (monoamine oxidase) attributable to the position of the methoxy groups.

Distal acidification defect induced by phosphate deprivation

The effect of phosphate deprivation on urinary acidification was investigated in rats fed a phosphate-deficient diet and in control rats fed the same diet supplemented with phosphate. Phosphate-deprived animals developed hypophosphatemia, hypercalcemia, and hypophosphaturia, but failed to develop hyperchloremic metabolic acidosis following 30 or 60 days of phosphate deprivation. Baseline urine pH was significantly higher in phosphate-deprived rats than in controls, but baseline urine HCO3 excretion was not significantly different between the two groups. The pattern of HCO3 reabsorption in phosphate-deprived rats was identical to that of controls at both low and high plasma HCO3 levels. During chronic NH4Cl administration, both 30- and 60-day phosphate-deprived rats had a sigificantly higher minimal urine pH and lower titratable acid and net acid excretion than seen in controls. NH4 excretion was significantly lower than controls in the 60-day phosphate-deprived rats only. During Na2SO4 administration the minimal urine pH was significantly lower in controls than in phosphate-deprived rats, but there was overlap of urine pH values. At comparable levels of urine pH, NH4 excretion was significantly lower in phosphate-deprived rats than in controls. Phosphate-deprived rats were able to raise urine-blood CO2 pressure to the same levels as controls during both HCO3 loading and Tris buffer administration. Phosphate-deprived rats had greater extrarenal buffering capacity than controls as evidenced by a lower decline in blood pH and HCO3 during HCl infusion in phosphate-deprived rats. These data demonstrate that phosphate deprivation is associated with distal acidification defect, impaired NH3 excretion, and increased extrarenal buffering capacity. The increased availability of buffer in phosphate deprivation may play an important role in acid-base homeostasis in this condition.

Cholinergic modulation of water transport in the toad bladder

The effect of carbachol on water transport by the toad bladder was studied. Carbachol caused a small increase in base-line water flow and inhibited, partially, vasopressin- (AVP) or cyclic AMP-stimulated water flow. The effect of carbachol on base-line or AVP-stimulated water flow was totally prevented by atropine, indicating that the effect of cabachol on water transport is mediated through a muscarinic receptor. Carbachol caused a significant increase in 45Ca uptake by toad bladder; this increase in calcium uptake could be prevented by atropine, pentobarbital, or lanthanum. The effect of carbachol on base-line and AVP-stimulated water flow was also prevented by pentobarbital or lanthanum, suggesting that the effect of carbachol is mediated, at least in part, by an increase in calcium uptake. The ionophore A-23187, an agent that increased 45Ca uptake, also enhanced base-line water flow and inhibited AVP-stimulated water flow. The effects of carbachol and the ionophore A-23187 on base-line water flow, AVP-stimulated water flow, and on calcium uptake were not additive, suggesting that both agents alter water transport by a similar mechanism. These data demonstrate that carbachol stimulates base-line water transport and inhibits AVP-stimulated water transport. They suggest that the alteration in water transport induced by carbachol is related to an increase in calcium uptake.

Effect of quinidine on Na, H+, and water transport by the turtle and toad bladders

The effect of quinidine on Na and H+ transport by the turtle bladder and water transport by the toad bladder was examined. Quinidine inhibited the short-circuit current and the potential difference in a dose-dependent fashion. The effect of quinidine on the short-circuit was not dependent on extracellular calcium concentration and was not reversible with removal of the drug. Quinidine inhibited H+ secretion in a dose-dependent fashion. The effect of quinidine on H+ secretion also was not dependent on extracellular calcium concentration and was not reversible, either with removal of the drug or with stimulation of H+ secretion with 5% CO2. The effect of quinidine on Na or H+ transport could not be elicited by an equivalent dose of tetracaine, suggesting that the inhibitory effect of quinidine is not dependent on its anesthetic properties. Quinidine also inhibited vasopressin and cyclic AMP stimulated water flow in the toad bladder. Quinidine did not alter calcium uptake by the turtle bladder but increased calcium efflux by the turtle and toad bladders. These observations suggest that a rise in cytosolic calcium is responsible for the inhibitory effect of quinidine on Na, H+, and water transport.