Steady-state Specific Activity Research Articles

Revisiting formic acid decomposition on metallic powder catalysts: Exploding the HCOOH decomposition volcano curve

This study revisits the classic volcano curve for HCOOH decomposition by metal catalysts by taking a modern catalysis approach. The metal catalysts (Au, Ag, Cu, Pt, Pd, Ni, Rh, Co and Fe) were prepared by H2 reduction of the corresponding metal oxides. The number of surface active sites (Ns) was determined by formic acid chemisorption. In situ IR indicated that both monodentate and bidentate/bridged surface HCOO* were present on the metals. Heats of adsorption (ΔHads) for surface HCOO* values on metals were taken from recently reported DFT calculations. Kinetics for surface HCOO* decomposition (krds) were determined with TPD spectroscopy. Steady-state specific activity (TOF=activity/Ns) for HCOOH decomposition over the metals was calculated from steady-state activity (μmol/g-s) and Ns (μmol/g). Steady-state TOFs for HCOOH decomposition weakly correlated with surface HCOO* decomposition kinetics (krds) and ΔHads of surface HCOO* intermediates. The plot of TOF vs. ΔHads for HCOOH decomposition on metal catalysts does not reproduce the classic volcano curve, but shows that TOF depends on both ΔHads and decomposition kinetics (krds) of surface HCOO* intermediates. This is the first time that the classic catalysis study of HCOOH decomposition on metallic powder catalysts has been repeated since its original publication.

Influence of valine flooding on channeling of valine into tissue pools and on protein synthesis.

Rates of valine incorporation into protein were measured under control and valine-"flooding" conditions and included correction for the degree of recycling of unlabeled valine derived from the steady-state breakdown of tissue protein into the precursor pool (tRNA bound). The correction factor lambda, which is the ratio of the steady-state specific activity of valine in the tissue tRNA-bound pool to that in the arterial plasma, was determined for each of the tissues. In controls, values of lambda ranged from 0.31 in adrenals to 0.54 in heart; in flooded animals, values were higher, but only in liver was the value of lambda close to 1.0. In control and flooded rats, rates of protein synthesis were highest in liver and adrenals and lowest in skeletal muscle, with intermediate values in brain and heart. Flooding resulted in increased rates of protein synthesis in liver and decreased rates in adrenals. Rates of protein synthesis in brain, heart, and skeletal muscle were not statistically significantly affected by flooding.

Regulation of galactose-1-phosphate uridyltransferase gene expression.

Analysis of both the human and rat galactose-1-phosphate uridyltransferase (GALT) genes reveal 5' regulatory consensus sequences suggestive of a housekeeping gene. This is in accord with the finding of GALT activity in all tissues. However, the complications seen in galactosemia, in particular ovarian dysfunction and verbal dyspraxia, suggest organ-specific sensitivity to lack of GALT activity. Analysis of steady-state GALT mRNA and specific activity levels in adult rat organs reveals a marked difference between organs that correlates with the degree of organ dysfunction in humans with galactosemia. The organ variation in GALT mRNA and activity thus appears to be due to genetic regulation. Discernment of the pathophysiologic basis for the organ-specific complications requires an understanding of the basis for the differences in organ regulation. The present state of knowledge of the regulation of the Leloir enzymes in general, GALT in particular, from prokaryotes to mammals is discussed.

Ovine fetal placental lactate exchange and decarboxylation at midgestation

In a study of the metabolic implications of the large placental-to-fetal mass ratio that characterizes early fetal development, fetal plasma lactate disposal rate and CO2 production from fetal plasma lactate by fetus and placenta were measured in six midgestation (71-80 days) pregnant sheep. A constant fetal intravenous infusion of L-[U-14C]lactate and 3H2O was given to establish fetal steady-state lactate specific activity and to measure uterine and umbilical blood flows. Fetal and placental weights were 158.6 +/- 19.7 and 441.9 +/- 32.7 g, respectively. There was a significant net lactate flux into the uterine circulation (31 +/- 4.3 mumol/min, P < 0.01) but no measurable umbilical uptake. Fetal plasma lactate disposal rate was 21.2 +/- 2.7 mumol/min, approximately one-half of which represented fetal lactate flux into the placenta (9.9 +/- 1.9 mumol/min). The oxidation rate of tracer lactate carbon to 14CO2 by placenta plus fetus was 72.6 +/- 4.7% of the infused tracer and was fairly evenly distributed between the placenta and the fetus (42.4 +/- 3.9 vs. 30.3 +/- 2.2%). The midgestation placenta metabolizes and produces fetal plasma lactate at rapid and nearly equal rates. This contrasts with the late-gestation placenta, which makes a small contribution to fetal lactate disposal and is a major net source of fetal lactate.

