Further Observations on the Effect of Synthetic Thyrotrophin-releasing Hormone in Man

Synthetic thyrotrophin-releasing hormone (TRH) given intravenously in doses of 50 μg or more causes a significant rise in serum thyroid-stimulating hormone (TSH) levels but has no effect on serum growth hormone, plasma luteinizing hormone, or plasma 11-hydroxycorticosteroids under carefully controlled basal conditions.The peak TSH response to intravenous TRH occurs at 20 minutes. The mild and transient side effects, which occur only after intravenous TRH, include nausea, a flushing sensation, a desire to micturate, a peculiar taste, and tightness in the chest. There is considerable variability in response to a given dose of TRH in the same subject on different occasions and in different subjects. Oral administration of TRH in doses of 1 mg and above causes a rise in serum TSH, maximal at two hours, a consistent response being obtained at doses of 20 mg and above. A rise in serum protein-bound iodine (P.B.I.) follows that of TSH, a consistent response being observed at 40-mg doses of TRH orally. Measurements of serum TSH after intravenous administration of TRH or of serum TSH or serum P.B.I. after oral TRH should prove useful tests of pituitary TSH reserve.     

The effect of dexamethasone on TSH and prolactin secretion after TRH stimulation

Serum thyrotropin (TSH) and prolactin levels were measured after intravenous administration of 400 μg of synthetic thyrotropin-releasing hormone (TRH) in 13 normal subjects and six hypothyroid patients before and after three days of administration of dexamethasone 2 mg per day. In the normal subjects dexamethasone suppressed baseline serum levels and secretion of TSH after TRH stimulation. On the other hand, it had no effect on the hypothyroid patients. In the control group dexamethasone also suppressed baseline serum levels but not secretion of prolactin after TRH stimulation. Dexamethasone had no effect on prolactin levels in the hypothyroid group. It is concluded that in normal patients short-term administration of dexamethasone has an inhibitory effect on TSH secretion at the pituitary level. As for prolactin, our results could indicate that TRH is a more potent stimulator of prolactin secretion than of TSH secretion, or that TSH and prolactin pituitary thresholds for TRH are different.     

Somatostatin in the physiologic feedback control of thyrotropin secretion

The addition of an antiserum against Somatostatin (SRIF) to a culture of dispersed rat pituitary cells incubated in the presence of hypothalamic tissue enhances thyrotropin (TSH) secretion into the medium. This indicates that SRIF is normally secreted in amounts sufficient to affect TSH secretion and suggests that SRIF may be the agent responsible for the unexplained rapid termination of TSH secretion by thyrotrophs which have been activated by Thyrotropin Releasing Factor (TRF).     

Long-term intramuscular administration of thyrotropin-releasing hormone tartrate in patients with cerebrovascular disease: effects on the pituitary-thyroid axis.

We studied the effects of long-term (30 days) refracted daily intramuscular administration of 4 mg TRH tartrate (TRH-T) on the pituitary-thyroid axis in 20 euthyroid patients affected by cerebrovascular disease (CVD). All subjects were assayed for T4, T3, FT4, FT3, TSH and TBG plasma levels before treatment (D0), after 15 and 30 treatment days (D15, D30), and after a 15-day washout (D45). In addition, TSH response to 200 micrograms intravenous TRH was assessed at D0, D30 and D45. We observed a significant increase in T4, FT4 and FT3 levels in the face of decreased TSH concentrations. A blunted TSH response to TRH bolus persisted at D30. These data demonstrate that the down-regulation mechanism may be partially overcome in vivo when thyrotrophs are chronically exposed to pharmacological TRH-T doses and that TSH pattern is mainly due to the negative feedback of thyroid hormones, even though pituitary TSH reserves may become depleted. Furthermore, prolonged TRH-T administration does not produce hyperthyroidism in euthyroid CVD patients.     

