Colchicine-binding protein and the secretion of thyroid hormone

The role of microtubules in the thyrotropin- or adenosine 3'',5'' cyclic monophosphate (cyclic AMP)-stimulated accumulation of cytoplasmic colloid droplets and secretion of iodine from the mouse thyroid gland has been investigated by means of different classes of agents that affect the stability of microtubules. The onset of inhibition of secretion by colchicine, the uptake of colchicine-³H by thyroid lobes, and the binding of colchicine-³H to thyroidal soluble protein are shown to have similar time courses Colloid droplet accumulation is also inhibited and does not readily resume upon removal of colchicine from the medium. This appears to be due to the slow washout of the drug (t _½ ∼ hr). Thyroids contain a soluble colchicine-binding protein that resembles microtubule proteins of other tissues with respect to apparent K_m for colchicine, pH optimum, and stability characteristics Colchicine analogues inhibit iodine secretion and colchicine binding in a parallel manner and as a function of their antimitotic potencies. Microtubule-stabilizing agents such as hexylene glycol and D₂O also inhibit secretion. Thus, inhibition of thyroid secretion by antimitotic agents appears to be mediated by an effect on microtubules. The inhibitory locus of colchicine inhibition occurs after the generation of cyclic AMP, since stimulation of secretion by this nucleotide is blocked by colchicine, whereas thyroid-stimulating hormone—induced accumulation of cyclic AMP is not affected. Thus, the functioning microtubule appears to play a role in the induction of colloid endocytosis.     

Influence of VIP on thyroid hormone secretion

Since VIP occurs in intrathyroidal nerves its role in thyroid hormone secretion has been investigated. It has been found that VIP is a stimulator of iodothyronine secretion in mice. In this respect VIP has a weaker potency than TSH, but shows a similar time characteristic. Also, VIP and TSH potentiate each others effects. In contrast to the effect of TSH, that of VIP is uninfluenced by alpha-adrenoceptor blockade. VIP, like TSH, stimulates thyroid cyclic AMP production. Thus, VIP nerves might, together with TSH, adrenergic and cholinergic nerves and other peptides such as somatostatin, participate in the complex regulation of iodothyronine secretion. Beside this, VIP has also been found to stimulate calcitonin secretion in rats. Other intrathyroidal neuropeptides, such as substance P and CCK-4, have been found to be without effects on iodothyronine secretion, but, like VIP, to stimulate calcitonin secretion.     

Impaired peripheral T3 production but normal induced thyroid hormone secretion in the sex-linked dwarf chick embryo

Plasma concentrations of thyroxine (T4), triiodothyronine (T3) and chicken GH (cGH), together with hepatic 5'-monodeiodination (5'-D) activity, were measured in normal (Dw) and dwarf chick (dw) embryos at incubation d 18. An injection of 10 micrograms of ovine GH (oGH) raised plasma concentrations of T3 in Dw embryos after 1 and 2 h and stimulated hepatic 5'-D activity after 2 h. A non-specific increase in T4 was also observed after 1 h in Dw animals probably due to the heterologous nature of the injection. These effects were not observed in dw embryos. An injection of 1 microgram of TRH was able to increase cGH levels after 15 min in Dw embryos, whereas the the observed increase in the dw group was not significant. In Dw embryos, 0.01, 0.1 and 1 microgram of TRH increased plasma concentrations of T3 in a dose-dependent way, whereas in dw embryos, no reaction to the TRH injections was seen, except for the highest dose used. Contrary to this observation, T4 was increased to the same level in both Dw and dw embryos following TRH injections. An injection of 1 microgram of ovine CRH increased corticosterone after 0.5 h and elevated T3 and T4 after 2 h to the same extent in Dw and dw embryos. It is concluded that the thyrotrophic activities of TRH and oCRH and the corticotropic activity of oCRH do not differ between normal and sex-linked dwarf embryos. However TRH and GH were unable to stimulate the T4-T3 conversion in the liver of dw embryos, presumably due to the lack of hepatic GH receptors in these animals.     

