Osmotic gradient dependence of osmotic water permeability in rabbit proximal convoluted tubule

Summary To assess steady-state transepithelial osmotic water permeability (P _f ), rabbit proximal convoluted tubules were perfused in vitro with the impermeant salt, sodium isethionate at 26°C. Osmotic gradients () were established by varying the bath concentration of the impermeant solute, raffinose. When lumen osmolality was 300 mOsm and bath osmolality was 320, 360 and 400 mOsm, apparentP _f decreased from 0.5 to 0.10 to 0.08 cm/sec, respectively. Similar data were obtained when lumen osmolality was 400 mOsm. Five possible causes of the dependence of apparentP _f were considered experimentally and/or theoretically: (1) external unstirred layer (USL); (2) cytoplasmic USL; (3) change in surface area; (4) saturation of water transport; (5) down-regulation ofP _f . ApparentP _f was inhibited 83% byp-chloromercuribenzene sulfonate (pCMBS) at 20 mOsm, but not at 60 mOsm , suggesting presence of a serial barrier resistance to water transport. Increases in perfusate or bath solution flow rate and viscosity did not alter apparentP _f , ruling out an external USL. A simple cytoplasmic USL, described by a constant USL thickness and solute diffusion coefficient, could not account for the dependence of apparentP _f according to a mathematical model. The activation energy (E _a ) for apparentP _f increased from 7.0 to 12.5 kcal/mol when was increased from 20 to 60 mOsm, not consistent with a simple USL or a change in membrane surface area with transepithelial water flow. These findings are most consistent with a complex cytoplasmic USL, where the average solute diffusion coefficient and/or the area available for osmosis decrease with increasing . These results (1) indicate that trueP _f (at physiologically low ) is very high (>0.5 cm/sec) in the rabbit proximal tubule; (2) provide an explanation for the wide variation inP _f values reported in the literature using different , and (3) suggest the presence of a flow-dependent cytoplasmic barrier to water flow.     

Diffusive water permeability in isolated kidney proximal tubular cells: Nature of the cellular water pathways

Summary The diffusive water permeability (P _d ) of the plasma membrane of proximal kidney tubule cells was measured using a¹H-NMR technique. The values obtained for the exchange time (T _ex) across the membrane were independent of the cytocrit and of the Mn²⁺ concentration (in the range 2.5 to 5mm). At 25°C the calculatedP _d value was (per cm² of outer surface area without taking into account membrane invaginations) 197±17 m/sec. This value equals 22.3±1.9 m/sec when the invaginations are taken into account. Cell exposure to 2.5mm parachloromercuribenzenesulfonic acid,pCMBS, (for 20 to 35 min) reducedP _d to 45% of its control value. Fivemm dithiothreitol, DTT, reverted this effect. The activation energy for the diffusive water flux was 5.2±1.0 kcal/mol under control conditions. It increased to 9.1±2.2 kcal/mol in the presence of 2.5mm pCMBS. Using our previous values for the osmotic water permeability (P _os) in proximal straight tubular cells theP _os/P _d ratio equals 18±1, under control conditions, and 3.2±0.3 in the presence ofpCMBS. These experimental results indicate the presence of pathways for water, formed by proteins, crossing these membranes, which are closed bypCMBS. Assuming laminar flow (within the pore), fromP _os/P _d of 13 to 18 an unreasonably large pore radius of 12 to 15 Å is calculated which would not hinder cell entry of known extracellular markers. Alternatively, for a single-file pore, 11 to 20 would be the number of water molecules which would be in tandem inside the pore. The water permeability remaining in the presence ofpCMBS indicates water permeation through the lipid bilayer. There are similarities between these results and those obtained in human red blood cells and in the apical cell membrane of the toad urinary bladder.     

