Ontogenesis of Thyroid Function in the Neonatal Rat. Thyroxine (T4) and Triiodothyronine (T3) Production Rates J. D. DUBOIS AND J. H. DUSSAULT Laborutoire de Recherches en Endocrinologie et Metabolisme, Le Centre Hospitalier de VUniversite Laval, Quebec G1V 4G2, Canada ABSTRACT. The ontogenesis of thyroxine (T4) and triiodothyronine (T3) production in the neonatal rat (from 5 days to adulthood) was examined by measuring [I25I]T4 and [125I]T3 kinetic parameters by single-compartmental analysis. T4 and T3 serum concentrations were measured by specific radioimmunoassay. The volume of distribution (VD) and the metabolic clearance rate (MCR) of T4, high at 5 days, increase to a peak at 22-26 days and decline to adult values. The T4 production rate (PR) low at 5 days, increases to peak values at 14 to 32 days and then declines toward adult values. The T4 fractional removal rate (K) is similar from birth to adulthood. No difference is observed for K, VD and MCR of T3 during the life of the rat. The T3 PR rises from minimal values at 5 days to attain peak values at 26 days. There is therefore a delay of about 12
T
HE ONTOGENESIS of thyroid function has been the subject of previous studies in fetal sheep (1,2), dog (3), and rat (4). In the sheep, there was a high rate of fetal T4 secretion and a relatively lower T 3 secretion during the last trimester of gestation. However, limited data are available for the developmental pattern of thyroid hormone kinetics for the neonatal rat, a species which develops its hypothalamo-pituitarythyroid axis after birth (4,5). Although studies of T4 kinetics have reported hyperactivity of thyroid function during late fetal and early neonatal life in sheep (6), and mice (7), the T 3 production rate in the fetal sheep was markedly diminished until birth (2). Beltz et al. (8) suggested that thyroid hormone turnover in the neonatal rat is slower than that of the adult animal.
days between the peak of T4 PR and the attainment of the peak T 3 PR. The T/T 3 PR ratio decreases from a maximal value at 5 days to values approaching those seen in the adult at 12 days. There is a significant rise at 14 days due to the marked rise in the T4 PR before the ratio returns to adult values by 26 days as a result of the increase in the T3 PR. The increasing T3 PR following the peak in the T4 PR may represent increasing peripheral T4 monodeiodination to T3 and may be analogous to the marked rise in T3 following birth in the sheep and human. These data confirm that the ontogenesis of thyroid hormone secretion occurs after birth in the rat and suggest that the first 20 days of life in the rat may correspond to the last trimester of gestation of the human fetus. (Endocrinology 101: 435, 1977)
For this reason, we were interested in studying the ontogenesis of T4 and T3 kinetics in the neonatal rat using the single compartmental model of Sterling and Chodos (9) as an extension of our previous studies which outlined the development of the hypothalamic-pituitary-thyroid axis (4). Furthermore, because of the similarity of this latter developmental process in fetal sheep (1,2) and man (10), we were interested in the comparison of the ontogenesis of thyroid function as reflected by T4 and T3 kinetics in these species. Materials and Methods
Virgin Sprague-Dawley rats were mated at the supply house (Canadian Breeding Farms, StConstant, Laprairie) and shipped 14-16 days later. Pregnant animals were housed in separate cages with free access to rat laboratory chow Received August 30, 1976. and water in a sound-attenuated, temperature Supported in part by MRC grant no. MA 5730. (24 ± 1 C) and light-controlled (14 hours light) Reprint requests to: J. H. Dussault, M.D., M.Sc, room. Laboratoire de Recherches en Endocrinologie et After birth, the neonates were kept with their Nfetabolisme, Le Centre Hospitalier de l'Universite Laval. 2705, boul. Laurier, Quebec G1Y 4G2, Can- mother until the time of experimentation or weanada. ing at 21 days. 435
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436
Endo i 1977 Vol 101 . No 2
DUBOIS AND DUSSAULT
[125I]T4 (SA, 72.3 /*Ci//*g) and [I25I]T3 (SA, 502 (jLCi/fxg), supplied by Abbott Laboratories, were purified prior to administration by the method of Bellabarba et al. (11). T4 and T3 kinetics experiments were performed at 5, 7, 12, 14, 22, 26, 32 and 40 days after birth. Adults weighing 200-250 g were also studied. Each group comprised 6 studies using both sexes distributed randomly. Older animals Adult and 32 and 40 day old animals received 1-2 ixCi [125I]T4 or [125I]T3 via an indwelling cannula in the superior vena cava. Lugol's solution (0.1 ml of a 10% solution in 0.9% NaCl) was injected ip to prevent thyroidal accumulation and reutilization of radioactive iodine. Blood (0.6 ml) was sampled via the cannula at 15, 30, 45 min, 2, 4, 6, 8, 12 and 24 h after injection for adults, and at 2, 4, 6, 8, 12 and 24 h for 32 and 40 day old rats. Blood was replaced with an equal volume of 0.9% NaCl to maintain the circulating blood volume. The blood was immediately centrifuged and the serum frozen at - 2 0 C until extraction.
