Determination of alveolar-capillary O2 partial pressure gradient by using 15NO

We propose an approach for determining the alveolar-mean capillary oxygen (O(2)) partial pressure gradient to evaluate the efficiency of O(2) equilibration between alveolar space and pulmonary capillary blood. For this purpose, measurements of the pulmonary [(15)N]nitric oxide diffusing capacity are to be interpolated into the recording of O(2) consumption. We expect the O(2) partial pressure gradient amounting to 3.3 mmHg for breathing room air at rest, a third of that commonly given. The simplicity of our method allows its application to children or even artificially ventilated patients. Therefore, it will enable a new insight into pulmonary O(2) equilibration.

Pulmonary 15NO uptake in interstitial lung disease

Because lung nitric oxide (NO) diffusing capacity (DL) represents alveolar-capillary gas diffusion, we queried as to whether disturbances of pulmonary gas exchange in interstitial lung disease (ILD) are appropriately reflected by using NO. In this pilot study, we applied the (15)N-labeled stable isotope (15)NO (relative abundance 0.37% of total NO) in order to ignore the endogenous NO production. In 10 ILD-outpatients, we measured DL (15)NO by performing the single-breath method. Lung function parameters as well as arterial oxygen partial pressure (PaO(2)) were also tested. Values of DL (15)NO ranged within 50-151 ml (15)NO/(mmHg min). Ratios of DL (15)NO/reference were between 43 and 108% of predicted data as taken from our previous work on healthy volunteers [Eur. J. Physiol. 446 (2003) 256]. We found a significant reduction of DL (15)NO/reference in five patients. Additionally, values of PaO(2) were significantly correlated to ratios of DL (15)NO/reference (adjusted R2 +/-SEE=0.407+/-8.051). In conclusion, (15)NO represents an appropriate indicator gas for reflecting an ILD-induced impairment of alveolar-capillary gas exchange.

Real‐Time Detection of Nitric Oxide Isotopes in Lung Function Tests

In lung function tests, the determination of the pulmonary diffusing capacity (D) using the single-breath method is a commonly applied technique. The calculation of D is performed on the basis of accurate measurements of indicator gas concentrations. In this chapter, we demonstrate the appropriateness of the stable nitric oxide (NO) isotopes 14NO and 15NO in revealing reliable data of D. We performed studies on animals (14NO) by using respiratory mass spectrometry (M3) and on humans (15NO) by applying laser magnetic resonance spectroscopy (LMRS). The equipment was characterized by sufficient detection limits of 70 parts/billion at [14NO] = 0.001% (M3) and 40 parts/billion at [15NO] = 0.002 % (LMRS), respectively. Lastly, we were able to show that D-values for 14NO indeed reveal the entire diffusive properties of the alveolar-capillary membrane and that 15NO is a useful indicator gas for reflecting disturbances of pulmonary gas exchange.

Pulmonary 15NO uptake in man

Nitric oxide (NO) is commonly thought to reveal more precise values of pulmonary gas uptake through alveolar-capillary membranes (DL) than the normally used carbon monoxide (CO). Since such measurements are influenced by a significant endogenous NO delivery within human airways, we propose the use of the naturally occurring (15)N-labelled stable nitric oxide isotope (15)NO. It occurs with a relative abundance of 0.37% of the dominating isotope (14)NO. Therefore, the endogenous (15)NO production can be neglected. In the present pilot study we demonstrate the workability of (15)NO in determining DL in healthy individuals. In seven female and 15 male volunteers, averaged values of DL increase with increasing mean alveolar volume as well as individual body height ( P=0.000001). Due to the very high significance level obtained from the multiple regression analysis, we conclude that the application of (15)NO establishes a novel approach to calculate standard values of DL. Such calculations can be employed to predict a reference for patients who suffer from pulmonary diffusion limitation.

