E.R. Kühn

Distribution and regulation of chicken growth hormone secretagogue receptor isoforms

Chicken ghrelin has recently been isolated as a hormone which stimulates growth hormone and corticosterone secretion in chicken. Ghrelin mediates these actions in mammals by binding to the growth hormone secretagogue receptor (GHS-R). In this study, we describe the partial cloning of two chicken GHS-R (cGHS-R) isoforms: cGHS-R1a and cGHS-R1c. cGHS-R1a and cGHS-R1c cDNA show, respectively, 81 and 78% homology with the corresponding parts of the human GHS-R1a cDNA. In contrast to the human GHS-R1b isoform, which is truncated after transmembrane domain 5 (TM-5), the chicken GHS-R1c isoform lacks 16 amino acids in TM-6 suggesting that this isoform is not active in ghrelin signal transduction. The cystein residues, N-linked glycosylation sites and potential phosphorylation sites, found in the human GHS-R1a, were also conserved in both chicken isoforms. RT-PCR analysis demonstrated cGHS-R1a and cGHS-R1c mRNA expression in all tissues tested, except liver and pancreas, with highest levels in the pituitary and the hypothalamus. Intermediate levels of expression were detected, in descending order, in the ovary, telencephalon, heart, adrenal gland, cerebellum, and optic lobes whereas low expression was detected in the brainstem, lung, kidney, proventriculus, duodenum, and colon. Very low expression was found in skin, stomach, and muscle. cGHS-R1c was expressed in lower amounts than cGHS-R1a in all analysed tissues. Administration of 1 microM chicken ghrelin to pituitaries in vitro resulted in a down-regulation of both cGHS-R isoforms within 15 min, whereas after 1h levels returned to control values. Growth hormone and corticosterone down-regulated cGHS-R1a and cGHS-R1c mRNA expression within 60 min of exposure, whereas growth hormone-releasing factor 1-29 (1 microM) only reduced cGHS-R1a mRNA expression after 60min. Thyrotropin-releasing hormone (1 microM) did not alter cGHS-R expression.

Comparative study of iodothyronine outer ring and inner ring deiodinase activities in five teleostean fishes

The presence of outer ring deiodinating (ORD) and inner ring deiodinating (IRD) activities was investigated in different tissues of Oreochromis niloticus (Nile tilapia), Clarias gariepinus (African catfish), Oncorhynchus mykiss (rainbow trout) and halmus maximus (turbot). High-Km rT3 ORD is present in the kidney of most of the fishes studied, except in catfish. In turbot, besides the kidney, rT3 ORD is also present in liver, heart and ovary. Low-Km T4 ORD is found in the liver and low-Km T3 IR the brain of all the fishes studied. In addition, low levels of low-Km T3 IRD were demonstrated in gill and skin of Nile tilapia, liver of rainbow trout and gill and kidney of turbot. For the different teleosts, the biochemical properties of the different rT3-deiodinating enzymes mentioned, T4 ORD in liver and T3 IRD in brain and tilapia gill were compared to those of the deiodinases formerly characterized in Oreochromis aureus (blue tilapia). In general, the different deiodinases demonstrate analogous sensitivities to iodothyronines and inhibitors, although minor differences occur. The various deiodinating enzymes all depend on addition of dithiothreitol and demonstrate maximal activity pH between 6.5 and 7. The optimal incubation temperature of rT3 ORD and T4 ORD in tilapia and catfish is 37 °C, in trout and turbot it varies, depending on the tissue, between 25 ° and 37 °C. For the different T3 IRD activities the optimal temperature is 37 °C in warmwater as well as in coldwater species. The apparent Km values for rT3 ORD lay in the μM range, for T4 ORD and T3 IRD they lay in the nM range. Vmax values are usually higher in tilapia as compared to the other teleosts studied. Based on the similarities in susceptibility to inhibition by different iodothyronines and inhibitors and the agreement of the apparent Km values, we conclude that the deiodinating enzymes in teleosts are more similar to mammalian deiodinases than is generally accepted.

Effect of dietary protein content on episodic growth hormone secretion and on heat production of male broiler chickens

1. The effect of the crude protein content (200 and 150 g/kg) of isoenergetic diets on episodic growth hormone (GH) release and on heat production was investigated in male broiler chickens. 2. Decreasing the crude protein content of isoenergetic diets from 200 g/kg (HP diet) to 150 g/kg (LP diet) resulted in depressed body weight gain, impaired food conversion efficiency and increased abdominal fat deposition. 3. The pattern of growth hormone secretion was markedly affected by dietary treatment. Broiler chickens fed on the LP diet had higher overall mean, amplitude, baseline and peak frequency than the HP chickens. 4. The LP chickens produced more heat per unit of metabolic body weight than the HP chickens. 5. The hypothesis relating the pattern of GH secretion to protein conversion efficiency was corroborated.

