Growth Hormone Induced Stimulation of the T4 to T3 Conversion in Fed and Fasting Dwarf Goats A. Iqbal1, A. M. Cheema2 and E. R. Kuhn1 Laboratory of Comparative Endocrinology, Zoological Institute, K. U. L., Leuven, Belgium Department of Zoology, University of the Punjab, Lahore, Pakistan
Fed and food deprived (7 days) adult dwarf goats were injected intravenously with 75 ug/kg b. w. of ovine growth hormone (o-GH). Blood samples were collected from the jugular vein — 2 , - 1 and 0 hr prior to and 30, 60, 90 and 120 min after injection and assayed for thyroxine (T4) and triiodothyronine (T3) concentrations. The 5'-monodeiodination (5'-D) activity was consequently determined in liver and kidney samples following slaughtering. Fasting alone increased plasma concentrations of T3 and T4, whereas injection of o-GH raised T3 additionally and more profoundly in food deprived animals compared to fed ones. A small increase in plasma T4 was also observed following O-GH injection, but only in starved goats. At the same time the hepatic, but not the kidney, 5'-D activity was stimulated in food deprived and G H injected animals. It is concluded that during prolonged fasting an increased peripheral T4 to T3 conversion is occurring contrary to the known decrease in T3 production during short periods of food deprivation. This increased conversion may be under the control of GH.
been reported for children (Sato, Suzuki, Taketani, Ishiguro, Masuyami, Takata, Sano, Kawashima, Koizumi and Nakajima 1977), cows (Peel, Frank, Baumon and Gorewit 1983) and lamb (Kuhn, Van Osselaer, Siau, Decuypere and Moreels 1986), whereas in the rat apparently the in vitro hepatic 5'-D activity is stimulated by G H without affecting plasma concentrations of thyroid hormone (Kuhn, Peeters and Pauwels 1986). As is known for mammals (Vagenakis, Burger, Portnay, Rudolph, O'Brian, Azizi, Arky, Nocid, Ingbar and Braverman 1975; Kaplan 1986), chicken react to a short period of fasting with a decrease in circulating levels of T3 and an increase in T4 together with a decrease in liver 5'-D (Decuypere and Kuhn 1984). In the chicken it has been shown that during a prolonged period of fasting plasma concentrations of T3 may become normal again, suggesting a reactivation of peripheral 5'-D (Verheyen, Decuypere, Kuhn, Fontaine and De Groote 1983). We have studied the possible reactivation of the thyroid function during prolonged food deprivation, together with the effect of G H on the T4 to T3 conversion and hepatic 5'-D in normal and starved dwarf goats.
Key words Fasting —Thyroid — GHinGoats
Introduction
Materials and Methods Adult dwarf goats of about 2 years old with a mean body weight of 25 kg were used. A group of 7 animals were food deprived for 7 days before the injection of GH but with free access to water. The other two groups of 7 animals each, were grazing animals with free access to water.
In lower vertebrates, growth hormone (GH) may control the peripheral conversion of thyroxine (T4) into triiodothyronine (T3) by stimulating the hepatic 5'-monodeiodination activity (5'-D). This has been demonstrated in eel (de Luze, Leloup and Olivereau 1984), salmonids (Farbridge and Leatherland 1988), the chick embryo (Kuhn, Vanderpooten, Huybrechts, Decuypere, Darras and Sharp 1988; Berghman, Darras, Huybrechts, Decuypere, Vandesande and Kuhn 1989) and the adult chicken (Kuhn, Verheyen, Chiasson, Huybrechts, Van den Steen and Decuypere 1987; Kuhn, Decuypere, Iqbal, Luysterborgh and Michielsen 1988). In mammals, however, conclusive data concerning this hormonal control are lacking. Indirect evidence for a hepatic stimulation of 5'-D by G H as judged by increased plasma concentrations of T3 has
One day before the injections of either saline or GH, the jugular vein of each animal was punctured with an injection needle and cannulated by inserting a silastic tubing. On the day of experiment the three groups, one for saline injection, second a non-fasted group for GH injection and third a fasted group for GH injection received either 0.5 ml physiological saline (0.9% NaCl) or GH (75 ug/kg b. w., NIAMDD-oGH-13, Bethesda, MD, U. S. A.) into the jugular vein. Blood samples were collected in heparin — 2 , - 1 and 0 hr prior to and 30, 60, 90 and 120 minutes after either saline or GH injection.
