Resonance assignments for latherin,a natural surfactant protein from horse sweat

Latherin is an intrinsically surfactant protein of ~23 kDa found in the sweat and saliva of horses. Its function is probably to enhance the translocation of sweat water from the skin to the surface of the pelt for evaporative cooling. Its role in saliva may be to enhance the wetting, softening and maceration of the dry, fibrous food for which equines are adapted. Latherin is unusual in its relatively high content of aliphatic amino acids (~25 % leucines) that might contribute to its surfactant properties. Latherin is related to the palate, lung, and nasal epithelium carcinoma-associated proteins (PLUNCs) of mammals, at least one of which is now known to exhibit similar surfactant activity to latherin. No structures of any PLUNC protein are currently available. ¹⁵N,¹³C-labelled recombinant latherin was produced in Escherichia coli, and essentially all of the resonances were assigned despite the signal overlap due to the preponderance of leucines. The most notable exceptions include a number of residues located in an apparently dynamic loop region between residues 145 and 154. The assignments have been deposited with BMRB accession number 19067.     

Isolation and characterization of latherin, a surface-active protein from horse sweat.

A protein, latherin, with unusual surface activity was isolated from horse sweat by gel filtration and ion-exchange chromatography. The protein has a Stokes radius, determined by gel filtration, of 2.47 nm, and in the ultracentrifuge sediments as a single species with S20,W 2.05 S, indicating an Mr of 24,400. On SDS/polyacrylamide-gel electrophoresis the molecule behaves as a single peptide chain of apparent Mr 20,000. Latherin contains a high proportion of hydrophobic amino acids (37.2%), and the leucine content (24.5%) is exceptionally high. The unusual composition of the protein may account for apparent anomalies in the Mr of latherin determined by empirical methods. Evidence indicating that latherin is responsible for much of the surface activity of horse sweat was obtained by a simple assay for surface tension and by contact-angle measurements. Latherin adsorbs very readily at hydrophobic surfaces, rendering them wettable. A possible role for latherin in thermoregulation is proposed.     

Biofoams and natural protein surfactants

Naturally occurring foam constituent and surfactant proteins with intriguing structures and functions are now being identified from a variety of biological sources. The ranaspumins from tropical frog foam nests comprise a range of proteins with a mixture of surfactant, carbohydrate binding and antimicrobial activities that together provide a stable, biocompatible, protective foam environment for developing eggs and embryos. Ranasmurfin, a blue protein from a different species of frog, displays a novel structure with a unique chromophoric crosslink. Latherin, primarily from horse sweat, but with similarities to salivary, oral and upper respiratory tract proteins, illustrates several potential roles for surfactant proteins in mammalian systems. These proteins, together with the previously discovered hydrophobins of fungi, throw new light on biomolecular processes at air-water and other interfaces. This review provides a perspective on these recent findings, focussing on structure and biophysical properties.     

Experimental determination of coefficient of evaporative heat loss in still air (author's transl)]

The authors have determined the coefficient of evaporative heat loss of the human body (he) by means of humidity steps in low air movement (Va less than or equal to 0,2 m/s). Such a determination requires a fully wetted skin and this implies therefore some loss of dripping sweat. The collection of this dripping sweat allows the determination of the total evaporation: this evaporation exists on the skin surface and around the drops during their fall from the skin to the oil pan where dripping sweat is collected. An estimation of this dripping sweat evaporation allows to assess the skin evaporation and, consequently, the evaporative coefficient he. In these experimental conditions: E = S - SNE - 0,0005 SNE (PsH2O - PaH2O) where E is the skin evaporative rate (g/h);S = total sweat rate (g/h);SNE = the nonevaporative sweat rate (g/h);PaH2O = the partial pressure of saturated water (at Ts) on skin (mb) and PaH2O the partial pressure of water vapor in ambient air (mb). The coefficient of evaporative heat loss in low air movement thus found, is 5,18 +/- 0,22 W/m2-mb.     