Calculations The per cent injected dose of [125I]T4 or [125I]Ts/ liter of serum was calculated for each extract and plotted semilogarithmically. The linear component of the disappearance curve so obtained was defined by the method of least mean squares and extrapolated to zero time. Assessement of the parameters of T4 and T3 kinetics was carried out using the single compartmental graphic analysis method of Sterling et al. (9) for all groups. Serum T4 (13) and T3 (14) concentrations were measured by specific radioimmunoassay on serum taken before injection for 32, 40 day old and adult rats and on other rats of the same litter for 5 to 26 day old rats. Statistical analyses were performed using Student's t test and analysis of variance (15).
Results
Thyroxine (TJ Figure 1 illustrates the disappearance curve of [125I]T4 in adult rats. There was an initial rapid rate of disappearance of the labelled hormone until 6 h when equilibrium, as repYounger animals resented by the linear component of the Since we observed no significant difference curve, was attained. The equilibrium time between the fate of iv and ip administered was similar for all groups and there were [125I]T4 and [125I]T3 in older animals, 5, 7, 12, 14, no significant differences observed among 22 and 26 day old rats received 1-2 fid [125I]T4 the curves. or [125I]T3 in a volume of 20 /xl and Lugol's Table 1 shows the T4 kinetic parameters solution ip. Six rats were sacrificed by decapita- as calculated by the single-compartmental tion at 2, 4, 6, 8, 12 and 24 h after administration analysis method of Sterling and Chodos of the tracer dose. Serum was stored at —20 C (9). No significant difference was observed until extraction. in the fractional removal rate (K) of the labelled hormone in any of the groups studied. Extraction The volume of distribution (VD) is signifSerum was extracted in duplicate using a icantly elevated at 5 days compared to adults modification of the method of Fisher et al. (12), (P < 0.001) and remains unchanged until 14 yielding a recovery of 95% organic iodide and days, after which time a significant increase less than 0.5% inorganic iodide, as verified by (P
T
HE ONTOGENESIS of thyroid function has been the subject of previous studies in fetal sheep (1,2), dog (3), and rat (4). In the sheep, there was a high rate of fetal T4 secretion and a relatively lower T 3 secretion during the last trimester of gestation. However, limited data are available for the developmental pattern of thyroid hormone kinetics for the neonatal rat, a species which develops its hypothalamo-pituitarythyroid axis after birth (4,5). Although studies of T4 kinetics have reported hyperactivity of thyroid function during late fetal and early neonatal life in sheep (6), and mice (7), the T 3 production rate in the fetal sheep was markedly diminished until birth (2). Beltz et al. (8) suggested that thyroid hormone turnover in the neonatal rat is slower than that of the adult animal.
