Reexamining Echidna Physiology: The Big Picture for Tachyglossus aculeatus acanthion.

The early divergence of monotremes and therian mammals has resulted in considerable interest in the comparative physiology of the short-beaked echidna (Tachyglossus aculeatus), the most common and widespread living monotreme. However, there are many and varied interpretations of its physiology, reflecting the many and varied studies, limitations and uncertainties of aspects of some previous studies, and potential differences between the various subspecies. Consequently, we thoroughly examine here the standardized physiology of the most widely distributed subspecies of short-beaked echidna (T. aculeatus acanthion) over a wide range of ambient temperatures to definitively assess its physiology in a comparative context. We conclude that the low and variable body temperature of the short-beaked echidna is physiologically "primitive," but it also reflects adaptation to its myrmecophagous niche. Other aspects of its physiology are more typically mammalian. A low metabolic rate reflects its low body temperature, and ventilatory variables are matched to accommodate a modest gas exchange requirement. Thermal conductance is typical for a mammal of equivalent mass. In contrast to previous studies, we demonstrate that short-beaked echidnas can enhance evaporative water loss above thermoneutrality, like other mammals, with a similar capacity for evaporative heat loss. Cooling of their nasal blood sinus with nasal mucous may contribute to this enhanced evaporative cooling. Their capacity to evaporatively cool explains how their distribution can include habitats where ambient temperature, even in shelters, exceeds their supposed critical thermal limit.

Thermoenergetics of pre-moulting and moulting kookaburras (Dacelo novaeguineae): they're laughing.

We examined the effect of temperature on resting metabolic rate in seven field-captured laughing kookaburras (Dacelo novaeguineae) during late winter and early spring. Basal metabolic rate averaged 201+/-3.4 ml O(2) h(-1) (0.603 ml O(2) g(-1) h(-1)). Overall thermal conductance (K(o)) declined with ambient temperature ( T(a)) and averaged 0.026 ml O(2) g(-1) h(-1) degrees C(-1) at T(a)s<10 degrees C. Day-night differences in body temperatures (2.6 degrees C) and in alpha-phase versus rho-phase minimum metabolic rates were much greater (33%) than predicted for 340-g nonpasserine birds and suggest that these animals operate as low-metabolic intensity animals in their rest phase, but normal-metabolic intensity animals during their active phase. Metabolic rate was measured in four of the same birds undergoing moult. Thermal conductance increased to 60% above pre-moult values about 6 weeks after moult began. Basal metabolic rate of moulting birds showing peak thermal conductance readings averaged 17 ml O(2) h(-1) higher than pre-moult measurements. Although this increase was not statistically significant, we believe the moult costs of kookaburras are too low to overcome the inherent variability of BMR determination. We suggest that moult costs of kookaburras are only somewhat higher than the measured costs of protein synthesis of other endotherms.

The echidna manifests typical characteristics of rapid eye movement sleep.

The failure to identify rapid eye movement sleep (REMS) in an early study of the echidna at an unmeasured ambient temperature (T(a)) was unexpected, as its brain stem structures resemble those that generate REMS in other mammals. However, typical mammalian REMS was evident in echidnas exposed to several T(a)s. The parallel presence of REMS in birds points to its reptilian origin.

Seasonal variations in thyroid hormone levels in free-living echidnas (Tachyglossus aculeatus).

Hulbert and Augee (1982) have suggested that the thyroid has little effect on energy metabolism in the echidna. In order to investigate whether thyroid status changes during hibernation, when metabolism drops dramatically, we measured levels of thyroid hormones in 31 free-living echidnas at various times during the year. Unlike eutherian hibernators, in which thyroid hormone levels may rise to seasonally high values in late hibernation, total and free T(4) and T(3) were all significantly depressed throughout hibernation. TT(4) from nonhibernating echidnas was 11.8 +/- 0.9 ng/ml (n = 23), confirming previously published values, but fell to half this level (5.9 +/- 0.7 ng/ml, n = 8) during hibernation. By contrast to the low TT(4) values, nonhibernating FT(4), TT(3), and FT(3) values were similar to normal values for eutherian mammals. Differences in the seasonal pattern of variation in thyroid hormones between echidnas and hibernating eutherians may be due to differences in thyroid hormone transporting proteins.

