The fine structure of the parabronchi and the gas exchange area of the Adelie penguin lung

The parabronchi of the Adelie penguin are endowed with wide atria forming pockets between a loose meshwork of bundles of smooth muscle cells lining the parabronchial lumen. The atrial epithelium is of variable thickness and bears numerous microvilli, which are overlain by/or embedded in sheets or whorls of lamellar material ("trilaminar substance", diameter of one lamella 8 ..10 nm) forming layers of very variable thickness. The cells contain either stacks or whorls of this material or roundish lamellated bodies, and are interconnected by desmosomal contacts as well as what presumably represent tight junctions. Underneath the epithelium and within the bundles of muscle cells regularly nerve fibres have been found. The diameter of the morphological air/blood barrier is about 165...210 nm in thin areas, excluding a 12...20 nm thick layer covering the luminal plasma membrane of the air capillary epithelium. The blood capillary endothelium ordinarily is markedly thicker (40...250 nm) than the air capillary epithelium (17...25 nm). The basal lamina between endo- and epithelium is a uniform structure measuring about 95...105 nm. The endothelial cells are interconnected by desmosomal and probably tight junctions.     

Urate/alpha-ketoglutarate exchange in avian basolateral membrane vesicles

Membrane transport pathways for transcellular secretion of urate across the proximal tubule were investigated in avian kidney. The presence of coupled urate/alpha-ketoglutarate exchange was investigated in basolateral membrane vesicles (BLMV) by [(14)C]urate and [alpha-(3)H]ketoglutarate flux measurements. An inward Na gradient induced accumulation of alpha-ketoglutarate of sufficient magnitude to suggest a Na-dicarboxylate cotransporter. An inward Na gradient also induced concentrative accumulation of urate in the presence of alpha-ketoglutarate but not in its absence, suggesting urate/alpha-ketoglutarate exchange. alpha-Ketoglutarate-dependent stimulation of urate uptake was not observed in brush-border membrane vesicles. An outward urate gradient induced concentrative accumulation of alpha-ketoglutarate. alpha-Ketoglutarate-coupled urate uptake was specific for alpha-ketoglutarate, Cl dependent, and insensitive to membrane potential. alpha-Ketoglutarate-coupled urate uptake was inhibited by increasing p-aminohippurate (PAH) concentrations, and alpha-ketoglutarate-coupled PAH uptake was observed. alpha-Ketoglutarate-coupled PAH uptake was inhibited by increasing urate concentrations, and an outward urate gradient induced concentrative accumulation of PAH. These results suggest a Cl-dependent, alpha-ketoglutarate-coupled anion exchange mechanism as a pathway for active urate uptake across the basolateral membrane of urate-secreting proximal tubule cells.     

Age, area and avian diversification

Using coarse resolution data on the spatial distribution of the entire New World avifauna, we test for phylogenclic patterns in the mean and total geographic range sizes of taxa. The analyses reveal that (i) the species-range size distribution is only approximately normalized, and remains significantly left-skewed, under logarithmic transformation. Most variance in range sizes is explained at the level of species within genera; (ii) there is no effect of the age of taxa on mean clade range size, although older taxa are more likely to have larger total range sizes; (iii) there is some evidence that taxa comprising more species have larger total range sizes; (iv) there is little or no evidence for a relationship between rate of cladogenesis and range size. The results suggest that geographic range size is a labile trait, at least for New World birds, and that the influence of evolutionary history is only weakly detectable in the range size variation of extant taxa, at least at the scale of analysis used here. In addition to these conclusions, two general and important procedural issues emerge.     

Phosphatidylinositol-inositol exchange in a rabbit lung

A microsomal fraction prepared from rabbit lung tissue was found to catalyze CDPdiacylglycerol-independent incorporation of [3H]inositol into phosphatidylinositol. This incorporation resulted from CMP-dependent phosphatidylinositol-inositol exchange and did not constitute a net synthesis of phosphatidylinositol. The phosphatidylinositol-inositol exchange activity was distinct from the phospholipid-base exchange enzymes and was specific for inositol. Optimal in vitro phosphatidylinositol-inositol exchange activity was observed at pH 8.5--8.8 and either Mn2+ or Mg2+ was essential for activity. Mercaptoethanol stimulated phosphatidylinositol-inositol exchange and Hg2+ inhibited this activity. In the absence of CMP, no phosphatidylinositol-inositol exchange was observed. CDP (and to a smaller extent CTP) also supported phosphatidylinositol-inositol exchange and this appeared to occur via the generation of CMP during incubations. The apparent Km values of the phosphatidylinositol-inositol exchange enzyme for CMP and inositol were 0.4 mM and 11 microM, respectively. When CDPdiacylglycerol was present at a concentration optimal for CDPdiacylglycerol : inositol transferase activity, CMP-dependent phosphatidylinositol-inositol exchange activity was still observed. However, in the presence of Hg2+ CDPdiacylglycerol inhibited phosphatidylinositol-inositol exchange activity. Several properties of the phosphatidylinositol-inositol exchange enzyme resemble those of CDPdiacylglycerol : inositol transferase, but the two enzymes appear distinct on the basis of different degrees of inhibition by either Ca2+, Hg/+ or heat, and on the basis of different changes in activity during lung development.     

