On the mechanism of diminished urinary carbon dioxide tension caused by amiloride

On the mechanism of diminished urinary carbon dioxide tension caused by amiloride. We investigated under both in vivo and in vitro conditions the mechanism whereby amiloride administration, a model of distal renal tubular acidosis in dogs, decreases the urine-to-blood PCO2 gradient (U-B PCO2) in alkaline urine. The results demonstrate that U-B PCO2 is reduced in amiloride-treated dogs as previously reported in rats. The reduction in U-B PCO2 could not be attributed to amiloride-induced reductions in urinary HCO-3 concentration since the reduction in U-B PCO2 was observed over the same range of urinary HCO-3 concentrations (150 to 250 mEq per liter) as that achieved prior to amiloride administration. U-B PCO2 correlated positively and linearly with urinary HCO-3 concentration both prior to (P < 0.001) and during amiloride infusion (P < 0.001). Amiloride administration significantly decreased the slope (Δ[U-B PCO2]/Δ[HCO-3]u) of the regression line (P < 0.005). The possibility that amiloride might lower urine PCO2 by catalyzing intraluminal dehydration of H2CO3 was excluded by demonstrating that amiloride does not possess carbonic anhydrase activity. The additional possibility that amiloride might facilitate dissipation of carbon dioxide gradients through diffusion (as reported for carbonic anhydrase) was excluded by in vitro studies of the effect of amiloride on carbon dioxide diffusion. These findings suggest that the U-B PCO2 lowering effect of amiloride is not caused by alterations in urinary [HCO-3, CO2 diffusibility, or alterations in the dehydration rate of H2CO3 thereby providing strong support for the interpretation that reductions in U-B PCO2 during amiloride administration represent an impairment in distal nephron hydrogen ion secretion.Sur le méchanisme de la diminution de la pression partielle de carbonique dans l'urine déterminée par l'amiloride. Nous avons examiné in vivo et in vitro le mécanisme par lequel l'administration d'amiloride, un modèle d'acidose tubulaire distale chez le chien, diminue le gradient urinesang de PCO2 (U-B PCO2) en urine alcaline. Les résultats démontrent que U-B PCO2 est réduit chez le chien traité par l'amiloride comme cela a été antérieurement montré chez le rat. Le diminution de U-B PCO2 ne peut pas être attribuée à la diminution de la concentration urinaire de HCO-3 déterminée par l'amiloride puisque la diminution de U-B PCO2 a été observée pour le même éventail de concentrations urinaires de HCO-3 (150 à 250 mEq par litre) que celui réalisé avant l'administration d'amiloride. U-B PCO2 est corrélé linéairement et positivement à la concentration urinaire de HCO-3 aussi bien avant (P < 0,001) que pendant la perfusion d'amiloride (P < 0,001). L'administration d'amiloride diminue régulièrement la pente (Δ[U-B PCO2]/Δ[HCO-3]u) de la droite de régression (P < 0,005). La possibilité que l'amiloride puisse diminuer la PCO2 urinaire en catalysant la déshydratation intraluminal de H2CO3 a été exclue par la démonstration de l'absence d'activité de type anhydrase carbonique de l'amiloride. La possibilité que l'amiloride puisse faciliter la dissipation par diffusion des gradients de carbonique a été exclue par des études in vitro. Ces constatations suggèrent que l'effet de l'amiloride de diminution de U-B PCO2 n'est pas lié à des modifications de [HCO-3] de l'urine, de la diffusibilité du carbonique, ou du débit de déshydratation de H2CO3, et par conséquent constituent un argument fort en faveur d'une altération de la sécrétion distale d'ion hydrogène