NO diffusing capacity of rabbit lungs increases with ventilator-driven lung expansion

In this animal study we evaluated the dependence of the pulmonary diffusing capacity (DL) of nitric oxide (DL,NO) on ventilator-driven increases in alveolar volume. According to the common hypothesis as to whether DL,NO reflects the diffusive properties of the alveolar-capillary membrane, DL,NO should be improved by lung expansion. However, the influence of a simultaneous raise in intrapulmonary pressure is unknown. DL,NO was determined by applying the single-breath method to four anaesthetised paralysed rabbits (weight 3.2-3.5 kg) at various alveolar volumes (38-69 ml) and intrapulmonary pressures (0.15-12.5 mmHg). The animals were ventilated with room air, using a computerised ventilatory servo-system that was also employed to perform the single-breath manoeuvres. Inflation procedures were always started from residual volume (12-13.7 ml), using 0.05% NO in N2 as the indicator gas mixture. End-tidal PNO was determined by respiratory mass spectrometry. DL,NO increased simultaneously with lung expansion and with increasing intrapulmonary pressure (P<0.001). We conclude that the DL,NO provides a measure of the pulmonary diffusive properties that is not influenced by a pressure-induced impairment of pulmonary capillary blood flow.

Rac1 inhibits myogenic differentiation by preventing the complete withdrawal of myoblasts from the cell cycle

The small GTPase protein Rac1 is involved in a wide range of biological processes, yet its role in cell differentiation is mostly unknown. Here we show that Rac1 activity is high in proliferating myoblasts and decreases during the differentiation process. To analyze the involvement of Rac1 in muscle differentiation, different forms of the protein were expressed in muscle cells. A constitutively activated form of Rac1 (Rac1Q61L) inhibited the activity of MyoD in promoting muscle differentiation, whereas a dominant negative form of Rac1 (Rac1T17N) induced the activity of MyoD in promoting muscle differentiation. Expression of Rac1T17N imposed myogenic differentiation on myoblasts growing under mitogenic conditions. In inquiring whether Rac1 affected the withdrawal of myoblasts from the cell cycle, we analyzed the expression of cyclin D1 and p21(WAF1) and the phosphorylation state of the retinoblastoma protein. According to these markers and bromodeoxyuridine incorporation, C2 myoblasts expressing Rac1T17N exited the cell cycle earlier than control C2 cells. Myoblasts expressing Rac1Q61L did not permanently withdraw from the cell cycle. An indication of the possible involvement of the mitogen-activated protein kinase (MAPK) pathway in Rac1-mediated myoblast proliferation was obtained by the use of MAPK kinase inhibitors U0126 and PD098059. These inhibitors arrested C2-Rac1Q61L cell cycling. Taken together, our results show that Rac1 activation interferes with myoblast exit from the cell cycle via or in concert with the MAPK pathway.

A biophysical explanation for Nd:YAG percutaneous laser disc decompression success

OBJECTIVE

To use pre- and postoperative CT density measurements to demonstrate changes achieved by Nd:YAG laser (1,064 nm) in protruded or extruded discs.

BACKGROUND DATA

Nonendoscopic percutaneous laser disc decompression and nucleotomy (PLDN) with the Nd:YAG laser is an established minimal invasive treatment of discogenic pain syndromes in patients with bulging, protruded, or extruded intravertebral discs. Extremely high success rates involving improvement in the flow of cerebrospinal fluid in the dural sac and enhanced venous flow by MRI myelography are 80% lumbar, 86.5% cervical, and 98% thoracic spine. However, verification of reduced intradisc and intraspinal pressure by imaging procedures is difficult.

METHODS

Twenty-one patients, four women and 17 men, with an average age of 43.67 (range, 24-59) were selected. The patients showed lumbar radicular pain syndromes with monosegmental disc protrusion or extrusion, and monoradicular clinical symptoms. Measurements were performed preoperatively and on postoperative day 1.

RESULTS

The preoperative density measurements averaged 83.43, in agreement with the Houndsfield units (HU) described in the literature. Postoperatively, the reduction down to 66.33 HU had a statistical significance of p = 0.001 verified in the t test. Standardizing the measurements prior to laser irradiation to 100% produced an average 20% postoperative density reduction.

CONCLUSION

PLDN with the Nd:YAG laser (1,064 nm) markedly reduces the postoperative density in protrusion and extrusion of intravertebral discs. The excellent therapeutic results achieved with this method in discogenic pain syndromes can be verified by imaging procedures.

