Physiological and morphological characteristics of the rhythmic contractions of the amnion in veiled chameleon (Chamaeleo calyptratus) embryogenesis

A morpho-functional study of the veiled chameleon (Chamaeleo calyptratus) amnion was performed as part of a general comparative study of amnion rhythmic contractions (ARC) in reptile and bird embryogenesis. Eggs incubated at 27.5 degrees C were used at different developmental stages from day 80 (D80) to D184 for the recording of ARC using a force transducer. Slow ARC, about 1 min in duration, were revealed from D88 (stage 31) to the near-hatching stages (incubation time was 183-198 days). The frequency and amplitude of slow ARC increased significantly towards the end of incubation. This ARC enhancement correlated with the differentiation of smooth muscle elements in the amniotic membrane from single spindle-shaped cells to complex "star-like" structures and with the development of the actin fibers, revealed by phalloidin, in the amniotic muscle layer. Short-term changes in temperature influenced ARC and heart rate (HR). Cooling to 25 degrees C from the control (27.5 degrees C) resulted in a significant decrease in both ARC frequency and HR. Heating to 30 degrees C significantly increased the embryonic HR, but not ARC frequency. Within the temperature range from 25 to 30 degrees C the temperature effect on ARC frequency and HR was reversible.     

Cardiac rhythms of late pre-pipped and pipped chick embryos exposed to altered oxygen environments

During the final stages of embryonic development in chickens, diffusive gas exchange through the chorioallantoic membrane (CAM) is progressively replaced by pulmonary respiration that begins with internal pipping (IP) of the CAM. Late chick embryos going through the transition from CAM respiration to pulmonary respiration were exposed to hyperoxic (100% O(2)) and hypoxic (10% O(2)/N(2)) environments for 2-h and the responses of baseline heart rate (HR), and HR fluctuation patterns were investigated. 16- and 18-day-old (referred to as 18-d) embryos and 20-d externally pipped (EP) embryos were examined as pre-pipped embryos and pipped embryos, respectively. 19-d embryos were divided into two groups: embryos that had not yet internally pipped (Pre-IP embryos) and embryos that had internally pipped (IP embryos). IP was identified by detecting the breathing signal with a condenser microphone attached hermetically on the eggshell (i.e. acoustorespirogram) on day 19 of incubation. In the hyperoxic environment, HR baseline of pre-pipped embryos remained unchanged and that of pipped embryos was depressed. In the hypoxic environment, HR baseline of 16-d pre-pipped embryos was depressed and that of pipped (IP and EP) embryos elevated. These different responses in pipped embryos might be partially attributed to increased cholinergic input from the vagus nerve in hyperoxia and increased adrenergic response in hypoxia. While hyperoxia did not induce marked modification of instantaneous heart rate (IHR) fluctuation patterns, hypoxia tended to augment transient decelerations of IHR in late pre-pipped embryos and markedly depressed HR fluctuations in pipped embryos.     

Gas exchange and energy metabolism of the ostrich (Struthio camelus) embryo.

We measured oxygen consumption (V(O(2))) and carbon dioxide emission (V(CO(2))) rates, air-cell gas partial pressures of oxygen (P(A)O(2)) and CO(2) (P(A)CO(2)), eggshell water vapour conductance and energy content of the ostrich (Struthio camelus) egg, 'true hatchling' and residual yolk, and calculated RQ and total oxygen consumption (V(O(2)tot)) for ostrich eggs incubated at 36.5 degrees C and 25% relative humidity. The V(O(2)) pattern showed a drop of approximately 5% before internal pipping. V(O(2)) just prior to internal pipping agrees with allometric calculations. Despite the higher incubation temperature compared to other studies, and the resultant shorter incubation duration (42 days), V(O(2)tot) (91.7 l kg(-1)) was similar to a previously reported value. RQ values during the second half of incubation (approx. 0.68) were lower than expected for lipid catabolism. Prior to internal pipping, P(A)O(2) and P(A)CO(2) were 98 and 48.3 torr (13.1 and 6.4 kPa), respectively. The growth pattern of the ostrich embryo is different from the typical precocial pattern, showing a time delay in the rapid growth phase. As a result, the lowered overall energy expenditure for tissue maintenance, as compared to other species, is reflected in the low yolk utilization and high residual yolk fraction of the whole hatchling dry mass. These could also result from the relatively short incubation period of the ostrich egg, thereby evading desiccation by excess water loss.     

