Corticotropin release: inhibition by intrahypothalamic implantation of atropine.

Anterior hypothalamic implantations of crystalline atropine markedly inhibit the adrenocortical responses evoked by surgical stress, ether anesthesia, or intravenolus injection of arginine vasopressin. Similar implants in nearby regions of the brain or sham implantations in the same region were ineffective. The data suggest that the hypothalamic control of pituitary corticotropin may have a cholinergic component.

Effects of various prostanoids on thyrotropin secretion by superfused anterior pituitary cells.

Continuously superfused rat anterior pituitary cells were used to study the effects of prostaglandins (PGs) and a thromboxane (TX) on the secretion of TSH. Indomethacin, a blocker of PG synthetase, inhibited the amount of TSH secreted in response to TRH. This reduction in TRH responsiveness was overcome by administration of PGE2 in combination with the TRH. Arachidonic acid, a prostanoid precursor, increased the amount of TSH released by TRH. Superfusion with TXB2 or imadazole, an inhibitor of TX synthetase, did not change TSH secretion. PGs A2, B2, D2, F1 alpha, F2 alpha, and endoperoxide analogs U-44069 and U-46619 had no effect on hormone release. PGE1 and E2 both increased TRH-stimulated TSH, but neither compound affected basal output; PGI2 was found to stimulate TSH release. Somatostatin inhibited TRH-induced TSH, but failed to block the effects of the PGs. These studies demonstrate that PGs, but no TXs, play a role in TSH secretion. PGE1 and PGE2 appear to modulate TRH responsiveness, while PGI2 directly stimulates hormone output.

Suppression of thyrotropic hormone secretion by prostaglandin synthesis inhibitors.

In order to determine whether endogenous prostaglandins (PGs) are involved in the secretion of thyrotropic hormone (TSH), we monitored plasma TSH levels in female rats receiving indomethacin (Ind) or aspirin (Asp) to inhibit PG synthesis. TSH secretion was induced by either exogenous thyrotropin-releasing hormone (TRH) or by throidectomy. On the basis of preliminary experiments, Ind was found to inhibit thyroid secretion directly. Subsequently, thyroidectomized rats receiving thyroxine (T4) replacement (2-4 mug/100 g BW/day) were used to avoid this complicating factor. These replacement regimens were judged to be adequate on the basis of the measurement of plasma triiodothyronine and T4 levels, and the lack of a compensatory rise in plasma TSH levels. Under these conditions, Ind significantly inhibited, but did not abolish, the TSH response to exogenous TRH (250 ng/100 g BW iv). Thyroidectomy-induced TSH secretion was abolished by Ind, and could be reversed upon cessation of Ind treatment. Aspirin was also found to inhibit significantly the compensatory TSH rise following thyroidectomy. These findings suggest that endogenous pituitary PGs mediate the stimulation of TSH secretion by TRH or by reduced feedback of thyroid hormones.

Effect of continuous thyroxine administration on thyrotropin secretion in thyroidectomized rats.

We have studied the effect of T4 itself (i.e. without extrapituitary conversion to T3) on the feedback regulation of pituitary TSH secretion in thyroidectomized rats receiving continuous T4 or T3 replacement. Continuous sc infusion of 1.0 or 2.0 microgram T4/100 g . day in 0.01 N NaOH, 20% 1,2-propanediol vehicle failed to maintain the plasma T4 concentration in thyroidectomized rats for more than 48 h, despite the verification of T4 delivery. Continuous sc infusion of 1.0 or 2.0 microgram T4/100 g . day in 0.01 N NaOH, 5% rat serum resulted in a dose-dependent elevation in the plasma T4 concentration and inhibition of the postthyroidectomy rise in plasma TSH, in the presence of low plasma T3 concentrations. Mimicking the low plasma T3 concentrations that resulted from T4 infusion by continuous replacement of 280 ng T3/100 g . day failed to inhibit the postthyroidectomy rise in plasma TSH. The pituitary responsiveness to TRH (250 ng/100 g, iv) after 96 h of thyroid hormone infusion was significantly (P less than 0.05) inhibited only in the group receiving 2.0 microgram T4/100 g . day. These results indicate that the plasma T4, concentration in addition to that of plasma T3, conveys feedback information to the pituitary. With regard to pituitary TSH secretion, T4 acts as both a prohormone giving rise to a protion of the plasma T3 by peripheral monodeiodination and as a hormone conveying feedback information via the blood to the pituitary.