Horm.metab.Res.22(1990)566-568 © Georg Thieme Verlag Stuttgart • New York
Received: 8 Dec. 1989
Plasma was separated by centrifugation at 4,000 g for 10 min and stored at — 20 °C prior to assay for T4 and T3. Liver and kidney samples were excised from each animal killed in the slaughterhouse just after the last blood sampling and immediately stored at — 20 °CtillassayforT4 5'-deiodination activity.
Accepted: 3 May 1990
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Summary
Horm. metab. Res. 22 (1990)
GHand T4 to T3 Conversion in Dwarf Goats
Fig. 2 Effect of a single i. v. injection of saline or GH (75 u.g/kg b. w.) on plasma concentrations of T4 in control (saline) ( • — • ) , non-fasted (GH injected) ( • - - - • ) and in fasted (GH injected) ( • - — • ) adult dwarf goats (x±SEM; n = 7).
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Fig. 1 Effect of a single i. v. injection of saline or GH (75 u.g/kg b. w.) on plasma concentrations of T3 in control (saline) ( • — • ) , non-fasted (GH injected) ( • - - - • ) and in fasted (Gh injected) ( • — • ) adult dwarf goats (x±SEM; n = 7). The arrow indicates the time of injection. As compared with saline control, *P < 0.05, **P < 0.01 and * " P < 0.001 (ANOVA, followed by Student's t-test). n. s. = not significant.
The T3 and T4 concentrations in plasma were assayed by using tracer obtained from Amersham International (U. K.), rabbit T3 antiserum from Mallinckrodt (Germany) and a laboratory-raised rabbit T4 antiserum. This T4 antiserum had a 0.16% cross-reactivity with T3. The sensitivity of our T4 and T3 assay was 3.12 ng/ml and 0.063 ng/ml respectively, with an inter- and intra-assay coefficient of variation of 6.2 and 2.9 respectively for T3 and 10.1 and 3.2 for T4. The hepatic and renal 5'-deiodination activity was measured according to the method used by Visser, Fekkes, Docter and Hennemann (1979) and modified by Decuypere, Scanes and Kiihn (1983). Briefly, livers and kidneys were homogenized in 1 ml of a phosphate buffer (0.15 M). After centrifugation 50 ul of the supernatant was incubated for 1 h in a water bath at 37 °C with 10 ul of T4 (5 uM) and 200 ul of dithiothreitol (DTT, 3 mM). To stop the reaction 1 ml of ice cold polyoxyethylene monolaurylether (Brij 35, 0.62%, Sigma) was added to each tube followed by T3 assay. Protein concentration in the liver and kidney homogenates was measured by the method used by Lowry, Rosebrough, Farr and Randall (1951). Statistical analysis of the data was performed by analysis of variance (ANOVA) followed by Student's t-test for comparison between specific pairs of means.
Fig. 3 Effect of a single i. v. injection of saline or GH (75 u,g/kg b. w.) on hepatic and renal 5'-D activity in fed and fasted adult dwarf goats (x + SEM; n = 7). As compared with respective saline group (S) **P < 0.01 (ANOVA, followed by Student's t-test). n. s. = not significant.
Results Fasting alone for this prolonged period increased plasma concentrations of T4 and T3 at the same time (P < 0.05). An injection of G H raised plasma concentrations of T3 in normal fed and food deprived animals. This increase was more pronounced in the latter group with T3 levels being still elevated after 2 hours compared to fed saline or G H injected goats (Fig. 1). G H injection also increased T4 concentrations in fasting goats but this significant increase in T4 was not observed in fed goats injected with GH, compared to saline control (Fig. 2). Finally no effect on the in vitro hepatic or renal 5'D activity could be found in fed animals killed after 2 hours, whereas this activity was still stimulated in the liver, but not in the kidney, of food deprived animals injected with G H (Fig. 3).
