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Biomedical subjects

B Sacktor

Publications and source records attributed to B Sacktor.

At least 55 records · Page 3Linked to original sources
PubMed1986

Kinetic studies on the stimulation of Na+-H+ exchange activity in renal brush border membranes isolated from thyroid hormone-treated rats.

Na+-H+ exchange activity in renal brush border membrane vesicles isolated from hyperthyroid rats was increased. When examined as a function of [Na+], treatment altered the initial rate of Na+ uptake by increasing Vm (hyperthyroid, 18.9 +/- 1.1 nmol Na+ X mg-1 X 2 sec-1; normal, 8.9 +/- 0.3 nmol Na+ X mg-1 X 2 sec-1), and not the apparent affinity KNa+ (hyperthyroid, 7.3 +/- 1.7 mM; normal, 6.5 +/- 0.9 mM). When examined as a function of [H+] and at a subsaturating [Na+] (1 mM), hyperthyroidism resulted in the proportional increase in Na+ uptake at every intravesicular pH measured. A positive cooperative effect on Na+ uptake was found with increased intravesicular acidity in vesicles from both normal and hyperthyroid rats. When the data were analyzed by the Hill equation, it was found that hyperthyroidism did not change the n (hyperthyroid, 1.2 +/- 0.06; normal, 1.2 +/- 0.07) or the [H+]0.5 (hyperthyroid, 0.39 +/- 0.08 microM; normal, 0.44 +/- 0.07 microM) but increased the apparent Vm (hyperthyroid, 1.68 +/- 0.14 nmol Na+ X mg-1 X 2 sec-1; normal 0.96 +/- 0.10 nmol Na+ X mg-1 X 2 sec-1). The uptake of Na+ in exchange for H+ in membrane vesicles from normal and hyperthyroid animals was not influenced by membrane potential. H+ translocation or debinding was rate limiting for Na+-H+ exchange since Na+-Na+ exchange activity was greater than Na+-H+ exchange activity. Hyperthyroidism caused a proportional increase and hypothyroidism caused a proportional decrease in Na+-Na+ and Na+-H+ exchange.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Glucocorticoids and metabolic acidosis-induced renal transports of inorganic phosphate, calcium, and NH4.

The initial rate (5 s) of Na+-dependent inorganic phosphate (Pi) uptake in brush-border membrane vesicles isolated from rat proximal tubule was decreased in metabolic acidosis, 0.42 +/- 0.02 vs. 0.59 +/- 0.05 nmol/mg protein, in vesicles from control animals. Phosphate, ammonium, and Ca2+ excretions were increased 100, 600, and 56%, respectively. These changes in brush-border Pi transport and urinary excretion of ions were largely dependent on intact adrenal glands. After adrenalectomy there were no significant changes in brush-border Pi transport, Pi, and Ca2+ excretion, whereas ammonium excretion increased only 300% compared with controls. When the glucocorticoid dexamethasone was administered to adrenalectomized animals, it mimicked the effects of metabolic acidosis both in the presence and the absence of metabolic acidosis. The initial rate of brush-border Pi transport was decreased by dexamethasone administration to 0.37 +/- 0.04 nmol/mg protein in adrenalectomized acidotic animals and 0.39 +/- 0.03 nmol/mg protein in adrenalectomized animals. Dexamethasone administered to adrenalectomized acidotic animals increased Pi, ammonium, and Ca2+ excretion 190, 690, and 23%, respectively. Dexamethasone administered to nonacidotic adrenalectomized animals increased Pi ammonium and Ca2+ excretion 165, 240, and 31%, respectively. We conclude that changes in Pi, ammonium, and Ca2+ excretion observed during metabolic acidosis were dependent on intact adrenal glands and that glucocorticoids administered to adrenalectomized acidotic or nonacidotic animals mimicked the changes observed in acidotic animals with intact adrenal glands.

Acidosis↗
PubMed1986

Effect of age on renal conservation of phosphate in the rat.

