Somatostatin inhibits rat hepatic T4-5'-deiodinase. The effect is independent of the associated hypoinsulinemia

Thyroid hormone availability and activity in avian species: a review

The intracellular thyroid hormone (TH) availability is influenced by different metabolic pathways. Some of the changes in intracellular TH availability can be linked to changes in local deiodination and sulfation capacities. The secretion of the chicken thyroid consists predominantly of thyroxine (T4). TH receptors (TRs) preferentially bind 3,5,3'-triiodothyronine (T3). Therefore, the metabolism of T4 secreted by the thyroid gland in peripheral tissues, resulting in the production and degradation of receptor-active T3, plays a major role in thyroid function. Food restriction in growing chickens increases hepatic type III deiodinase (D3) levels but decreases growth hormone (GH)-dependent variables such as plasma insulin-like growth factor-I (IGF-I) and T3 concentrations. Refeeding restores hepatic D3 and plasma T3 to control levels within a few hours. It can be concluded that the tissue and time dependent regulation of the balance between TH activating and inactivating enzymes plays an essential role in the control of local T3 availability and hence in TH activity. Two separate genes encode multiple TR isoforms, i.e. TRalpha and TRbeta. These TRs consist of a DNA-binding domain, a ligand-binding domain, a hinge region and an amino-terminal (A/B) domain. TRs mediate their effects on transcription by binding as homodimers or heterodimers to the TH response elements (TREs). Also, unliganded TRs can bind to TREs and may so modulate transcription of target genes.

Acute cold exposure, leptin, and somatostatin analog (octreotide) modulate thyroid 5'-deiodinase activity

We investigated the effect of acute cold exposure, leptin, and the somatostatin analog octreotide (OCT) on thyroid type I (D1) and II (D2) deiodinase activities. Microsomal D1 and D2 activities were measured by the release of (125)I from (125)I-reverse triiodothyronine (rT(3)) under different assay conditions. Rats exposed to 4 degrees C (15, 30, 60, and 120 min) showed progressive reduction in thyroidal D1 and D2, reaching approximately 40% at 2 h (P < 0.05) despite increased circulating TSH (P < 0,05) associated with the higher thyroid D1 and D2 in hypothyroid rats. A single injection of leptin (8 microg/100 g body wt sc) induced increased thyroid and liver D1 (P < 0.05), but not thyroid D2, activities at 30 and 120 min, independently of the serum TSH rise shown only at 2 h. OCT (1 microg/kg body wt sc) increased D1 and D2 activity significantly 24 h after a single injection, with no changes in serum TSH. Therefore, leptin and somatostatin are potential physiological upregulators of thyroid deiodinases, and their low secretion during acute cold exposure may be a potential mechanism contributing to cold-induced reduction in thyroid deiodinase activity.

The somatostatin analogue octreotide modulates Iodothyronine deiodinase activity and pituitary neuromedin B

Somatostatin inhibits growth hormone and thyrotropin (TSH) secretion. It also enhances the inhibitory effect of thyroid hormone (TH) on TSH by poorly understood mechanisms. We investigated the acute effect of the long-acting somatostatin analogue, octreotide (OCT), on anterior pituitary type 1 (D1) and 2 (D2) deiodinase activity, on liver D1, and on pituitary content of neuromedin B (NB), an autocrine inhibitor of TSH secretion, which is positively regulated by thyroid hormones. Euthyroid or hypothyroid rats were sacrificed at different times after a single subcutaneous injection of OCT (1 microg/kg body weight [BW]). D1 and D2 activities were measured by the release of 125I from 125I reverse triiodothyronine (rT3) under different assay conditions. NB, TSH, T3, and thyroxine (T4) were quantitated by radioimmunoassay (RIA). In euthyroid rats, liver and pituitary D1 activities were decreased (50%) 6 hours after OCT injection; pituitary D2 and NB remained unchanged. In hypothyroid rats, OCT increased near to the level of normal rats both pituitary D1 activity (but not liver) and NB content, at 24 hours and at 6 and 24 hours, respectively (p < 0.05). Pituitary D2, greatly increased by hypothyroidism, showed a small (25%) but significant reduction at 3 hours, persisting at 24 hours (p < 0.01), although it remained higher than that of euthyroid control. Serum thyroid hormones were not affected by OCT injection. The results show that octreotide acutely regulates pituitary deiodinases and NB content, both representing mechanisms that potentially can contribute to somatostatin and octreotide actions on pituitary growth hormone (GH) and TSH secretion and to modulate these cells sensitivity to thyroid hormone action.