Thyrotropin-like immunoreactivity in the pituitary and three brain regions of the female rat: diurnal variations and the effect of thyroidectomy.

A method for extracting and assaying TSH-like immunoreactivity (TSH-LI) in regions of the central nervous systems (CNS) of individual rats was developed and validated. Serial dilutions of tissue extracts paralleled dilutions of purified rat TSH standard in the RIA, and tissue TSH-LI comigrated electrophoretically with TSH standard. TSH-LI was measured in the pituitary (PIT), the medial basal hypothalamus (MBH), the remainder of the hypothalamus (HYPO), and the cerebral cortex (CC). Female rats were maintained on a normal 12-h light, 12-h dark cycle (LD; onset of light, 0730 h) or an inverted 12-h light, 12-h dark cycle (DL; onset of light, 1930 h) for 3 weeks and then killed at 4-h intervals throughout the day. Another group of rats on the LD photoperiod were either thyroidectomized or sham thyroidectomized and killed 3 weeks later at 1000 h. Diurnal variations of TSH-LI were present in the PIT, MBH, HYPO, and CC on the LD photoperiod (P less than 0.05). The acrophases of MBH, HYPO, and CC TSH-LI diurnal variations occurred soon after the onset of darkness, whereas the PIT TSH-LI variation peaked soon after the onset of light. Inverting the photoperiod inverted the PIT TSH-LI diurnal variation, but central nervous system TSH-LI variations were not phase shifted after 3 weeks on the inverted photoperiod. Thyroidectomy elevated plasma and MBH TSH-LI levels (P less than 0.05), but PIT, HYPO, and CC TSH-LI levels were not significantly different from those in sham-operated controls. These results indicated that brain TSH concentrations could change under different physiological states independently of changes in PIT or plasma TSH. Although brain TSH exhibited diurnal variations, the data suggested that the phase of these variations was not set by the light-dark cycle.

Diurnal variations of plasma thyrotropin, thyroxine, and triiodothyronine in female rats are phase shifted after inversion of the photoperiod.

The present study examined plasma TSH, T4, and T3 concentrations in rats throughout the day to determine if diurnal variations of these hormones occurred. Female rats on a 12-h light, 12-h dark cycle (onset of light, 0730 h) were sampled by cardiac puncture at 2-h intervals throughout the day on 2 days, 1 week apart. Significant diurnal variations of plasma TSH, T4, and T3 were detected (P less than 0.01), Peak TSH concentrations occurred soon after the onset of light, whereas T4 and T3 concentrations peaked 3-4 h later. After these variations were detected, the effect of inverting the photoperiod was examined. Female rats were placed on 12-h light,, 12-h dark cycles, with the onset of light at 0730 h (LD) or 1930 h (DL). After 3 weeks, rats from each group were killed by decapitation at 4-h intervals throughout the day, and trunk blood was collected. Diurnal variations in plasma TSH, T4, and T3 (P less than 0.01) were similar to those found with 2-h sampling intervals in the previous experiment, and plasma corticosterone and PRL rhythms peaked near the onset of darkness (P less than 0.05). Inversion of the photoperiod phase shifted all diurnal variations, such that they maintained a similar phase relationship to the light-dark cycle on both LD and DL photoperiods. These results indicated that diurnal variations of plasma TSH, T4, and T3 concentrations could be repeatedly detected with different sampling protocols. These variations were phase shifted by inverting the photoperiod, which indicated that some aspect of the light-dark cycle can act to set the phase of these diurnal variations in the pituitary-thyroid axis.

Release of immunoreactive thyroid stimulating hormone from the rat hypothalamus in vitro.

Thyroid stimulating hormone (TSH) has been identified in the hypothalamus and other brain areas of the rat. However, the alteration of the release of brain TSH has not been demonstrated. Therefore, we examined the release of immunoreactive TSH (IR-TSH) in vitro from hypothalamic tissue obtained from hypophysectomized, thyroidectomized and intact control rats. Whereas TRH (10(-5) M) and PGE2 (10(-4) M) did not alter hypothalamic IR-TSH release, depolarizing concentrations of potassium (60 mM) or veratridine (5 mM) stimulated the release of IR-TSH from hypothalamic tissue from all groups. These data suggest that IR-TSH synthesized in the hypothalamus is stored in a releasable form.

Direct pituitary stimulation of thyrotropin secretion by opioid peptides.