Nonshivering thermogenesis in marsupials: absence of thermogenic response to beta 3-adrenergic agonists.

The status of nonshivering thermogenesis (NST) in marsupials remains controversial. Although morphological studies have failed to find evidence for the presence of brown adipose tissue (BAT) in adults or juveniles of species from all extant families of marsupial, a number of studies have investigated the metabolic response of marsupials to noradrenaline (NA) and yielded conflicting results. In eutherian mammals, NA stimulates NST in BAT by acting on beta 3-receptors, and in the experiments reported here we investigated the response of adult and juvenile brush tail possums (Trichosurus vulpecula), a Brazilian opossum (Monodelphis domestica), adult and juvenile red-necked (Bennett's) wallabies (Macropus rufogriseus) and the laboratory rat to selective beta 3-agonists (ICI D7114 and BRL 35135) and to NA. Wallabies were tested with the beta 3-agonists only. Although NA and both beta 3-agonists caused an 85% increase in oxygen consumption in rats, there was no significant effect on any of the marsupials. These results clearly indicate no beta 3-stimulated NST in these marsupials. All reports of metabolic responses to NA are from macropods, and a recent study demonstrates that NA and other alpha-adrenergic agonists stimulate thermogenesis in a small macropod, the bettong (Bettongia gaimardi), by acting on alpha 1-receptors. Thermogenic responses to NA seems to be restricted to macropods, showing the danger of characterising the response of any one marsupial species as being representative of marsupials as a group.

Transthyretin expression evolved more recently in liver than in brain.

1. Transthyretin was found to be synthesized and secreted by choroid plexus from rats, echidnas, and lizards, but not toads. 2. Transthyretin was observed in blood from placental mammals, birds, and marsupials, but not reptiles and monotremes. 3. The obtained data suggest that transthyretin synthesis by the liver evolved independently in the lineage leading to the placental mammals and marsupials and in that leading to the birds. 4. It is proposed that transthyretin gene expression in mammalian liver appeared about 200 million years later than its first occurrence in the choroid plexus of the stem reptiles.

Haematology, red cell metabolism and blood chemistry of the black-faced cormorant Leucocarbo fuscescens.

1. Haematology, red cell metabolism and blood chemistry of five fledgeling black-faced cormorants Leucocarbo fuscescens were studied and the results were compared with previously reported data on several other sea-birds. 2. The mean erythrocyte count of the cormorant is similar to that of penguins but lower than that of flying, non-diving sea-birds. The cormorant's red cell mean cell volume (MCV) is lower than that of penguins but higher than that of non-diving sea-birds. 3. Leucocyte numbers are within expected limits for avian species. 4. Red cell enzymes: glucose phosphate isomerase, phosphofructokinase, aldolase and enolase are higher in the cormorant than in the little penguin; glyceraldehyde phosphate dehydrogenase, monophosphoglyceromutase, pyruvate kinase, lactate dehydrogenase and glutathione reductase are lower. 5. Haemoglobin electrophoresis showed a typical avian haemoglobin pattern.

Measurement of gastric secretion as a student teaching exercise.

Teaching gastrointestinal physiology to preclinical medical students presents problems in finding suitable practical exercises to demonstrate the physiology of gastric acid secretion. In our course, students measure their own gastric secretory activity by the use of nasogastric tubes. Gastric secretion can be stimulated by insulin-induced hypoglycemia or by pentagastrin, a synthetic gastrin analogue. The time course of the secretory responses, i.e., volume, acid output, and pH, are followed by collecting control and poststimulatory secretions into 15-min samples. The effect of antiulcer drugs, such as cimetidine, can be easily studied in such experiments. The results of these experiments are very reproducible, allowing year-to-year comparisons of treatments. Examples of results of various experimental protocols are shown. We believe this to be a useful class exercise not only because of the excellent results it yields but because of the experience and insights it produces.

MRATE: a Pascal program for on-line calculation of metabolic rate using wet gases.