Osmotic water permeabilities of human placental microvillous and basal membranes

Summary Literature data suggest that water accumulation by the human fetus is driven by osmotic gradients of small solutes. However, the existence of such gradients has not been supported by prior measurements. Attempts to estimate the size of the gradient necessary to drive net water movement have been seriously hampered by the lack of permeability data for the syncytiotrophoblast membranes. Stopped-flow light scattering techniques were employed to measure the osmotic water permeability (P _f )of microvillous (MVM) and basal membrane (BM) vesicles isolated from human term placenta. At 37°C, the P _f was determined to be 1.9±0.06 × 10^+–3 cm/sec for MVM and 3.1±0.20 × 10^+–3 cm/sec for BM (mean ±SD, n = 6). At 23°C, P _f was reduced to 0.7±0.04 × 10^+–3 cm/sec in MVM and 1.6±0.05 × 10^+–3 cm/sec in BM. These P _f values are comparable to those observed in membranes where water has been shown to permeate via a lipid diffusive mechanism. Arrhenius plots of P _f over the range 20–40°C were linear, with activation energies of 13.6 ± 0.6 kcal/mol for MVM and 12.9±1.0 kcal/mol for BM. Water permeation was not affected by mercurial sulfhydryl agents and glucose transport inhibitors. These data clearly suggest that water movement across human syncytiotrophoblast membranes occurs by a lipid diffusion pathway. As noted in several other epithelial tissues, the basal membrane has a higher water permeability than the microvillous membrane. It is speculated that water accumulation by the human fetus could be driven by a solute gradient small enough to be within the error of osmolarity measurements.We thank the staff of the labor and delivery ward at University of San Francisco Medical Center for help in obtaining placental tissue. This work was supported by NIH grant HD 26392. Dr. Jansson was supported by the Sweden-America Foundation, The Swedish Society of Medicine, The Swedish Society for Medical Research, and the Swedish Medical Research Council.     

Permeability and the mechanism of transport of boric acid across the plasma membrane of Xenopus laevis oocytes

Boron is an essential element for vascular plants and for diatoms, cyanobacteria, and a number of species of marine algal flagellates. Boron was recently established as an essential micronutrient for frogs (Xenopus laevis) and preliminary evidence suggests that it may be essential for all animals. The main form of B, which is available in the natural environment, is in the form of undissociated boric acid. The permeability coefficient and the mechanism of transport of boric acid, however, have not been experimentally determined across any animal membrane or cell. In the experiments described here, the permeability coefficient of boric acid in Xenopus oocytes was 1.5 × 10⁻⁶ cm/s, which is very close with the permeability across liposomes made with phosphatidylcholine and cholesterol (the major lipids in the oocyte membrane). Moreover, we investigated the mechanism of boric acid movement across the membrane of Xenopus oocytes and we compared it with the transport across artificial liposomes. The transport of boric acid across Xenopus oocytes was not affected by inhibitors such as HgCl₂, phloretin, or 4,4-diisothiocyanatostilbene-2,2′-d-sulfonic acid (DIDS). The kinetics of B uptake was linear with concentration changes, and the permeability remained the same at different external boric acid concentrations. These results suggest that B transport occurs via simple passive diffusion through the lipid bilayer in Xenopus oocytes.     

Evidence for water channels in renal proximal tubule cell membranes

Summary Water transport mechanisms in rabbit proximal convoluted cell membranes were examined by measurement of: (1) osmotic (P _f ) and diffusional (P _d ) water permeabilities, (2) inhibition ofP _f by mercurials, and (3) activation energies (E _a ) forP _f .P _f was measured in PCT brush border (BBMV) and basolateral membrane (BLMV) vesicles, and in viable PCT cells by stopped-flow light scattering;P _d was measured in PCT cells by proton NMR T_i relaxation times using Mn as a paramagnetic quencher. In BLMV,P _f (0.019 cm/sec, 23°C) was inhibited 65% by 5mm pCMBS and 75% by 300 m HgCl₂ (K _l =42 m);E _a increased from 3.6 to 7.6 kcal/mole (15–40°C) with 300 m HgCl₂. In BBMV,P _f (0.073 cm/sec, 23°C,E _a =2.8 kcal/mole, <33°C and 13.7 kcal/mole, >33°C) was inhibited 65% with HgCl₂ withE _a =9.4 kcal/mole (15–45°C). Mercurial inhibition in BLMV and BBMV was reversed with 10 m mercaptoethanol. Viable PCT cells were isolated from renal cortex by Dounce homogenization and differential seiving. Impedence sizing studies show that PCT cells are perfect osmometers (100–1000 mOsm). Assuming a cell surface-to-volume ratio of 25,000 cm^–1,P _f was 0.010±0.002 cm/sec (37°C) andP _d was 0.0032 cm/sec.P _f was independent of osmotic gradient size (25–1000 mOsm) withE _a 2.5 kcal/mole (<27°C) and 12.7 kcal/mole (>27°C). CellP _f was inhibited 53% by 300 m HgCl₂ (23°C) withE _a 6.2 kcal/mole. These findings indicate that cellP _f is not restricted by extracellular or cytoplasmic unstirred layers and that cellP _f is not flow-dependent. The high BLMV and BBMVP _f , inhibition by HgCl₂, lowE _a which increases with inhibition, and the measuredP _f /P _d >1 in cells in the absence of unstirred layers provide strong evidence for the existence of water channels in proximal tubule brush border and basolateral membranes. These channels are similar to those found in erythrocytes and are likely required for rapid PCT transcellular water flow.     