Study of metabolite composition of eccrine sweat from healthy male and female human subjects by 1H NMR spectroscopy

The aim of the study was to evaluate metabolite variability in human eccrine sweat using a metabonomics based approach. Eccrine sweat is a dilute electrolyte solution whose primary function is to control body temperature via evaporative cooling. Although the composition of sweat is primarily water, previous studies have shown that a diverse array of organic and inorganic compounds are also present. Human eccrine sweat samples from 30 female and 30 male subjects were analysed using high-resolution ¹H nuclear magnetic resonance (NMR) spectroscopy in conjunction with statistical pattern recognition. High-resolution ¹H NMR spectroscopy produced spectra of the sweat samples that readily identified and quantified many different metabolites. The major metabolite classes found to be present were lactate, amino acids and lipids, with lactate being by far the most dominant metabolite found in all samples. Principal Components Analysis, Principal Components-Discriminant Analysis and Partial Least Squares-Discriminant Analysis of the eccrine sweat samples, revealed no significant differences in metabolite composition and concentration between female and male subjects. Also, the variation between subjects did not appear to be correlated with any other clinical information provided by the subjects. Overall, the spectra data set demonstrates the large physiological variability in terms of number of metabolites present and concentrations between subjects i.e. human eccrine sweat samples exhibit a high degree of inter-individual variability.     

Temperature regulation by evaporative cooling in a desert grasshopper, Calliptamus barbarus (Ramme, 1951)

1. 1. We examined the desert-dwelling grasshopper, Calliptamus barbarus, to determine whether it used evaporative cooling, and if differences existed in the use of evaporative cooling between the small males and larger females. Male C. barbarus are the smallest grasshoppers tested for their use of evaporative cooling.

2. 2. Calliptamus barbarus use evaporative cooling at high ambient temperatures to keep their body temperature below lethal levels. This has been shown in insects such as cicadas, bees and other grasshoppers. Maximal water loss rates for C. barbarus are similar (8–10% of body mass per hour) to those of other grasshoppers.

3. 3. Male C. barbarus weigh 370 mg on average, and are 20% of the females' mass. At low ambient temperatures males evaporated 13.31 ± 1.14 mg water/h (n = 12), a similar rate to that in females, who evaporated 17.53 ± 2.03 mg water/h (n = 29), but a considerably greater fraction of body mass per unit time. At high ambient temperatures, the males lost less in absolute terms, but a similar amount relative to body mass. The differences are partially accounted for by scaling effects, but for the most part, the reasons for these differences are unclear. They may be linked to differences in ventilatory patterns between males and females or differences in cuticular permeability, the two major pathways of water loss in insects.

     

Is the evaporative water loss of Knot Calidris canutus higher in tropical than in temperate climates?

To test whether Knot Calidris canutus wintering in the tropics suffer higher rates of water loss through evaporation than do Knot wintering at temperate latitudes, we tried to develop a physically realistic model to predict evaporative heat loss from air temperature, wind and humidity. In separate experiments, involving respirometry and double-labelled water, we tried to estimate relevant parameters. In both sets of experiments, we were able to show significant effects of air temperature on evaporative water loss only. Knot which were able to eat and drink had an evaporative water loss three times that of postabsorptive Knot unable to drink when in a metabolic chamber. Water turnover rates of Knot feeding on bivalves under simulated field conditions were high and did not correlate with predicted evaporative water loss. Over 32 experimental bird-days, the average contribution of predicted evaporative water loss to daily water turnover was 20%. A comparison of predicted evaporative water loss in the north-temperate Dutch Wadden Sea and in the tropical Banc d'Arguin in Mauritania in midwinter showed that Knot wintering in the tropics may need only marginally more water for evaporative cooling than Knot wintering in the Dutch Wadden Sea. Knot foraging on intertidal invertebrates are able to maintain high water turnover rates with little need to drink seawater.     

Temperature control in a continuously mixed bioreactor for solid-state fermentation

A continuously mixed, aseptic paddle mixer was used successfully for solid-state fermentation (SSF) with Aspergillus oryzae on whole wheat kernels. Continuous mixing improved temperature control and prevented inhomogeneities in the bed. Respiration rates found in this system were comparable to those in small, isothermal, unmixed beds, which showed that continuous mixing did not cause serious damage to the fungus or the wheat kernels. Continuous mixing improves heat transport to the bioreactor wall, which reduces the need for evaporative cooling and thus may help to prevent the desiccation problems that hamper large-scale SSF. However, scale-up calculations for the paddle mixer indicated that wall cooling becomes insufficient at the 2-m(3) scale for a rapidly growing fungus like Aspergillus oryzae. Consequently, evaporative cooling will remain important in large-scale mixed systems. Experiments showed that water addition will be necessary when evaporative cooling is applied in order to maintain a sufficiently high water activity of the solid substrate. Mixing is necessary to ensure homogeneous water addition in SSF. Automated process control might be achieved using the enthalpy balance. The enthalpy balance for the case of evaporative cooling in the paddle mixer was validated. This work shows that continuous mixing provides promising possibilities for simultaneous control of temperature and moisture content in solid-state fermentation on a large scale.     