Effects of dexamethasone on renal and systemic acid-base metabolism

Effects of dexamethasone on renal and systemic acid-base metabolism. We carried out long-term balance studies in adrenalectomized (ADX) dogs to evaluate the effects of small amounts of a glucocorticoid steroid with minimal mineralocorticoid potency (dexamethasone; DEX) on renal and systemic acid-base metabolism under conditions of constant mineralocorticoid replacement and both normal and increased systemic acid loads. We investigated the effects of low and high dosages of dexamethasone (0.2 mg/day [normal-DEX] vs. 0.8 mg/day [high-DEX]) before and during hydrochloric acid feeding (5 mmol/kg/day) in paired studies on ADX dogs (N = 7) maintained on constant mineralocorticoid replacement (deoxycorticosterone, corticosterone, aldosterone). Prior to hydrochloric acid feeding, no differences in plasma acid-base composition were observed between the two dosages despite greater endogenous acid production with the higher dosage of DEX, evidenced by greater rates of both net acid excretion (NAE) and the excretion of urinary anions other than chloride, bicarbonate, and phosphate (urine anion gap). During hydrochloric acid feeding, mean plasma bicarbonate (PHCO3) decreased from 21.2 ± 0.4 to 13.7 ± 0.5 (normal-DEX) and from 21.1 ± 0.4 to 15.8 ± 0.4 mEq/liter (high-DEX). The difference in the decrements in PHCO3 between groups was significant (P < 0.05). With continued hydrochloric acid feeding in both groups, increasing the DEX dosage from 0.2 to 0.8 mg/day in the normal-DEX group resulted in a. significant increase in NAE (ΣΔNAE, +161 mEq, P < 0.02) and in PHCO3 (+3.6 ± 0.5 mEq/liter, P < 0.01) to steady-state levels by day 10, which were values not significantly different from those in high-DEX. The DEX dose-related increase in NAE was greater than the corresponding increase in endogenous acid production estimated from the change in urine anion gap, and was due largely to an increase in ammonium excretion, which, because urine pH did not decrease, could not be attributed to increased intraluminal trapping of ammonia as a result of more acidic tubular fluid. These studies indicate that the severity of hydrochloric acid-induced metabolic acidosis in mineralocorticoid-replete ADX dogs can be mitigated by increasing the dosage of exogenous glucocorticoid and suggest that this acidosis mitigating effect is mediated in part by the increased NAE associated with the stimulation of renal ammonia production. These studies further indicate that the rate of production of fixed acids of metabolism increases with an increased dosage of exogenous glucocorticoid, but that this acidosis-producing effect is more than offset by independent stimulation of renal net acid excretion, such that metabolic acidosis is prevented (basal condition) or if present (hydrochloric acid feeding) is significantly ameliorated.Effets de la dexaméthasone sur le métabolisme acido-basique rénal et systémique. Les études de bilan qui sont rapportées ont été réalisées chez des chiens surrénalectomisés (ADX) pour évaluer les effets de faibles quantités d'un stéroïde glucocorticoïde, ayant une activité minéralocorticoïde faible (dexaméthasone; DEX), sur le métabolisme acido-basique rénal et systémique dans des conditions de traitement substitutif permanent de minéralocorticoïdes et de charge acide soit normale soit élevée. Nous avons étudié les effets de doses de dexaméthasone (0,2 mg/jour; normale-DEX) ou 0,8 mg/jour (élevée-DEX) avant et pendant l'administration d'acide chlorhydrique à raison de 5 mmol/kg/jour dans des études appariées chez des chiens ADX (N = 7) recevant un traitement substitutif par les minéralocorticoïdes (deoxycorticosterone acetate, corticosterone, aldosterone). Avant l'administration d'acide chlorhydrique, il n'y avait pas de différence dans la composition acido-basique du plasma selon les doses de DEX malgré l'augmentation de production endogène d'acide sous l'effet de la dose la plus élevée de DEX, augmentation traduite par une élévation de l'état stationnaire d'excrétion nette d'acide (NAE) et de la somme des débits d'excrétion des anions urinaires autres que le chlore, le bicarbonate et le phosphate (trou anionique urinaire). Au cours de l'administration d'acide chlorhydrique la concentration plasmatique moyenne de bicarbonate (PHCO3) a diminué de 21,2 ± 0,4 à 13,7 ± 0,5 (normale-DEX) et de 21,1 ± 0,4 à 15,8 ± 0,4 mEq/litre (élevée-DEX). La différence des diminutions de bicarbonate était significative (P < 0,05). Au cours de l'administration prolongée d'acide chlorhydrique aux deux groupes l'augmentation de la dose de DEX de 0,2 à 0,8 mg/jour dans le groupe normale-DEX a eu pour résultat une augmentation significative de NAE (ΣΔNAE, +161 mEq, P < 0,02) et de PHCO3 (+ 3,6 ± 0,5 mEq/litre, P < 0,01), jusqu'à de nouveaux états stationnaires, le jour 10, non sigmficativement différents de ceux observés dans le groupe élevée-DEX. L'augmentation de NAE dépendant de la dose de DEX a été plus grande que l'augmentation correspondante de la production endogène d'acide estimée à partir de la modification du trou anionique urinaire, elle était principalement due à une augmentation de l'excrétion d'ammonium qui, du fait que le pH de l'urine n'a pas diminué, ne peut pas être attribuée à une augmentation de la captation intraluminale d'ammonia. Ces résultats indiquent que la sévérité de l'acidose métabolique déterminée par acide chlorhydrique chez les chiens ADX recevant des minéralocorticoïdes peut être atténuée par l'augmentation de la dose de glucocorticoïdes exogènes et suggère que cet effet d'atténuation a pour médiateur partiel l'augmentation de NAE associée à la stimulation de la production rénale d'ammonia. Ces résultats indiquent, de plus, que le débit de production des acides fixes augmente en même temps que la dose de glucocorticoïdes exogènes mais que cet effet de production d'acidose est plus que compensé par la stimulation indépendante de l'excrétion rénale nette d'acide, de telle sorte que l'acidose métabolique est empêchée (conditions basales) ou significativement améliorée au cours de l'administration acide chlorhydrique