Effects of PCMBS on the water and small solute permeabilities in frog urinary bladder

Summary It has been reported that PCMBS (p-chloromercuribenzene sulfonate) blocks the water permeability of red cells and of the tubular kidney membranes. In this study we compare the effects of this mercurial compound on the permeability of water and other small solutes in the frog urinary bladder.We observed that: (i) 5mm PCMBS applied at pH 5.0 to the mucosal side inhibited the net and unidirectional water fluxes induced by oxytocin without changing the P _f/P _d ratio. (ii) The oxytocin-induced urea and Na⁺ influxes were also inhibited by PCMBS. (iii) The unidirectional Cl^– movement was first reduced and then increased during the course of PCMBS treatment. (iv) The short-circuit measured at low mucosal Na⁺ concentration (10mm), diminished continuously, whereas the transepithelial resistance first increased and then diminished. (v) Mannitol, raffinose, -methyl-glucose, antipyrine, caffeine and Rb⁺ movements were not changed significantly during the first 26 min of the water permeability inhibition. In conclusion: (i) The ADH-sensitive water, urea and Na⁺ transport systems were inhibited by PCMBS, (ii) PCMBS did not induce a nonspecific and general effect on the permeability of the membrane during the development of the water permeability inhibition, and (iii) in terms of water channels, the inhibition of water transport with the maintenance of a highP _f/P _d ratio suggests that PCMBS closes the water channels in an all or none manner, reducing their operative number in the apical border of frog bladder.     

Osmotic water permeabilities of brush border and basolateral membrane vesicles from rat renal cortex and small intestine

Summary The osmotic water permeabilityP _f of brush border (BBM) and basolateral (BLM) membrane vesicles from rat small intestine and renal cortex was studied by means of stopped-flow spectrophotometry. Scattered light intensity was used to follow vesicular volume changes upon osmotic perturbation with hypertonic mannitol solutions. A theoretical analysis of the relationship of scattered light intensity and vesicular volume justified a simple exponential approximation of the change in scattered light intensity. The rate constants extracted from fits to an exponential function were proportional to the final medium osmolarity as predicted by theory. For intestinal membranes, computer analysis of optical responses fitted well with a single-exponential treatment. For renal membranes a double-exponential treatment was needed, implying two distinct vesicle populations.P _f values for BBM and BLM preparations of small intestine were equal and amount to 60 m/sec. For renal preparations,P _f values amount to 600 m/sec for the fast component, BBM as well as BLM, and to 50 (BBM) and 99 (BLM) m/sec for the slow component. The apparent activation energy for water permeation in intestinal membranes was 13.3±0.6 and in renal membranes, 1.0±0.3 kCal/mole, between 25 and 35°C. The mercurial sulfhydryl reagentpCMBS inhibited completely and reversibly the highP _f value in renal brush border preparations. These observations suggest that in intestinal membranes water moves through the lipid matrix but that in renal plasma membranes water channels may be involved. From the highP _f values of renal membrane vesicles a transcellular water permeability for proximal tubules can be calculated which amounts to 1 cm/sec. This value allows for an entirely transcellular route for water flow during volume reabsorption.     