Foreign DNA integration--perturbations of the genome--oncogenesis

We have been interested in the consequences of foreign DNA insertion into established mammalian genomes and have initially studied this problem in adenovirus type 12 (Ad12)-transformed cells or in Ad12-induced hamster tumors. Since integrates are frequently methylated de novo, it appears that they might be modified by an ancient defense mechanism against foreign DNA. In cells transgenic for the DNA of Ad12 or for the DNA of bacteriophage lambda, changes in cellular methylation and transcription patterns have been observed. Thus, the insertion of foreign DNA can have important functional consequences that are not limited to the site of foreign DNA insertion. These findings appear to be relevant also for tumor biology and for the interpretation of data derived from experiments with transgenic organisms. For most animals, the main portal of entry for foreign DNA is the gastrointestinal tract. Large amounts of foreign DNA are regularly ingested with the supply of nutrients. Starting in 1987/1988, we have been investigating the fate of orally administered foreign DNA in mice. Naked DNA of bacteriophage M13 and the cloned gene for the green fluorescent protein (GFP) of Aequorea victoria have been used as test molecules. Moreover, the plant-specific gene for the ribulose-1,5-bisphosphate carboxylase (rubisco) has been followed in mice after feeding soybean leaves. At least transiently, food-ingested DNA can be traced to different organs and, after transplacental transfer, to fetuses and newborns. There is no evidence for germ line transmission or for the expression of orally administered GFP DNA.

Foreign DNA integration. Genome-wide perturbations of methylation and transcription in the recipient genomes

In hamster cells transgenic for the DNA of adenovirus type 12 (Ad12) or for the DNA of bacteriophage lambda, the patterns of DNA methylation in specific cellular genes or DNA segments remote from the site of transgene insertion were altered. In the present report, a wide scope of cellular DNA segments and genes was analyzed. The technique of methylation-sensitive representational difference analysis (MS-RDA) was based on a subtractive hybridization protocol after selecting against DNA segments that were heavily methylated and hence rarely cleaved by the methylation-sensitive endonuclease HpaII. The MS-RDA protocol led to the isolation of several cellular DNA segments that were indeed more heavily methylated in lambda DNA-transgenic hamster cell lines. By applying the suppressive subtractive hybridization technique to cDNA preparations from nontransgenic and Ad12-transformed or lambda DNA-transgenic hamster cells, several cellular genes with altered transcription patterns were cloned from Ad12-transformed or lambda DNA-transgenic hamster cells. Many of the DNA segments with altered methylation, which were isolated by a newly developed methylation-sensitive amplicon subtraction protocol, and cDNA fragments derived from genes with altered transcription patterns were identified by their nucleotide sequences. In control experiments, no differences in gene expression or DNA methylation patterns were detectable among individual nontransgenic BHK21 cell clones. In one mouse line transgenic for the DNA of bacteriophage lambda, hypermethylation was observed in the imprinted Igf2r gene in DNA from heart muscle. Two mouse lines transgenic for an adenovirus promoter-indicator gene construct showed hypomethylation in the interleukin 10 and Igf2r loci. We conclude that the insertion of foreign DNA into an established mammalian genome can lead to alterations in cellular DNA methylation and transcription patterns. It is conceivable that the genes and DNA segments affected by these alterations depend on the site(s) of foreign DNA insertion.

The fate of foreign DNA in mammalian cells and organisms

We have investigated the consequences of foreign DNA insertions into the genomes of mammalian cells in transgenic cell lines, in adenovirus type 12 (Ad12)-transformed cells, in Ad12-induced tumor cells or in transgenic mice. We have reported previously on the de novo methylation of integrated foreign genomes and on extensive changes in cellular patterns of DNA methylation upon foreign DNA insertion. These studies have been extended and several independent methods have been applied to document these alterations in cellular DNA methylation and gene expression patterns in transgenic cell lines and in transgenic mice. These data are relevant for the mechanism of (viral) oncogenesis and for the interpretation of data gathered in experiments with transgenic animals.

