Con~p. Biochem.Physiol,Vol. 103A,No. 3, pp. 541-543,1992 0
Printed in Great Britain
0300-9629/92 $5.00+ 0.00 1992 Pergamon Press Ltd
THE TRANSFER OF THYROXINE FROM THE MOTHER TO THE YOUNG OF THE MARSUPIALS, THE BANDICOOT (ISOODON MACROURUS) AND THE POSSUM (TRICHOSURUS VULPECULA) R. T. GEMMELL and C. SERNIA Departments of Anatomy, and Physiology and Pharmacology, University of Queensland, Brisbane, 4072 Australia. Tel.: 61-7-365-l 111 Fax: 61-7-365-l 199 (Received 17 February 1992; accepted 18 March 1992) Abstract-l. The young of the bandicoot and possum are of similar weight at birth (200-250 mg) but by day 50 post-partum have weights of 100 and 20 g, respectively. The aim of this study was to determine whether the disparate growth rate was associated with a difference in the transfer of thyroxine from mother to young. 2. Radioactive thyroxine was injected intramuscularly into the lactating female and 24 hr later a blood sample was obtained from the mother and the pouch young removed. The amount of radioactive thyroxine remaining in the blood and within the young was determined. 3. The data obtained indicated that milk is a source of thyroid hormones in early pouch life. 4. The hypothesis that greater quantities of thyroxine are transferred in the milk of bandicoots than that of brushtail possums is not supported.
The bandicoots, Peramelidae, have very short periods of oependence, with short gestation, short lactation, small weight at weaning and rapid growth (Lee and Cotkburn, 1985). This rapidity of bandicoot growth and development has been remarked upon previously (Mackerras and Smith, 1960; Lyne, 1964) but is not fully appreciated until comparison is made with other marsupials of similar or lesser size (Russell, 1982). The bandicoot, Isoodon macrourus, has a gestation length of 12.5 days (Lyne, 1974), and a lactation period of 59 days (Gemmell, 1982; Gemmell and Johnston, 1985). By comparison, the gestation and lactation periods of the brushtail possum Trichosurus uufpecufa are 17.5 and 150 days, respectively (Pilton and Sharman, 1962). In our colonies the adult female bandicoot I. macrourus weighs about 1.2 kg and gives birth to a litter of three young, each weighing between 200 and 250 mg (Lyne, 1964; Gemmell et al., 1988), whereas the adult female possum T. vulpecula weighs approximately 1.8 kg and gives birth to a single young, weight 200mg (Gemmell et al., 1988). The young of I. macrourus are weaned at day 59 post partum when they each weigh between 90 and 250 g (Gemmell et al., 1984). A young possum weighs about 20 g at day 60 post partum (Lyne and Verhagen, 1957) and in our colony between 780 and 1031 g when weaned at day 170-180 post partum. Recently, Janssens et al. (1990) have presented evidence that thyroxine may be transferred in the milk from the mother to the developing tammar wallaby Macropus eugenii. Although the young of the bandicoot and the possum are of a similar size at birth, the young of the bandicoot grow faster than those of the brushtail possum both in terms of absolute weight and relative to adult weight. The aim CBPA
of the present study was to determine whether a greater rate of transfer of thyroxine occurs in the bandicoot which could explain the faster growth rate in this family of marsupials.