Flight of the honey bee

Summary Using manometric and gas analytical methods oxygen consumption , carbon dioxide production , respiratory quotientRQ, (Fig. 1A-C) and thorax surface temperature difference T _ts (Fig. 3) were determined in single bees. The animals were either sitting in respiratory chambers or were suspended by the scutum, in which case they were resting, walking (turning a small polystyrene ball) or flying in a closed miniature wind tunnel.During resting (sitting in Warburg vessels) at an ambient temperatureT _a=10°C,RQ was 1.01±0.2 (n=905) with variations due to method (Fig. 1D, E).RQ values during walking were determined in single cases. In no case were they significantly different from 1.00. After the first 10 min of flight meanRQ was 1.00±0.04. It was significantly smaller than 1.00 (RQ=0.97) only during the last 5% of long-time flights (mean flight duration 58.8±28.8 min). With the exception of near-exhaustion conditions no signs of fuels other than carbohydrates were found.Metabolic rateP _m was 19.71±21.38 mW g^–1 during resting at 20°CT _a30°C indicating that many resting bees actively thermoregulate at higherT _a. After excluding bees which were actively thermoregulating, by an approximationP _m was 5.65±2.44 mW g^–1 at 20°CT _a30°C. True resting metabolic rate for sitting bees atT _a=10°C was 1.31±0.53 mW g^–1 (Fig. 2A, B).A significant negative correlation was found between relative (specific) oxygen consumption rel and body massM _b at 85 mgM _b150 mg.At 0°CT _ts16.5°C a significant (-0.01) positive correlation was found between and T _ts in single resting bees: T _Ts+0.099, or betweenP _m and T _ts:P _m=1.343 T _ts+0.581 (Fig. 3D) in ml h^–1,P _m in mW,T in °C).During walking (duration 13.15±5.71 min,n=13) at 12.5°CT _a21°C a stable T _ts of 11.41±3.37°C, corresponding to 167 mW g^–1, was reached for 80 to 90% of the walking time (Fig. 4B).During wind tunnel flights of tethered animals the minimal metabolic power measured in exhaustion experiments was 240 mW g^–1. Calculation of factors of increase inP _m is of limited value in poikilotherms, in which true resting conditions are not exactly defined.     

Temperature functions of thermal death in yeasts and their relation to the maximum temperature for growth

Summary Parameters of thermal death were determined in 10 strains of yeast species whose maximum temperatures for growth (T _max) ranged from 22 to 49°C. Arrhenius plots of the specific thermal death rates (k _d) formed a positional sequence at the level of the experimental points that corrresponded in all but one case to the sequence of the respective T _max values. Extrapolated k _d values at higher or lower temperatures no longer formed this sequence.The correlation of the temperature functions with T _max could be characterized in terms of a new activation parameter, for which the name thermal death activation constant is introduced. It has the following form: T.D.A. – S where H and S are respectively the apparent heat and entropy of activation of thermal death and n is the number of degrees above T _max (expressed in °K) at which the T.D.A. constant exists.Seven mesophilic yeasts had a T.D.A. constant between 72 and 79 calxmol^-1 degree^-1 at n values between 1 and 4°. This suggested that the destructive process that limits k _d in these strains is of the same species as one that contributes to the establishment of T _max. Two psychrophilic yeasts apparently had a similar T.D.A. constant but at a high n value (about 12.5°C) which suggested that in these strains T _max is governed by a destructive process unrelated to the one that underlies thermal death. The strain of the nearly thermophilic Hansenula angusta (T _max 49°C) did not fit in either group.The significance of the T.D.A. constant is discussed and expressions for H and S in terms of bond activation parameters are proposed.     

Temperature insensitive O2 in blood of the tree frogChiromantis petersi

Summary Respiratory gas exchange and blood respiratory properties have been studied in the East-African tree frogChiromantis petersi. This frog is unusually xerophilous, occupies dry habitats and prefers body temperatures near 40°C and direct solar exposure. Total O₂ uptake was low at 81 l O₂·g^–1·h^–1±19.0 (SD) at 25°C increasing to 253.5 l O₂·g^–1·h^–1±94.8 (SD) at 40°C giving aQ ₁₀ value of 2.1. Skin O₂ uptake at 25°C was 38.5% of total. The gas exchange ratio was 0.71 for whole body gas exchange, 0.61 for the lungs and 1.02 for the skin at 25°C.Blood O₂ affinity was low with aP ₅₀ of 47.5 mmHg at 25°C and pH 7.65. Then _H-value at 25°C increased from 2.7 aroundP ₅₀ to 5.0 at O₂ saturations exceeding 70–80%. Surprisingly, blood O₂ affinity was nearly insensitive to temperature expressed by a H value of ±1.0 kcal·mole between 25 and 40°C.The adaptive significance of the low O₂ affinity, the increase ofn _H with O₂ saturation and the temperature insensitive O₂-Hb binding is discussed in relation to the high and fluctuating body temperatures ofChiromantis.     