We have studied the effects of opioid peptides in vitro on the release of TSH from superfused anterior pituitary tissue. The opioid peptide beta-endorphin was found to increase TSH secretion in a dose-related manner from either dispersed pituitary cells or pituitary fragments. This effect of beta-endorphin was not blocked by a wide range of doses (10(-9)-10(-5) M) of naloxone. In contrast, somatostatin (10(-9) M) significantly decreased beta-endorphin-stimulated TSH secretion. In addition to beta-endorphin, the opioid peptides met-enkephalin, leu-enkephalin, dynorphin, and gamma-endorphin all increased, whereas des-Tyr-gamma-endorphin did not affect, TSH secretion. These results indicate that the opioid peptides may participate in the regulation of TSH secretion via a direct pituitary action.

Effects of vasoactive intestinal peptide on thyroid blood flow and circulating thyroid hormone levels in the rat.

The effects of vasoactive intestinal peptide (VIP) on thyroid blood flow and hormone levels were studied in rats. Tissue blood flow was determined from the distribution of radioactive microspheres after injection by cardiac puncture directly into the left ventricle of anesthetized rats. Initial results indicated that the systemic infusion of 6.25 micrograms VIP iv resulted in increased thyroid blood flow, but was also associated with hypotension, as measured by left ventricular pressure. In contrast, topical administration of VIP to the left of the thyroid increased blood flow to that lobe, but not to the right lobe, and produced no systemic cardiovascular effects. In a further set of experiments, graded doses of VIP were administered iv. Infusions of 6.25 and 0.625 micrograms VIP were associated with 2- to 3-fold increases in thyroid and pancreatic blood flows, but lower doses were ineffective. Blood flows to the adrenals, brain, small intestine, kidneys, and spleen were not altered by any dose of VIP. Mean left ventricular pressure was again reduced by the 6.25-micrograms dose of VIP, but was not affected by lower doses. The infusions of VIP had no effect on plasma TSH, T3, or T4 levels either 20 min or 2 h after treatment. These results suggest that thyroid blood flow is, in part, controlled by VIP and indicate that changes in thyroid blood flow can occur at doses of VIP that have no apparent effect on circulating thyroid hormone levels.

Effects of thyrotropin on the vascular conductance of the thyroid gland.

It is well established that TSH from the anterior pituitary is the principal stimulatory agent in the physiological regulation of the thyroid gland. Chronic elevations of plasma TSH induce hyperplasia and hypertrophy of thyroid follicular cells and enlargement of blood capillaries. At low plasma TSH levels the thyroid gland atrophies. We have examined the vascular conductance (C = blood flow/mean arterial pressure) of the thyroid gland and several other tissues over a wide range of endogenous plasma TSH concentrations and after treatment with bovine TSH (bTSH) in rats. Tissue blood flows were determined using 15 +/- 5-microns diameter 141Ce-labeled microspheres in a modification of the reference sample microsphere technique. The microspheres were injected directly into the left cardiac ventricle via a 23-gauge needle passed through the chest wall, while the reference blood sample was collected and systemic arterial blood pressure was monitored through femoral arterial catheters. After the animals were killed, tissues were cleaned and weighed, and the tissue radioactivity was determined. Blood samples for determination of plasma hormone levels were obtained from the jugular vein before the injection of microspheres. In the first series of experiments, the vascular C per mass of thyroid gland was significantly decreased 4 and 8 days after hypophysectomy. Treatment of hypophysectomized rats with bTSH (185 mU/100 g.day as a continuous iv infusion for 2 or 6 days) restored thyroid vascular C per mass of tissue to control levels. In the second series of experiments, we manipulated circulating plasma TSH levels in intact rats by 6 days of treatment with propylthiouracil (2.0 mg/day, ip), thyroid hormones (1.5 micrograms T4, 0.4 micrograms T3 or 3.0 micrograms T4, plus 0.8 micrograms T3/100 g.day, sc by continuous infusion), TRH (240 micrograms/day, iv, by continuous infusion), bTSH (800 mU/day, iv, by continuous infusion), or combinations of these treatments. The vascular C per mass of thyroid gland was significantly decreased at very low chronic plasma TSH levels and increased at very high chronic plasma TSH levels. Thyroid vascular C per mass was unchanged, however, over a broad intermediate range of plasma TSH concentrations encompassing normal values, despite alterations in the size and function of the thyroid gland. At these intermediate levels of TSH stimulation, the thyroid gland may respond by adding or subtracting functional units without changing the blood flow per unit. The amount of blood flow per functional unit may be altered only at very high or very low levels of TSH stimulation.