Embryonic heart rate and oxygen pulse in two procellariiform seabirds,Diomedea immutabilis and Puffinus pacificus

Developmental patterns of embryonic heart rate were measured non-invasively in two procellariiform seabirds, the Laysan albatross (Diomedea immutabilis) and wedge-tailed shearwater (Puffinus pacificus), during prepipping and after pipping. The O₂ pulse, defined as the O₂ consumption per single heart beat, was calculated using the previously reported O₂ consumption for these species. The embryonic heart rate of the albatross was not changed by internal pipping (initial pipping event in this species), remained unchanged during the prolonged internal pipping period and tended to increase, although insignificantly, with the initiation of external pipping (second pipping event). Heart rate in the shearwater remained unchanged during the late prepipping stages, did not change with external pipping (initial pipping event), but increased during the prolonged internal pipping period (second pipping event) and reached a maximum on the last day of incubation. The developmental pattern of heart rate in the shearwater was very similar to that reported previously for the brown noddy, a member of the order Charadriiformes but with the same pipping sequence. Developmental patterns of embryonic O₂ pulse were also different between the two procellariiform seabirds. However, those of the shearwater and the noddy were similar. The sequence of access to atmospheric O₂ during development and hatching may be an important factor determining the developmental patterns of embryonic heart rate and O₂ pulse.Abbreviations A-VO₂ difference, arteriovenous oxygen difference - BCG ballistocardiogram of egg - CV coefficient of variation - EP external pipping - HR heart rate - IP internal pipping - mass mass of freshly laid egg - SD standard deviation     

Scaling the amplitudes of the circadian pattern of resting oxygen consumption, body temperature and heart rate in mammals

We questioned whether the amplitudes of the circadian pattern of body temperature (T(b)), oxygen consumption (V (O(2))) and heart rate (HR) changed systematically among species of different body weight (W). Because bodies of large mass have a greater heat capacitance than those of smaller mass, if the relative amplitude (i.e., amplitude/mean value) of metabolic rate was constant, one would expect the T(b) oscillation to decrease with the increase in the species W. We compiled data of T(b), V (O(2)) and HR from a literature survey of over 200 studies that investigated the circadian pattern of these parameters. Monotremata, Marsupials and Chiroptera, were excluded because of their characteristically low metabolic rate and T(b). The peak-trough ratios of V (O(2)) (42 species) and HR (35 species) averaged, respectively, 1.57+/-0.08, and 1.35+/-0.07, and were independent of W. The daily high values of T(b) did not change, while the daily low T(b) values slightly increased, with the species W; hence, the high-low T(b) difference (57 species) decreased with W (3.3 degrees C.W(-0.13)). However, the decrease in T(b) amplitude with W was much less than expected from physical principles, and the high-low T(b) ratio remained significantly above unity even in the largest mammals. Thus, it appears that in mammals, despite the huge differences in physical characteristics, the amplitude of the circadian pattern is a fixed (for V (O(2)) and HR), or almost fixed (for T(b)), fraction of the 24-h mean value. Presumably, the amplitudes of the oscillations are controlled parameters of physiological significance.     

Respiratory and cardiovascular responses to acute hypoxia and hyperoxia in internally pipped chicken embryos.