Effect of Loading Doses of l‐Valine on Relative Contributions of Valine Derived from Protein Degradation and Plasma to the Precursor Pool for Protein Synthesis in Rat Brain

"Flooding" amino acid pools with high doses of labeled amino acids of low specific activity has been proposed to minimize the effects of recycling of amino acids derived from protein degradation on the specific activity of the amino acid precursor pool for protein synthesis. We have examined the influence of recycling on the precursor pool for protein synthesis under conditions in which plasma valine concentrations were normal (0.19 mM) and "flooded" (10-28 mM) by comparing the steady-state specific activity of the tRNA-bound valine with that of the plasma valine. Under normal and "flooding" conditions, the relative contributions of valine from protein degradation to the precursor pool were 63 and 26%, respectively; "flooding" with a plasma level of 28 mM raised the brain acid-soluble pool level to 3.1 mM but was no more effective in decreasing the relative contribution of valine from protein degradation to the precursor pool than "flooding" with a plasma level of 17 mM valine, which raised the brain acid-soluble level only to 2.3 mM. The results of these studies show that "flooding" amino acid pools does indeed reduce the effect of recycling on the precursor amino acid pool for protein synthesis, but it does not totally eliminate it.

Estimation of glucose kinetics in fetal-maternal studies: potential errors, solutions, and limitations

We investigated whether errors occur in the estimation of ovine maternal-fetal glucose (Glc) kinetics using the isotope dilution technique when the Glc pool is rapidly expanded by exogenous (protocol A) or endogenous (protocol C) Glc entry and sought possible solutions (protocol B). In protocol A (n = 8), after attaining steady-state Glc specific activity (SA) by [U-14C]glucose (period 1), infusion of Glc (period 2) predictably decreased Glc SA, whereas. [U-14C]glucose concentration unexpectedly rose from 7,208 +/- 367 (means +/- SE) in period 1 to 8,558 +/- 308 disintegrations/min (dpm) per ml in period 2 (P less than 0.01). Fetal endogenous Glc production (EGP) was negligible during period 1 (0.44 +/- 1.0), but yielded a physiologically impossible "negative" value of -2.1 +/- 0.72 mg.kg-1.min-1 during period 2. When the fall in Glc SA during Glc infusion was prevented by addition of [U-14C]glucose admixed with the exogenous Glc (protocol B; n = 7), EGP was no longer "negative." In protocol C (n = 6), sequential infusions of four increasing doses of epinephrine serially decreased SA, whereas tracer Glc increased from 7,483 +/- 608 to 11,525 +/- 992 dpm/ml plasma (P less than 0.05), imposing an obligatory underestimation of EGP. Thus a tracer "mixing" problem leads to erroneous estimations of fetal Glc utilization and Glc production via the three-compartment model in sheep when the Glc pool is expanded exogenously or endogenously. These errors can be minimized by maintaining the Glc SA relatively constant.

Cellular progesterone receptor phosphorylation in response to ligands activating protein kinases

Progesterone receptors were immunoprecipitated with monoclonal antibodies KD68 from lysates of human breast carcinoma T47D cells labelled to steady state specific activity with 32Pi. The 120 kDa 32P-labelled progesterone receptor band was resolved by polyacrylamide gel electrophoresis and identified by autoradiography. Phosphoamino acid analysis revealed serine phosphorylation, but no threonine or tyrosine phosphorylation. Treatment of the 32Pi-labelled cells with EGF, TPA or dibutyryl cAMP had no significant quantitative effect on progesterone receptor phosphorylation, though the EGF receptor and the cAMP-dependent protein kinases have been reported to catalyze phosphorylation of purified avian progesterone receptor preparations in cell free systems. Progesterone receptor phosphorylation on serine residues was increased by 2-fold in cells treated with 10 nM progesterone; EGF had no effect on progesterone-mediated progesterone receptor phosphorylation.