Changes in plasma T3 during fasting/refeeding in tilapia (Oreochromis niloticus) are mainly regulated through changes in hepatic type II iodothyronine deiodinase

Fasting and refeeding have considerable effects on thyroid hormone metabolism. In tilapia (Oreochromis niloticus), fasting results in lower plasma T3 and T4 concentrations when compared to the ad libitum fed animals. This is accompanied by a decrease in hepatic type II (D2) and in brain and gill type III (D3) activity. No changes in kidney type I (D1) activity are observed. Refeeding results in a rapid restoration of plasma T4 values but not of plasma T3. Plasma T3 remains low for two days of refeeding before increasing to normal levels. Liver D2 and gill D3 also do not increase until two days after refeeding. Brain D3, on the other hand, rises immediately upon refeeding. These results suggest that the change in hepatic D2 activity is one of the main factors responsible for the changes in plasma T3 observed during starvation and refeeding in tilapia. This finding supports the hypothesis that, in contrast to mammals and birds, liver D2 is the primary source of plasma T3 in fish. Although the deiodinases important for the gross regulation of plasma T3 during fasting/refeeding differ (mammals: D1 and D3, birds: D3, fish: D2), they all occur in the liver, suggesting that the organ itself may play a crucial role. In addition, the changes in brain and gill D3 suggest that these enzymes constitute a fine tuning mechanism for regulation of T3 availability at the cellular or plasma levels, respectively.

Effects of experimental hypo- and hyperthyroidism on iodothyronine deiodinases in Nile tilapia, Oreochromis niloticus

In the present study, we examined the effects of experimentally-induced increases or decreases in plasma concentrations of thyroid hormones on iodothyronine deiodinases in tilapia, Oreochromis niloticus. To obtain hyperthyroid tilapia, fish were injected with porcine follicle stimulating hormone (pFSH) 36 hours before sampling or fed on demand for 11 days with tilapia pellets containing 12 ppm T3. Tilapias were made hypothyroid by providing them food containing 0.2% methimazole for 11 days. Plasma T4 and T3 and the in vitro deiodinase activity in liver, kidney, brain and gill were measured at the end of the treatment period. Injection with pFSH caused an increase in plasma T4 but had no influence on plasma T3 levels. A small increase in plasma T3 was observed in T3-fed fish. Plasma levels of both T4 and T3 were decreased by methimazole treatment. We observed no changes in kidney type I deiodinase (D1), whereas liver type II deiodinase (D2) was increased during hypothyroidism and decreased during hyperthyroidism. Hypothyroidism resulted in a significant decrease in brain, gill and liver type III deiodinase (D3). An pFSH-induced increase in T4 stimulated brain and gill D3 but not liver D3, whereas the opposite was true in T3-fed fish. We conclude that the regulation of D1 and D3 in tilapia is probably different compared to mammals.

Sexual dimorphism in two pure cichlid species, Oreochromis niloticus niloticus (Linnaeus 1758) and Sarotherodon melanotheron melanotheron Rüppel 1852, and their intergeneric hybrids

Growth performances and sexual growth dimorphism were compared in two pure species of tilapia, Oreochromis niloticus niloticus (OO) and Sarotherodon melanotheron melanotheron (SS), and their reciprocal intergeneric hybrids (male O. n. niloticus × female S. m. melanotheron [OS] and male S. m. melanotheron × female O. n. niloticus [SO]). Fish obtained from artificial reproduction were reared on artificial diets over 10 weeks at 25 ± 2 °C, under light regimes of 12 h light:12 h darkness. Growth was measured on a weekly basis. Social interactions were recorded with a video camera. Pure O. n. niloticus achieved the fastest growth rates (Mean Specific Growth Rate (SGR) of 2.7 ± 0.6% d–1 for males and 2.3 ± 0.4% d–1 for female) and S. m. melanotheron the slowest (1.3 ± 0.3% d–1 for males and 1.4 ± 0.3% d–1 for females). The SGR of the intergeneric hybrids fell between that of the two pure strains. OS females grew faster (1.7 ± 0.4% d–1) than SO females (1.3 ± 0.2% d–1), whereas no difference was observed between males. Aggressive behaviour emerged first among faster-growing fish (O. niloticus and SO). The role of parental components in behavioural and physiological traits of tilapia is discussed.

Effect of triiodothyronine supplementation on thyrotropin-releasing hormone-induced growth-hormone secretion in sex-linked dwarf and normal chicks

The effect of a dietary triiodothyronine (T3) supplement, of either 0.1 or 0.5 microgram/g of feed, was studied on the thyrotropin-releasing hormone (TRH)-induced growth hormone (GH) secretion in sex-linked dwarf (dw) or normal (Dw) chicks of both sexes. In normal chicks, 0.1 microgram/g T3 decreased plasma GH levels before TRH as well as the GH increase after TRH, and 0.5 microgram/g T3 totally suppressed any response to TRH, either at 4 or at 7 weeks of age. Dwarf chicks were more sensitive to TRH than normals when receiving either 0 or 0.1 microgram/g T3; 0.5 microgram/g T3 abolished the difference between genotypes at 4 weeks of age but not so clearly at 7 weeks of age, where dwarf females showed a slight but still significant GH increase after TRH. Interactions between genotype, TRH injection, and T3 treatments were often significant at 4 weeks of age and even more at 7 weeks of age. Dwarf chicks receiving 0.1 microgram/g T3, expected to have normal plasma T3 levels, showed a higher GH response after TRH. This suggests that other hormones may be involved in the regulation of this response, particularly IGF-I, which is known to remain at a low level in T3-treated dwarf chicks.