Karyotype and nuclear DNA content of hexa-, octo-, and duodecaploid lines of Bromus subgen. Ceratochloa

The subgenus Ceratochloa of the genus Bromus includes a number of closely related allopolyploid forms or species that present a difficult taxonomic problem. The present work combines data concerning chromosome length, heterochromatin distribution and nuclear genome size of different 6x, 8x and 12x accessions in this subgenus. Special attention is paid to the karyotype structure and genomic constitution of duodecaploid plants recently found in South America. Hexaploid lineages possess six almost indistinguishable genomes and a nuclear DNA content between 12.72 pg and 15.10 pg (mean 1Cx value = 2.32 pg), whereas octoploid lineages contain the same six genomes (AABBCC) plus two that are characterized by longer chromosomes and a greater DNA content (1Cx = 4.47 pg). Two duodecaploid accessions found in South America resemble each other and apparently differ from the North American duodecaploid B. arizonicus as regards chromosome size and nuclear DNA content (40.00 and 40.50 pg vs. 27.59 pg). These observations suggest that the South American duodecaploids represent a separate evolutionary lineage of the B. subgenus Ceratochloa, unrecognized heretofore

Bone refilling in cortical bone multicellular units: Insights into tetracycline double labelling from a computational model

Bone remodelling is carried out by `bone multicellular units' (BMUs) in which active osteoclasts and active osteoblasts are spatially and temporally coupled. The refilling of new bone by osteoblasts towards the back of the BMU occurs at a rate that depends both on the number of osteoblasts and on their secretory activity. In cortical bone, a linear phenomenological relationship between matrix apposition rate (MAR) and BMU cavity radius is found experimentally. How this relationship emerges from the combination of complex, nonlinear regulations of osteoblast number and secretory activity is unknown. Here, we extend our previous mathematical model of cell development within a single BMU to investigate how osteoblast number and osteoblast secretory activity vary along the BMU's closing cone. MARs predicted by the model are compared with data from tetracycline double labelling experiments. We find that the linear phenomenological relationship observed in these experiments between MAR and BMU cavity radius holds for most of the refilling phase simulated by our model, but not near the start and end of refilling. This suggests that at a particular bone site undergoing remodelling, bone formation starts and ends rapidly. Our model also suggests that part of the observed cross-sectional variability in tetracycline data may be due to different bone sites being refilled by BMUs at different stages of their lifetime. The different stages of a BMU's lifetime depend on whether the cell populations within the BMU are still developing or have reached a quasi-steady state while travelling through bone. We find that due to their longer lifespan, active osteoblasts reach a quasi-steady distribution more slowly than active osteoclasts. We suggest that this fact may locally enlarge the Haversian canal diameter (due to a local lack of osteoblasts compared to osteoclasts) near the BMU's point of origin.Comment: 16 pages, 6 figures, 3 tables. V3: minor changes: added 2 paragraphs (BMU cavity in Section 2 and Model Robustness in Section 4), references [52,54