Effects of somatostatin and adrenergic blockade on glucagon, insulin and glucose in exercising sheep

This study was conducted to characterize the mechanisms of hyperglycaemia in exercising sheep. Sheep were run on a treadmill for 45 min (5.5 km h-1, 8% incline) during adrenergic blockade (propranolol or phentolamine mesylate infusions) and during suppression of the rise in glucagon by infusion of somatostatin (SRIF). Propranolol did not alter the glucagon, insulin or glucose responses, except it tended to increase the metabolic clearance of glucose, presumably as a result of blocking the beta-adrenergic inhibition of glucose uptake. Phentolamine mesylate administration was associated with a suppression of the rise in glucagon concentrations, a reversal of alpha-adrenergic inhibition of insulin release and a reduction in glucose appearance during exercise. SRIF prevented the rise in glucagon and reduced insulin concentrations to below resting values. Propranolol and phentolamine mesylate did not alter the glucagon, insulin or glucose response to SRIF. However, SRIF prevented the insulin rise that occurred during phentolamine administration. The increment in glucose appearance produced in response to exercise was the same for SRIF, plus phentolamine mesylate and phentolamine mesylate in the first 25 min of exercise, but was significantly less than in the controls. During the last 20 min of exercise, glucose appearance was not significantly different from the control for any of the groups. The depression by SRIF and alpha-adrenergic blockade of the increment in glucose appearance due to exercise was associated with an impairment of the glucagon response. It appears, therefore, that glucagon may stimulate glucose production early in exercise in sheep directly, as well as by having a permissive effect.     

A physiological role of E and F series prostaglandins in the regulation of TSH secretion

The regulation of TSH secretion by E1, E2, E1 alpha and F2 alpha prostaglandins was studied by means of a monolayer culture system of dispersed rat anterior pituitary cells which was appropriately responsive to TRH, T3 and SRIF. PGEs and Fs induced significant increases in basal TSH release of the order of 30% at 10(-9) or 10(-8) to 10(-5) or 10(-4) M. Only PGEs accentuated the TSH release induced by a half maximal dose of TRH (10(-9) M) of the order of 60% in a dose dependent manner (10(-9) to 10(-6) M of PGEs), whereas PGFs did not. SRIF (10(-8) or 10(-9) M) alone failed to alter basal TSH release but did completely inhibit the TSH response to TRH (10(-9) M). SRIF also significantly inhibited both the increase in basal TSH release and the accentuation of the TSH response to TRH induced by PGEs (10(-6) M) but did not diminish the enhancement of basal TSH release induced by PGFs (10(-6) M). 7-oxa-13-prostynoic acid (PY1), a prostaglandin antagonist, which can act as an agonist in some systems, itself exhibited agonistic properties of PGEs with respect to basal and TRH induced TSH release. PY1 failed to inhibit the TSH release induced by all PGs, but partially inhibited the accentuated TSH response to TRH induced by PGEs. Indomethacin, PG synthetase inhibitor, did not affect basal or TRH induced TSH release in our system. These data suggest that PGs of the E and F series probably modulate TSH release via different mechanisms and that the PGE effect on basal TSH release differs from its augmentation of TRH induced TSH response. It is speculated that these effects of PGs may have physiological significance.     

Serum free thyrotropin subunit in congenital isolated thyrotropin deficiency

In two patients with congenital isolated thyrotropin (TSH) deficiency, serum TSH determined by a sensitive immunoradiometric assay (IRMA) was consistently undetectable. The basal levels of serum free TSH-alpha subunit (TSH-alpha) determined by a specific radioimmunoassay (RIA) were elevated in the hypothyroid state, and decreased to the undectable level during displacement therapy with thyroid hormone. The serum free TSH-alpha significantly increased following intravenous administration of thyrotropin releasing hormone (TRH). Serum free TSH-beta subunit (TSH-beta) was undectable. These findings suggest that TSH deficiency in this disease is not due to absence of thyrotroph in the pituitary gland or deficiency of TSH-alpha, but to abnormalities of the TSH-beta gene.     

Plasma somatostatin in normal subjects and in various diseases: increased levels in somatostatin-producing tumors