Influence of hydration and airflow on thermoregulatory control in the heat

Exercise-heat exposure results in significant sweat losses due to large biophysical requirements for evaporative heat loss. Progressive body water losses will increase plasma tonicity and decrease blood volume (hypertonic–hypovolemia). The result is reduced dry and evaporative heat exchange through alterations in the core temperature threshold for initiation of skin blood flow and sweating as well as changes in the sensitivity of these thermo-effectors. Regulation of reduced sweating conserves body water, which reduces heat loss and increases exercise hyperthermia, but the magnitude of this effect is modified by environmental heat transfer capabilities. The focus of this paper is to (1) examine the major mechanisms by which hypohydration alters thermoregulatory responses in the heat, and (2) illustrate how important differences in environmental airflow characteristics between laboratory and field settings may modify these effects.     

Is cardiac filling pressure the limiting factor in adjusting to heat stress?

The concept that a specific level of central venous pressure (CVP) limits man's adjustment to heat stress has been debated. Evidence was presented that identifies a true limit of adjustment as being more related to factors affecting evaporative cooling, such as level of hydration, release of active vasodilation substance (AVS), and sweat gland fatigue. However, it was conceded that decreases in CVP and subsequent low-pressure baroreceptor activation modify cutaneous blood flow and subsequently reduce conductance of heat from the core to the periphery. It was suggested that CVP merely reflects a downstream pressure, which must be allowed to reach a pressure lower than that observed in the peripheral venous bed during active cutaneous vasodilation, to insure adequate venous return. However, a loss of evaporative cooling has been observed during prolonged progressive dehydration of subjects in the supine position, resulting in 3 to 4 percent loss of total body weight. This loss of evaporative cooling was not apparent when euhydration was maintained. As it was unlikely that CVP was reduced in these experiments in the supine position, it was concluded that CVP was not the limiting factor in man's adjustment to heat stress.     

Compensatory hyperhidrosis following thoracic sympathectomy: a biophysical rationale

A side-effect of endoscopic thoracic sympathectomy (ETS) is compensatory hyperhidrosis (CH), characterized by excessive sweating from skin areas with intact sudomotor function. The physiological mechanism of CH is unknown, but may represent an augmented local sweat rate from skin areas with uninterrupted sympathetic innervation based on evaporative heat balance requirements. For a given combination of activity and climate, the same absolute amount of evaporation (if any) is needed to balance the rate of metabolic heat production both pre- and post-ETS. However, the rate of local sweating per unit of skin surface area with intact sudomotor activity must be greater post-ETS as evaporation must be derived from a smaller skin surface area. Under conditions with high evaporative requirements, greater degradations in sweating efficiency associated with an increased dripping of sweat should also occur post-ETS, further pronouncing the sweat rate required for heat balance. In conclusion, in addition to the potential role of psychological stimuli for increased sudomotor activity, the existence of CH post-ETS can be described by the interplay between fundamental thermoregulatory physiology and altered heat balance biophysics and does not require a postoperative alteration in physiological control.     

Quantifying irrigation cooling benefits to maize yield in the US Midwest

Irrigation is an important adaptation strategy to improve crop resilience to global climate change. Irrigation plays an essential role in sustaining crop production in water‐limited regions, as irrigation water not only benefits crops through fulfilling crops' water demand but also creates an evaporative cooling that mitigates crop heat stress. Here we use satellite remote sensing and maize yield data in the state of Nebraska, USA, combined with statistical models, to quantify the contribution of cooling and water supply to the yield benefits due to irrigation. Results show that irrigation leads to a considerable cooling on daytime land surface temperature (?1.63°C in July), an increase in enhanced vegetation index (+0.10 in July), and 81% higher maize yields compared to rainfed maize. These irrigation effects vary along the spatial and temporal gradients of precipitation and temperature, with a greater effect in dry and hot conditions, and decline toward wet and cool conditions. We find that 16% of irrigation yield increase is due to irrigation cooling, while the rest (84%) is due to water supply and other factors. The irrigation cooling effect is also observed on air temperature (?0.38 to ?0.53°C) from paired flux sites in Nebraska. This study highlights the non‐negligible contribution of irrigation cooling to the yield benefits of irrigation, and such an effect may become more important in the future with continued warming and more frequent droughts.     