Epidermal growth factor stimulates tyrosine phosphorylation of human glucocorticoid receptor in cultured cells

Human breast epithelial HBL100 cells, which bind both epidermal growth factor (EGF) and glucocorticoids, were labelled to steady state specific activity with 32Pi and the glucocorticoid receptor was immunoprecipitated from cell lysates with polyclonal antiserum GR884. Immunoprecipitated receptor was resolved by NaDodSO4-polyacrylamide gel electrophoresis and identified by autoradiography. Immunoprecipitated receptor also was characterized by western blot analysis and affinity labelling with [3H]dexamethasone-21-mesylate. Phosphoamino acid analysis of 32P-glucocorticoid receptor revealed 89% phosphoserine and 11% phosphotyrosine. Treatment of steady state 32Pi-labelled cells with EGF stimulated total and alkali-stable phosphorylation in the 97 kDa receptor band by about 35%. Prior incubation with dexamethasone inhibited EGF stimulated, alkali-stable phosphorylation of the 97 kDa glucocorticoid receptor band.

Studies on the specific activity of [γ- 32P]ATP in adipose and other tissue preparations incubated with medium containing [ 32P]phosphate

1. The specific activity of the γ- 32P position of ATP was measured in various tissue preparations by two methods. One employed HPLC and the enzymatic conversion of ATP to glucose 6-phosphate and ADP. The other was based on the phosphorylation of histone by catalytic subunit of cAMP-dependent protein kinase (Hawkins, P.T., Michell, R.H. and Kirk, C.J. (1983) Biochem. J. 210, 717–720). The HPLC method also allowed the incorporation of 32P into the ( α + β)-positions of ATP to be determined. 2. In rat epididymal fat-pad pieces and fat-cell preparations the specific activity of [γ- 32P]ATP attained a steady-state value after 1–2 h incubation in medium containing 0.2 mM [ 32P]phosphate. Addition of insulin or the β-agonist isoprenaline increased this value by 5–10% within 15 min. 3. Under these conditions the steady-state specific activity of [γ- 32P]ATP was 30–40% of the initial specific activity of the medium [ 32P]phosphate. However, if allowance was made for the change in medium phosphate specific activity during incubations the equilibration of the γ-phosphate position of ATP with medium phosphate was greater than 80% in both preparations. The change in medium phosphate specific activity was a combination of the expected equilibration of [ 32P]phosphate with exchangeable intracellular phosphate pools plus the net release of substantial amounts of tissue phosphate. At external phosphate concentrations of less than 0.6 mM the loss of tissue phosphate to the medium was the major factor in the change in medium phosphate specific activity. 4. It is concluded that little advantage is gained in employing external phosphate concentrations of less than 0.6 mM in experiments concerned with the incorporation of phosphate into proteins and other intracellular constituents. Indeed, a low external phosphate concentration may cause depletion of important intracellular phosphorus-containing components.

Tritium dynamics in juvenile rainbow trout, Salmo gairdneri.

Juvenile rainbow trout, Salmo gairdneri, were maintained in tritiated water, fed a tritiated diet or both, and the specific activity of 3H in the fish was determined during and following 3H exposure. Tissue water 3H-equilibrated rapidly with the ambient water (estimated half-life less than 30 min) and maintained a steady-state specific activity similar to ambient. In contrast, the steady-state specific activity of organically bound 3H was significantly affected by that of the diet. The steady-state specific activity of organically bound 3H of trout maintained in tritiated water but fed non-tritiated diets was approximately 20% of that of the ambient water. This steady-state specific activity was achieved relatively quickly (half-life approximately 1 d) and presumably reflects the exchangeable H of the fish. For trout fed a tritiated diet, in either tritiated or non-tritiated water, steady-state specific activities of organically bound 3H were on the order of 80% of that of the diet. These steady-state specific activities were reached much more slowly (estimated half-lives of 18-32 d) and are likely to represent the accumulation of non-exchangeable 3H in the organic material of the trout.

Indomethacin and salicylate modulate effect of insulin on glucose kinetics in dogs.

We studied insulin's effects on glucose production (Ra) and utilization (Rd) in trained, conscious dogs before and during treatment with indomethacin (Indo) and salicylate (S). Ra and Rd (mg X kg-1 X min-1) were calculated by isotope dilution using [3-3H]glucose. Animals were treated with either oral Indo or acetylsalicylic acid for 1 day before the respective studies. On the study day, experimental animals were given a continuous infusion of either saline (control), Indo (5 mg/kg bolus followed by 0.05 mg X kg-1 X min-1), or sodium salicylate (0.45 mg X kg-1 X min-1) for 330 min on separate days; each animal participated in all three protocols. After establishing steady-state specific activity, control (C) and experimental animals (n = 6/group) received insulin, 0.275 mU X kg-1 X min-1 for 150 min, raising serum insulin levels two- to threefold above basal. During insulin infusion in C, plasma glucose (G) fell from 99 +/- 2 to 82 +/- 6 ml/dl (P less than 0.01), associated with a transient fall in Ra from 2.5 +/- 0.3 to 1.9 +/- 0.2 (P less than 0.01) at 30 min, returning to base line at 45 min; Rd did not change. In the Indo and S groups, G also fell by a similar extent. In contrast to C, however, the fall in G was associated with a rise in Rd, commencing at 30 min in the Indo group (P less than 0.05) and at 45 min in the S group (P less than 0.01); Ra did not fall and actually rose above basal (P less than 0.05), although it did not match the rise in Rd.(ABSTRACT TRUNCATED AT 250 WORDS)