Renal handling of phosphate (Pi) was examined in male Wistar-derived rats, 2-3, 6, 12, 18, and 24 mo of age. We observed a significant age-related phosphaturia [i.e., elevated urinary excretion (UPi V) and fractional excretion (FEPi)] in rats fed a normal phosphorus diet (NPD; 0.5% Pi). Concomitantly, plasma Pi decreased significantly and progressively with age. The mechanism of this age-related decrement in Pi conservation was examined by determining the initial (5 s) rate of Na+ gradient-dependent uptake of Pi in renal brush-border membrane vesicles (BBMV). Pi uptake significantly declined with increasing age. No consistent age-related decrease was seen in the Na+ gradient-dependent uptakes of glucose and proline by the same BBMV preparations, demonstrating the specificity of the Pi transport decrement. Pi transport kinetics revealed a significant age-related decrease in Vmax. No difference in Km of Pi was seen between age groups. These kinetic findings suggest either a decreased number of Pi carriers or decreased turnover of Pi carriers. Elevated parathyroid hormone did not explain the alteration in Pi conservation since urinary cAMP was not elevated in the intact senescent rat, and Pi uptake was not normalized in 24-mo-old rats 3 days after parathyroidectomy. The senescent 24-mo-old rat as well as the young adult 6-mo-old animal adapted to a low-phosphorus diet (LPD; 0.1% Pi) with a striking (greater than 100%) increase in Pi uptake by BBMV compared with NPD. thus the senescent kidney retained the capacity to respond appropriately to a LPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed1986

Calcium and renal adaptation to a phosphate load in the thyroparathyroidectomized rat.

Infusion of phosphate into thyroparathyroidectomized rats on a normal phosphorus diet caused a decrease in net phosphate reabsorption, even though the plasma concentration of phosphate continued to rise. This response was expressed at the level of the proximal tubule brush-border membrane and was coincident with a decrease in sodium-dependent phosphate uptake in membrane vesicles. Kinetic experiments indicated that the increased phosphate load caused a decrease in the Vmax of the membrane uptake system with no change in the apparent Km for phosphate. The infusion of phosphate resulted in a lowered plasma calcium concentration, and it was previously hypothesized that the inhibition of maximal phosphate reabsorption was mediated by the hypocalcemia. When the fall in plasma calcium was prevented by the simultaneous infusions of calcium and phosphate, the reduction in maximal phosphate reabsorption was blunted; however, the phosphate infusion-induced inhibition of brush-border membrane vesicle phosphate uptake was still evident. Thus a major discrepancy was found to the general concept that renal phosphate reabsorption in vivo correlated positively with sodium-dependent phosphate uptake activity in proximal tubule brush-border membrane vesicles. Several possible explanations to account for this anomaly were discussed. It was also found that calcium infusion into saline-infused thyroparathyroidectomized rats slightly increased maximal phosphate reabsorption but did not affect phosphate uptake in the membrane vesicles.

Absorption↗
PubMed1985

Thyroid hormones increase Na+-H+ exchange activity in renal brush border membranes.

Na+-H+ exchange activity, i.e., amiloride-sensitive Na+ and H+ flux, in renal proximal tubule brush border (luminal) membrane vesicles was increased in the hyperthyroid rat and decreased in the hypothyroid rat, relative to the euthyroid animal. A positive correlation was found between Na+-H+ exchange activity and serum concentrations of thyroxine (T4) and triiodothyronine (T3). The thyroid status of the animal did not alter amiloride-insensitive Na+ uptake. The rate of passive pH gradient dissipation was higher in membrane vesicles from hyperthyroid rats compared to the rate in vesicles from hypothyroid animals, a result which would tend to limit the increase in Na+ uptake in vesicles from hyperthyroid animals. Na+-dependent phosphate uptake was increased in membrane vesicles from hyperthyroid rats; Na+-dependent D-glucose and L-proline uptakes were not changed by the thyroid status of the animal. The effect of thyroid hormones in increasing the uptake of Na+ in the brush border membrane vesicle is consistent with the action of the hormones in enhancing renal Na+ reabsorption. Further, the regulation of transtubular Na+ flux has now been shown to be concomitant with modulation of the entry of Na+ into the tubular cell across its luminal membrane, mediated by the exchange reaction, and with the previously reported control of the pumping of Na+ out of the cell across its basolateral membrane, mediated by the Na+,K+-ATPase.

Animals↗
PubMed1985

Isolation and characterization of four forms of trehalase from rabbit kidney cortex.

Four forms of renal trehalase were isolated and purified to homogeneity. Hydrophobic interaction chromatography separated two forms; A-form and B-form. Both forms were subdivided further on Con A-Sepharose and were stained with periodic acid-Schiff reagent, indicating that they are glycoproteins. The four forms of renal trehalase showed no significant difference in Km values for trehalose and K1 values for various inhibitors. The optimum pH of the four forms was pH 6.0 in phosphate buffer. Apparent molecular weights on gel filtration of the four forms were the same, 175,000. Furthermore, the four forms showed the same antigenicity on double immunodiffusion. However, isoelectric point (pI), susceptibility to HgCl2, stability at -80 degrees C and Na+ activation behavior were different. Glycoprotein forms were more susceptible to HgCl2 and showed lower Na+ activation than nonglycoprotein forms. The pI of less hydrophobic forms (A1, A2) was more acidic than that of more hydrophobic forms (B1, B2). On the basis of these results, it is likely that four forms of renal trehalase are "isozymes."