MRATE is a Pascal program designed for on-line calculation of metabolic rate in an open circuit system, when outlet gases are not dried. Voltages from O2 and CO2 analysers, humidity and temperature probes and mass flow meter are digitised and used to calculate outlet flow, oxygen consumption, carbon dioxide production, respiratory quotient and metabolic rate. The program may be easily modified to suit a variety of experimental set-ups.

Ventilation and oxygen extraction in the little penguin (Eudyptula minor), at different temperatures in air and water.

Oxygen consumption, (VO2), carbon dioxide production (VCO2) and ventilation were measured in little penguins (Eudyptula minor) in air and water over a range of ambient temperatures. Barometrically measured minimum ventilation was very low (112 +/- 23 ml.min-1 over the range 15-20 degrees C, n = 9), principally due to a lower than predicted respiratory frequency (f) (6.9 +/- 1.1 min-1). This low ventilation resulted in a very high oxygen extraction (EO2) and low air convection requirement (VI/VO2). Despite large increases in metabolic rate in birds in cold water, VI/VO2 and hence EO2 did not vary in non-heat exposed penguins over a range of air and water temperatures. Mean extraction was 53.2 +/- 10.8% (n = 82) corresponding to an air convection requirement of 11.3 +/- 2.5 L.L-1. Above thermoneutrality there was a dramatic increase in ventilation and fall in EO2 resulting from increases in both f and tidal volume (VT). End-expired gases were measured at thermoneutrality and during heat exposure. At thermoneutrality FE'CO2 was 0.074 +/- 0.005, and FE'O2 0.115 +/- 0.009. During heat exposure end expired gases approached atmospheric levels.

Haematology and metabolism of the blood of the little penguin, Eudyptula minor.

1. We studied the haematology of the little penguin, Eudyptula minor, and measured red cell enzymes, and the rate of O2 and glucose consumption and CO2 and lactate production of whole penguin blood. 2. Little penguins have a very low RBC count (1.66 x 10(12) l-1) and very large red cells (MCV = 229 microns 3), resulting in a normal haemoglobin and haematocrit. 3. Despite the lower body temperature of penguins, the rate of O2 consumption by whole blood is close to that predicted from studies on other birds.

Comparison of barometric and pneumotachographic measurements of resting ventilation in the little penguin (Eudyptula minor).

1. We compared the use of the barometric method and pneumotachography for measurement of ventilation in the little penguin Eudyptula minor. 2. Simultaneous use of both techniques showed the barometric method to give a reliable estimate of tidal volume. 3. Comparison of birds with and without masks for pneumotachography showed wearing masks to produce a significant increase in ventilation, principally through a raised respiratory frequency.

High performance liquid chromatographic profiles of nucleosides, bases and tryptophan in the plasma of the Tasmanian devil and four other marsupial species.

Plasma profiles of nucleosides, bases and trytophan of five marsupial species were established using the reversed-phase mode of high performance liquid chromatography (RHPLC). Within each species, the profiles were highly reproducible and between species there were distinct differences. In the Tasmanian devil, the circulating levels of constituents examined with one exception, were generally lower than in the other marsupials. The exception was a constituent present in large amounts and having the characteristics of a purine nucleoside derivative which was found only in the plasma of the devil.

Acetylation of sulphanilamide in four marsupials and a monotreme.

1. The urinary metabolites of sulphanilamide (100 mg/kg, i.p.) have been studied in four marsupials (the Tasmanian devil, brushtail possum, pademelon and barred bandicoot), a monotreme (the echidna) and a eutherian (the rat). 2. All species excreted some unchanged sulphanilamide (20-30% of dose in 24 h). The major urinary metabolite in the devil, possum and pademelon was N4-acetylsulphanilamide (6-17%). This was less than that excreted by the rat (40%). These three marsupials and the rat also excreted small amounts of N1-acetyl and N1, N4-diacetylsulphanilamide. 3. The bandicoot and echidna were virtually unable to acetylate sulphanilamide, unlike the 16 other species of animals and birds in which this has been studied. The reason for this metabolic defect is unknown.

Effects of decortication and carotid sinus nerve section on ventilation of the rat.