During the first day of hatching, the developing chicken embryo internally pips the air cell and relies on both the lungs and chorioallantoic membrane (CAM) for gas exchange. Our objective in this study was to examine respiratory and cardiovascular responses to acute changes in oxygen at the air cell or the rest of the egg during internal pipping. We measured lung (VO2(lung)) and CAM (VO2(CAM)) oxygen consumption independently before and after 60 min exposure to combinations of hypoxia, hyperoxia, and normoxia to the air cell and the remaining egg. Significant changes in VO2(total) were only observed with combined egg and air cell hypoxia (decreased VO2(total)) or egg hyperoxia and air cell hypoxia (increased VO2(total)). In response to the different O2 treatments, a change in VO2(lung) was compensated by an inverse change in VO2(CAM) of similar magnitude. To test for the underlying mechanism, we focused on ventilation and cardiovascular responses during hypoxic and hyperoxic air cell exposure. Ventilation frequency and minute ventilation (V(E)) were unaffected by changes in air cell O2, but tidal volume (V(T)) increased during hypoxia. Both V(T) and V(E) decreased significantly in response to decreased P(CO2). The right-to-left shunt of blood away from the lungs increased significantly during hypoxic air cell exposure and decreased significantly during hyperoxic exposure. These results demonstrate the internally pipped embryo's ability to control the site of gas exchange by means of altering blood flow between the lungs and CAM.     

Hypohydration adversely affects lactate threshold in endurance athletes

The purpose of this investigation was to observe the effect of hypohydration (-4% body mass) on lactate threshold (LAT) in 14 collegiate athletes (8 men and 6 women; age, 20.9 +/- 0.5 years; height, 171.1 +/- 2.4 cm; weight, 64.8 +/- 2.3 kg; V(O)2 max, 62.8 +/- 1.9 ml x kg(-1) x min(-1); percentage of fat, 11.4 +/- 1.5%). Subjects performed 2 randomized, discontinuous treadmill bouts at a dry bulb temperature (T(db)) of 22 degrees C to volitional exhaustion in 2 states of hydration, euhydrated and hypohydrated. The hypohydrated condition was achieved in a thermally neutral environment (T(db), 22 degrees C; humidity, 45%), with exercise conducted at a moderate intensity as defined by rating of perceived exertion (RPE, approximately 12) 12-16 hours before testing. On average, subjects decreased 3.9% of their body mass before the hypohydration test. Blood lactate, hematocrit, V(O)2, minute ventilation (VE), R value, heart rate (HR), and RPE were measured during each 4-minute stage of testing. In the hypohydrated condition, LAT occurred significantly earlier during exercise and at a lower absolute V(O)2, VE, respiratory exchange ratio, RPE, and blood lactate concentration. Also, the blood lactate concentration was significantly lower in the hypohydrated condition (6.7 +/- 0.8 mmol) compared with the euhydrated condition (10.2 +/- 0.9 mmol) at peak exercise. There were no differences in HR or percentage of maximum HR at LAT nor did plots of V(CO2):V(O)2 reveal differences in bicarbonate buffering during exercise between the 2 conditions. From these results, we speculate that hypohydration did not significantly alter cardiovascular function or buffering capacity but did cause LAT to occur at a lower absolute exercise intensity.     

Temperature effect on contractile activity of the Ambystoma dumerilii heart previously treated with isoproterenol

The spontaneous heart rate (HR) and ventricular (V) and atrium (A) tensions (T) were evaluated through isolated organ assays at different temperatures in hearts from Ambystoma dumerilii control and treated with isoproterenol (ISO) [(150 mg/kg i.p. each 24 h, for 3 days)] on days 1, 5, 30 and 90 after ISO. In control hearts, the HR increased and the T decreased when temperature was augmented. One day after ISO the HR (43-24%) and T (50-25%) decreased with respect to control, between 8 and 24 degrees C. Five, 30 and 90 days after ISO, HR showed a gradual recovery with similar effect when the temperature was changed; but the AT increased and VT decreased at temperatures between 8 and 12 degrees C and were only recovered at temperatures above 12 degrees C. Our results indicate that the HR recovers after ISO in A. dumerilii independently of temperature. The recovery of AT and VT is similar to HR at temperatures higher than 12 degrees C and the increases in VT could be compensating the decrease in VT caused by ISO, at temperatures lower than 12 degrees C. The changes in heart contractile activity of A. dumerilii after insult show the thermic plasticity that is observed in ectothermic vertebrates.     

A comparison of the energetics of annexin I and annexin V.