Discussion These results confirm the previous observations made in chicken {Verheyen et al. 1983) that plasma concentrations of T3 are not decreased after prolonged fasting but may even be higher than in control fed animals. The initial lowering of circulating T3 which is present in all birds and mammals studied has been explained as a means of reducing oxygen consumption and metabolism in order to maintain nutritional reserves {Harris, Fang, Azizi, Lipworth, Vagenakis and Braverman 1978). An increased peripheral T4 intoT3 conversion apparently is required again when food supply is lacking for a longer period. The present results indicate that G H , which is known to be released during fasting {Kaplan 1986), may be re-
Horm. metab. Res. 22 (1990) sponsible for this conversion. This m a y be also s u p p o r t e d by the observation that food deprived a n d G H injected goats d o have an increased hepatic 5'-D activity c o m p a r e d to fed saline or G H injected animals, whereas in birds (Decuypere and Kiihn 1984) a n d m a m m a l s (Kaplan 1986) this activity is decreased during fasting. References Berghman, L., V. M. Darras, L. M. Huybrechts, E. Decuypere, F. Vandesande, E. R. Kiihn: Evidence for chicken G H as the only hypophyseal factor responsible for the stimulation of hepatic 5'monodeiodination activity in the chick embryo. Reprod. Nutr. Develop.29:197-202(1989) de Luze, A., J. Leloup, M. Olivereau: Fish growth hormone enhances peripheral conversion of thyroxine to triiodothyronine in the eel (Anguilla anguilla L.). Gen. Comp. Endocrinol. 56: 308—312 (1984) Decuypere, E., E. R. Kiihn: Effect of fasting and feeding time on circadian rhythms of serum thyroid hormone concentration, glucose, liver monodeiodinase activity and rectal temperature in growing chickens. Domestic Animal Endocrinology 1:251 —262 (1984) Decuypere, E., C. G. Scanes, E. R. Kiihn: Effects of glucocorticoids on circulating concentrations of thyroxine (T4) and triiodothyronine (T3) and on peripheral monodeiodination in pre- and posthatching chicken. Horm. Metab. Res. 15:233-236 (1983) Farbridge, K. J., J. F. Leatherland: Interaction between ovine growth hormone and triiodo-L-thyronine on metabolic reserves of rainbowtrout. Fish Physiology and Biochemistry 5:141 — 151 (1988) Harris, A. R. C, F. Fang, L. Azizi, L. Lipworth, A. G. Vagenakis, L. E. Braverman: Effect of starvation on hypothalamic-pituitary-thyroid function in rats. Metabolism 27:1027-1083(1978) Kaplan, M. M.: Regulatory influences on iodothyronine deiodination in animal tissues. In: Thyroid Hormone Metabolism. (Ed. G. Hennemann), New York. Marcel Dekker Inc. (1986), pp. 231—253 Kiihn, E. R., E. Decuypere, A. Iqbal, D. Luysterborgh, R. Michielsen: Thyrotropic and peripheral activities of thyrotrophin and thyrotrophin-releasing hormone in the chick embryo and adult chicken. Horm. Metab. Res. 20:158-162 (1988) Kiihn, E. R., R. Peelers, J. Pauwels: Stimulation of the hepatic 5'-monodeiodination activity in the adult male rat following injection of ovine growth hormone. International Research and Communication System Medical Science 14: 804 (1986) Kiihn, E. R., A. Vanderpooten, L. M. Huybrechts, E. Decuypere, V. Darras, P. J. Sharp: Hypothalamic hormones that release growth hor-
A. Iqbal, A. M. Cheema andE. R. Kiihn mone stimulate hepatic 5'-monodeiodination activity in the chick embryo. J. Endocrinol. 118:233-236(1988) Kiihn, E. R., P. Van Osselaer, O. Siau, E. Decuypere, A. Moreels: Thyroid function in newborn lambs: influence of prolactin and growth hormone. J. Endocrinol. 