Influence of hydrothermal ambient conditions on sweat evaporation efficiency]

Sweat efficiency is defined as the ratio between evaporative and sweat rates. The work was carried out on two resting subjects acclimatised to humid heat. Body sweat rate and rate of sweat loss by dripping were recorded separately by continuous weighing. Evaporation from the skin was obtained by the difference between the two weight loss curves. The subjects were exposed for 75 minutes to increases in humidity levels as constant air temperatures (42, 44, 46, or 48 degrees C). The amplitude of the increases was successively equal to 7.5, 15.0, 22.5 or 50.0 mb of water vapor pressure. During the 75 minutes preceding each increase the water vapor pressure of the air was maintained at 20.0 mb. 1. Sweat efficiency decreases prior to complete wetting of the skin surface. The inter-individual mean value of the wetted skin area threshold over which sweat efficiency is less than 1 is around 60%. 2. Sweat efficiency is linearly related to the reciprocal of the required wetted skin area (see article). These results are compared with those of other authors. The differences observed are explained in terms of physiological or physical variables involved in the sweat rate control or in the evaporative sweat loss. These include wetness of skin, posture, activity of subjects and the velocity of air over the skin surface.     

Secretory proteins as potential semiochemical carriers in the horse

Two soluble proteins were isolated as major secretory products of horse sweat and of the parotid gland and characterized for structural and functional properties. The first protein, lipocalin allergen EquC1, was characterized for its glycosylation sites and bound glycosidic moieties. Only one (Asn53) of the two putative glycosylation sites within the sequence was post-translationally modified; a different glycosylation pattern was determined with respect to data previously reported. When purified from horse sweat, this protein contained oleamide and other organic molecules as natural ligands. Ligand binding experiments indicated good protein selectivity toward volatile compounds having a straight chain structure of 9-11 carbon atoms, suggesting a role of this lipocalin in chemical communication. The second protein, here reported for the first time in the horse, belongs to the group of parotid secretory proteins, part of a large superfamily of binding proteins whose function in most cases is still unclear. This protein was sequenced and characterized for its post-translational modifications. Of the three cysteine residues present, two were involved in a disulfide bridge (Cys155-Cys198). A model, built up on the basis of similar proteins, indicated a general fold characterized by the presence of a long hydrophobic barrel. Binding experiments performed with a number of different organic compounds failed to identify ligands for this protein with a well-defined physiological role.     

A model of evaporation from the skin while wearing protective clothing

 A simple model was developed to describe the transport of water vapour from subjects working in hot environments while wearing chemical-protective clothing. The goal of the modelling was to obtain a better estimate of evaporative cooling of the subjects, as it was hypothesised that calculations of evaporative heat loss based on changes in dressed weight over-estimate the actual benefit experienced by the subjects. The model employed measured values of vapour pressure within the clothing ensemble to estimate the skin vapour pressure. The resistance of the clothing ensemble to water vapour transport was calculated from measurements of the physical properties of the materials in conjunction with estimates of the resistance of air layers between the clothing layers. The model predicts mean evaporation rates from the skin that are approximately 60% of those calculated from measured changes in dressed weight. Error analysis failed to account for the magnitude of this difference and possible explanations for the difference are advanced. A brief examination of the effect of wicking suggests that some of the difference results from a reduction of the resistance of the garment to water vapour due to wicking of liquid sweat through fabric layers. Received: 4 June 1997 / Accepted: 21 October 1997     

Stomatal Control and Leaf Thermal and Hydraulic Capacitances under Rapid Environmental Fluctuations

Leaves within a canopy may experience rapid and extreme fluctuations in ambient conditions. A shaded leaf, for example, may become exposed to an order of magnitude increase in solar radiation within a few seconds, due to sunflecks or canopy motions. Considering typical time scales for stomatal adjustments, (2 to 60 minutes), the gap between these two time scales raised the question whether leaves rely on their hydraulic and thermal capacitances for passive protection from hydraulic failure or over-heating until stomata have adjusted. We employed a physically based model to systematically study effects of short-term fluctuations in irradiance on leaf temperatures and transpiration rates. Considering typical amplitudes and time scales of such fluctuations, the importance of leaf heat and water capacities for avoiding damaging leaf temperatures and hydraulic failure were investigated. The results suggest that common leaf heat capacities are not sufficient to protect a non-transpiring leaf from over-heating during sunflecks of several minutes duration whereas transpirative cooling provides effective protection. A comparison of the simulated time scales for heat damage in the absence of evaporative cooling with observed stomatal response times suggested that stomata must be already open before arrival of a sunfleck to avoid over-heating to critical leaf temperatures. This is consistent with measured stomatal conductances in shaded leaves and has implications for water use efficiency of deep canopy leaves and vulnerability to heat damage during drought. Our results also suggest that typical leaf water contents could sustain several minutes of evaporative cooling during a sunfleck without increasing the xylem water supply and thus risking embolism. We thus submit that shaded leaves rely on hydraulic capacitance and evaporative cooling to avoid over-heating and hydraulic failure during exposure to typical sunflecks, whereas thermal capacitance provides limited protection for very short sunflecks (tens of seconds).     

