Does leptin signal adiposity in the egg-laying mammal, Tachyglossus aculeatus?

Leptin is a peptide hormone best known for its role in feedback regulation of adiposity in eutherian mammals. Normally an increase in adipose tissue mass leads to an increase in circulating leptin which increases energy expenditure and limits food intake, but in hibernating eutherian mammals this relationship may change to allow prehibernatory fattening. The echidna (Tachyglossus aculeatus) is a monotreme mammal which accumulates significant fat reserves before entering hibernation, and mates immediately at the end of hibernation. We hypothesised that echidnas would show a strong relationship between body mass and plasma leptin for most of the year which would change during the pre-hibernatory period. We measured plasma leptin and body mass in free-ranging echidnas over several reproductive and hibernation cycles. There were significant seasonal variations in plasma leptin in both sexes, with the highest levels occurring in hibernation and in mating females. The lowest levels were found in males when they were foraging maximally after the reproductive period. We used mass%, body mass at the time of sampling as a percentage of long term mean mass, as a proxy for adiposity. There was a weak negative relationship between mass% and plasma leptin, from which we infer a weak negative relationship between adiposity and plasma leptin as has been found in reptiles and birds, rather than the strong positive relationship found in other mammals.

Effect of diet on serum 25-hydroxyvitamin D concentration in the short-beaked echidna (Tachyglossus aculeatus).

BACKGROUND: Elevated concentrations of 25-hydroxyvitamin D (25(OH)-D) were diagnosed in captive short-beaked echidnas (Tachyglossus aculeatus) from three different zoological facilities within Australia. RESULTS: The mean serum 25(OH)-D concentration in the wild echidnas was 24.7 nmol/L and was significantly higher in captive echidnas from all three facilities: Facility 1, mean 335.5 nmol/L (P < 0.001); Facility 2, mean 187.2 nmol/L (P = 0.003); Facility 3, mean 194 nmol/L (P = 0.005). Animals did not appear to have clinical manifestations of vitamin D toxicosis. The increased serum 25(OH)-D concentration was attributed to excessive dietary intake and a reduction in the amount of vitamin D(3) in the diet of echidnas from Facility 1 resulted in a marked decrease in the serum 25(OH)-D concentrations (mean 33 nmol/L). The reduction in serum 25(OH)-D concentration was statistically significant (P = 0.002) and the resulting concentrations were similar to those of wild echidnas (P = 0.212). CONCLUSION: It is not known what effect an elevated serum 25(OH)-D concentration has on echidnas.

Assessment of the hemorheological profile of koala and echidna.

Koala, a marsupial, and echidna, a monotreme, are mammals native to Australia. Blood viscosity (62.5-1250s(-1)), red blood cell (RBC) deformability, RBC aggregation, aggregability and surface charge, and hematological parameters were measured in blood samples from six koalas and six echidnas and compared to adult human blood. Koala had the largest RBC mean cell volume (107.7+/-2.6fl) compared to echidna (81.3+/-2.6fl) and humans (88.4+/-1.2fl). Echidna blood exhibited the highest viscosity over the entire range of shear rates. Echidna RBC were significantly less deformable than koala RBC but more deformable than human RBC. Echidna RBC had significantly lower aggregability (i.e., aggregation in standardized dextran medium) than koala or human RBC, while aggregation in autologous plasma was similar for the three species. Erythrocyte surface charge as indexed by RBC electrophoretic mobility was similar for human and echidna cells but was 40% lower for koala RBC. Data obtained during this preliminary study indicate that koala and echidna have distinct hemorheological characteristics; investigation of these properties may reveal patterns relevant to specific behavioral and physiological features of these animals.

Body temperature-related structural transitions of monotremal and human hemoglobin.

In this study, temperature-related structural changes were investigated in human, duck-billed platypus (Ornithorhynchus anatinus, body temperature T(b) = 31-33 degrees C), and echidna (Tachyglossus aculeatus, body temperature T(b) = 32-33 degrees C) hemoglobin using circular dichroism spectroscopy and dynamic light scattering. The average hydrodynamic radius (R(h)) and fractional (normalized) change in the ellipticity (F(obs)) at 222 +/- 2 nm of hemoglobin were measured. The temperature was varied stepwise from 25 degrees C to 45 degrees C. The existence of a structural transition of human hemoglobin at the critical temperature T(c) between 36-37 degrees C was previously shown by micropipette aspiration experiments, viscosimetry, and circular dichroism spectroscopy. Based on light-scattering measurements, this study proves the onset of molecular aggregation at T(c). In two different monotremal hemoglobins (echidna and platypus), the critical transition temperatures were found between 32-33 degrees C, which are close to the species' body temperature T(b). The data suggest that the correlation of the structural transition's critical temperature T(c) and the species' body temperature T(b) is not mere coincidence but, instead, is a more widespread structural phenomenon possibly including many other proteins.

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.

Comparative erythrocyte metabolism in marsupials and monotremes.

