Er Kuhn

Effect of fasting and feeding time on circadian rhythms of serum thyroid hormone concentrations, glucose, liver monodeiodinase activity and rectal temperature in growing chickens

Abstract Rhode Island Red chickens fasted during three consecutive days showed significantly lower triiodothyronine (T 3 ), but higher thyroxine (T 4 ) serum values compared with those of controls. The differences are due, at least in part, to a lower 5′-monodeiodinase activity as shown by an in vitro liver test. The T 3 -cyclicity disappeared after one day of fasting. Serum T 4 -rhythmicity did not disappear, but a shift in acrophase from early morning to the late afternoon or evening was observed. Refeeding immediately restored the usual cycle of both T 3 and T 4 . Shifts in time of meal-eating were paralleled by shifts in circadian rhythms of glucose, rectal temperature, and serum T 4 and T 3 levels; it is concluded that the serum rhythms of T 3 and T 4 are totally or partly due to meal-time-related shifts in 5′-monodeiodinase activity. In conclusion, feeding and time of feeding are important in thyroid hormone rhythmicity; however, they are not the only determining factors. Moreover, rhythmicities of circulating T 3 and T 4 may be determined by totally different processes.

Immunocytochemical identification and localization of the different cell types in the pituitary of the seabass (Dicentrarchus labrax).

Antisera raised against chum salmon prolactin (PRL), trout growth hormone (GH), mammalian adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH), and alpha-melanophore-stimulating hormone (alpha-MSH) were used to localize PRL, GH, ACTH, gonadotropic, TSH, and MSH cells in the hypophysis of the teleost Dicentrarchus labrax using the unlabeled peroxidase anti-peroxidase method. In the rostral pars distalis, ACTH cells stained very intensively with anti-ACTH; so did the MSH cells in the pars intermedia. The prolactin cells stained very specifically with anti-prolactin without staining the growth hormone cells. In the proximal pars distalis anti-GH, anti-TSH beta, and anti-LH stained selectively the corresponding cells; with these antisera no cross-reaction with any other cell type was observed. Anti-alpha-MSH only stained cells in the pars intermedia. Some cells in the pars intermedia did not react at all; these could correspond to the PAS-positive cells. A characteristic feature was positive staining with anti-LH in some cell groups encircling the pars intermedia, indicating the fact that in the seabass some cells of the proximal pars distalis surround the pars intermedia.

Serum levels of triiodothyronine and thyroxine in the domestic fowl following mild cold exposure and injection of synthetic thyrotropin-releasing hormone

Abstract Radioimmunoassays of triiodothyronine (T 3 ) and thyroxine (T 4 ) were carried out on sera collected from the domestic fowl (Rhode Island Red strain). In all experiments serum T 4 levels were consistently lower in the afternoon (1500 hr) than in the morning (1000 hr). A gradual decline in ambient temperature from 26.5 to 17.5° elevated serum T 3 in 40-day-old chicks. Increases in T 4 occurred when the temperature was lowered further to 12.5°. Intravenous injections of thyrotropin-releasing hormone (TRH) into 40-day-old cockerels increased T 3 within 1 hr. Pretreatment with CB-154 markedly enhanced the capacity of TRH to increase T 3 . In the last experiment the effect of an iv dose of 25 ng of TRH was investigated in 40-day-old cockerels over a number of hours. Both T 3 and T 4 were increased. The release of T 4 , but not of T 3 , was biphasic suggesting possible effects on both release and synthesis. Cold treatment (10°) was also capable of initiating this biphasic response. CB-154 pretreatment potentiated the release of T 4 after iv TRH, the cockerels having a higher serum level but a lower thyroidal T 4 content. It was concluded that, in the domestic fowl, both cold and TRH can stimulate thyroid function in a manner similar to that in mammals. The potentiating effect of CB-154 may be reconciled with the presumed goitrogenic effect of prolactin in lower vertebrates.