How to calculate the single-breath nitric oxide diffusing capacity in rabbits

Nitric oxide (NO) is a novel indicator gas for investigating alveolar capillary gas exchange conditions. In clinical practice, pulmonary gas uptake is determined by measuring the single-breath diffusing capacity (DL,NO). Different algorithms are employed to calculate DL,NO. To compare the accuracy of those most commonly used, we performed single-breath experiments on 12 artificially ventilated rabbits. In each animal four manoeuvres, executing breath-holds of 2, 4, 6 and 8 s, were carried out. In each case we administered 55 ml of an indicator gas mixture containing 0.05% NO. Alveolar gas was analysed by respiratory mass spectrometry. The two algorithms for calculating DL,NO based on the conventional solution of the breath-holding equation [Ogilvie et al. (1957) J Clin Invest 36:1-17 and Jones and Meade (1961) Q J Exp Physiol 46:131-143], were compared with the three-equation technique [Graham et al. (1980) IEEE Trans Biomed Eng 27:221-227] as the reference. The deviation between DL,NO calculated from the conventional methods and the reference decreased linearly with increasing duration of NO uptake (deltat). The mean deviations declined from 16.6% (Jones and Meade) or 7.7% (Ogilvie) at deltat=4 s to 5.7% (Jones and Meade) or 2.4% (Ogilvie) at deltat=10 s. The larger mean values are due to the conventional solution where three-tenths of the inflation time is subtracted from deltat. These findings qualify the common prediction that the latter method yields DL,NO values of the highest accuracy. We therefore recommend Ogilvie's procedure if the three-equation technique cannot be employed.

An algebraic solution to dead space determination according to Fowler's graphical method

According to Fowler's method, anatomical dead space (VD) can be determined graphically or computer-aided by iteration procedures by which phase III of a fraction-volume expirogram F(V) is back-extrapolated by a straight line R(V). Whereas Fowler visually partitioned phase II into two equal areas bordered by F(V), R(V), and VD, in the present paper the area between F(V) and R(V) is set equal to the area of a trapezoid, one side of which is the unknown VD to be determined. We obtained two algebraic equations for both possible conditions, nonsloping and sloping alveolar plateau, and, as the main result, an even more general third equation that includes both Bohr's and Fowler's solution. The formulas exactly represent Fowler's graphical method and can be applied to all gases which are applicable in dead space determination. The derived equations were tested in experimental situations, showing equality between values of dead space determined by using the algebraic solution and the graphical method. Their major advantage is facilitating and speeding up computer-aided on-line determinations of VD.

Insertion of foreign DNA into an established mammalian genome can alter the methylation of cellular DNA sequences