Gas exchange across avian eggshells oscillates in phase with heartbeat

Rahn et al. (J. Appl. Physiol. 69: 1546-1548, 1990) showed that the gas pressure in a plethysmograph containing an intact egg oscillates in phase with electrocardiogram (ECG) and that this pressure variation could be used as a noninvasive way to determine the heart rate of an avian embryo. One possible mechanism to account for the pressure oscillation is the mechanical movement of the embryonic heart, which leads to volume shifts of gas within the plethysmograph. Another possibility is that the oscillation of gas pressure with heartbeat is pulsatile gas exchange resulting from pulsatile blood flow. If gas exchange were transiently stopped, a pressure signal dependent on gas exchange should disappear, while a pressure signal dependent on cardiovascular motion should persist. Using a number of late-age hen eggs (at days 15-20 of incubation), we tested these hypotheses by suddenly changing the gas composition surrounding an egg and measuring the effect of the pressure oscillation. We found that 1) after 5% CO2-95% N2 was flushed into the plethysmograph (presumably halting gas exchange), pressure oscillations went almost to zero and the ECG signal remained; after air was flushed back to the plethysmograph, the pressure signal returned to control level; 2) after 20% CO2-20% O2-60% N2 was flushed into the plethysmograph (presumably increasing net gas exchange), the pressure signal increased 2.5-fold compared with that in air; and 3) after 1% CO2-99% N2 was flushed into the plethysmograph (presumably reversing gas exchange), the oscillation pressure decreased to one-fourth of that in air and the phase of pressure relative to ECG reversed compared with the phase in air.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sister chromatid exchange in the centromere and centromeric area

Central and peripheral sister chromatid exchanges (SCE) were evaluated separately in human phytohemagglutinin (PHA)-stimulated lymphocytes after culture for 72 h in 5-bromodeoxyuridine (BrdU) containing medium. At the same time, the length of chromosome No. 1 was measured in 10 metaphases per case and the mean value taken as a representative parameter for the contraction of chromosomes. The statistical analysis of regression revealed a close relationship between the percentage of SCE observed in the centromere and the contraction state of chromosomes (P less than or equal to 0.01). A statistically significant increase of central exchanges was seen in more condensed chromosomes, due to the difficulty in differentiating clearly between centric and pericentric exchanges. Consequently, if exchanges in the centromere are omitted from evaluation, this would lead to spuriously low SCE rates in more contracted chromosomes. In order to exclude the variable factor of chromosome contraction in SCE studies, we highly recommend inclusion of counts of central exchanges. Results obtained on chromosomes with twisted chromatids, a situation which tends to stimulate SCE, should be omitted.     

Gas fluxes in avian eggs: Driving forces and the pathway for exchange

• 1.1. In this study, we review reported values for fluxes of water vapor and oxygen across the pores in avian eggshells, pore numbers per egg, the changes in O₂ and CO₂ tensions in the air space during development, and the absolute humidity of bird nests in various climates.
• 2.2. With egg mass from 1 to 1500 g as the independent variable we use regression analysis of daily water vapor loss, O₂ uptake at the preinternal pipping (PIP) stage of development, and pore numbers to show that O₂, CO₂ and water vapor fluxes per pore are 68, 50 and 49 μl/day, respectively, independent of egg mass. When these fluxes are divided by the invariant pore conductance previously established by Ar and Rahn (Respir. Physiol. 61, 1–20, 1985), predicted air cell O₂ and CO₂ tensions of ca 100 and 40 Torr just prior to the initiation of lung function are obtained, values which agree well with mean measured gas tensions in 25 species.
• 3.3. Our analysis complements the model proposed by Ar and Rahn, in which pores serve as basic respiratory units for most bird eggs. In this model, pore number per egg is matched to O₂ demand at the PIP stage of development to produce the commonly observed air cell gas tensions. Average, total diffusive water loss in 117 species is 15%, SD 2.6, of initial egg mass. To achieve this value requires the proper combination of pore number, egg temperature and nest water vapor tension; the latter is also a function of nest construction and incubation behavior. Examples of nest absolute humidity are cited for 20 species which incubate in diverse climates with ambient absolute humidities from 4 to 22 Torr. Exceptions to the model are seen in eggs incubated under environmental conditions which are unusual in temperature, humidity, or altitude.