Effect of thyrotropic hormone on the membrane potential of thyroid gland cells and thyroid hormone secretion with aging

Effect of thyrotropic hormone (TTH) on membrane potential (MP) of thyroid cells and thyroid hormone secretion was studied in experiments on male rats of two age groups (7--12- and 27--32-month-old animals). It was found that during the first 3 hours after TTH administration (5 U/100 g i. v.) the depolarization of secretory cell membranes of adult rats was done pronounced and developed more rapidly than in old ones and that an increase in free thyroxin (T4) correlation with MP changes with time. In a dose of 0.5 U/100 g TTH caused a significant rise in T4 secretion only in old rats. The cAMP level in the thyroid gland declined with aging. In a dose of 5 U/100 g TTH provoked a significant increase in the cAMP content in adult rats and had no effect on its content in old ones. A relationship between the MP level of thyroid secretory cells and thyroid hormone secretion is discussed.     

Effect of methadone on TSH and thyroid hormone secretion

The hypothalamus-pituitary-thyroid function was studied in 15 male patients on chronic methadone treatment (40 mg/day). No significant variations of TSH, T4, T3 and rT3 levels were documented, either in basal conditions or after TRH stimulation; however a reduced TSH pituitary response was recorded in some patients (6 out of 15).     

Cholinergic and VIPergic effects on thyroid hormone secretion in the mouse

The thyroid gland is known to harbor cholinergic and VIPergic nerves. In the present study, the influences of cholinergic stimulation by carbachol, cholinergic blockade by methylatropine and stimulation with various VIP sequences on basal, TSH-induced and VIP-induced thyroid hormone section were investigated in vivo in mice. The mice were pretreated with 125I and thyroxine; the subsequent release of 125I is an estimation of thyroid hormone secretion. It was found that basal radioiodine secretion was inhibited by both carbachol and methylatropine. Furthermore, TSH-induced radioiodine secretion was inhibited already by a low dose of carbachol. Moreover, a high dose of carbachol could inhibit VIP-induced radioiodine secretion. Methylatropine did not influence TSH- or VIP-stimulated radioiodine secretion, but counteracted the inhibitory action of carbachol on TSH- and VIP-induced radioiodine release. In addition, contrary to VIP, six various synthesized VIP fragments had no effect on basal or stimulated radioiodine release. It is concluded that basal thyroid hormone secretion is inhibited by both cholinergic activation and blockade. Furthermore, TSH-induced thyroid hormone secretion is more sensitive to inhibition with cholinergic stimulation than is VIP-induced thyroid hormone secretion. In addition, the VIP stimulation of thyroid hormone secretion seems to require the full VIP sequence.     

Neuropeptide control of thyroid blood flow and hormone secretion in the rat

The effects of peptide HI (PHI), neuropeptide Y (NPY), and substance P (SP) on thyroid blood flow and hormone levels were studied in anesthetized rats. Regional blood flows were determined using radioactive microspheres. No change in heart rate or mean left ventricular pressure occurred during these neuropeptide infusions (0.625 micrograms iv over 2 min). PHI treatment resulted in a four-fold increase in thyroid blood flow. Blood flows to the pancreas and salivary gland also increased during PHI treatment. Infusions of NPY or SP did not significantly alter thyroid blood flow. However, SP decreased blood flow to the spleen and small intestine. These neuropeptides had no effect on blood flows to the adrenal, kidney, brain, heart, and adipose tissues. Following PHI, NPY, and SP infusions, plasma triiodothyronine and thyroxine levels were not different from values in saline-treated rats. This study demonstrates that PHI, like vasoactive intestinal peptide, is a potent thyroidal vasodilator at a dose that does not affect circulating thyroid hormone secretion.     

A model for the pulsatile secretion of gonadotropin-releasing hormone from synchronized hypothalamic neurons

Cultured gonadotropin-releasing hormone (GnRH) neurons have been shown to express GnRH receptors. GnRH binding to its receptors activates three types of G-proteins at increasing doses. These G-proteins selectively activate or inhibit GnRH secretion by regulating the intracellular levels of Ca2+ and cAMP. Based on these recent observations, we build a model in which GnRH plays the roles of a feedback regulator and a diffusible synchronizing agent. We show that this GnRH-regulated GnRH-release mechanism is sufficient for generating pulsatile GnRH release. The model reproduces the observed effects of some key drugs that disturb the GnRH pulse generator in specific ways. Simulations of 100 heterogeneous neurons revealed that the synchronization mediated by a common pool of diffusible GnRH is robust. The population can generate synchronized pulsatile signals even when all the individual GnRH neurons oscillate at different amplitudes and peak at different times. These results suggest that the positive and negative effects of the autocrine regulation by GnRH on GnRH neurons are sufficient and robust in generating GnRH pulses.     