Functional differentiation of thermoregulation, with particular references to seasonal variation of whole body sweat rate of exercising man

We investigated the seasonal variation of sweating response during exercise. Four adult healthy men repeated a moderate bicycle exercise (60 watts) in a climatic chamber of an ambient temperature of 30 degrees C (relative humidity, 45%) in winter, spring, summer, and fall. In summer, sweat rate immediately increased as soon as the exercise started, whereas in winter in a few minutes. The mean sweat rate during exercise was significantly different between winter and summer. The transient reduction of the Tsk was observed at the beginning of the exercise in winter. The Tsk decreased in proportion to increasing of sweat rate in each season. Significantly negative correlations were found between sweat rate and the rate of change of Tsk during exercise in each season. The slope and intercept of regression line were significantly different between winter and summer. The index of sweating was made available for the relative value, changing rate against annual mean value of total sweat loss (delta SR, %). The relative value rather than the absolute value (i.e., expressed as g.m-2.h-1) corrected well with skin temperature. It is suggested that the present results may reflect adapted changes in the thermoregulatory mechanisms to seasonal acclimatization. Moreover, the fall in skin temperature during exercise may be not due to increased evaporative cooling, but may be the result of vasoconstriction probably caused by non-thermal factors.     

Consequences of partial body warming and cooling for the drives to local sweat rates

Climatic chamber experiments were carried out on young, healthy male students. The ambient temperature was 36 degrees C, while local warming of one extremity was compensated for by heatflow-equivalent cooling of the ipsilateral extremity by on-line calculation of the heat balance. When warming the arm and cooling the leg (type 1 experiments), a slight, but not statistically significant increase of local sweat rates at these extremities was recorded. However, when cooling the arm and warming the leg (type 2 experiments), both corresponding local sweat rates declined. The divergent results are interpreted in terms of previously reported different central weighting factors for skin temperatures as determined: (1) by the weighting for the area, or (2) by the weighting for the area and the sensitivity of the local sweat rate to warming and cooling. This means that the central processing of the mean skin temperature may be different for cooling and warming and that in both cases values can be different from recorded (area weighted) skin temperature. Calculating this modified mean skin temperature, we conclude that type 1 experiments may be interpreted by the hypothesis that the central regulator has a status very near an overall heat-balance, whereas type 2 experiments, although also carried out at heat-balance, may be centrally evaluated as predominant cooling. In these experiments again the central drives representing the whole body thermal state seem to override both the direct and centrally mediated local drives.     

Bat thermoregulation in the heat: Limits to evaporative cooling capacity in three southern African bats

High environmental temperatures pose significant physiological challenges related to energy and water balance for small endotherms. Although there is a growing literature on the effect of high temperatures on birds, comparable data are scarcer for bats. Those data that do exist suggest that roost microsite may predict tolerance of high air temperatures. To examine this possibility further, we quantified the upper limits to heat tolerance and evaporative cooling capacity in three southern African bat species inhabiting the same hot environment but using different roost types (crevice, foliage or cave). We used flow-through respirometry and compared heat tolerance limits (highest air temperature (T_a) tolerated before the onset of severe hyperthermia), body temperature (T_b), evaporative water loss, metabolic rate, and maximum cooling capacity (i.e., evaporative heat loss/metabolic heat production). Heat tolerance limits for the two bats roosting in more exposed sites, Taphozous mauritianus (foliage-roosting) and Eptesicus hottentotus (crevice-roosting), were T_a = ~44 °C and those individuals defended maximum T_b between 41 °C and 43 °C. The heat tolerance limit for the bat roosting in a more buffered site, Rousettus aegyptiacus (cave-roosting), was T_a = ~38 °C with a corresponding T_b of ~38 °C. These interspecific differences, together with a similar trend for higher evaporative cooling efficiency in species occupying warmer roost microsites, add further support to the notion that ecological factors like roost choice may have profound influences on physiological traits related to thermoregulation.