The insertion of adenovirus type 12 (Ad12) DNA into the hamster genome and the transformation of these cells by Ad12 can lead to marked alterations in the levels of DNA methylation in several cellular genes and DNA segments. Since such alterations in DNA methylation patterns are likely to affect the transcription patterns of cellular genes, it is conceivable that these changes have played a role in the generation or the maintenance of the Ad12-transformed phenotype. We have now isolated clonal BHK21 hamster cell lines that carry in their genomes bacteriophage lambda and plasmid pSV2neo DNAs in an integrated state. Most of these cell lines contain one or multiple copies of integrated lambda DNA, which often colocalize with the pSV2neo DNA, usually in a single chromosomal site as determined by the fluorescent in situ hybridization technique. In different cell lines, the loci of foreign DNA insertion are different. The inserted bacteriophage lambda DNA frequently becomes de novo methylated. In some of the thus-generated hamster cell lines, the levels of DNA methylation in the retrotransposon genomes of the endogenous intracisternal A particles (IAP) are increased in comparison to those in the non-lambda-DNA-transgenic BHK21 cell lines. These changes in the methylation patterns of the IAP subclone I (IAPI) segment have been documented by restriction analyses with methylation-sensitive restriction endonucleases followed by Southern transfer hybridization and phosphorimager quantitation. The results of genomic sequencing experiments using the bisulfite protocol yielded additional evidence for alterations in the patterns of DNA methylation in selected segments of the IAPI sequences. In these experiments, the nucleotide sequences in >330 PCR-generated cloned DNA molecules were determined. Upon prolonged cultivation of cell lines with altered cellular methylation patterns, these differences became less apparent, perhaps due to counterselection of the transgenic cells. The possibility existed that the hamster BHK21 cell genomes represent mosaics with respect to DNA methylation in the IAPI segment. Hence, some of the cells with the patterns observed after lambda DNA integration might have existed prior to lambda DNA integration and been selected by chance. A total of 66 individual BHK21 cell clones from the BHK21 cell stock have been recloned up to three times, and the DNAs of these cell populations have been analyzed for differences in IAPI methylation patterns. None have been found. These patterns are identical among the individual BHK21 cell clones and identical to the patterns of the originally used BHK21 cell line. Similar results have been obtained with nine clones isolated from BHK21 cells mock transfected by the Ca2+-phosphate precipitation procedure with DNA omitted from the transfection mixture. In four clonal sublines of nontransgenic control BHK21 cells, genomic sequencing of 335 PCR-generated clones by the bisulfite protocol revealed 5'-CG-3' methylation levels in the IAPI segment that were comparable to those in the uncloned BHK21 cell line. We conclude that the observed changes in the DNA methylation patterns in BHK21 cells with integrated lambda DNA are unlikely to preexist or to be caused by the transfection procedure. Our data support the interpretation that the insertion of foreign DNA into a preexisting mammalian genome can alter the cellular patterns of DNA methylation, perhaps via changes in chromatin structure. The cellular sites affected by and the extent of these changes could depend on the site and size of foreign DNA insertion.

Nitric oxide used to test pulmonary gas exchange in rabbits

To evaluate whether nitric oxide (NO) is an appropriate test gas for assessing pulmonary gas exchange, we determined the rates of disappearance from the alveolar space (lambda) of NO and singly and doubly 18O-labelled carbon dioxide (C16O18O, C18O2) by performing single-breath manoeuvres on seven artificially ventilated rabbits. By exploiting unique features of both isotopic species and by analysing pulmonary gas transport and lambda values with a commonly used model, we found that diffusion forms 98+/-6% (mean +/- SD) of the overall resistance to alveolar-capillary NO transfer. This means that measurements of pulmonary NO uptake reveal the entire diffusive properties of the alveolar-capillary membrane, because the extremely fast binding of NO to haemoglobin negates the "reactive" component within red blood cells of pulmonary capillaries.

Single-breath CO diffusing capacity influenced by initial alveolar partial pressure of CO

The purpose of this study was to assess the influence of incorrect determinations of the initial alveolar partial pressure of carbon monoxide (CO) at the beginning of breath holding (PIACO) on the pulmonary CO diffusing capacity of the lung (DLCO). Single-breath maneuvers were performed on 14 anesthetized and artificially ventilated rabbits, using 0.2% CO in nitrogen as the indicator gas mixture. Inflation and deflation procedures were carried out in an identical manner on each animal, with inflation always starting from residual volume. End-tidal partial pressure of CO was determined by respiratory mass spectrometry and was used to calculate DLCO values with the application of the three-equation (method 1), as well as the conventional (method 2), solution. In each rabbit, method 2 caused DLCO values to be overestimated when compared with method 1, and this overestimation decreased with increasing time intervals of CO uptake. Because we were able to recalculate this deviation using PIACO values that were obtained by taking the diffusive removal of CO during inflation into account, we concluded that errors in estimating PIACO by applying method 2 significantly contribute to the discrepancy between both methods.