Evidence for permanent water channels in the basolateral membrane of an ADH-sensitive epithelium

Summary The transepithelial water permeability in frog urinary bladder is believed to be essentially dependent on the ADH-regulated apical water permeability. To get a better understanding of the transmural water movement, the diffusional water permeability (P _d) of the basolateral membrane of urinary bladder was studied. Access to this post-luminal barrier was made possible by perforating the apical membrane with amphotericin B. The addition of this antibiotic increasedP _d from 1.12±0.10×10^–4 cm/sec (n=7) to 4.08±0.33×10^–4 cm/sec (n=7). The effect of mercuric sulfhydryl reagents, which are commonly used to characterize water channels, was tested on amphotericin B-treated bladders. HgCl₂ (10^–3 m) decreasedP _d by 52% andpara-chloromercuribenzoic acid (pCMB) (1.4×10^–4 m) by 34%. The activation energy for the diffusional water transport was found to increase from 4.52±0.23 kcal/mol (n=3), in the control situation, to 9.99±0.91 kcal/mol (n=4) in the presence of 1.4×10^–4 m pCMB. Our second approach was to measure the kinetics of water efflux, by stop-flow light scattering, on isolated epithelial cells from urinary bladders.pCMB (0.5 or 1.4×10^–4 m) was found to inhibit water exit by 91±2%. These data strongly support the existence of proteins responsible for water transport across the basolateral membrane, which are permanently present.     

Response of effective quantum yield of photosystem 2 to in situ temperature in three alpine plants

The response of effective quantum yield of photosystem 2 (F/F_m) to temperature was investigated under field conditions (1 950 m a.s.l.) in three alpine plant species with contrasting leaf temperature climates. The in situ temperature response did not follow an optimum curve but under saturating irradiances [PPFD >800 µìmol(photon) m^–2s^–1] highest F/F_m occurred at leaf temperatures below 10°C. This was comparable to the temperature response of antarctic vascular plants. Leaf temperatures between 0 and 15°C were the most frequently (41 to 56%) experienced by the investigated species. At these temperatures, F/F_m was highest in all species (data from all irradiation classes included) but the species differed in the temperature at which F/F_m dropped below 50% (Soldanella pusilla >20°C, Loiseleuria procumbens >25°C, and Saxifraga paniculata >40°C). The in situ response of F/F_m showed significantly higher F/F_m values at saturating PPFD for the species growing in full sunlight (S. paniculata and L. procumbens) than for S. pusilla growing under more moderate PPFD. The effect of increasing PPFD on F/F_m, for a given leaf temperature, was most pronounced in S. pusilla. Despite the broad diurnal leaf temperature amplitude of alpine environments, only in S. paniculata did saturating PPFD occur over a broad range of leaf temperatures (43 K). In the other two species it was half of that (around 20 K). This indicates that the setting of environmental scenarios (leaf temperature×PPFD) in laboratory experiments often likely exceeds the actual environmental demand in the field.This revised version was published online in March 2005 with corrections to the page numbers.     

Very low osmotic water permeability and membrane fluidity in isolated toad bladder granules

Summary Osmotic water permeability of the apical membrane of toad urinary epithelium is increased greatly by vasopressin (VP) and is associated with exocytic addition of granules and aggrephores at the apical surface. To determine the physiological role of granule exocytosis, we measured the osmotic water permeability and membrane fluidity of isolated granules, surface membranes and microsomes prepared from toad bladder in the presence and absence of VP.P _f was measured by stopped-flow light scattering and membrane fluidity was examined by diphenylhexatriene (DPH) fluorescence anisotropy. In response to a 75mm inward sucrose gradient, granule size decreased with a single exponential time constant of 2.3±0.1 sec (sem, seven preparations, 23°C), corresponding to aP _f of 5×10^–4 cm/sec; the activation energy (E _a ) forP _f was 17.6±0.8 kcal/mole. Under the same conditions, the volume of surface membrane vesicles decreased biexponentially with time constants of 0.13 and 1.9 sec; the fast component comprised 70% of the signal. Granule, surface membrane and microsome time constants were unaffected by VP. However, in surface membranes, there was a small decrease (6±2%) in the fraction of surface membranes with fast time constant. DPH anisotropies were 0.253 (granules), 0.224 (surface membrane fluidity is remarkably lower than that of surface and microsomal membranes, and (4) rapid water transport occurs in surface membrane vesicles. The unique physical properties of the granule suggests that apical exocytic addition of granule membrane may be responsible for the low water permeability of the unstimulated apical membrane.     