The annexins comprise a family of soluble Ca2+- and phospholipid-binding proteins. Although highly similar in three-dimensional structure, different annexins are likely to exhibit different biochemical and functional properties and to play different roles in various membrane related events. Since it must be expected that these functional differences arise from differences in the characteristic thermodynamic parameters of these proteins, we performed high-sensitivity differential scanning microcalorimetry (DSC) and isothermal guanidinium hydrochloride (GdnHCl)-induced unfolding studies on annexin I and compared its thermodynamic parameters with those of annexin V published previously. The DSC data were analyzed using a model that permits quantitative treatment of the irreversible reaction. It turned out, however, that provided a heating rate of 2 K min-1 is used, unfolding of annexin I can be described satisfactorily in terms of a simple two-state reaction. At pH 6.0 annexin I is characterized by the following thermodynamic parameters: t1/2=61.8 degrees C, DeltaHcal=824 kJ mol-1 and DeltaCp=19 kJ mol-1 K-1. These parameters result in a stability value of DeltaG0D (20 degrees C)=51 kJ mol-1. The GdnHCl induced isothermal unfolding of annexin I in Mes buffer (pH 6.0), yielded DeltaG0D (buffer) values of 48, 60 and 36 kJ mol-1 at 20, 12 and 5 degrees C, respectively. These DeltaG0D values are in reasonable agreement with the values obtained from the DSC studies. The comparison of annexin I and annexin V under identical conditions (pH 8.0 or pH 6.0) shows that despite the pronounced structural homology of these two members of the annexin familiy, the stability parameters are remarkably different. This difference in stability is consistent with and provides a thermodynamic basis for the potential different in vivo functions proposed for these two annexins.     

Global analysis of the thermal and chemical denaturation of the N-terminal domain of the ribosomal protein L9 in H2O and D2O. Determination of the thermodynamic parameters, deltaH(o), deltaS(o), and deltaC(o)p and evaluation of solvent isotope effects.

The stability of the N-terminal domain of the ribosomal protein L9, NTL9, from Bacillus stearothermophilus has been monitored by circular dichroism at various temperatures and chemical denaturant concentrations in H2O and D2O. The basic thermodynamic parameters for the unfolding reaction, deltaH(o), deltaS(o), and deltaC(o)p, were determined by global analysis of temperature and denaturant effects on stability. The data were well fit by a model that assumes stability varies linearly with denaturant concentration and that uses the Gibbs-Helmholtz equation to model changes in stability with temperature. The results obtained from the global analysis are consistent with information obtained from individual thermal and chemical denaturations. NTL9 has a maximum stability of 3.78 +/- 0.25 kcal mol(-1) at 14 degrees C. DeltaH(o)(25 degrees C) for protein unfolding equals 9.9 +/- 0.7 kcal mol(-1) and TdeltaS(o)++(25 degrees C) equals 6.2 +/- 0.6 kcal mol(-1). DeltaC(o)p equals 0.53 +/- 0.06 kcal mol(-1) deg(-1). There is a small increase in stability when D2O is substituted for H2O. Based on the results from global analysis, NTL9 is 1.06 +/- 0.60 kcal mol(-1) more stable in D2O at 25 degrees C and Tm is increased by 5.8 +/- 3.6 degrees C in D2O. Based on the results from individual denaturation experiments, NTL9 is 0.68 +/- 0.68 kcal mol(-1) more stable in D2O at 25 degrees C and Tm is increased by 3.5 +/- 2.1 degrees C in D2O. Within experimental error there are no changes in deltaH(o) (25 degrees C) when D2O is substituted for H2O.     