Animals↗
PubMed1985

Phosphate excretion in uremic rats: effects of parathyroidectomy and phosphate restriction.

As progressive renal failure develops, phosphate excretion per functioning nephron increases, thus preserving homeostasis. To test whether dietary phosphate supply might contribute to the regulation of renal phosphate excretion in the uremic setting, groups of male Sprague-Dawley rats that were either parathyroidectomized (PTX) or sham PTX (S-PTX) and either five-sixths nephrectomized (Nx) or sham Nx (S-Nx) were studied following a 4-wk dietary regimen consisting of 0.1 or 0.7% phosphate. For Nx rats fed the 0.7% phosphate diet the fractional excretion of phosphate (FEPi) was enhanced (47 +/- 6 vs. 21 +/- 3%) and the maximum tubular reabsorption of phosphate per milliliter GFR (TmPi/GFR) was suppressed (1.65 +/- 0.19 vs. 2.33 +/- 0.19 mumol/ml). FEPi was unchanged by PTX in these Nx animals (42 +/- 6 vs. 47 +/- 6%). TmPi/GFR remained suppressed in PTX, NX animals when compared with S-Nx, PTX controls (3.38 +/- 0.33 vs. 5.07 +/- 0.41 mumol/ml). For rats fed the 0.1% phosphate diet Nx did not affect TmPi/GFR in either S-PTX (5.40 +/- 0.43 vs. 4.97 +/- 0.34 mumol/ml) or PTX (7.03 +/- 0.23 vs. 6.98 +/- 0.21 mumol/ml) animals. For both S-Nx and Nx animals the effects of PTX and dietary phosphate restriction on TmPi/GFR were independent and additive. In all groups of animals, tubular reabsorption of phosphate per milliliter GFR (TRPi/GFR) dropped acutely with continued infusion of phosphate once TmPi/GFR was achieved. Thus, a resetting of TRPi/GFR occurs among Nx rats in response to both chronic dietary phosphate deprivation and acute intravenous phosphate loading.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption↗
PubMed1985

Glucocorticoid activation of Na+/H+ exchange in renal brush border vesicles: kinetic effects.

Administration of the synthetic glucocorticoid dexamethasone to adrenalectomized rats increased Na+/H+ exchange activity in isolated renal brush border membrane vesicles. Treatment altered the initial rate of Na+ uptake by increasing Vmax (19.90 +/- 2.17 vs. 27.32 +/- 1.50 nmol.mg protein-1.5 s-1) and not the apparent affinity KNa+ (8.33 +/- 1.11 vs. 7.94 +/- 1.60 mM). Dexamethasone treatment resulted in a proportional increase in 1 mM Na+ uptake at every intravesicular pH measured. When these data were analyzed by the Hill equation, it was found that dexamethasone treatment did not change the apparent number of H+ binding sites (1.24 vs. 1.26) or the [H+]0.5 (0.33 vs. 0.32 microM) but increased the apparent Vmax (0.98 vs. 0.55 nmol.mg protein-1.2 s-1). It was also found that dexamethasone injections of 60 micrograms/100 g body wt resulted in maximum stimulation of exchange activity and that a significant increase in amiloride-sensitive Na+ uptake was detected within 12 h after a single dose of dexamethasone.

Adrenalectomy↗
PubMed1985

Forskolin and antidiuretic hormone stimulate a Ca2+-activated K+ channel in cultured kidney cells.

Single channels in the apical cell membrane of primary cultured chick kidney cells were studied using the patch clamp technique. Cell-attached recordings revealed the presence of a 107 +/- 6 pS channel that increased fractional open time upon depolarization. Experiments with inside-out excised patches indicated that the channel is K+ selective, Ca2+ activated, and inhibited by Ba2+. The addition of forskolin or antidiuretic hormone (ADH) to the bath during cell-attached recordings caused an increase in the fractional open time of the channel. The activation of a K+ channel by increases in cAMP may be one way in which K+ secretion in the kidney is stimulated by ADH in vivo.

Animals↗
PubMed1984

Sodium-dependent transport of inorganic sulfate by rabbit renal brush-border membrane vesicles. Effects of other ions.