     

Gas-blood PCO2 gradients during avian gas exchange.

The avian respiratory system is a crosscurrent gas exchange system. One of the aspects of this type of gas exchange system is that end-expired PCO2 is greater than arterial PCO2, the highest possible value being equal to mixed venous PCO2. We made steady-state measurements of arterial, mixed venous, and end-expired PCO2 in anesthetized, spontaneously breathing chickens during inhalation of room air or 4-8% CO2. We found end-expired PCO2 to be higher than both arterial and mixed venous PCO2, the sign of the differences being such as to oppose passive diffusion. The observation that end-expired PCO2 was higher than arterial PCO2 can be explained on the basis of crosscurrent gas exchange. However, the observation that end-expired PCO2 exceeded mixed venous PCO2 must be accounted for by some other mechanism. The positive end-expired to mixed venous PCO2 gradients can be explained if it is postulated that the charged membrane mechanism suggested by Gurtner et al. (Respiration Physiol. 7: 173-187, 1969) is present in the avian lung.     

Role of exchange and dipolar interactions in the radical pair model of the avian magnetic compass

It is not yet understood how migratory birds sense the Earth's magnetic field as a source of compass information. One suggestion is that the magnetoreceptor involves a photochemical reaction whose product yields are sensitive to external magnetic fields. Specifically, a flavin-tryptophan radical pair is supposedly formed by photoinduced sequential electron transfer along a chain of three tryptophan residues in a cryptochrome flavoprotein immobilized in the retina. The electron Zeeman interaction with the Earth's magnetic field (∼50 μT), modulated by anisotropic magnetic interactions within the radicals, causes the product yields to depend on the orientation of the receptor. According to well-established theory, the radicals would need to be separated by >3.5 nm in order that interradical spin-spin interactions are weak enough to permit a ∼50 μT field to have a significant effect. Using quantum mechanical simulations, it is shown here that substantial changes in product yields can nevertheless be expected at the much smaller separation of 2.0 ± 0.2 nm where the effects of exchange and dipolar interactions partially cancel. The terminal flavin-tryptophan radical pair in cryptochrome has a separation of ∼1.9 nm and is thus ideally placed to act as a magnetoreceptor for the compass mechanism.     

Organizing principles of spectro-temporal encoding in the avian primary auditory area field L

The organization of postthalamic auditory areas remains unclear in many respects. Using a stimulus based on properties of natural sounds, we mapped spectro-temporal receptive fields (STRFs) of neurons in the primary auditory area field L of unanesthetized zebra finches. Cells were sensitive to only a subset of possible acoustic features: nearly all neurons were narrowly tuned along the spectral dimension, the temporal dimension, or both; broadly tuned and strongly orientation-sensitive cells were rare. At high stimulus intensities, neurons were sensitive to differences in sound energy along their preferred dimension, while at lower intensities, neurons behaved more like simple detectors. Finally, we found a systematic relationship between neurons' STRFs, their electrophysiological properties, and their location in field L input or output layers. These data suggest that spectral and temporal processing are segregated within field L, and provide a unifying account of how field L response properties depend on stimulus intensity.     

A test of the mechanisms behind avian generalized individuals–area relationships

Aim Questions related to abundances of organisms are central to ecological research. A priori, a scale independent estimation of abundances would be expected. However, we find estimates of numbers of bird individuals from all over the world to increase less than proportionately with increasing plot size. At the whole assemblage level, the pattern holds across biogeographical regions and habitats. The slope of the interspecific and, for the majority of species, the intraspecific individuals–area relationship is also significantly shallower than 1. The question arises as to which mechanisms cause these patterns. Location Global. Methods At the assemblage, interspecific and intraspecific levels, we tested three mechanisms that could be responsible for these patterns by comparing the slope of the individuals–plot area relationship for subsets of a database compiled from the literature. Spatial autocorrelation was controlled for. Results There was no evidence for an influence of plot area choice in order to sample a constant number of individuals. Evidence for higher survey efficiency was available only with increasing number of visits at the intraspecific level. Evidence for influences of habitat heterogeneity was present at the assemblage, interspecific and intraspecific levels. This mechanism can work only if small plots are delimited non‐randomly in homogeneous habitat. Main conclusions Avian population size estimates without indication of the area over which they were obtained are of substantially less value than those coupled with that information. Ecologists planning to compare avian abundances between plots varying in some other factor of interest should minimize variations in their areas and/or account for them in data analyses. Population viability analyses, regional and global population size estimates, site prioritization and the scaling of ecosystem and species energy utilization need to address the plot area effect on assemblage and individual species abundances.