Physiological development of the embryonic and larval crayfish heart

The cardiovascular system is the first system to become functional in a developing animal and must perform key physiological functions even as it develops and grows. The ontogeny of cardiac physiology was studied throughout embryonic and larval developmental stages in the red swamp crayfish Procambarus clarkii using videomicroscopic dimensional analysis. The heart begins to contract by day 13 of development (at 25 degrees C, 20 kPa O(2)). Cardiac output is primarily regulated by changes in heart rate because stroke volume remains relatively constant throughout embryogenesis. Prior to eclosion, heart rate and cardiac output decreased significantly. Previous data suggest that the decrease in cardiac parameters prior to hatching may be due to an oxygen limitation to the embryo. Throughout development, metabolizing mass and embryonic oxygen consumption increased, while egg surface area remained constant. The surface area of the egg membrane is a constraint on gas exchange; this limitation, in combination with the increasing oxygen demand of the embryo, results in an inadequate diffusive supply of oxygen to developing tissues. To determine if the decrease in cardiac function was the result of an internal hypoxia experienced during late embryonic development, early and late-stage embryos were exposed to hyperoxic water (PO(2) = 40 kPa O(2)). Heart rate in late-stage embryos exposed to hyperoxic water increased significantly over control values, which suggests that the suppression in cardiac function observed in late-stage embryos is due to a limited oxygen supply.     

Study on temperature-dependent changes in hydrogen bonds in cellulose Ibeta by infrared spectroscopy with perturbation-correlation moving-window two-dimensional correlation spectroscopy

Infrared (IR) spectra were measured for cellulose Ibeta prepared from the mantle of Halocynthia roretzi over a temperature range of 30-260 degrees C to explore the temperature-dependent changes in hydrogen bonds (H-bonds) in the crystal. Structural changes at the phase transition temperature of 220 degrees C are elucidated at the functional group level by perturbation-correlation moving-window two-dimensional (PCMW2D) correlation spectroscopy. The PCMW2D correlation spectra show that the intensities of bands arising from O3-H3...O5 and O2-H2...O6 intrachain H-bonds dramatically decrease at 220 degrees C, whereas the intensity changes of bands due to interchain H-bonds are not observed adequately. These results suggest that the phase transition is induced by the dissociation of the O3-H3...O5 and O2-H2...O6 intrachain H-bonds. However, the interchain H-bonds are not so much responsible for the transition directly.     

Rectal and rectal vs. esophageal temperatures in paraplegic men during prolonged exercise

This study investigated the rectal (Tre), esophageal (Tes), and skin (Tsk) temperature changes in a group of trained traumatic paraplegic men pushing their own wheelchairs on a motor-driven treadmill for a prolonged period in a neutral environment. There were two experiments. The first experiment (Tre and Tsk) involved a homogeneous group (T10-T12/L3) of highly trained paraplegic men [maximum O2 uptake (VO2max) 47.5 +/- 1.8 ml.kg-1.min-1] exercising for 80 min at 60-65% VO2max.Tre and Tsk (head, arm, thigh, and calf) and heart rate (HR) were recorded throughout. O2 uptake (VO2), minute ventilation (VE), CO2 production (VCO2), and heart rate (HR) were recorded at four intervals. During experiment 1 significant changes in HR and insignificant changes in VCO2, VE, and VO2 occurred throughout prolonged exercise. Tre increased significantly from 37.1 +/- 0.1 degrees C (rest) to 37.8 +/- 0.1 degrees C after 80 min of exercise. There were only significant changes in arm Tsk. Experiment 2 involved a nonhomogeneous group (T5-T10/T11) of active paraplegics (VO2max 39.9 +/- 4.3 ml.kg-1.min-1) exercising at 60-65% VO2max for up to 45 min on the treadmill while Tre and Tes were simultaneously recorded. Tes rose significantly faster than Tre during exercise (dT/dt 20 min: Tes 0.050 +/- 0.003 degrees C/min and Tre 0.019 +/- 0.005 degrees C/min), and Tes declined significantly faster than Tre at the end of exercise. Tes was significantly higher than Tre at the end of exercise. Our results suggest that during wheelchair propulsion by paraplegics, Tes may be a better estimate of core temperature than Tre.     