Seasonal variation in physiological responses to mild cold air in young and older men

Eight men aged 60–65 years and six men aged 20–25 years, wearing only swimming trunks, were exposed to an air temperature of 17° C and 45% R.H. in each of the four seasons. The increase in the rate of metabolic heat production for the older group in the cold test was significantly higher in summer and autumn than in winter and spring (P<0.05), but did not differ in the young group between seasons. Compared to the young group the was significantly greater for the older group (due to a marked increase in four individuals) in summer and autumn (P<0.04). At the end of the period of cold exposure, the decrements of rectal temperature (T _re), mean skin temperature ( ; due to a marked decrease in four individuals) and foot skin temperature (T _foot) were significantly greater for the older group compared to the young group at all times of the year (P<0.003). Seasonal variations in the two groups were similar, e.g., theT_re gradually became smaller from summer to winter (P<0.05) and then increased slightly in the spring (P=0.07).T _foot for both groups decreased from summer to autumn (P<0.01) and remained unchanged subsequently. No seasonal variations were observed for in either group. The increase in diastolic blood pressure (BP_d) during the test was significantly smaller in winter in both groups (P<0.05). BP_d became larger again during spring in the older group (P<0.01), but remained low in the young group. The BP_d was significantly greater for the older group than the young group in winter and spring (P<0.05). Compared to young men these results suggest that older men may lose the tolerance acquired by earlier cold acclimatization as seen by the BP_d responses, and have a somewhat lower thermoregulatory capability in coping with mild cold air in all seasons.     

Electrolytes control flows of water across the apical barrier in toad skin: The hydrosmotic salt effect

Summary The reversible dependence of skin osmotic water permeability (L _PD ) upon the ionic concentration of the outer bathing solution — which we have called hydrosmotic salt effect (HSE) — was studied in the isolated skin of the toadBufo marinus ictericus. The skin osmotic water flow (J _V ) was measured as a function of outer bathing solution osmolality (O _e ).L _PD , calculated as (J _v /) _P=0 (where and P are the osmotic and hydrostatic pressure differences across the skin, respectively) was constant whenO _e was altered with sucrose, a nonelectrolyte. In contrast,L _PD increased continuously in the hypotonic range asO _e was raised from zero (distilled water) with NaCl or KCl. The HSE could also be evoked in the condition of reversed osmotic volume flow, with the outer bathing medium made hypertonic with sucrose.Diffusional¹⁴C-sucrose permeability, measured in theJ _v =0 condition to prevent solvent drag of sucrose in the paracellular pathways, indicate that the hydrosmotic salt effect cannot be explained by assuming a paracellular permeability increase, due to tight junction opening, but might be interpreted as due to changes in the osmotic water permeability of the apical membranes of the most superficial cells of the epithelium.The hydrosmotic salt effect can be elicited in control skins and in vasopressin-stimulated skins, on top of the hormonal response.The time course of the hydrosmotic salt effect is substantially different from that of the hydrosmotic response to vasopressin. Its half-time is 4 to 5 times faster than that of vasopressin action, with individual values as short as 1.5 min.The time courses of the hydrosmotic salt-effect onset and reversibility are exponential, clearly contrasting with the typical sigmoidal shape of vasopressin onset and washout time courses.Based on time course data and on speed of response we postulate that the mechanism underlying the hydrosmotic salt effect is due to modifications of existing water pathways in the apical membrane, rather than to incorporation and removal of water permeability units in this structure.     