Glucagon-induced changes in plasma thyroid hormone concentrations in healthy dogs resemble "euthyroid sick syndrome"

We recently demonstrated that glucagon infusion induced a decline in T3 and a rise in rT3 in anesthetized dogs. These changes in T3 and rT3 may be attributed, at least in part, to anesthesia itself, since general anesthesia is known to cause lowering of T3 and an elevation in rT3 during the perioperative period. Therefore, to eliminate the contribution, if any, of anesthesia to these changes in T3 and rT3, we assessed plasma glucose, T3 resin uptake (T3RU), T4, free T4, T3 and rT3 concentrations following intravenous glucagon (0.5 mg) or normal saline (0.5 ml) administration at frequent intervals for 3 h in 6 conscious dogs fasted for 16 h. No significant alterations were noted in T4, free T4, and T3RU levels during either study. However, glucagon infusion alone induced a significant fall in T3 (0.33 +/- 0.06 in nmol/l vs -0.03 +/- 0.03 nmol/l with normal saline; p less than 0.01) and marked elevations in glucose (3.66 +/- 0.22 mmol/l vs 0.61 +/- 0.11 nmol/l with normal saline, p less than 0.001) and rT3 concentrations (0.11 +/- 0.02 nmol/l vs 0.005 nmol/l; p less than 0.001). Furthermore, the integrated responses of T3 and rT3 as assessed by cumulative changes and areas under the curves were markedly greater during glucagon infusion when compared with saline administration (p less than 0.01 for all comparisons). Since the elevations in levels of stress hormones known to ensue during anesthesia do not occur during a conscious resting state, we believe that hyperglucagonemia may be a major contributor of thyroid hormone alterations observed in several euthyroid sick states, not associated with stress, and may enhance these changes during euthyroid sick syndrome associated with stressful crises. Finally, these changes may be attributed to altered metabolism of iodothyronines in peripheral tissues as reflected by lowered T3/T4 and increased rT3/T4 ratios.

Effect of immunization against somatostatin in the pregnant ewe on growth and endocrine status of the neonatal lamb

Absorption of somatostatin (SRIF) specific antibodies from colostrum of ewes actively immunized against SRIF may improve growth rate of the neonatal lamb by neutralizing the inhibitory effects of SRIF on pituitary and thyroid function. Growth and endocrine parameters in the offspring of SRIF immunized (SI) and control (C) crossbred ewes were examined. Lamb weight was recorded at birth and twice each week to 24 days of age. Blood samples were collected prior to first suckle and twice each week. At 21 to 24 days of age, in separate experiments, lambs were infused with glucose (0.29 g/kg), arginine (0.25 g/kg) or thyrotropin-releasing hormone (TRH; 0.33 microgram/kg). A strong correlation (R = 0.88; P less than .01) was observed between anti-SRIF titre in the ewe at parturition and in the lamb at 3 days of age. No effect on lamb birth weight (SI 4.28 +/- 0.27 kg; C 4.35 +/- 0.23 kg) was observed. At 24 days of age cumulative gain in SI lambs (5.4 +/- 0.32 kg) was greater (P less than .05) than in C lambs (4.5 +/- 0.32 kg). The growth hormone secretory responses to glucose or arginine were not affected by treatment. Plasma IGF-I, plasma thyroxine (T4) and the plasma thyrotropin and T4 responses to TRH were not different between treatments. Plasma triiodothyronine (T3) was higher (P less than .05) in SI (2.46 +/- .10 ng/ml) than in C (2.01 +/- .05 ng/ml) lambs, however, the plasma T3 response to TRH was lower in SI lambs. Plasma glucose (mg/dl) was higher (P less than .05) in SI (118.4 +/- 1.7) than in C (106.0 +/- 4.0) lambs. Plasma insulin was not affected by treatment. Increased plasma T3 and glucose concentrations during SRIF immunoneutralization in the neonate lamb may be important factors contributing to the growth response observed.