Opiate modulation of glucose turnover in dogs

We examined the effect of opiate infusion and of opiate blockade on glucose turnover in the basal state, using isotope dilution techniques in trained conscious dogs (n = 5). After a primed-continuous infusion of 3- 3H glucose to steady state specific activity (90 minutes), infusion of one of the following was given: D-met 2 pro 5 enkephalinamide (DMPE), a potent morphine-like opiate, 0.5 μg/kg/min; naloxone, an opiate antagonist, 1.25 mg followed by 10 μg/min; or saline control. Infusion of DMPE led to a fall in glucose from 92 ± 3 to 87 ± 3 mg/dL by 60 minutes ( P < 0.05), associated with a rise in glucose utilization (Rd) from 3.0 ± 0.4 to 3.9 ± 0.6 mg/kg/min by 30 minutes ( P < 0.005); a transient rise in glucose production (Ra; from 3.2 ± 0.4 to 4.3 ± 0.4 mg/kg/min) occurred at 5 minutes ( P < 0.05). Infusion of naloxone did not result in a change of plasma glucose, but caused transient falls at 5 minutes in both Ra (3.5 ± 0.7 to 1.3 ± 0.3 mg/kg/min) and Rd (3.6 ± 0.7 to 1.6 ± 0.4 mg/kg/min; P < 0.05). Changes in counterregulatory hormones did not account for these findings: insulin was unchanged during all infusions; glucagon showed small late rises at 75 minutes during both DMPE and naloxone infusion; cortisol rose by 30 and 15 minutes, respectively, of DMPE and naloxone infusion; epinephrine rose transiently after 5 minutes of naloxone but was unchanged during DMPE, and norepinephrine was unchanged throughout. Saline infusion had no effects on any of these indices. We conclude that a potent opiate with morphine-like effects (DMPE) can lower glucose in dogs by enhancing peripheral glucose utilization independently of hormonal changes.

Indomethacin and salicylate decrease epinephrine-induced glycogenolysis

Epinephrine (E) produces an immediate (0–30 minutes) rise in hepatic glucose production (Ra), largely due to activation of glycogenolysis; thereafter, E-stimulated gluconeogenesis becomes the major factor maintaining glucose production. To investigate the possible role of arachidonic acid metabolites on Ra during E stimulation, we infused E in trained conscious dogs before and during administration of two inhibitors of arachidonic acid metabolism, indomethacin (INDO) and salicylate (S). On separate days, experimental animals were treated with both oral and IV INDO and oral acetylsalicylic acid and IV sodium salicylate. Ra and glucose utilization (Rd), both in mg · kg −1min −1, were calculated by isotope dilution using 3- 3H-glucose. After achieving steady state specific activity, control (C) and experimental animals (n = 6 per group) received E (0.1 ug · kg −1min −1) for 150 minutes, raising plasma levels to approximately 1500 pg/mL in each group. In C, plasma glucose (G; mg/dL) rose by 17 ± 5 at 10 minutes and 19 ± 3 at 20 minutes due to an initial spike in Ra (2.7 ± 0.2 to 4.9 ± 0.5; P < 0.01) at 10 minutes. INDO and S treatment attenuated this initial (10–20 minutes) rise in G ( P < 0.05) due to a lower stimulated Ra at 10 minutes (3.3 ± 0.1 with INDO; 3.0 ± 0.5 with S; P < 0.05). After 20 minutes Ra was not different in the 3 groups; no overall differences in Rd, glucose clearance, or plasma insulin levels occurred with INDO or S treatment. In conclusion, INDO and S blunt the early (0–20 minutes) E-induced rise in G and Ra without changing Rd, glucose clearance, or insulin secretion. These results suggest that arachidonic acid metabolites may modulate E-induced glycogenolysis.