Luteinizing hormone-releasing hormone as a potent stimulator of the thyroidal axis in ranid frogs

Plasma concentrations of T4, measured by radioimmunoassay, were raised significantly 2 and 4 hr after intravenous injection of synthetic luteinizing hormone-releasing hormone (LHRH) in Rana ridibunda (1 and 10 micrograms on 2 consecutive days) and in Rana esculenta (10 micrograms). A dose of 1 microgram LHRH was not so effective as 50 micrograms synthetic thyrotropin-releasing hormone (TRH) when injected in Rana ridibunda in November. However 10 micrograms LHRH was equipotent to 50 micrograms TRH. In February somewhat less than half of the Rana temporaria group was responsive to LHRH. There is no clear indication that fluctuating plasma T3 concentrations were caused by LHRH or TRH. Preinjection levels of T3 and T4 were higher during the breeding season (April) in R. esculenta (resp. 35.4 +/- 1.4 pg/ml; 744 +/- 134 pg/ml; n = 22) compared to the basal concentrations in the very closely related Rana ridibunda (November) (resp. 15.2 +/- 1.1; 162 +/- 24 pg/ml; n = 28). Four days after removal of the pars distalis plasma T4 concentrations were significantly decreased in Rana esculenta, whereas T3 could stay longer in circulation. T3 and T4 content of the thyroids was not altered by the short-term hypophysectomy. Injection of 10 micrograms LHRH had no influence on plasma T4 nor testosterone concentrations in these frogs, contrary to the sham-ectomized animals in which plasma testosterone remained elevated longer than T4. The results suggest that the stimulatory effect of intravenous injected LHRH on thyroid (and gonadal) activity in the frog is primarily mediated through the hypophysis. They also point to a possible correlation between the gonadal and thyroidal axis.

Thyroid hormone feedback regulation of the secretion of bioactive thyrotropin in the frog

Thyrotropin-releasing hormone (TRH), ovine corticotropin-releasing hormone (oCRH) (both 268 nM), and mammalian gonadotropin-releasing hormone (mGnRH) (268 and 2680 nM) stimulated the secretion of bioactive thyrotropin (TSH) by Rana esculenta pituitaries (pars distalis) in vitro. Preincubation of the pituitaries with 50 ng/ml (64 nM) thyroxine (T4) for 6 hr suppressed the TRH- and oCRH-induced (268 nM) secretion of bioactive TSH, but did not affect the response of the pituitaries to 268 nM mGnRH. Triiodothyronine (T3) (64 nM) reduced both the TRH- and mGnRH-stimulated release of bioactive TSH; the response of TSH to TRH even decreased toward basal levels while a significant TSH response to mGnRH remained. In a separate experiment, pituitaries were preincubated for 6 hr with different equimolar doses of T3 and T4 (6.4, 32, and 64 nM); neither treatment affected the mGnRH-stimulated secretion of bioactive TSH. On the other hand, T4 suppressed the TSH response to TRH in a dose-dependent manner. The inhibitory effects of thyroid hormones on the TRH-induced release of bioactive TSH was present for at least 4 hr after their removal from the incubation medium. These results suggest that thyroid hormones exert a negative feedback control on the secretion of bioactive TSH in adult frogs by a direct action on the pars distalis. There may also be differences in thyroid hormone sensitivities of the TSH responses to mGnRH and TRH.

Circadian variations in plasma osmolality, electrolytes, glucose, and cortisol in carp (Cyprinus carpio)

Male carps (Cyprinus carpio) of about 1.5 kg were used in all experiments performed in December and February. Blood was taken by heart-puncture within 1 min or less every 4 hr from different fishes during a 24-hr period to establish circadian rhythmicities of all parameters. The water temperature was 11-12 degrees. In December circadian variations could be calculated for cortisol, Na+ and plasma osmolality in partly fed carps and for cortisol, Ca2+, glucose, and plasma osmolality in food deprived animals. No difference in cortisol level, amplitude, or acrophase was present between both experiments but plasma Na+ and osmolality was more elevated when feeding. Also the acrophase of the osmolality rhythm differed between both groups. In February carps, cortisol levels were still comparable to December ones, but a 4 hr shift in acrophase had occurred (from +/- 02 hr to 06 hr). Levels of all other parameters also were comparable to December, except for glucose with only 1/5th of the December values. There was also a shift in acrophase present from the morning to around midnight. The level of cortisol in February carps acclimated to 18 degrees for 1 week was twice the one found in the 11 degrees group. At the same time a significant increase in amplitude and shift in acrophase to 22 hr was seen. Other parameters, except for glucose of which the level was significantly lower than in the 11 degrees group, remained unchanged. Also no correlation between the individual 24-hr data of all parameters could be found. It is therefore concluded that cortisol is not responsible for the observed circadian rhythmicities of Na+, Ca2+, glucose, or plasma osmolality.