Intraspecific allometry of standard metabolic rate in green iguanas, Iguana iguana

To study the allometric relationship between standard metabolic rate and body mass (mass range 16-3627 g) in green iguanas, Iguana iguana (n=32), we measured rates of oxygen consumption (V(O(2))) at 30 degrees C during scotophase. The relationship could be described as: V(O(2))(ml h(-1))=0.478W(0.734). The resulting mass exponent was similar to the 3/4 power commonly used in interspecific curves (P>0.05), but differed from a proposed intraspecific value of 2/3 (P<0.05). The mass exponents of male (n=8) and female (n=11) iguanas did not differ (P>0.05). The mass adjusted V(O(2)) was higher than predicted from generalized squamate curves. The mean mass exponent of intra-individual allometric equations of iguanas (n=7) at varying masses during ontogeny did not differ from that of the pooled equation, indicating that scaling of V(O(2)) is similar for both between and within individuals. Thermal acclimation, compensatory changes in V(O(2)) with prolonged exposure to a constant temperature, was not observed in juvenile iguanas (n=11) between 1 and 5 weeks of acclimation at 30 degrees C.     

Bronchial reactivity of healthy subjects: 18-20 h postexposure to ozone.

Exposure of humans to ambient levels of ozone (O(3)) causes inflammatory changes within lung tissues. These changes have been reported for the "initial" (1- to 3-h) and "late" (18- to 20-h) postexposure periods. We hypothesized that at the late period, when protein and cellular markers of inflammation at the airway surface remain abnormal and the integrity of the epithelial barrier is compromised, bronchial reactivity would be increased. To test this, we measured airway responsiveness to cumulative doses of methacholine (MCh) aerosol in healthy subjects 19+/-1 h after a single exposure to O(3) (130 min at ambient levels between 120 and 240 parts/billion and alternate periods of rest and moderate exercise) or filtered air. Exposures were conducted at two temperatures: mild (22 degrees C) and moderate (30 degrees C). At the late period, bronchial reactivity to MCh increased, i.e., interpolated dose of MCh leading to a 50% fall in specific airway conductance (PC(50)) was less after O(3) than after filtered air. PC(50) for O(3) at 22 degrees C was 27 mg/ml (20% less than the PC(50) after filtered air), and for O(3) at 30 degrees C it was 19 mg/ml (70% less than the PC(50) after filtered air). The forced expiratory volume in 1 s (FEV(1)) at the late time point after O(3) was slightly but significantly reduced (2.3%) from the preexposure level. There was no relationship found between the functional changes observed early after exposure to O(3) and subsequent changes in bronchial reactivity or FEV(1) at the late time point. These results suggest that bronchial reactivity is significantly altered approximately 1 day after O(3); this injury may contribute to the respiratory morbidity that is observed 1-2 days after an episode of ambient air pollution.     

Evidence for the association of ultraviolet-C and H(2)O(2)-induced apoptosis with acid sphingomyelinase activation

We measured the temperature dependence of oxygen evolution in thylakoids from tobacco using mass spectrometry and high resolution polarography. We determined the initial S-state distribution and the efficiency of the transition between these states including the probability of the O(2) yield through a fast mode. We observed discontinuous changes of the parameters at the temperatures 11 degrees C, 15 degrees C and 21 degrees C. Due to the mass spectroscopy data we think that the irregularity observed at 11 degrees C is due to conformational changes within the water catalytic site. We show that the different contributions of the slow and fast modes of oxygen evolution and of the water molecule exchange are correlated and that their behavior can be explained in terms of the H(2)O accessibility to the water splitting enzyme.     

The physiological strain index applied to heat-stressed rats.