MATERIALS AND METHODS Eleven female bandicoots, Isoodon macrourus, and 10 female brushtail possums, Trichosururuulpecula,with young were bred in captivity. The bandicoots were kept in large outdoor enclosures, each measuring 30 x 30 m. They were provided with running water and fed on dog food (Drimeat Dog Ration, Provincial Traders Pty., Queensland) ad lib. (Gemmell, 1982; 1989). Adult females were examined weekly and the ages of pouch young not present the previous week were estimated from the length of the head according to the method described by Lyne (1964) for Perumeles nasura. The possums were housed in fully enclosed yards each measuring 2 x 10 m and were also examined weeklv (Gemmell ef al.. 1987; Gemmell et al.. 1988). The ages of pouch young were estimated from the length of the head according to the method described by Lyne and Verhagen (1957). When required female bandicoots were trapped in rectangular baited wire cages approximately 23 x 23 x 52cm in size. Female possums were captured and transported in hessian bags. All female marsupials were anaesthetized with a halothane:oxygen mixture to allow examination of the pouch. Each female received an intramuscular injection of 20 PCi of L-[3’,5’izSIlthyroxine (Amersham International, Amersham, U.K.. IM14. Lot 748, 7.4 Ba. 200 uCi/ml). and the animals returned to their respective holdiug yards .after recovery from the anaesthetic. Exactly 24 hr later the animals were recaptured, anaesthetized and a 3 ml blood sample obtained from the mother by cardiac puncture. The young were removed and weighed. A I ml blood sample and the developing marsupial were each placed in the well of an LKB gamma counter and the amount of radioactivity determined. 541
E” P 4000
0 bandsoots 0 possums
days post partum
Fig. 1. The weight of possums and bandicoots against days post partum. With one female bandicoot the radioactive thyroxine was administered 1 day prior to birth and the young removed for counting on the day of birth. The young had suckled milk from the mother. The body weight of the young was recorded for the day of removal. To determine the radioactivity present as free iodine and that as thyroxine, plasma (1 ml} was d~roteiniz~ with ethanol (3 ml), the supematant evaporated to dryness and the residue suspended in 0.3ml of potassium phosphate buffer (30mmol/l, pH 7.4). Thyroxine was separated from free iodine by gel permeation chromatography on a column (1.2 cm x 10 cm) of Sephadex G-10 equilibrated with phosphate buffer (30 mmol/l, pH 7.4). Preliminary runs with [‘251]thyroxine were used to determine the elution profile of authentic thyroxine. Radioactivity in the pouch young was extracted by homogeni~tion in 5 ml of 70% ethanol evap-
orated to dryness and chromatography on Sephadex G-10. Results were corrected for losses during extraction and chromatography procedures.
The growth rates of the bandicoot and the possum were similar until approximately day 12 post partum. After day 12 there was an increase in growth rate in the bandicoot and by day 20 post partum the developing bandicoot was twice the weight of the develop-
days post paftum
2. The percentage ratio of radioactivity in the developing possum and bandicoot against the days post partum.
ing possum (Fig. 1; Table 1). The increase in the weights of the bandicoot and possum when analysed by linear regression could be described by: wt = 288.2D - 552 (correlation coefficient 0.96) and wt = 157.9D - 127.1 (correlation coefficient 0.99) respectively, where wt = weight in g and D = days post partum. These equations illustrate the greater growth of the bandicoot. Although a standard 20 PCi of radioactive thyroxine was injected into each female marsupial, the amount of radioactivity in the blood of the mother 24 hr iater varied from 1331 to 29166 cpm/ml of blood (Table 1). In bandicoots and possums the amount of radioa~ti~ty in the developing marsupial increased with age. The regression lines for the ratio of the radioactivity in the young relative to that in the blood of the mother was analysed by linear regression and was described by R = 21.8D - 33.2 (correlation coefficient of 0.94) and R = 24.8D - 91.9 (correlation coefficient of 0.91) for bandicoots and possums, respectively, where R = cpm in young/cpm in 1 ml of mother’s blood, and D = days post partum.
DISCUSSION Table 1, Age, weight, radioactivity in blood of the mother, within the developing marsupial and the percentage ratio of radioactivity in the young with respect to the mother Days post partum
Mother’s blood 1 ml (cpm)
303 319 388 848 978 1090 1620 2093 3183 4665 6110
4507 3495 29,166 2450 5804 19,354 7340 10,126 1311 7476 3357
807 1311 2134 2494 4123 11,557 12,501 16,332 3687 34,438 12,068
19.1 37.5 7.3 102.0 71.0 59.1 170.3 161.3 281.3 460.6 357.8
348 416 490 589 658 1190 1245 2070 2416 3142
1431 4935 6245 6406 4126 6164 6551 4118 4628 2128
147 I228 1218 I693 955 6419 3273 5768 18,318 10,044
10.3 22.9 19.5 26.4 23.2 104.0 49.9 140.1 387.2 472.0
2 3 4 6 I 10 12 13 18 20 POsSumS 2 3 4 5 6 8 9 I5 I6 20
This study has shown that ‘251-1abe11edthyroxine can pass via the milk from the female bandicoot and possum to the pouch young. In both species, the transfer of radioactive thyroxine occurred throughout the 3-week study period, with marked increases in the later part of the study. A recent report in the tammar wallaby (Janssens et al., 1990) showed the presence of high thyroxine levels in pouch young less than 160 days post-partum (Setchell, 1974). Milk was suggested as a possible source of thyroxine in the early pouch young; a suggestion which is now supported by this study. Sharman (1965) commented that the bandicoots had a short gestation but had allantoic placentation and gave birth to relatively large young. This statement has been interpreted to mean that the chorioallantoic placenta is a more efficient organ of exchange than the yolk sac placenta of other marsupials (Russell, 1982; Tyndale-Biscoe and Renfree, 1987). However this interpretation is not supported by the similarity in body wt and organ development at birth of I. mucrourus and T. uulpeculu (Gemmell and Rose, 1989; Hughes et al., 1989). These two species have a
Thyroxine transfer in marsupials G;
authors thank the Research Council for financial assistance.