Role of the kallikrein-kinin system in glandular secretion

Microsomes were prepared from liver, lung and kidney of rats and rabbits using a Ca⁺² aggregation method. Microsomal protein yield from the lung of both species was higher by this method of preparation as compared with ultracentrifuged samples. Specific activities of rat and rabbit pulmonary p-chloro-N-methylaniline (CMA) demethylase, biphenyl 4-hydroxylase and rat pulmonary TPNH cytochrome c reductase also were decreased. Specific activities of rabbit hepatic TPNH cytochrome c reductase, CMA N-demethylase, UDP-glucuronyltransferase and biphenyl hydroxylase were decreased by calcium aggregation Renal enzyme activities were unchanged by this method of preparation. These data indicate an apparent species and organ difference in microsomal enzyme response to calcium aggregation.     

Thyrotropin-releasing hormone gene expression in normal thyroid parafollicular cells

The rat TRH gene encodes a 255-amino-acid precursor polypeptide, preproTRH, containing five copies of TRH and seven non-TRH peptides. Expression of this gene is well documented in the central nervous system, particularly in the hypothalamus. Thyroids also contain TRH immunoreactivity, but it is unknown whether this immunoreactivity results from expression of the TRH gene or from other genes encoding TRH-like products. Since the CA77 neoplastic parafollicular cell line expresses the TRH gene, we investigated whether TRH gene expression also occurs in normal thyroid parafollicular cells. Northern analysis of total thyroid RNA with a preproTRH-specific RNA probe identified a single hybridizing band the same size as authentic TRH mRNA found in hypothalamus and CA77 cells. Gel filtration analysis of thyroid extracts identified the same 7-kilodalton and 3-kilodalton species of immunoreactive preproTRH53-74 previously identified in hypothalamus and CA77 cells. Immunoreactive preproTRH115-151, not previously identified, was found in all three tissues. Part of this immunoreactivity comigrated with the synthetic preproTRH115-151 standard on gel filtration and reversed-phase HPLC. PreproTRH53-74 was localized to thyroid parafollicular cells by immunostaining. These findings demonstrate authentic TRH gene expression by normal rat thyroid parafollicular cells and establish the CA77 cell line as the only model system of a normal TRH-producing tissue. In addition to expanding the range of neuroendocrine peptides known to be produced by parafollicular cells, these results also suggest a potential paracrine regulatory role for TRH gene products within the thyroid.     

Atrial natriuretic factor (ANF) inhibits thyroid hormone secretion in the mouse

Recently, thyroid follicular cells were shown to exhibit atrial natriuretic factor (ANF)-like immunoreactivity and high affinity ANF receptors. In this study, we therefore examined the effects of synthetic rat ANF1-28 on basal and stimulated thyroid hormone secretion in the mouse, according to the McKenzie technique. Iodine deficient mice were pretreated with 125I and thyroxine. ANF (3 nmol/animal) was found to inhibit the increase in blood radioiodine levels that was induced by TSH or vasoactive intestinal polypeptide (VIP). Furthermore, ANF and norepinephrine additively inhibited the TSH-induced increase in blood radioiodine levels. It is concluded that ANF inhibits thyroid hormone secretion, which, therefore, might be locally regulated by intrathyroidal ANF.     

Effect of potassium iodide and perchlorate on the process of thyroid hormone secretion]

The influence of potassium iodide and perchlorate on the parameters characterizing the thypoid hormones secretion, such as the cAMP level in the gland tissue and the number of intracellular colloid droplets under condition of stimulation by thyrotropic hormone was studied. It was shown that the abovementioned parameters were depressed by an excess of iodide, but perchlorate administration prevented the inhibitory effect of iodide. The results obtained favour the conception on the sensitivity of the thyroid adenylate cyclase system to the organic iodine concentration. Apparently and excess of iodide depressed the capacity of perchlorate to influence its concentration in the gland, and thereby the process of iodine organification and of the thyroid hormone secretion maintained at the optimal leve.