109:215-219(1986) Kiihn, E. R., G. Verheyen, R. B. Chiasson, L. Huybrechts, P. Van den Steen, E. Decuypere: Growth hormone stimulates the peripheral conversion of thyroxine into triiodothyronine by increasing the liver 5'-monodeiodinase activity in the fasted and normal fed chicken. Horm. Metab. Res. 19:304-308 (1987) Lowry, O. H., N. T. Rosebrough, A. L. Fan, R. J. Randall: Protein measurement with Folin reagent. J. Biol. Chem. 193: 265—275 (1951) Peel, C. J., T. J. Frank, D. E. Baumon, R. C. Gorewit: Effects of exogenous growth hormone in early and late lactation on lactation performance of dairy cows. J. Dairy Sci. 66:776—782 (1983) Sato, T., Y. Suzuki, T. Taketani, K. Ishiguro, T. Masuyami, Y. Takata, M. Sano, H. Kawashima, S. Koizumi, H. Nakajima: Enhanced peripheral conversion of thyroxine to triiodothyronine during h-GH therapy in G H deficient children. Journal of Clinical Endocrinology and Metabolism 45:324-329 (1977) Vagenakis, A. G., A. Burger, G. I. Portnay, M. Rudolph, J. T. OBrian, F. Azizi, R. A. Arky, P. Nocid, S. H. Ingbar, L. A. Braverman: Diversion of peripheral thyroxine metabolism from activating to inactivating pathways during complete fasting. J. Clin. Endocrinol. 42: 197-200(1975) Verheyen, G., E. Decuypere, E. R. Kiihn, G. Fontaine, G. De Groote: Arret de la ponte par induction chez la poule. Effet de differentes methodes sur certains parametres de production et sur les concentrations en hormones thyroi'diennes, en prolactine, en Ca, P, Na et en protei'nes dans le serum sanguin. Revue de 1'Agriculture 36: 1535-1559(1983) Visser, T. J., D. Fekkes, R. Docter, G. Hennemann: Kinetics of enzymic reductive deiodination of iodothyronines. Biochem. J. 179: 489— 495(1979)
Requests for reprints should be addressed to: Prof. E. R. Kiihn Laboratory of Comparative Endocrinology Katholieke Universiteit Leuven Naamsestraat 61 B-3000 Leuven (Belgium)
Downloaded by: NYU. Copyrighted material.
568
Fed and food deprived (7 days) adult dwarf goats were injected intravenously with 75 ug/kg b. w. of ovine growth hormone (o-GH). Blood samples were collected from the jugular vein — 2 , - 1 and 0 hr prior to and 30, 60, 90 and 120 min after injection and assayed for thyroxine (T4) and triiodothyronine (T3) concentrations. The 5'-monodeiodination (5'-D) activity was consequently determined in liver and kidney samples following slaughtering. Fasting alone increased plasma concentrations of T3 and T4, whereas injection of o-GH raised T3 additionally and more profoundly in food deprived animals compared to fed ones. A small increase in plasma T4 was also observed following O-GH injection, but only in starved goats. At the same time the hepatic, but not the kidney, 5'-D activity was stimulated in food deprived and G H injected animals. It is concluded that during prolonged fasting an increased peripheral T4 to T3 conversion is occurring contrary to the known decrease in T3 production during short periods of food deprivation. This increased conversion may be under the control of GH.
been reported for children (Sato, Suzuki, Taketani, Ishiguro, Masuyami, Takata, Sano, Kawashima, Koizumi and Nakajima 1977), cows (Peel, Frank, Baumon and Gorewit 1983) and lamb (Kuhn, Van Osselaer, Siau, Decuypere and Moreels 1986), whereas in the rat apparently the in vitro hepatic 5'-D activity is stimulated by G H without affecting plasma concentrations of thyroid hormone (Kuhn, Peeters and Pauwels 1986). As is known for mammals (Vagenakis, Burger, Portnay, Rudolph, O'Brian, Azizi, Arky, Nocid, Ingbar and Braverman 1975; Kaplan 1986), chicken react to a short period of fasting with a decrease in circulating levels of T3 and an increase in T4 together with a decrease in liver 5'-D (Decuypere and Kuhn 1984). In the chicken it has been shown that during a prolonged period of fasting plasma concentrations of T3 may become normal again, suggesting a reactivation of peripheral 5'-D (Verheyen, Decuypere, Kuhn, Fontaine and De Groote 1983). We have studied the possible reactivation of the thyroid function during prolonged food deprivation, together with the effect of G H on the T4 to T3 conversion and hepatic 5'-D in normal and starved dwarf goats.