Gemmell R. T. (1982) Breeding bandicoots in Brisbane (Marsupialia: Peramelidae). Rust. Mammal. 5, 187-194. Gemmell R. T. (1989) Breeding season and litter size of the bandicoot Zsoodon macrourus (Marsupialia; Peramelidae), in captivity. Aust. Mammal. 12, 77-79. Gemmell R. T., Hughes R. L. and Jenkin G. (1987) Comparative studies on the hormonal profiles of progesterone and prostaglandin F metabolite in the possum Trichosu~ vul~~lo, pp. 279-291. Royal Zoological Society of New South Wales and Surrey, Beatty and Sons, Sydney. Gemmell R. T. and Johnston G. (1985) The development of thermoregulation and the emergence from the pouch of the marsupial bandicoot, Isoodon macrourus. Physiol. Zool. 58, 299-302.
Gemmell R. T., Johnson G. and Barnes A. (1984) The unifo~ity of growth within the litter of the marsupial Isoodon macrourus. Growth 48, 221-233.
Fig 3. The separation of radioactive thyroxine from free radioactive iodine with gel chromatography. The first peak which’ occurred at volume 5 is thyroxine and the second larger peak at volume 20 is free iodine. (a) 12.4% of the radioactivity was thyroxine in plasma, (b) 20.1% of the radioactivity was thyroxine in the pouch young.
Gemmell R. T., Johnston G. and Bryden M. M. (1988) Osteogenesis in two marsupial species, the bandicoot Isoodon macrourus, and the possum Trichosurus vulpecula. J. Anat. 159, 155-164. Gemmell R. T. and Rose R. W. (1989) Organ development in some newborn marsupials with particular reference to the rat kangeroos. In Kangeroos, Woiiabies und Rat Kangeroos (Edited by Griag G., Jarman P. and Hulme I.), pp. 349-354. Surrey Beat& and Sons, Sydney. Huehes R. L.. Hall L. S.. Tvndale-Biscoe C. H. and Hinds L. A. ‘(1989) Evolutionary implication of macropodid organogenesis. In Kungeroos Wallabies and Rat Kangeroos (Edited by Grigg G., Jarman P. and Hulme I.), pp~ 377405. Surrey Beatty and Sons, Sydney. Janssens P. A., Grigg J. A., Dove H. and Hulbert A. J. (1990) Thyroid hormones during development of a marsupial, the tammar wallaby, Mocropus eugenii. J. Endocr. 127, 427-436.
Johnston G. M. and Gemmell R. T. (1987) Thyroid development in the marsupial bandicoot, Isoodon macrourus.
similar growth rate until approximately day 12 post par-turn (Fig. 1). Subs~u~ntly the development of the bandicoot is more rapid than that of the possum (Gemmell et al., 1988). It is unlikely that thyroxine produced by the young is responsible for the increased growth rate since the follicles of the thyroid are still forming at day 12 and do not show secretory activity until day 30 post-partum (Johnston and Gemmell, 1987). After day 30, the body wt of the bandicoot increases dramatically as does that of the possum about day 100, suggesting the involvement of the thyroid at this stage of growth. Our results show that the rate of transfer of radioactive thyroxine from the mother to the young is similar in both the bandicoot and possum (Figs 2 and 3). Since these results reflect the transfer of maternal non-radioactive thyroxine, and the thyroid of the young is inactive, then the increase in growth rate of the bandicoot, compared to the possum, after day 12 appears not to be a result of thyroid hormone actton. However, possible increases in maternal plasma concentration of thyroxine or thyroid hormone receptor activity in the young need to be excluded before a firm conclusion may be drawn.
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