Epidermal growth factor and potent phorbol tumor promoters induce epidermal growth factor receptor phosphorylation in a similar but distinctively different manner in human epidermoid carcinoma A431 cells.

When human epidermoid carcinoma A431 cells labeled with 32Pi to steady state specific activity were treated either with epidermal growth factor (EGF) or with active phorbol ester tumor promoters such as 12-O-tetradecanoylphorbol 13-acetate (TPA) or phorbol 12,13-dibutyrate, labeling of 160 kDa EGF receptors isolated by immunoprecipitation with monoclonal anti-EGF receptor IgG was increased 2- to 3-fold. These treatments produced no significant increase in 32Pi labeling of acid-precipitable material present in detergent extracts of the cells. Phosphoamino acid analysis of radiolabeled EGF receptors isolated from these cells revealed several differences: the relative abundance of phosphotyrosine in EGF receptors was increased in cells treated with EGF, but decreased in cells treated with TPA; the overall relative abundance of phosphothreonine in EGF receptors was decreased in cells treated with EGF, but remained constant within the limits of experimental detection in cells treated with TPA. Two-dimensional mapping of the radiolabeled phosphopeptides produced from EGF receptors isolated by immunoprecipitation and treated with trypsin revealed 9 independent labeled regions, 2 of which contained phosphothreonine and were present only in EGF- or TPA-treated cells. These two phosphopeptide regions were more highly labeled in cells treated with TPA than with EGF.

Rates of protein synthesis and turnover in fetal life.

Uniformly labeled [14C]lysine was infused at constant rate into the inferior vena cava of eight ovine fetuses with gestational ages ranging from 110 to 145 days. The infusion lasted 9 to 13 h and produced a steady-state specific activity of free lysine in the fetal plasma. In the steady state, approximately 9% of the infused radioactivity was excreted by the fetus as 14CO2, indicating fetal catabolism of lysine. At the end of the infusion, the fetal carcass was analyzed for its total content of labeled and unlabeled lysine. The rate of protein synthesis was calculated from the carcass-to-plasma lysine specific activity ratio. The fractional rate constant (Ks) for the unidirectional flux of lysine into fetal proteins was inversely related (r = -0.88) to fetal age: Ks = 0.584 - 0.0036 age (days). In each fetus, Ks was 2-4 times greater than the estimated fractional rate of fetal protein accretion (KG). The discrepancy between Ks and KG demonstrates that a large fraction of protein synthesis in the ovine fetus is devoted to protein turnover.

The expression of β-galactosidase by Escherichia coli during continuous culture

We have used the technique of continuous culture to study the expression of β-galactosidase in Escherichia coli. In these experiments the cultures were grown on carbon-limited media in which half of the available carbon was supplied as glycerol, glucose, or glucose 6-phosphate, and the other half as lactose. Lactose itself provided the sole source of inducer for the lac operon. The steady-state specific activity of the enzyme passed through a maximal value as a function of dilution rate. Moreover, the rate at which activity was maximal (0.40 h −1) and the observed specific activity of the enzyme at a given growth rate were found to be identical in each of the three media tested. This result was unexpected, since the steady-state specific activity can be shown to be equal to the differential rate of enzyme synthesis, and since it is known that glycerol, glucose, and glucose-6- P-cause different degrees of catabolite repression in batch culture. The differential rate of β-galactosidase synthesis was an apparently linear function of the rate of lactose utilization per milligram protein regardless of the composition of the input medium. That is, it is independent of the rate of metabolism of substrates other than lactose which are concurrently being utilized and the enzyme level appears to be matched to the metabolic requirement for it. If this relationship is taken to indicate the existence of a fundamental control mechanism, it may represent a form of attenuation of the rate of β-galactosidase synthesis which is independent of cyclic AMP levels.

Crystallite size effects in the low-temperature oxidation of ammonia over supported platinum

Abstract The effect of crystallite size on the specific catalytic activity of supported platinum catalysts in ammonia oxidation with molecular oxygen was determined. Rates of ammonia oxidation were measured in a differential, fixed bed, flow reactor between 393 and 473 °K, and catalysts containing average crystallite sizes of 2.0, 2.7 and 15.5 nm (1 nm = 10 A) were used. Nitrogen and nitrous oxide were the only nitrogen-containing products with about four times as much nitrogen as nitrous oxide. The initial specific catalytic activity of the large crystallite catalyst was higher than the 2.0 and 2.7 nm catalyst by factors of 5.7 and 3.7, respectively. All catalysts showed a marked decline in activity in the first 6 hr of operation with steady-state activity being reached in about 12 hr. Smaller crystallites were more severely deactivated so that the resultant steady-state specific activity of the large crystallite catalyst was greater by factors of 14 and 8. Both the specific catalytic activity and the selectivity, also a function of crystallite size, are explained in terms of the changes in average surface concentration of active oxygen with changes in crystallite size. The reaction rate data are best represented by a Langmuir-Hinshelwood model involving dissociative adsorption of both reactants.