Key words Fasting —Thyroid — GHinGoats
Introduction
Materials and Methods Adult dwarf goats of about 2 years old with a mean body weight of 25 kg were used. A group of 7 animals were food deprived for 7 days before the injection of GH but with free access to water. The other two groups of 7 animals each, were grazing animals with free access to water.
In lower vertebrates, growth hormone (GH) may control the peripheral conversion of thyroxine (T4) into triiodothyronine (T3) by stimulating the hepatic 5'-monodeiodination activity (5'-D). This has been demonstrated in eel (de Luze, Leloup and Olivereau 1984), salmonids (Farbridge and Leatherland 1988), the chick embryo (Kuhn, Vanderpooten, Huybrechts, Decuypere, Darras and Sharp 1988; Berghman, Darras, Huybrechts, Decuypere, Vandesande and Kuhn 1989) and the adult chicken (Kuhn, Verheyen, Chiasson, Huybrechts, Van den Steen and Decuypere 1987; Kuhn, Decuypere, Iqbal, Luysterborgh and Michielsen 1988). In mammals, however, conclusive data concerning this hormonal control are lacking. Indirect evidence for a hepatic stimulation of 5'-D by G H as judged by increased plasma concentrations of T3 has
One day before the injections of either saline or GH, the jugular vein of each animal was punctured with an injection needle and cannulated by inserting a silastic tubing. On the day of experiment the three groups, one for saline injection, second a non-fasted group for GH injection and third a fasted group for GH injection received either 0.5 ml physiological saline (0.9% NaCl) or GH (75 ug/kg b. w., NIAMDD-oGH-13, Bethesda, MD, U. S. A.) into the jugular vein. Blood samples were collected in heparin — 2 , - 1 and 0 hr prior to and 30, 60, 90 and 120 minutes after either saline or GH injection.
Horm.metab.Res.22(1990)566-568 © Georg Thieme Verlag Stuttgart • New York
Received: 8 Dec. 1989
Plasma was separated by centrifugation at 4,000 g for 10 min and stored at — 20 °C prior to assay for T4 and T3. Liver and kidney samples were excised from each animal killed in the slaughterhouse just after the last blood sampling and immediately stored at — 20 °CtillassayforT4 5'-deiodination activity.
Accepted: 3 May 1990
Downloaded by: NYU. Copyrighted material.
Summary
Horm. metab. Res. 22 (1990)
GHand T4 to T3 Conversion in Dwarf Goats
Fig. 2 Effect of a single i. v. injection of saline or GH (75 u.g/kg b. w.) on plasma concentrations of T4 in control (saline) ( • — • ) , non-fasted (GH injected) ( • - - - • ) and in fasted (GH injected) ( • - — • ) adult dwarf goats (x±SEM; n = 7).
Downloaded by: NYU. Copyrighted material.
Fig. 1 Effect of a single i. v. injection of saline or GH (75 u.g/kg b. w.) on plasma concentrations of T3 in control (saline) ( • — • ) , non-fasted (GH injected) ( • - - - • ) and in fasted (Gh injected) ( • — • ) adult dwarf goats (x±SEM; n = 7). The arrow indicates the time of injection. As compared with saline control, *P < 0.05, **P < 0.01 and * " P < 0.001 (ANOVA, followed by Student's t-test). n. s. = not significant.