Increases in thyroid gland blood flow after hemithyroidectomy in the rat.

After subtotal thyroidectomy, the thyroid gland remnant undergoes compensatory alterations in function and morphology. Under the trophic stimulation of elevated plasma TSH concentrations, the thyroid remnant responds with an increase in hormone synthesis and secretion and, in addition, increases in mass. We have examined the alterations in thyroid blood flow which accompany increased secretion and growth after hemithyroidectomy (HTX) in male Sprague-Dawley rats (200-220 g). At various times after surgical HTX (1, 2, and 3 weeks), blood samples for the determination of plasma hormone concentrations were obtained and tissue blood flows were determined using 15 +/- 5 microns diameter 141Ce-labeled microspheres in a modification of the reference sample microsphere technique. The microspheres were injected directly into the left cardiac ventricle via a 23-gauge needle passed through the chest wall while a reference blood sample was collected. After the animals were killed, tissues were cleaned and weighed, then tissue and reference blood sample radioactivities were determined. In addition, thyroidal immunoreactive vasoactive intestinal peptide was measured after acetic acid (0.67 N) extraction. After HTX, plasma TSH concentrations were significantly elevated. The plasma concentrations of T4 and T3 fell, but by less than the expected 50%. The mass of the remaining thyroid lobe increased progressively over the 3 weeks post thyroidectomy, reaching approximately 70% of the total thyroid gland weight of sham-operated controls. Thyroid blood flow per gram of tissue was significantly elevated at all times post HTX. HTX did not induce any alterations in thyroidal immunoreactive vasoactive intestinal peptide concentration. Thus, after HTX, the well documented compensatory alterations in thyroid remnant growth and secretion were accompanied by a prompt and striking increase in thyroid blood flow.

Immunocytochemical studies of the peptidergic innervation of the thyroid gland in the Brattleboro rat.

An indirect immunofluorescence technique was used to study the peptidergic innervation of the thyroid gland in homozygous Brattleboro rats (DI) and normal Long-Evans rats (LE). The primary goal of this study was to determine whether the previously demonstrated decrease in thyroid responsiveness to TSH in DI might be due to an abnormality in the innervation of the thyroid. Thyroids from both types of rats were found to contain nerve fibers containing immunoreactivity for vasoactive intestinal peptide (VIP), substance P (SP), neuropeptide Y (NPY), and peptide HI (PHI). All four types of fibers were found in close association with both follicle cells and blood vessels. Well developed networks of fibers surrounding blood vessels were particularly apparent in the case of NPY. The density of fibers associated with follicle cells in DI was at least as great as that in LE in regard to SP, NPY, and PHI. Fibers containing VIP were found in greater abundance in DI than in LE. Additional studies revealed no evidence of thyroid fibers containing either somatostatin or neurophysin, which was used as a marker for vasopressin. We conclude that the reduced responsiveness of the thyroid in DI is not due to an inadequate supply of any of the neuropeptides included in this study. Since VIP is known to enhance thyroid secretion, we suggest that the apparent proliferation of VIP-containing fibers in DI may be a reflection of a neural mechanism attempting to compensate for a thyroid gland deficiency analogous to the humoral mechanism by which TSH secretion increases in response to thyroid deficiency.

The effects of the duration of severe hypothyroidism and aging on the metabolic clearance rate of thyrotropin (TSH) and the pituitary TSH response to TSH-releasing hormone.