Role of reaction resistance in limiting carbon monoxide uptake in rabbit lungs

The contribution of reaction resistance to overall resistance to pulmonary carbon monoxide (CO) uptake [DLCO/(ThetaCO . Vc), where DLCO is lung CO diffusing capacity, ThetaCO is CO uptake conductance of erythrocytes, and Vc is pulmonary capillary blood volume] was determined in 10 anesthetized, paralyzed, and artificially ventilated rabbits. On the basis of the classical double-reciprocal equation of F. G. W. Roughton and R. E. Forster (J. Appl. Physiol. 11: 290-302, 1957), DLCO/(ThetaCO . Vc) was obtained by solving the relation DLCO/(ThetaCO . Vc) = 1 - 2/(DLNO/DLCO), where DLNO/DLCO represents the ratio between the respective single-breath diffusing capacities (DL) of nitric oxide (NO) and CO pulmonary capillary blood. The lungs of eight rabbits were inflated, starting from residual volume, by using 55 ml of indicator gas mixture (0.2% CO and 0.05% NO in nitrogen). DL values were calculated by taking the end-tidal partial pressures of CO and NO as analyzed by using a respiratory mass spectrometer. The overall value was DLCO/(ThetaCO . Vc) = 0.4 +/- 0.025 (mean +/- SD). Because of the use of O2-free indicator gas mixtures, the end-tidal O2 partial pressures were approximately 21 Torr. In one other rabbit, the application of 0.2% CO and 0.001% NO yielded DLCO/(ThetaCO . Vc) = 0.39; in the tenth rabbit, however, inspiratory volume was varied, and an identical value was found at functional residual capacity. We conclude that the contribution of reaction resistance to overall resistance to pulmonary CO uptake is independent of the inspiratory NO concentration used, including, with respect to the pertinent literature, the conclusion that in rabbits, dogs, and humans this contribution amounts to 40% when determined at functional residual capacity.

Respiratory physiology teaching: determination of residual volume by applying the indicator-dilution technique

Apart from the current teaching of spirometric methods in laboratory courses on respiratory physiology, we have included an experiment in which medical students determine their own residual volume by applying the indicator-dilution technique. For hygienic reasons we used a bag-in-the-box system to dilute helium within alveolar space by performing the single-breath method. Although each participant independently underwent only one single-breath maneuver, we gained a reliable relationship between residual volume and subjects' height and body weight in 68 female (r = 0.6, P < 0.0001) and 99 male (r = 0.42, P < 0.0001) students. From this successful outcome and with the opportunity to discuss the limitations of the single-breath method as well, we inferred that this experiment affords a transparent and instructive approach to interpreting the determination of lung volumes on the basis of the indicator-dilution technique.

Endotoxemia affects lung vascular prostanoid metabolism in rabbits

The following is the abstract of the article discussed in the subsequent letter:

Steudel, Wolfgang, Hans-Joachim Krämer, Daniela Degner, Simone Rosseau, Hartwig Schütte, Dieter Walmrath, and Werner Seeger. Endotoxin priming of thromboxane-related vasoconstrictor responses in perfused rabbit lungs. J. Appl. Physiol. 83(1): 18–24, 1997.—In prior studies of perfused lungs, endotoxin priming markedly enhanced thromboxane (Tx) generation and Tx-mediated vasoconstriction in response to secondarily applied bacterial exotoxins. The present study addressed this aspect in more detail by employing precursor and intermediates of prostanoid synthesis and performing functional testing of vasoreactivity and measurement of product formation. Rabbit lungs were buffer perfused in the absence or presence of 10 ng/ml endotoxin. Repetitive intravascular bolus applications of free arachidonic acid provoked constant pulmonary arterial pressor responses and constant release reactions of TxA2 and prostaglandin (PG) I2 in nonprimed lungs. Within 60–90 min of endotoxin recirculation, which provoked progressive liberation of tumor necrosis factor-α but did not effect any hemodynamic changes by itself, both pressor responses and prostanoid release markedly increased, and both events were fully blocked by cyclooxygenase (Cyclo) inhibition with acetylsalicylic acid (ASA). The unstable intermediate PGG2 provoked moderate pressor responses, again enhanced by preceding endotoxin priming and fully suppressed by ASA. Vasoconstriction also occurred in response to the direct Cyclo product PGH2, again amplified after endotoxin pretreatment, together with markedly enhanced liberation of TxA2 and PGI2. In the presence of ASA, the priming-related increase in pressor responses and the prostanoid formation were blocked, but baseline vasoconstrictor responses corresponding to those in nonprimed lungs were maintained. Pressor responses to the stable Tx analog U-46619 were not significantly increased by endotoxin pretreatment, but some generation of TxA2 and PGI2 was also noted under these conditions. We conclude that endotoxin priming exerts profound effects on the lung vascular prostanoid metabolism, increasing the readiness to react with Tx-mediated vasoconstrictor responses to various stimuli, suggesting that enhanced Cyclo activity is an important underlying event.