A Na+ gradient (extravesicular greater than intravesicular) increased the rate of inorganic sulfate (SO24-) uptake into renal brush-border membrane vesicles and energized the transient accumulation of the anion against its concentration gradient, indicating a secondary active transport system. Stimulation of SO24- uptake was specific for Na+. The anions, SO23-, S2O23-, SeO24-, MoO24-, CrO24-, and WO24-, but not HPO24-, cis-inhibited and trans-stimulated SO24- uptake, suggesting that these divalent anions shared the SO24- carrier. The Na+/SO24- co-transport and Na+. The apparent Km for SO24- was 0.6 mM at 100 mM Na+. The relationship between Na+ concentration and rate of SO24- uptake was sigmoidal. From a Hill analysis of the data a [Na+]0.5 of 36 mM and an n value of 1.6 were calculated. Comparisons of the effects of a K+ diffusion potential (inside positive), of a H+ diffusion potential (inside negative), of Na+ salts of anions of different conductances on the Na+-dependent uptakes of SO24- and D-glucose, and of the responses of a membrane potential-sensitive fluorescent probe concomitant with the uptakes indicate that Na+/SO24- co-transport was electroneutral. The simplest stoichiometry consistent with an electroneutral mechanism would be the co-transport of two Na+ and one SO24-. Na+ gradient-dependent SO24- uptake was enhanced by intravesicular Cl-. cis-Cl- inhibited the efflux as well as the influx of SO24-. These findings suggest that Cl- was an inhibitor of SO24- transport. Intravesicular K+ stimulated Na+ gradient-dependent SO24- uptake. The co-transport of Na+/SO24- appeared not to be coupled to the transmembrane flux of K+. It is hypothesized that the co-transport system contained an internal site activated by K+.

Animals↗
PubMed1984

Renal trehalase: two subsites at the substrate-binding site.

Phlorizin, phloretin, Tris and beta-methylglucoside are competitive inhibitors, with respect to the substrate trehalose, of purified renal trehalase. Mercuric chloride is a noncompetitive inhibitor. The active site of trehalase was examined further by multi-inhibition kinetic studies involving combinations of inhibitors. Phlorizin vs. phloretin and phlorizin vs. Tris were mutually non-competitive. In contrast, phloretin vs. Tris was mutually competitive. These findings suggest that the binding site of phlorizin to the enzyme differed from that of phloretin or Tris, and that phloretin and Tris might bind at a common site. These findings suggest a model in which trehalase has two binding sites at the substrate-binding site, a phlorizin (glucosyl) and a phloretin (phenyl) binding site, analogous to the model proposed previously for the glucose carrier. In addition, mercuric chloride vs. beta-methylglucoside was mutually competitive, although mercuric chloride and beta-methylglucoside, respectively, were noncompetitive and competitive inhibitors with respect to the substrate. Thus, it is suggested that the substrate binding and the SH-inhibitor binding sites are located very close to each other.

Animals↗
PubMed1984

Na+-H+ exchange in isolated renal brush-border membrane vesicles in response to metabolic acidosis. Kinetic effects.

Chronic metabolic acidosis increased the Na+-H+ exchange activity in isolated renal brush-border membrane vesicles. Treatment altered the initial rate of Na+ uptake by increasing Vm (acidotic, 15.3 +/- 0.7 nmol of Na+ X mg-1 X 2 s-1; normal, 11.3 +/- 0.9 nmol of Na+ X mg-1 X 2 s-1), and not the apparent affinity KNa+ (acidotic, 10.2 +/- 0.5 mM; normal 10.2 +/- 0.6 mM). Metabolic acidosis resulted in the proportional increase in 1 mM Na+ uptake at every intravesicular pH measured. A positive cooperative effect on Na+ uptake was found with increased intravesicular acidity in vesicles from both normal and acidotic rats. When the data were analyzed by the Hill equation, it was found that metabolic acidosis did not change the n (acidotic, 1.33 +/- 0.13; normal, 1.43 +/- 0.07) or the K'H+ (acidotic, 0.27 +/- 0.05 microM; normal, 0.28 +/- 0.06 microM), but increased the apparent Vm (acidotic, 1.10 +/- 0.08 nmol of Na+ X mg-1 X 2 s-1; normal, 0.81 +/- 0.07 nmol of Na+ X mg-1 X 2 s-1). The uptake of Na+ in exchange for H+ in membrane vesicles from normal and acidotic animals was not influenced by membrane potential. We conclude that metabolic acidosis leads to either an increase in the number of functioning exchangers or an increase in the turnover rate of the limiting step in the exchange.