We have studied the effects of the duration of severe hypothyroidism (thyroidectomy) and aging on the pituitary TSH response to exogenous TRH and the TSH MCR in female rats. Rats were thyroidectomized (THYREX) or sham operated (SHAM) 4, 14, or 68 days before experimentation. All rats were prepared with chronic intraatrial catheters through which TRH (0, 250, or 1000 ng/100 g BW) or [125I]TSH was administered, and sequential blood samples were obtained. The MCR of TSH was determined in each group from the plasma disappearance curves of injected [125I]iodo-TSH and were found to be decreased in 4-day-THYREX [0.23 +/- 0.01 (+/- SE) vs. 0.27 +/- 0.01 ml/min X 100 g; P less than 0.05], but not in 14-day-THYREX or 68-day-THYREX rats compared with age-matched SHAM controls. In addition, the TSH MCR was found to be decreased in 68-day-SHAM compared to 4- or 14-day-SHAM rats (0.15 +/- 0.01 vs. 0.27 +/- 0.01 ml/min X 100 g; P less than 0.05). The amount of TSH secreted (MCR X area under delta plasma TSH curve X BW) in response to each dose of TRH was calculated and was found to be decreased in 4-day-THYREX rats (4.5 +/- 1.5 vs. 30.0 +/- 4.5 micrograms; P less than 0.05), but not different in 14-day- or 68-day-THYREX rats compared with age-matched SHAM controls. The amount of TSH secreted in response to TRH was also reduced in 68-day-SHAM compared with 4- or 14-day-SHAM rats (16.0 +/- 1.0 vs. 30.0 +/- 4.5 micrograms; P less than 0.05). These results indicate that 1) the MCR of TSH is decreased by aging, but not by severe hypothyroidism in the rat; 2) the pituitary TSH response of the rat to exogenous TRH is decreased by aging, and 3) during severe hypothyroidism, the pituitary TSH response of the rat to a midrange or maximal dose of TRH is decreased (4-day-THYREX) or not different (14- and 68-day-THYREX) compared with that of age-matched euthyroid rats.

In vivo inhibition of thyroid secretion by indomethacin.

Indomethacin (Ind) was administered to adult female rats to reduce endogenous prostaglandin (PG) synthesis in order to investigate the role of PGs in thyroid hormone secretin. This treatment markedly reduced thyroidal PGF levels (667.7 vs. 1822.1 pg/my, P less than .001). Although the plasma TSH concentrations were normal in the Ind-treated group (41.14 vs. 40.01 mug/100 ml), dramatic decreases were observed in plasma T3 (24.5 ca. 6.7 nf/100 ml, P less than .001) and T4 (5.6 vs. 0.7 mug/100 ml, P less than .001) levels...

Effects of food deprivation on the feedback regulation of the hypothalamic-pituitary-thyroid axis of the rat.

We have studied the effects of food deprivation on the plasma concentrations of T4, T3, and TSH, the TSH MCR, the pituitary TSH content, and the pituitary TSH response to TRH in female rats. Two days before the beginning of the experiment all rats were prepared with chronic intraatrial catheters through which TRH (100 ng/100 g BW) or [125I]iodo-TSH was administered, and sequential blood samples were obtained. The plasma concentrations of T4 and T3 in food-deprived rats were significantly (P less than 0.05) reduced compared to those in control rats 6, but not 2, days after food removal. Despite the fall in plasma concentrations of T4 and T3, no compensatory rise in the plasma TSH concentration was observed in the food-deprived rats. Plasma TSH concentrations did not differ between groups at any time. The change in the plasma TSH concentration (delta plasma TSH) was significantly greater 15, 30, and 45 min after iv bolus injection of TRH in food-deprived rats than in control rats. To further evaluate the basal TSH secretion rate (MCR X plasma TSH concentration) and the amount of TSH secreted in response to TRH (MCR X area under delta plasma TSH curve), the MCR of TSH was determined in control and food-deprived rats from the plasma disappearance curves of injected [125I]iodo-TSH. No differences were found in the TSH MCR or the calculated basal plasma TSH secretion rate (micrograms per day/100 g BW) of control or 6-day food-deprived rats. However, the change in the amount of TSH secreted in response to TRH was significantly greater in food-deprived rats than in control rats (17.7 +/- 2.1 vs. 11.0 +/- 1.0 micrograms/100 g BW; P less than 0.05). In addition, pituitary weight was significantly decreased in food-deprived rats (8.3 +/- 0.3 vs. 10.8 +/- 0.5 mg; P less than 0.05), but the pituitary TSH concentration (micrograms per mg tissue) was unchanged. These results are consistent with the development of tertiary hypothyroidism (i.e. decreased TSH stimulatory input to the pituitary by TRH or some other agent) during food deprivation in the rat.

Alterations in thyroid blood flow induced by varying levels of iodine intake in the rat.