days between the peak of T4 PR and the attainment of the peak T 3 PR. The T/T 3 PR ratio decreases from a maximal value at 5 days to values approaching those seen in the adult at 12 days. There is a significant rise at 14 days due to the marked rise in the T4 PR before the ratio returns to adult values by 26 days as a result of the increase in the T3 PR. The increasing T3 PR following the peak in the T4 PR may represent increasing peripheral T4 monodeiodination to T3 and may be analogous to the marked rise in T3 following birth in the sheep and human. These data confirm that the ontogenesis of thyroid hormone secretion occurs after birth in the rat and suggest that the first 20 days of life in the rat may correspond to the last trimester of gestation of the human fetus. (Endocrinology 101: 435, 1977)
For this reason, we were interested in studying the ontogenesis of T4 and T3 kinetics in the neonatal rat using the single compartmental model of Sterling and Chodos (9) as an extension of our previous studies which outlined the development of the hypothalamic-pituitary-thyroid axis (4). Furthermore, because of the similarity of this latter developmental process in fetal sheep (1,2) and man (10), we were interested in the comparison of the ontogenesis of thyroid function as reflected by T4 and T3 kinetics in these species. Materials and Methods
Virgin Sprague-Dawley rats were mated at the supply house (Canadian Breeding Farms, StConstant, Laprairie) and shipped 14-16 days later. Pregnant animals were housed in separate cages with free access to rat laboratory chow Received August 30, 1976. and water in a sound-attenuated, temperature Supported in part by MRC grant no. MA 5730. (24 ± 1 C) and light-controlled (14 hours light) Reprint requests to: J. H. Dussault, M.D., M.Sc, room. Laboratoire de Recherches en Endocrinologie et After birth, the neonates were kept with their Nfetabolisme, Le Centre Hospitalier de l'Universite Laval. 2705, boul. Laurier, Quebec G1Y 4G2, Can- mother until the time of experimentation or weanada. ing at 21 days. 435
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 08 July 2015. at 13:55 For personal use only. No other uses without permission. . All rights reserved.
436
Endo i 1977 Vol 101 . No 2
DUBOIS AND DUSSAULT
[125I]T4 (SA, 72.3 /*Ci//*g) and [I25I]T3 (SA, 502 (jLCi/fxg), supplied by Abbott Laboratories, were purified prior to administration by the method of Bellabarba et al. (11). T4 and T3 kinetics experiments were performed at 5, 7, 12, 14, 22, 26, 32 and 40 days after birth. Adults weighing 200-250 g were also studied. Each group comprised 6 studies using both sexes distributed randomly. Older animals Adult and 32 and 40 day old animals received 1-2 ixCi [125I]T4 or [125I]T3 via an indwelling cannula in the superior vena cava. Lugol's solution (0.1 ml of a 10% solution in 0.9% NaCl) was injected ip to prevent thyroidal accumulation and reutilization of radioactive iodine. Blood (0.6 ml) was sampled via the cannula at 15, 30, 45 min, 2, 4, 6, 8, 12 and 24 h after injection for adults, and at 2, 4, 6, 8, 12 and 24 h for 32 and 40 day old rats. Blood was replaced with an equal volume of 0.9% NaCl to maintain the circulating blood volume. The blood was immediately centrifuged and the serum frozen at - 2 0 C until extraction.
Calculations The per cent injected dose of [125I]T4 or [125I]Ts/ liter of serum was calculated for each extract and plotted semilogarithmically. The linear component of the disappearance curve so obtained was defined by the method of least mean squares and extrapolated to zero time. Assessement of the parameters of T4 and T3 kinetics was carried out using the single compartmental graphic analysis method of Sterling et al. (9) for all groups. Serum T4 (13) and T3 (14) concentrations were measured by specific radioimmunoassay on serum taken before injection for 32, 40 day old and adult rats and on other rats of the same litter for 5 to 26 day old rats. Statistical analyses were performed using Student's t test and analysis of variance (15).
Results
Thyroxine (TJ Figure 1 illustrates the disappearance curve of [125I]T4 in adult rats. There was an initial rapid rate of disappearance of the labelled hormone until 6 h when equilibrium, as repYounger animals resented by the linear component of the Since we observed no significant difference curve, was attained. The equilibrium time between the fate of iv and ip administered was similar for all groups and there were [125I]T4 and [125I]T3 in older animals, 5, 7, 12, 14, no significant differences observed among 22 and 26 day old rats received 1-2 fid [125I]T4 the curves. or [125I]T3 in a volume of 20 /xl and Lugol's Table 1 shows the T4 kinetic parameters solution ip. Six rats were sacrificed by decapita- as calculated by the single-compartmental tion at 2, 4, 6, 8, 12 and 24 h after administration analysis method of Sterling and Chodos of the tracer dose. Serum was stored at —20 C (9). No significant difference was observed until extraction. in the fractional removal rate (K) of the labelled hormone in any of the groups studied. Extraction The volume of distribution (VD) is signifSerum was extracted in duplicate using a icantly elevated at 5 days compared to adults modification of the method of Fisher et al. (12), (P < 0.001) and remains unchanged until 14 yielding a recovery of 95% organic iodide and days, after which time a significant increase less than 0.5% inorganic iodide, as verified by (P