The effect on ventilation of exposure to hypoxic, hypercapnic and hypoxic/hypercapnic gas mixtures was studied before and after functional decortication of intact rats and rats in which the carotid chemoreceptors had been disconnected. Unanaesthetized rats responded to both hypoxia and hypercapnia with an increase in minute ventilation (V) through increases in both frequency (f) and tidal volume (VT). Decortication led to a greater V response to CO2. This was through an effect on f, rather than VT. Carotid sinus nerve section (CSNS) caused a lessening in the V response to gas mixtures, f and VT being equally affected. Decortication, following CSNS, increased the V response but this time through increased VT rather than f. This effect on VT was not specific to any particular gas mixture. It is concluded that the carotid body chemoreceptors, together with the bulbopontine rate controller, influence the response to CO2. It is further suggested that this integration takes place in the reticular formation and is normally under some degree of inhibition from the cerebral cortex.

The respiratory frequency response to carbon dioxide inhalation in conscious rabbits.

1. The respiratory responses to CO(2) inhalation were measured in New Zealand White rabbits. Prior to testing, the rabbits either received drinking water ad libitum (C), or were subjected to 72 hr water deprivation (WD). The rabbits were tested in a climatic chamber at either 20 degrees C, (C20, WD20) or 30 degrees C (C30, WD30).2. CO(2) exposure caused increases in both tidal volume (V(T)) and minute volume (V(E)). The direction and magnitude of changes in respiratory frequency (f), however, were dependent upon treatment.3. Linear regressions were drawn for f vs.% CO(2) in inhaled gas. Treatment C20 showed zero slope, treatment C30 a large negative slope, and treatments WD20 and WD30 a significant positive slope.4. The slopes of the regression lines obtained from plotting f vs.% CO(2) were plotted against their intercepts for each rabbit under each treatment condition. This yielded a highly significant linear regression from which it could be concluded that the frequency response to CO(2) exposure is dependent upon the initial frequency when the rabbits are breathing air.5. It is suggested that frequency is shifted during hypercapnia towards an optimal value which is dependent upon the existing% CO(2) in the inspired gas.6. Comparison of the results obtained for the rabbit with results reported for the conscious dog and sheep suggest that a similar relationship may hold for these species.

Responses of conscious rabbits to CO2 at ambient temperatures of 5, 20, and 35 degrees C.

Conscious rabbits were exposed to atmospheric air or to 6% CO2 in air at ambient temperatures (Ta) of 5, 20 and 35 degrees C. Measurements were made of rectal temperature (Tre), metabolic rate (MR), respiratory frequency (f), tidal volume (VT), and minute volume (VE). CO2 exposure did not affect Tre at any Ta and only affected MR at 35 degrees C when it caused an increase. At each Ta hypercapnia caused an increase in VT and a decrease in f. At 5 degrees C VE was increased by CO2, at 35 degrees C VE decreased, and at 20 degrees C the results were variable. The data were examined in the light of theories relating to the relative contributions of inputs from brain stem and from pulmonary stretch receptors, in response to body temperature and CO2 partial pressure. It was concluded that hypercapnia stimulates an increase in VT via the brain stem, whereas at the same time removing a hypocapnic drive which, along with central thermal inputs, stimulates f.

Panting in small mammals: a comparison of two marsupials and the laboratory rabbit.

By use of a barometric technique, tidal volume (VT), minute volume (VE), respiratory frequency (f), and respiratory evaporative heat loss (Eex) were measured from conscious unrestrained potoroos (Potorous tridactylus), barred bandicoots (Perameles gunnii), and New Zealand white rabbits (Oryctolagus cuniculus) at temperatures in and above the thermoneutral zone (TNZ). Rectal temperature (Tre) and oxygen consumption were also measured. VT initially decreased with rising Ta, but in the potoroo and rabbit it then increased past the resting level. VE increased much more in the marsupials than in the rabbit, and higher Eex maxima were also found for the marsupials. The marsupials had high Q10's above the TNZ, and had a panting efficiency of 80%. The rabbits had a Q10 of 1.9 above the TNZ and 100% panting efficiency. The high VE and Eex and low panting efficiency of the marsupials may be due to their lower Tre which allows transfer of heat from the environment to the animal.