Thyroid hormone biosynthesis depends upon the presence of adequate amounts of thyroidal iodine, and during fluctuations in dietary iodine intake, relatively constant thyroid hormone levels are maintained by various homeostatic mechanisms. These mechanisms include an enhancement of iodide pump efficiency and organification when iodine intake is limited, and significant decreases in iodide uptake and hormone synthesis when excess iodine intake occurs. The present study was designed to determine whether acclimation to different dietary iodine regimens is associated with changes in thyroid blood flow and to assess the time course of any such alterations in relation to pituitary-thyroid axis hormone levels. Male Sprague-Dawley rats were fed a diet containing low (LID), high (HID), or normal (CTR) iodine concentrations. Three, 7, 14, or 133 days after starting these dietary regimens, the animals were anesthetized with ketamine/pentobarbital, and thyroid blood flows were assessed using the reference sample version of the microsphere technique. At the same times and at weekly intervals throughout the 133 days of treatment, blood samples for the determination of TSH, T4, and T3 levels were obtained. Additionally, thyroidal immunoreactive vasoactive intestinal peptide (VIP) was measured at the end of the experiments. LID treatment increased thyroid blood flows to 240%, 350%, and 240% of levels in control rats at 7, 14, and 133 days of treatment, respectively. Thyroid weight was also elevated above levels in control animals at each of these times. A slight decrease in plasma T4 levels occurred over the 133 days of LID treatment; however, this dietary regimen did not alter circulating levels of T3 or TSH or thyroidal VIP concentration. HID treatment had opposite effects, in general, to those of LID. Thyroid blood flows were decreased by 34%, 56%, 46%, and 35% after 3, 7, 14, and 133 days of treatment with HID, respectively. Circulating levels of T4 were increased over the 133 days of HID treatment, whereas plasma levels of T3 and TSH and thyroid weights remained unchanged from those in control rats over this period of study. A small decrease in thyroidal VIP concentrations coincident with the decrease in thyroid blood flow was observed at the beginning of the HID treatment. Neither LID nor HID had any effect on blood pressure, cardiac output, or blood flow in other organs. These data demonstrate that acclimation to changes in dietary iodine intake in the rat include alterations in thyroid blood flow which are reciprocal to the iodine intake level and appear to be independent of circulating TSH levels.

Thyroid vascular conductance: differential effects of elevated plasma thyrotropin (TSH) induced by treatment with thioamides or TSH-releasing hormone.

We have reported previously that thyroid gland blood flow, expressed as vascular conductance (C) per mass, is decreased at very low and increased at very high chronic plasma TSH concentrations, but is apparently unchanged over a broad range of plasma TSH concentrations encompassing normal levels. The aim of the present study was to examine the apparently very steep dose-response relationship between elevated plasma TSH and thyroid vascular C/mass. In the first series of experiments, endogenous plasma TSH concentrations were manipulated by treating male Sprague-Dawley rats (250-280 g) for 6 days as follows: 1) controls (0.5 ml saline/day, ip), 2) propylthiouracil injections (2.0 mg PTU/day, ip), 3) PTU plus partial thyroid hormone replacement (2.0 mg PTU/day and 0.3-0.9 microgram T4 plus 0.075-0.225 microgram T3/100 g.day via continuous sc infusion), or 4) TRH (9-1200 micrograms TRH/100 g.day via continuous iv infusion). The vascular C values of the thyroid gland, salivary gland, kidney, and pancreas were determined using the reference sample version of the radioactive microsphere technique. PTU treatment led to the expected hypothyroidism, increased plasma TSH concentrations (959 +/- 66 vs. 154 +/- 22 ng/dl), increased thyroid weight (9.19 +/- 0.36 vs. 4.60 +/- 0.15 mg/100 g), and increased thyroid vascular C/mass (495 +/- 51 vs. 127 +/- 20 microliters/mm Hg.g/min). PTU-treated rats receiving partial thyroid hormone replacement demonstrated a dose-related suppression of plasma TSH, thyroid weight, and thyroid vascular C. Although, TRH treatments resulted in increased plasma TSH concentrations (e.g. 1200 micrograms TRH, 706 +/- 46 ng/dl) and thyroid weight (e.g. 1200 micrograms TRH, 7.45 +/- 0.41 mg/100 g), thyroid vascular C per tissue mass was not significantly increased after any TRH treatment (e.g. 1200 micrograms TRH, 166 +/- 19 microliters/mm Hg.g/min). Thus, at similarly elevated plasma TSH concentrations, the thyroid vascular C/mass of PTU- and TRH-treated rats constituted separate populations. Both PTU- and TRH-induced thyroid growth were accompanied by similar alterations in thyroid gland morphology (i.e. increased cellular mass with little change in the total amount of colloid). To investigate the mechanisms involved, groups of rats were treated for 6 days as follows: 1) control, 2) PTU or methimazole (25 mg MMI/day, ip), 3) PTU or MMI plus thyroid hormone replacement (1.2 micrograms T4 plus 0.3 microgram T3/d.100 g), 4) TRH (12 micrograms/100 g.day), and 5) PTU or MMI, thyroid hormones, and TRH.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological studies of the involvement of hypothalamic prostaglandins in the regulation of thyrotropin secretion.