Thyroid function in sex-linked and autosomal dwarf chickens

Thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) elevated plasma thyroxine (T4) and triiodothyronine (T3) concentrations in a (euthyroid) control line of chickens and in an autosomal dwarf strain. These agents were ineffective in sex-linked dwarf (SLD) chickens. Similarly, while somatostatin (SRIF) lowered plasma T4 and T3 concentrations in autosomal dwarf (ADW) chickens and controls, it had no inhibitory effect in the SLD strain. These results suggest that an impairment in the hypothalamus-pituitary-thyroid axis is at least partly responsible for hypothyroidism in the SLD strain.

Somatostatin inhibits thyroid function in fowl

Plasma concentrations of thyroxine (T4) and triiodothyronine (T3) were marginally (less than 25%) lowered 10 and 60 min, respectively, following somatostatin (SRIF) administration (at doses of 10-30 micrograms/kg) in immature domestic fowl. When plasma iodothyronine levels were elevated by the administration of anti-SRIF (1.0 ml/kg) 10 min prior to SRIF challenge, the inhibitory effect of SRIF on T4 and T3 concentrations was greatly augmented. The relative reduction in the T3 concentration was greater than the decline in the T4 level, resulting in a decline in the T3:T4 ratio. These data indicate that SRIF inhibits thyroid function in fowl, possibly by direct effects on the thyroid gland and by effects on the peripheral conversion of T4 to T3.

Comparative study of pituitary-thyroid hormone economy in fasting and hypothyroid rats

Starvation in laboratory rodents results in significant alterations in thyroid hormone economy characterized by decreased circulating levels of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) and a decline in serum thyrotropin (TSH) concentration. To investigate this apparent paradox, we have compared in fasted and hypothyroid animals the intracellular parameters mediating thyroid hormone action in the anterior pituitary gland. In vitro saturation analysis combined with quantitation of nuclear T3 content by radioimmunoassay allowed for characterization of pituitary nuclear T3 receptors and estimation of the endogenous fractional receptor occupancy. In rats, thyroidectomized 4 wk earlier, the 10-fold increase in serum TSH levels and decline in peripheral thyroid hormone concentrations were accompanied by a 61% decrease in pituitary nuclear T3 content and a marked decline in fractional T3 receptor occupancy as compared with control animals. In euthyroid animals subjected to short-term starvation (72 h), serum T3, T4, and TSH levels declined by 52, 43, and 48%, respectively. Despite these marked decreases in circulating thyroid hormone levels, pituitary nuclear T3 content in fasted rats declined by only 15% (P less than 0.05) relative to control levels. This modest decline in nuclear T3 content, combined with a 23% decrease in total T3 receptor number, resulted in an estimated fractional receptor occupancy in fasted animals which was equal to or greater than that noted in controls. The effects of fasting and hypothyroidism on the pituitary were further investigated by quantifying low Michaelis constant (Km) T4 5'-deiodinase activity in the crude cytosol fraction of pituitary homogenates. In thyroidectomized animals, maximum velocity was increased ninefold, whereas fasting resulted in a 37% decrease (P less than 0.025) in this parameter compared with controls. Km values were similar in all experimental groups (4.7 +/- 0.6 nM). These results demonstrate that, despite significant reductions in circulating thyroid hormone concentrations and pituitary T4 5'-deiodinase activity, nuclear T3 levels are maintained at relatively normal levels in the pituitary of the fasted animal and fractional T3 receptor occupancy may actually increase. These findings are in marked contrast to those noted in thyroidectomized animals and suggest that the suppression of TSH secretion accompanying starvation in the rat is mediated, at least in part, by local pituitary mechanisms that serve to maintain and possibly enhance nuclear T3 receptor occupancy.