Phenylalanine hydroxylase in dilute lethal mice

The steady-state specific activity of hepatic phenylalanine hydroxylase in the 19–20-day-old dilute lethal mouse d 1/ d 1 is 2-fold higher than in its wild-type control D/ D. The apparent K m values for phenylalanine and tetrahyfropteridine are identical for mutant and control enzymes. Acrylamide gel electrophoresis results in three peaks for each of the genotypes D/ D, D/ d 1 and d 1/ d 1. The ratios of these peaks vary for each genotype. The slowest migrating peak is most abundant in d 1/ d 1, however, it is also unstable.

Influences of various amino acids on tryptophan-mediated control of the tryptophan biosynthetic enzymes in Escherichia coli.

Lysates of Escherichia coli Ymel obtained from cultures grown in the absence of tryptophan in minimal medium supplemented with 0.1% casein hydrolysate show an approximate fivefold increase in steady-state specific activity of both anthranilate synthetase and tryptophan synthetase A protein relative to cultures grown in nonsupplemented medium. In the presence of repressing levels of exogenous tryptophan, growth of cultures in casein hydrolysate-supplemented medium results in a noncoordinate enhancement of repression of 10-fold for anthranilate synthetase and twofold for tryptophan synthetase A protein. Similar, but less pronounced, effects are shown for strain W3110. Strains possessing tryptophan regulator gene mutations do not exhibit this first effect, but do yield an approximate twofold decrease in specific activity of both enzymes when grown in medium supplemented with tryptophan and casein hydrolysate. A stimulation of derepression of both enzymes in strain Ymel equivalent to that induced by casein hydrolysate can be reproduced by growth in minimal medium supplemented with threonine, phenylalanine, tyrosine, serine, glutamic acid, and glutamine. Doubling time in this medium is not significantly different from that in minimal medium. An enhancement of repression which partially mimics that observed on growth in medium supplemented with tryptophan plus casein hydrolysate is obtained when Ymel is grown on medium supplemented with tryptophan plus methionine. Threonine or phenylalanine plus tyrosine as separate medium supplements are independently capable of producing a 1.4-fold or 3.4-fold stimulation, respectively, but in combination only the phenylalanine plus tyrosine effect is manifested unless serine and glutamic acid or glutamine are included. Our data show that expression of the tryptophan biosynthetic enzymes can be significantly influenced in vivo as a result of growth in medium supplemented with a variety of amino acids.

Kinetics of potassium in colonic mucosa of normal and sodium‐depleted rats

1. The transport of K across the mucosal epithelium of the colon of normal and Na-depleted rats has been investigated in vivo by methods involving I.V. administration of solutions containing (42)K and by labelling the mucosa with (42)K from the lumen and observing the ;soaking-out' rates into radionucleide-free solutions.2. During the infusions, the specific activity of the mucosal K was always less than the simultaneous specific activity of the secreted K. On stopping the infusion the specific activity of the mucosal K fell although the plasma specific activity remained at all times greater than that of the mucosa. The blood flow rate to the mucosa estimated from these results was 0.95 ml./min.g wet wt.3. Experiments with (42)K in the lumen showed that the steady-state specific activity of the mucosal K was considerably less than expected if the K had been in a single compartment. The (42)K efflux curves were also consistent in showing two components. The curves were analysed on the basis of a model having two compartments in the mucosa with half-times of 6 and 33 min.4. Na depletion produced two significant changes, (a) K secretion rate doubled, (b) the steady-state specific activity of the mucosal K after luminal exposure to (42)K was increased. The half-times of the compartments were unaffected.5. The results were discussed in terms of a working hypothesis which represented the mucosal K in two compartments. Most of the K exchanges across the mucosa took place through the smaller compartment which probably contained 5% or less of the mucosal K, but which had a rapid turnover rate. The change of K permeability of the luminal face produced by Na depletion and probably due to increased aldosterone action, may well be an important factor in producing the rise of K secretion rate.