A physiological strain index (PSI) based on heart rate (HR) and rectal temperature (Tre) was recently suggested to evaluate exercise-heat stress in humans. The purpose of this study was to adjust PSI for rats and to evaluate this index at different levels of heat acclimation and training. The corrections of HR and Tre to modify the index for rats are as follows: PSI = 5 (Tre t - Tre 0). (41.5 - Tre 0)-1 + 5 (HRt - HR0). (550 - HR0)-1, where HRt and Tre t are simultaneous measurements taken at any time during the exposure and HR0 and Tre 0 are the initial measurements. The adjusted PSI was applied to five groups (n = 11-14 per group) of acclimated rats (control and 2, 5, 10, and 30 days) exposed for 70 min to a hot climate [40 degrees C, 20% relative humidity (RH)]. A separate database representing two groups of acclimated or trained rats was also used and involved 20 min of low-intensity exercise (O2 consumption approximately 50 ml. min-1. kg-1) at three different climates: normothermic (24 degrees C, 40% RH), hot-wet (35 degrees C, 70% RH), and hot-dry (40 degrees C, 20% RH). In normothermia, rats also performed moderate exercise (O2 consumption approximately 60 ml. min-1. kg-1). The adjusted PSI differentiated among acclimation levels and significantly discriminated among all exposures during low-intensity exercise (P < 0.05). Furthermore, this index was able to assess the individual roles played by heat acclimation and exercise training.     

Factorial scopes of cardio-metabolic variables remain constant with changes in body temperature in the varanid lizard, Varanus rosenbergi

The majority of information concerning the cardio-metabolic performance of varanids during exercise is limited to a few species at their preferred body temperature (T(b)) even though, being ectotherms, varanids naturally experience rather large changes in T(b). Although it is well established that absolute aerobic scope declines with decreasing T(b), it is not known whether changes in cardiac output (V(b)) and/or tissue oxygen extraction, (Ca(O2) - Cv(O2)), are in proportion to the rate of oxygen consumption (Vo(2)). To test this, we studied six Rosenberg's goannas (Varanus rosenbergi) while at rest and while maximally exercising on a treadmill both at 25 and 36 degrees C. During maximum exercise both at 25 and 36 degrees C, mass-specific rate of oxygen consumption (Vo(2kg)) increased with an absolute scope of 8.5 ml min(-1) kg(-1) and 15.7 ml min(-1) kg(-1), respectively. Interestingly, the factorial aerobic scope was temperature-independent and remained at 7.0 which, at each T(b), was primarily the result of an increase in V(bkg), governed by approximate twofold increases both in heart rate (f(H)) and cardiac stroke volume (V(Skg)). Both at 25 degrees C and 36 degrees C, the increase in V(bkg) alone was not sufficient to provide all of the additional oxygen required to attain maximal Vo(2kg), as indicated by a decrease in the blood convection requirement V(bkg)/Vo(2kg); hence, there was a compensatory twofold increase in (Ca(O2) - Cv(O2)). Although associated with an increase in hemoglobin-oxygen affinity, a decrease in T(b) did not impair unloading of oxygen at the tissues and act to reduce (Ca(O2) - Cv(O2)); both Ca(O2)) and Cv(O2)) were maintained across T(b). The change in Vo(2kg) with T(b), therefore, is solely reliant on the thermal dependence of V(bkg). Maintaining a high factorial aerobic scope across a range of T(b) confers an advantage in that cooler animals can achieve higher absolute aerobic scopes and presumably improved aerobic performance than would otherwise be achievable.     

Body cooling and the diving capabilities of muskrats (Ondatra zibethicus): a test of the adaptive hypothermia hypothesis

We tested the hypothesis that immersion hypothermia enhances the diving capabilities of adult and juvenile muskrats by reducing rates of oxygen consumption (V O2). Declines in abdominal body temperature (T(b)) comparable to those observed in nature (0.5-3.5 degrees C) were induced by pre-chilling animals in 6 degrees C water. Pre-chilling did not reduce diving V O2 of any animal tested in 10 degrees C or 30 degrees C water, irrespective of the nature of the dive. Most behavioural indices of dive performance, including average and cumulative dive times, were unaffected by T(b) reduction in adults, but depressed in hypothermic juveniles (200-400 g). Hypothermia reduced diving heart rate only on short (<25s) dives (16% reduction, P=0.01), but did not affect the temporal onset of diving bradycardia. Post-immersion V O2 was higher for pre-chilled than for normothermic muskrats, but the difference became insignificant on longer (>90 s) dives. Our findings suggest that the mild hypothermia experienced by muskrats in nature has minimal effect on diving and post-immersion metabolic costs, and thus has little impact on the dive performance of this northern semi-aquatic mammal.