Influence of the temperature on batch cultivation of Candida utilis IZ-1840 on a synthetic medium containing glycerol as the main carbon source

Summary The highest values of the specific growth rate at the exponential phase (0.144 h^-1) and of the yeast cells productivity (0.80 g.L^-1.h^-1) were obtained at 34°C and 30°C, respectively. The cells yield factor decreased from 0.495 to 0.275 when the temperature was increased from 26°C to 42°C.Nomenclature P yeast cells productivity - P yeast cells productivity - r correlation coefficient - S glycerol concentration - t time - t_f duration of the test - T temperature - X yeast cells concentration, dry matter - X₀ initial value of X - X_f final value of X - Y_x/s yeast cells yield - t duration of the exponential phase - _m specific growth rate at the exponential phase     

Brain temperature in pigeons: Effects of anterior respiratory bypass

Summary During heat stress in domestic pigeons (Columba livia, mean mass 0.43 kg) brain temperature (T _B) varied in parallel with colonic temperature (T _c). The difference between these (T _C–T _B=T) averaged 0.7°C and was not significantly altered when the animal breathed through a trachael cannula bypassing the buccopharyngeal cavity. When we sealed the nares and beak in bypass animals, T was significantly reduced but was nevertheless maintained at 0.4°C. When the eyes were sealed as well, however, T was reversed, amounting to –0.4°C. Conversely, with eyes sealed but beak and nares open, T was indistinguishable from that in controls. These results suggest a role for the cornea in evaporative cooling, at least when respiratory evaporation is impaired, and are consistent with the hypothesis that buccopharyngeal and corneal evaporation are coupled to brain cooling. The probable mechanism for this coupling is the flow of venous blood from evaporative surfaces through theretia mirabilia in the temporal areas. Here heat is transferred from the warmer arterial blood flowing through theretia toward the brain to the centrally flowing, cooler venous blood.     

Diurnal changes in the radius of a subalpine Norway spruce stem: their relation to the sap flow and their use to estimate transpiration

Diurnal changes in the stem radius of a subalpine mature Norway spruce were measured simultaneously with the flow of sap in xylem. Matric potentials in the soil were > -35 kPa. The kinetics of the flow were closely related to the changes in the radius of the stem resulting from depletion of its extensible tissues. The radius of the stem oscillated daily and, fairly independently of this, fluctuated over several days. The daily shrinkage (_d) was correlated with the daily flow through its base (Q_d). When the crown transpired little and was nearly saturated during rainy days, ASd tended to increase relative to Q_d. Using a linear relation, the estimates of flow by _d deviated less than ± 10% from the values measured by heat balance, provided that the periods of calibration in their ratio of dry to rainy days were comparable to those estimated. If the two periods differed in this respect, the estimates of flow deviated up to 42%. A quadratic relation yielded estimates that depended less on weather. It reduced maximal deviations to ±22%. Since _d additionally may represent the time pattern of the daily transpiration better than Q_d, analysing changes in the radius of stems may supplement or partly replace measurements of sap flow in stems.     

Lack of Aquaporin 3 in bovine erythrocyte membranes correlates with low glycerol permeation

In general, erythrocytes are highly permeable to water, urea and glycerol. However, expression of aquaporin isoforms in erythrocytes appears to be species characteristic. In the present study, human (hRBC) and bovine (bRBC) erythrocytes were chosen for comparative studies due to their significant difference in membrane glycerol permeability.Osmotic water permeability (P_f) at 23 °C was (2.89 ± 0.37) × 10⁻² and (5.12 ± 0.61) × 10⁻² cm s⁻¹ for human and bovine cells, respectively, with similar activation energies for water transport. Glycerol permeability (P_gly) for human ((1.37 ± 0.26) × 10⁻⁵ cm s⁻¹) differed in three orders of magnitude from bovine erythrocytes ((5.82 ± 0.37) × 10⁻⁸ cm s⁻¹) that also showed higher activation energy for glycerol transport. When compared to human, bovine erythrocytes showed a similar expression pattern of AQP1 glycosylated forms on immunoblot analysis, though in slight higher levels, which could be correlated with the 1.5-fold larger P_f found. However, AQP3 expression was not detectable. Immunofluorescence analysis confirmed the absence of AQP3 expression in bovine erythrocyte membranes.In conclusion, lack of AQP3 in bovine erythrocytes points to the lipid pathway as responsible for glycerol permeation and explains the low glycerol permeability and high E_a for transport observed in ruminants.