A case is made for the involvement of pituitary prostaglandins (PGs) in the regulation of thyrotropin (TSH) secretion by citing recent evidence that TSH release in vivo and in vitro is enhanced by treatment with exogenous PGs and is inhibited by drugs (e.g., indomethacin) that block PG synthesis. Pharmacological studies were then performed to test the hypothesis that hypothalamic PGs also affect TSH secretion indirectly via the appropriate hypothalamic hormones that regulate pituitary secretion. The inhibition of thyroidectomy-induced TSH secretion was used as an endpoint in choosing the best of several drugs purported to inhibit PG synthesis. The established effectiveness of indomethacin and aspirin were used for reference in testing the following drugs: naproxen, mefenamic acid, tranylcypromine, and phenelzine. Only naproxen was found to be effective, but since it was no more potent than indomethacin, the latter drug was used for subsequent work. Indomethacin was stereotaxically implanted into several hypothalamic regions known to regulate TSH secretion, and sequential plasma samples were analyzed for TSH by radioimmunoassay. Bilateral implants of indomethacin in the anterior hypothalamic area increased TSH secretion throughout the 72 hr period of study. Sham inplants at this site and indomethacin implants in other nearby sites were ineffective. These findings suggest that endogenous PGs play an inhibitory role in the hypothalamic regulation of pituitary secretion.

Factors modulating the secretion of thyrotropin and other hormones of the thyroid axis.

The first portion of this paper is devoted to an overview of the normal function of the hypothalamo-pituitary-thyroid axis. This section emphasizes areas of current research interest and it identifies several sites and mechanisms that are potentially important interfaces with toxins or toxic mechanisms. We then describe an in vitro technique for the continuous superfusion of enzymatically dispersed pituitary cells; this approach is particularly valuable in studying the dynamics of the TSH responses to the factors known (or suspected) to regulate TSH secretion in vivo. Using this technique, we have found that 10(-5)M prostaglandin (PG)I2 stimulates TSH secretion without altering the response to TRH (10(-8)M), and that this stimulation is not due to its rapid conversion to 6-keto PGF1 alpha. In contrast PGs of the E series (PGE1 and PGE2, 10(-5)M) increase responsiveness to TRH but have no effect alone. We found no effects of any of the other prostanoids tested (PGs A2, B2, F1 alpha, F2 alpha, thromboxanes A2 and B2, and the endoperoxide analog, U-44069. Somatostain (10(-9)M inhibits TRH-induced TSH secretion, but does not alter the responsiveness to PGI2. These findings suggest that somatostatin blocks TSH secretion at a point that is functionally prior to the involvement of the PGs, and perhaps does so by blocking synthesis or limiting availability of selected PGs.

Lack of effect of vasoactive intestinal peptide antagonists on blood flow in the rat thyroid.

We used three putative vasoactive intestinal peptide (VIP) antagonists: 1) [4C1-D-Phe6,Leu17]VIP, 2) [N-Ac-Tyr1,D-Phe2] GRF(1-29)-NH2, and 3) VIP(10-28) to assess the involvement of endogenous VIP in the regulation of thyroid hormone secretion and thyroid blood flow (BF). We measured thyroid BF in ketamine-pentobarbital-anesthetized rats using the microsphere technique. Increases in thyroid BF induced by VIP administration (30 pmol-1.5 nmol/100 g b.wt.) were not affected by any of the three compounds tested at doses 10-100 times higher than that of VIP. These compounds (3-15 nmol/100 g b.wt.) also failed to affect basal thyroid BF or hormone secretion. Increases in pancreatic and salivary gland BFs induced by VIP (30 pmol/100 g b.wt.) were also not affected by [4C1-D-Phe6,Leu17]VIP or [N-Ac-Tyr1,D-Phe2]GRF(1-29)-NH2 (3 nmol/100 g b.wt.). These results indicate that the three compounds tested are not effective inhibitors of VIP receptors in the thyroid vasculature and, therefore, they cannot be used in the investigation of the functional significance of endogenous VIP in the regulation of thyroid BF.