Concentrations of ATP and DPG, activities of 10 enzymes and the glycolytic rates were measured in the erythrocytes of 11 species of marsupials and two species of monotremes. Mean DPG concentrations were greater in the erythrocytes of marsupials than those of eutherian mammals. The opposite is true of ATP. Significant findings from the results of enzyme activities were: high activity of hexokinase (7.39 +/- 0.82 EU/g Hb) in the short-beaked echidna, pyruvate kinase (37.49 +/- 1.0 EU/g) Hb in bridled nailtail wallaby and glucose-6-phosphate dehydrogenase (G6PD; 41.66 +/- 1.24 EU/g Hb) in black-striped wallaby. About 6- to 7-fold difference in the activity of G6PD levels between the two species of wombats was confirmed. Glucose phosphate isomerase activity was also shown to be twice as high in the red cells of the common wombat compared with those of the southern hairy nosed wombat. There were wide variations in the glycolytic rate among the species examined.

Active cation transport and Na+K+Mg ATPase of the monotreme erythrocytes.

The erythrocytes of the echidna (Tachyglossus aculeatus) and platypus (Ornithorhynchus anatinus), which are practically devoid of intracellular ATP content (1), were examined for active Rb86 influx and for the presence of Na+K+Mg ATPase. We found that intact erythrocytes of both species possess the ability to actively transport cations. Ouabain sensitive Rb86 influx in the echidna was approximately 0.17 mumoles/ml cells x hr, whereas the platypus exhibited a higher value of 0.43 mumoles/ml cells x hr. Surprisingly, ouabain sensitive Na+K+Mg ATPase activity of isolated membranes was high amounting to some 15 to 25 fold higher than the human erythrocyte counterpart determined under identical conditions. These findings suggest that a trace amount of ATP is sufficient to maintain active cation transport across the monotreme cell membranes.

Erythrocyte phosphates and hemoglobin function in monotremes and some marsupials.

Hematologic values, red blood cell (RBC) organic phosphate composition, hemoglobin function, and hemoglobin composition have been determined on blood from the monotremes, the duckbill platypus and the echidna, and three species of marsupials, the Tasmanian devil, the wallaby, and the brush-tail possum. Blood from the platypus had a RBC count of 8.63 X 10(6)/mm3, a mean corpuscular volume of 49.1 millemicron3, and a white blood cell count of 26.0 X 10(3)/mm3. The RBCs from the monotremes and the three marsupials exhibited hemoglobin polymorphism, each with three hemoglobin components. Addition of ATP, 2,3-bisphosphoglycerate (2,3-P2-glycerate), or inositol pentakisphosphate (inositol-P5) to phosphate-free hemoglobin from each species decreased hemoglobin oxygen affinity; the order of effect of these compounds was ATP less than 2,3-P2-glycerate less than inositol-P5. The RBCs of all species had concentrations of 2,3-P2-glycerate ranging from 6.02 mumol/ml RBCs in the wallaby to 10.39 mumol/ml RBCs in the possum. The RBCs from the three species of marsupials had concentrations of ATP ranging from 0.24 mumol/ml RBCs in the possum to 0.80 mumol/ml RBCs in the Tasmanian devil. The level of ATP in RBCs of the platypus and echidna were 0.06 and 0.03 mumol/ml RBCs, respectively.

Adenosine triphosphate-deficient erythrocytes of the egg-laying mammal, echidna (tachyglossus aculeatus).

The erythrocytes of the short-beaked echidna (Tachyglossus aculeatus), an egg-laying mammal, were examined for the presence of phosphorylated compounds. The erythrocytes contained only 0.03 +/- 0.01 micromoles of adenosine 5'-triphosphate per milliliter of cells. This amount is two orders of magnitude less than that in human cells. Although the echidna erythrocytes had an abundance of 2,3-diphosphoglycerate and other glycolytic intermediates, no other energy-rich pyridine and purine compounds were detected.

Blood corticosteroids in Australian marsupial and placental mammals and one monotreme.

Peripheral blood corticosteroid levels were determined in nine species of Australian marsupial (Eastern grey kangaroo, black-tailed, Bennett's and pademelon wallabies, quokka, wombat, koala and Western native and tiger cats), one species of monotreme (echidna) and one placental Australian mammal (dingo). Animals were obtained or bled with minimal disturbance and came from areas considered to have adequate sodium content of the vegetation. Aldosterone, corticosterone, cortisol, 11-deoxycorticosterone and 11-deoxycortisol were measured and levels found to be similar to five introduced eutherian species (sheep, cow, dog, fox and man) with the exception of the koala and the wombat. Cortisol was the predominant corticosteroid, except in the koala, which produced corticosterone in relatively the greatest quantity, and the wombat which produced more 11-deoxycortisol. Steroid levels were generally low in the wombat. ACTH administered to the koala changed its pattern of corticosteroid secretion from predominantly corticosterone to cortisol. In the dingo, administration of ACTH caused rises in corticosteroid levels similar to those seen in most other eutherian mammals.