Decreasing diffusion limitation on exercise in lower-graded interstitial lung disease

In this study we investigated the contribution of diffusion limitation to the exercise-induced hypoxaemia in interstitial lung disease (ILD). We applied isotopic analysis to the composition of the stable isotopic oxygen molecules 16O2 and 16O18O in expiratory gas mixtures obtained from six ILD patients and six healthy subjects at rest and during ergometer work (60 W). The changes in the 16O18O/16O2 ratios were interpreted by using the overall fractionation factor of respiration (alpha O) which would be increased towards 1.03 on increasing diffusion limitation. In addition, the O2 partial pressures of alveolar gas and arterial blood (PAO2, PaO2) were determined. In the patients, alpha O was significantly reduced from 1.0066 +/- 0.0004 (mean +/- SD) at rest to 1.0035 +/- 0.0004 during exercise and in the healthy subjects from 1.0072 +/- 0.0008 to 1.0044 +/- 0.0004. Furthermore, the exercise-induced reduction of PaO2 (from 77 to 69 mmHg) was due to a drop of alveolar PO2 found in each patient, whereas in each healthy subject PaO2 was increased on exercise. On the basis of a resistance model we conclude that the patients' data were inconsistent with increasing diffusion limitation but showed an increasing impairment of O2 transport by ventilation.

Theta values for C16O18O and C18O2 related to respective pulmonary diffusing capacities

The single-breath diffusing capacities for singly and doubly 18O-labeled CO2, DLC16O18O and DLC18O2, as well as for NO, were determined in seven anesthetized rabbits to investigate whether the theoretically predicted ratio of specific blood uptake rates of both isotopic CO2 species, theta C18O2/theta C16O18O = 2.0, can be derived from the measured values of DLC16O18O and DLC18O2. Data of DL were obtained by inflating the lungs with gas mixtures containing 0.35% C16O18O or 0.8% C18O2 or 0.05% NO in nitrogen, with breath-holding periods of 0.05-0.5 s and 2-12 s for the CO2 and NO tests, respectively. theta C18O2/theta C16O18O was calculated by applying the double-reciprocal Roughton-Forster equation to DL values obtained in each animal and by assuming that NO diffusing capacity represents the gas conductance of the alveolar-capillary membrane. The measured ratio was theta C18O2/theta C16O18O = 1.9 +/- 0.2 (mean +/- SD), thus comparing reasonably with the predicted one. Therefore, our findings provide evidence that the greater value of DLC18O2 is mainly due to the twofold higher probability (or theta value) for C18O2 than for C16O18O to disappear within red blood cells via isotopic exchange reactions.

TFIID (TBP) stabilizes the binding of MyoD to its DNA site at the promoter and MyoD facilitates the association of TFIIB with the preinitiation complex

The myogenic determination factor MyoD activates the transcription of muscle-specific genes by binding to consensus DNA sites found in the regulatory sequences of these genes. The interaction of MyoD with the basal transcription machinery is not known. Several activators induce transcription by recruiting TFIID and/or TFIIB to the promoter. We asked whether MyoD interacted functionally with TFIID and TFIIB in transcription. We reconstituted in vitro DNA binding and transcription systems of MyoD and basal transcription factors, and found that MyoD function in transcription occurred during the assembly of the preinitiation complex. Interestingly, MyoD activated transcription without affecting the binding of TFIID to the promoter. However, TFIID or TBP dramatically stabilized the binding of MyoD to its recognition site. MyoD and TBP interacted in solution. Deletion analysis of MyoD suggested that interaction of MyoD with TBP is needed for its activity in transcription. At a later stage of assembly, MyoD stabilized the binding of TFIIB to the preinitiation complex. These findings suggest that MyoD is involved in two steps of preinitiation; first, TFIID stabilizes MyoD binding to its DNA recognition site and at a later stage MyoD facilitates the association of TFIIB with the preinitiation complex.