The T3 and T4 concentrations in plasma were assayed by using tracer obtained from Amersham International (U. K.), rabbit T3 antiserum from Mallinckrodt (Germany) and a laboratory-raised rabbit T4 antiserum. This T4 antiserum had a 0.16% cross-reactivity with T3. The sensitivity of our T4 and T3 assay was 3.12 ng/ml and 0.063 ng/ml respectively, with an inter- and intra-assay coefficient of variation of 6.2 and 2.9 respectively for T3 and 10.1 and 3.2 for T4. The hepatic and renal 5'-deiodination activity was measured according to the method used by Visser, Fekkes, Docter and Hennemann (1979) and modified by Decuypere, Scanes and Kiihn (1983). Briefly, livers and kidneys were homogenized in 1 ml of a phosphate buffer (0.15 M). After centrifugation 50 ul of the supernatant was incubated for 1 h in a water bath at 37 °C with 10 ul of T4 (5 uM) and 200 ul of dithiothreitol (DTT, 3 mM). To stop the reaction 1 ml of ice cold polyoxyethylene monolaurylether (Brij 35, 0.62%, Sigma) was added to each tube followed by T3 assay. Protein concentration in the liver and kidney homogenates was measured by the method used by Lowry, Rosebrough, Farr and Randall (1951). Statistical analysis of the data was performed by analysis of variance (ANOVA) followed by Student's t-test for comparison between specific pairs of means.
Fig. 3 Effect of a single i. v. injection of saline or GH (75 u,g/kg b. w.) on hepatic and renal 5'-D activity in fed and fasted adult dwarf goats (x + SEM; n = 7). As compared with respective saline group (S) **P < 0.01 (ANOVA, followed by Student's t-test). n. s. = not significant.
Results Fasting alone for this prolonged period increased plasma concentrations of T4 and T3 at the same time (P < 0.05). An injection of G H raised plasma concentrations of T3 in normal fed and food deprived animals. This increase was more pronounced in the latter group with T3 levels being still elevated after 2 hours compared to fed saline or G H injected goats (Fig. 1). G H injection also increased T4 concentrations in fasting goats but this significant increase in T4 was not observed in fed goats injected with GH, compared to saline control (Fig. 2). Finally no effect on the in vitro hepatic or renal 5'D activity could be found in fed animals killed after 2 hours, whereas this activity was still stimulated in the liver, but not in the kidney, of food deprived animals injected with G H (Fig. 3).
Discussion These results confirm the previous observations made in chicken {Verheyen et al. 1983) that plasma concentrations of T3 are not decreased after prolonged fasting but may even be higher than in control fed animals. The initial lowering of circulating T3 which is present in all birds and mammals studied has been explained as a means of reducing oxygen consumption and metabolism in order to maintain nutritional reserves {Harris, Fang, Azizi, Lipworth, Vagenakis and Braverman 1978). An increased peripheral T4 intoT3 conversion apparently is required again when food supply is lacking for a longer period. The present results indicate that G H , which is known to be released during fasting {Kaplan 1986), may be re-
Horm. metab. Res. 22 (1990) sponsible for this conversion. This m a y be also s u p p o r t e d by the observation that food deprived a n d G H injected goats d o have an increased hepatic 5'-D activity c o m p a r e d to fed saline or G H injected animals, whereas in birds (Decuypere and Kiihn 1984) a n d m a m m a l s (Kaplan 1986) this activity is decreased during fasting. References Berghman, L., V. M. Darras, L. M. Huybrechts, E. Decuypere, F. Vandesande, E. R. Kiihn: Evidence for chicken G H as the only hypophyseal factor responsible for the stimulation of hepatic 5'monodeiodination activity in the chick embryo. Reprod. Nutr. Develop.29:197-202(1989) de Luze, A., J. Leloup, M. Olivereau: Fish growth hormone enhances peripheral conversion of thyroxine to triiodothyronine in the eel (Anguilla anguilla L.). Gen. Comp. Endocrinol. 