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Corticosteroid secretion and excretion by the Australian monotreme Tachyglossus aculeatus was investigated by assay of peripheral blood, adrenal venous blood, urine and faeces.Free corticosteroids could be demonstrated in only one out of five samples of peripheral whole blood, in which trace amounts of cortisol and corticosterone were detected at concentrations of the order of 0\m=.\1\p=n-\0\m=.\2 \g=m\g./100 ml.Quantitatively, the major corticosteroid detected in adrenal venous blood of three males and one female was identified as corticosterone, at concentrations of 8\p=n-\25 \g=m\g./100 ml. whole blood. Lesser amounts of cortisol were detected in samples of adrenal venous blood from two males and the female, the ratios 'F:B' being 0\m=.\25and 0\m=.\40in the males, and 0\m=.\13in the female. The highest rate of corticosterone secretion was only 2\m=.\6 \g=m\g./100mg. adrenal/hr. or 1\m=.\17 \g=m\g./kg.body weight/hr. No free or conjugated corticosteroids were detected in either urine or faeces. Zimmermann chromogens were found in both urine and faeces, and total daily excretion varied unpredictably from undetectable levels to a maximum of only 25 \g=m\g./kg.body weight/day.

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Adrenal venous and peripheral blood was collected from five adult and two immature kangaroos, anaesthetized with pentobarbitone.Seven C-21 \ g=D\ 4, 3-oxo steroids were identified in extracts of adrenal venous blood from all animals. The major component\p=m-\72\p=n-\92% of detected corticosteroids\p=m-\was cortisol. The remainder were 11 -deoxy and 21 -deoxy cortisol, corticosterone, aldosterone, 11\g=b\-OH and 17\g=a\-OH progesterone. In four out of six experiments, i.v. corticotrophin caused variable degrees of increase in corticosteroid secretion. Maximum cortisol secretion rates in all the adults and the immature male were in the range 29\p=n-\70\g=m\g./hr./100mg. adrenal, or 28\p=n-\58 \ g=m\ g. / hr. / kg. body weight and 98 \g=m\g./hr./100mg. or 114 \g=m\g./hr./kg. in the immature female.Average aldosterone secretion rate in the adults was 0\m=.\27 \g=m\g./hr./100mg. adrenal, or 0\m=.\18\g=m\g./hr./kg.body weight. It was significantly higher in the immature male. However, the Na status of the animals was not known.Cortisol was the major corticosteroid in peripheral blood plasma at concentrations of 2\ m=. \ 0\ p=n-\ 8\ m=. \ 1 \g=m\g./100 ml.The unusual secretion of 21-deoxy cortisol suggests there may be differences in some aspects of adrenocortical biosynthesis, compared with eutherians.

Adrenal venous and peripheral blood was collected from three Tasmanian devils and three Eastern native cats. Ten \g=D\4-3-oxosteroids were identified in the adrenal venous blood of the devils and nine from the native cats. Cortisol was the major steroid and its maximum corticotrophin (ACTH) stimulated secretion rate averaged 61 \g=m\g/h/100mg adrenal in devils and 36 \ g=m\ g in cats. The cortisol : corticosterone ratio in two devils and two cats was approx. 2\m=.\5and 5, respectively. Aldosterone secretion rates in devils ranged from 0\m=.\06 to 0\m=.\12 \ g=m\ g/ h/ 100mg adrenal and in cats from 0\m=.\12to 0\m=.\93, the response to ACTH varied in different animals. 21-Deoxycortisol was found in all animals, it increased during ACTH stimulation (maximum range in devils: 8\m=.\5-19\m=.\5 \g=m\g/h/100mg adrenal, and 0\m=.\6-5\m=.\4in cats). Another unusual steroid isolated from adrenal venous blood was identified as 17\g=a\-hydroxy-20\g=b\-dihydroprogesterone. Its rate of secretion was higher in devils (range approx. 2-6 \g=m\g/h/100mg adrenal). No relationship was found between adrenal blood flow and corticosteroid secretion. In the peripheral blood plasma only cortisol and corticosterone were identified. The female of both species had a significantly higher concentration of cortisol ofthe order of 7 \g=m\g/100 ml plasma. The combined adrenal weights/ kg body weight averaged 168 mg/kg in the devils and 132 mg in the cats. Since all three Superfamilies of Australian marsupials have now been investigated a comparison of the results obtained from seven representative species is made and they are also considered in relation to eutherian mammals.

The formation of corticosteroids from [14C]pregnenolone or [14C]progesterone by platypus and echidna adrenal homogenates was studied. Corticosteroids were assayed in the peripheral blood plasma of the platypus. In the platypus nine labelled conversion products were identified. The combined yields of cortisol and cortisone averaged 70 % and the combined yields of corticosterone and 11-dehydrocorticosterone averaged 14%. Aldosterone was identified in each experiment in yields below 0\m=.\1 %. In the echidna ten labelled conversion products were identified. Those in highest yield were 11-deoxycorticosterone and 11-deoxycortisol which contained, respectively, an average of 35 and 11% of the added substrate. The yield of corticosterone was always less than 1% and that of cortisol was consistently lower. Aldosterone was identified in all cases in yields below 0\m=.\05%.The yields of total 11\g=b\-hydroxylated conversion products from platypus adrenal exceeded 80%, whereas they were below 1 % from glands of male echidnas and 7 % from female echidna.The yield\p=m-\time curves for labelled conversion products from [ 14C] \ x=r eq-\ pregnenolone indicated that the biosynthetic pathways of corticosteroid production by the monotreme adrenal are similar to those in eutherian mammals.From stored endogenous precursors of a platypus adrenal incubated for 30 min, corticosteroids were formed in the following amounts (per 100 mg adrenal): cortisone, 5\m=.\1\g=m\g;cortisol, 1\m=.\3\g=m\g;11-dehydrocorticosterone, 1\m=.\5 \g=m\g. In the peripheral plasma of platypus the concentration of combined cortisol and cortisone was 14\g=m\g/100ml and the combined corticosterone and 11-dehydrocorticosterone was 1\m=.\8 \g=m\g. The mean ratio of adrenal weight (mg) to body weight (kg) was 39\m=.\5in the echidna and 257 in the platypus. The marked contrast in adrenocortical biosynthetic capacity of the two monotremes is discussed and a comparison of the results is made in relation to higher mammals.

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