Acidosis↗
PubMed1984

Regulation of pyruvate oxidation in blowfly flight muscle mitochondria: requirement for ADP.

Blowfly (Phormia regina) flight muscle mitochondria oxidized pyruvate ( + proline) in the presence of either ADP (coupled respiration) or carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP-uncoupled respiration). There was an absolute requirement for ADP (Km = 8.0 microM) when pyruvate oxidation was stimulated by FCCP in the presence of oligomycin. This requirement for ADP was limited to the oxidation of pyruvate; uncoupled alpha-glycerolphosphate oxidation proceeded maximally even in the absence of added ADP. Atractylate inhibited uncoupled pyruvate oxidation whether added before (greater than 99%) or after (95%) initiation of respiration with FCCP. In the presence of FCCP, oligomycin, and limiting concentrations of ADP (less than 110 microM), there was a shutoff in the uptake of oxygen. This inhibition of respiration was completely reversed by the addition of more ADP. Plots of net oxygen uptake as a function of the limiting ADP concentration were linear; the observed ADP/O ratio was 0.22 +/- 0.025. An ADP/O ratio of 0.2 was predicted if phosphorylation occurred only at the succinyl-CoA synthetase step of the tricarboxylate cycle. Experiments performed in the presence of limiting concentrations of ADP, and designed to monitor changes in the mitochondrial content of ADP and ATP, demonstrated that the shutoff in oxygen uptake was not due to the presence of a high intramitochondrial concentration of ATP. Indeed, ATP, added to the medium prior to the addition of FCCP, inhibited uncoupled pyruvate oxidation; the apparent KI was 0.8 mM. These results are consistent with the hypothesis that it is the intramitochondrial ATP/ADP ratio that is one of the controlling factors in determining the rate of flux through the tricarboxylate cycle. Changes in the mitochondrial content of citrate, isocitrate, alpha-ketoglutarate, and malate during uncoupled pyruvate oxidation in the presence of a limiting concentration of ADP were consistent with the hypothesis that the mitochondrial NAD + -linked isocitric dehydrogenase is a major site for such control through the tricarboxylate cycle.

Adenosine Diphosphate↗
PubMed1984

Sodium gradient-dependent calcium uptake in renal basolateral membrane vesicles. Effect of parathyroid hormone.

The Na+/Ca2+ exchange system in rat renal cortex basolateral membrane vesicles was studied. Uptake and efflux of Ca2+ in the membrane vesicles were stimulated by trans-Na+. The enhancement of Ca2+ uptake by the intravesicular greater than extravesicular Na+ gradient was inhibited by ionophores that dissipated the gradient, and was increased by an outside negative membrane potential. Na+-dependent Ca2+ uptake was saturable with respect to both Ca2+ and Na+. A [Ca2+]0.5 of 8 microM was calculated. The relationship between Na+ concentration and rate of Ca2+ efflux was sigmoidal; a [Na+]0.5 of 15 mM and a Hill coefficient of 2.5 were estimated. Removal of parathyroid glands from the rats resulted in a 40% decrease in the Na+-dependent Ca2+ uptake. Infusion of parathyroid hormone (the synthetic tetratriacontapeptide) into these animals fully restored the activity. The isolated basolateral membrane possessed parathyroid hormone-sensitive adenylate cyclase. These findings may suggest a mechanism by which parathyroid hormone regulates the reabsorption of Ca2+ in the kidney.

Adenylyl Cyclases↗
PubMed1984

Calcium inhibition of the NAD+-linked isocitrate dehydrogenase from blowfly flight muscle mitochondria.

Free Ca2+ was shown to inhibit the NAD+-isocitrate dehydrogenase from blowfly flight muscle mitochondria. Inhibition by free Ca2+ concentrations of 40 microM or greater was found in the absence or presence of ADP and citrate, two known activators of the enzyme. Calcium decreased the affinity of the enzyme for its substrate, the magnesium DL-isocitrate chelate; no change in the apparent V of the reaction was observed. Calcium was inhibitory when activity was measured in the presence of fixed concentrations of magnesium DL-isocitrate chelate in the presence of several fixed concentrations of either free isocitrate3-, an activator, or free Mg2+, an inhibitor of the enzyme. That NAD+-isocitrate dehydrogenase from blowfly flight muscle mitochondria was not activated by micromolar free Ca2+ is consistent with the view that calcium does not play a role in regulating the flux through the tricarboxylate cycle in this species.

Adenosine Diphosphate↗