56: 308—312 (1984) Decuypere, E., E. R. Kiihn: Effect of fasting and feeding time on circadian rhythms of serum thyroid hormone concentration, glucose, liver monodeiodinase activity and rectal temperature in growing chickens. Domestic Animal Endocrinology 1:251 —262 (1984) Decuypere, E., C. G. Scanes, E. R. Kiihn: Effects of glucocorticoids on circulating concentrations of thyroxine (T4) and triiodothyronine (T3) and on peripheral monodeiodination in pre- and posthatching chicken. Horm. Metab. Res. 15:233-236 (1983) Farbridge, K. J., J. F. Leatherland: Interaction between ovine growth hormone and triiodo-L-thyronine on metabolic reserves of rainbowtrout. Fish Physiology and Biochemistry 5:141 — 151 (1988) Harris, A. R. C, F. Fang, L. Azizi, L. Lipworth, A. G. Vagenakis, L. E. Braverman: Effect of starvation on hypothalamic-pituitary-thyroid function in rats. Metabolism 27:1027-1083(1978) Kaplan, M. M.: Regulatory influences on iodothyronine deiodination in animal tissues. In: Thyroid Hormone Metabolism. (Ed. G. Hennemann), New York. Marcel Dekker Inc. (1986), pp. 231—253 Kiihn, E. R., E. Decuypere, A. Iqbal, D. Luysterborgh, R. Michielsen: Thyrotropic and peripheral activities of thyrotrophin and thyrotrophin-releasing hormone in the chick embryo and adult chicken. Horm. Metab. Res. 20:158-162 (1988) Kiihn, E. R., R. Peelers, J. Pauwels: Stimulation of the hepatic 5'-monodeiodination activity in the adult male rat following injection of ovine growth hormone. International Research and Communication System Medical Science 14: 804 (1986) Kiihn, E. R., A. Vanderpooten, L. M. Huybrechts, E. Decuypere, V. Darras, P. J. Sharp: Hypothalamic hormones that release growth hor-
A. Iqbal, A. M. Cheema andE. R. Kiihn mone stimulate hepatic 5'-monodeiodination activity in the chick embryo. J. Endocrinol. 118:233-236(1988) Kiihn, E. R., P. Van Osselaer, O. Siau, E. Decuypere, A. Moreels: Thyroid function in newborn lambs: influence of prolactin and growth hormone. J. Endocrinol. 109:215-219(1986) Kiihn, E. R., G. Verheyen, R. B. Chiasson, L. Huybrechts, P. Van den Steen, E. Decuypere: Growth hormone stimulates the peripheral conversion of thyroxine into triiodothyronine by increasing the liver 5'-monodeiodinase activity in the fasted and normal fed chicken. Horm. Metab. Res. 19:304-308 (1987) Lowry, O. H., N. T. Rosebrough, A. L. Fan, R. J. Randall: Protein measurement with Folin reagent. J. Biol. Chem. 193: 265—275 (1951) Peel, C. J., T. J. Frank, D. E. Baumon, R. C. Gorewit: Effects of exogenous growth hormone in early and late lactation on lactation performance of dairy cows. J. Dairy Sci. 66:776—782 (1983) Sato, T., Y. Suzuki, T. Taketani, K. Ishiguro, T. Masuyami, Y. Takata, M. Sano, H. Kawashima, S. Koizumi, H. Nakajima: Enhanced peripheral conversion of thyroxine to triiodothyronine during h-GH therapy in G H deficient children. Journal of Clinical Endocrinology and Metabolism 45:324-329 (1977) Vagenakis, A. G., A. Burger, G. I. Portnay, M. Rudolph, J. T. OBrian, F. Azizi, R. A. Arky, P. Nocid, S. H. Ingbar, L. A. Braverman: Diversion of peripheral thyroxine metabolism from activating to inactivating pathways during complete fasting. J. Clin. Endocrinol. 42: 197-200(1975) Verheyen, G., E. Decuypere, E. R. Kiihn, G. Fontaine, G. De Groote: Arret de la ponte par induction chez la poule. Effet de differentes methodes sur certains parametres de production et sur les concentrations en hormones thyroi'diennes, en prolactine, en Ca, P, Na et en protei'nes dans le serum sanguin. Revue de 1'Agriculture 36: 1535-1559(1983) Visser, T. J., D. Fekkes, R. Docter, G. Hennemann: Kinetics of enzymic reductive deiodination of iodothyronines. Biochem. J. 179: 489— 495(1979)
Requests for reprints should be addressed to: Prof. E. R. Kiihn Laboratory of Comparative Endocrinology Katholieke Universiteit Leuven Naamsestraat 61 B-3000 Leuven (Belgium)
Downloaded by: NYU. Copyrighted material.
568