Thyroxine and triiodothyronine in plasma and thyroids of the neotenic and metamorphosed axolotl Ambystoma mexicanum: influence of TRH injections

Circulating levels of T3 and T4, as well as T3 and T4 content of the thyroid glands were measured by radioimmunoassay in the neotenic and metamorphosed axolotl Ambystoma mexicanum. In the two experiments which were performed plasma T4 concentrations were more elevated in metamorphosed axolotls, especially in the first experiment (2.12 +/- 0.40 ng/ml vs. 369 +/- 30 pg/ml). T3 plasma values which were only estimated in the second experiment were about five times higher in metamorphosed animals (63.2 +/- 7.4 pg/ml vs. 12.5 +/- 0.8 pg/ml). Also the thyroid hormone content of the glands was higher after metamorphosis. Nevertheless the neotenic gland still contained considerable amounts of T3 (14.7 +/- 1.8 ng and 48.3 +/- 4.8 ng/thyroid, respectively, in the first and second experiment) and T4 (530 +/- 61 ng; 2173 +/- 291 ng/thyroid). Because of the higher T3/T4 ratio found in the plasma compared to the thyroid gland, it was suggested that circulating T3 may be derived partly from peripheral T4 conversion, mainly after metamorphosis. An intravenous injection of 10 micrograms synthetic TRH was able to induce a very significant increase of the plasma T4 concentration (which was maintained during 24 hr) in the metamorphosed axolotls of the first experiment, however, not in those of the second experiment nor in the neotenic animals. Following an injection of 10 mU bovine TSH (first experiment) circulating levels of T4 were raised in both groups. The opposing TRH results could be related with the different control levels of T4 in the two experiments. However, the results indicate that TRH is capable of functioning as a possible thyrotropin-releasing factor in the metamorphosed axolotl.

Differences in serum iodohormone concentration between chick embryos with and without the bill in the air chamber at different incubation temperatures.

Serum levels of triiodothyronine (T3) and thyroxine (T4) were measured, by RIA, in developing chick embryos of the Rhode Island Red strain incubated at different temperatures in a forced-draught laboratory incubator. A low incubation temperature resulted in a longer incubation period, whereas eggs incubated at a higher temperature hatched sooner. In all temperature groups serum levels of T3 and T4 increased during the incubation period studied. Whatever the total duration of incubation within the experimental conditions, maximal T3 and T4 serum levels were always obtained on the day of “pipping”. Embryos having perforated the air space membrane the day before pipping showed elevated serum levels of T3, but not of T4, compared with embryos that had not perforated the air space. The presence of high levels of T3 in serum, in chick embryos after perforation of the air space membrane, and the sharp increase in ratio before the event of “pipping”, appear to suggest that T3 has an important role in the processes of “pipping” and hatching.

Fetal plasma prolactin levels and fetal growth in relation to maternal CB-154 treatment in the rat.

Pregnant Wistar rats received daily injections of 1 mg of CB-154 starting Day 14 of gestation. The CB-154 treatment did not suppress maternal plasma prolactin levels, but a significant decrease in fetal prolactin was seen. This suppression of prolactin levels in fetal plasma however did not influence fetal growth. These results do indicate that prolactin does not act as a growth promoting hormone in fetal rats.

Difference in thyroid function between male and female frogs (Rana temporaria L.) with increasing temperature

Abstract In Rana temporaria acclimation to higher environmental temperatures results in an increased 125I uptake by the thyroid glands but only in males. Both male and female frogs have comparable excretion rates for 125I which increase with consecutive elevations of temperature. December frogs had the lowest thyroidal uptake rates and the increased uptake percentage following acclimation to higher temperature was less pronounced compared to October or January frogs. In males 125I uptake was more pronounced in skin, stomach, and liver, whereas in females up to 21.5% of iodine injected was accumulated in the ovaries. Thyroxine content of thyroids in December frogs was about five times as high as in March frogs. No triiodothyronine was detectable in thyroids of December frogs and in March frogs only minimal amounts could be found. In frogs tested in March, the thyroxine content of male thyroids at 22° was only half that at 4°, whereas the level of thyroxine remained unchanged at both temperatures in females. The results support the hypothesis that the lower 125I thyroid uptake in females is not caused by an accumulation of 125I in the ovaries but by competition at the thyroidal site with iodine released from the ovaries.