The plasma levels of somatostatin (SRIF) were studied in normal subjects and patients with various disorders by a sensitive and specific radioimmunoassay. In 45 normal subjects, the fasting plasma SRIF concentrations were 13.3 +/- 5.3 pg/ml (mean +/- SD). Very high concentrations of plasma SRIF, ranging from 125.0 pg/ml to 400.0 pg/ml, were found in all four patients with medullary carcinoma of the thyroid examined and the SRIF levels were changed in parallel with their clinical course after resection of the tumor. A case of pheochromocytoma also showed a relatively high SRIF concentration in plasma (47.0 pg/ml), but the plasma SRIF level decreased to 8.7 pg/ml after removal of the tumor. In normal subjects, plasma SRIF levels did not fluctuate during 2 hr-observation period in basal state. Glucagon (1 mg, iv) and secretin (3 CHRU/kg B.W., iv infusion over 30 min) had no effect on the SRIF levels in the peripheral blood plasma of normal subjects. On intravenous infusion of arginine (0.5 g/kg B.W.) over 30 min, all 6 normal subjects showed a significant increase in plasma SRIF 30-45 min after the start of the infusion (basal value, 11.6 +/- 1.5 pg/ml; peak value, 27.2 +/- 3.0 pg/ml; p less than 0.005). Two cases of medullary thyroid carcinoma showed exaggerated responses after the arginine administration (increases of 103 pg/ml and 157 pg/ml, respectively), suggesting that SRIF was released from the tumor. The findings indicate that plasma SRIF determination in the basal state and after arginine administration is useful for detecting and following up SRIF-producing tumors.     

Neuropeptidergic control of cyclic AMP accumulation in human thyroid cell

Somatostatin (SRIF), cholecystokinin (CCK), gastrin and substance P, as single agents, do not influence baseline cellular cAMP levels in human thyroid cultures. SRIF inhibits TSH-induced cAMP accumulation in human thyroid cell, while CCK, gastrin and substance P do not modify cAMP response to TSH. Vasoactive intestinal peptide (VIP) increases cellular cAMP levels in human thyroid cultures and its effect is additive to increases produced by norepinephrine (NE) and isoproterenol (ISO). Neither SRIF nor the other tested peptides influence adrenergic and VIP-ergic cAMP stimulation.     

Interaction of human pancreatic growth hormone-releasing factor, thyrotropin-releasing hormone, and somatostatin on growth hormone release in chickens

The effects of synthetic somatostatin (SRIF) on serum growth hormone (GH) concentrations stimulated by exogenous administration of synthetic thyrotropin-releasing hormone (TRH) and/or human pancreatic GH-releasing factor (hpGRF) were investigated in 4-week-old cockerels. In addition, the additive effects of TRH and hpGRF on serum GH were examined. TRH and hpGRF, when given in combination intravenously, produced an additive effect on serum GH concentration that peaked 10 min after the injection. The somatostatin did not significantly affect basal GH concentrations when given alone, but did significantly decrease the magnitude of the GH response to hpGRF. In contrast, SRIF did not significantly decrease the stimulatory effects of TRH on GH release. These results suggest that TRH and hpGRF are potent GH releasers in vivo and that their stimulating effects on GH release are additive, suggesting different mechanisms for their stimulation. The results obtained from the combination studies suggest that the main site of the stimulatory action of hpGRF is at the pituitary, and that SRIF significantly inhibited the rise in serum GH induced by a synthetic hpGRF, but not that induced by TRH.     

The role of cholecystokinin octapeptide (CCK-8) in regulating thyrotropin secretion in rats. In vivo studies

The effect of cholecystokinin octapeptide (CCK-8) on basal and TRH-stimulated secretion of TSH was investigated in 67 male Sprague-Dawley rats. Blood for TSH determinations was sampled by cannulation of right heart auricle in urethane narcosis before and four times during 60 minutes following CCK-8 administration. It was found that CCK-8 administered to the lateral brain ventricle at a dose of 0.5 microgram per animal caused a decrease in blood serum TSH concentration but did not change the response of TSH to TRH. Intravenous administration of CCK-8 at doses of 2 and 20 micrograms/kg had no effect on blood serum concentration of TSH.     

Involvement of central somatostatin in the alteration of GH secretion in starved rats

In order to determine the central or peripheral origin of the starvation-induced modifications of growth hormone (GH) and thyroid-stimulating hormone (TSH) secretions, the effects of starvation were studied in freely moving male rats with hypothalamo-hypophyseal disconnection. Five days after the disconnection GH secretion exhibited lower maximal values and higher trough levels and ultradian pulsatile secretion was lost as compared to controls. TSH levels were also decreased. The lesion did not modify pituitary somatostatin (SRIF) receptors as assessed by 125I-Tyr-O-D-Trp-8-SRIF binding or inhibition of adenylate cyclase activity. On the other hand, the growth hormone releasing factor (GRF) capacity to stimulate adenylate cyclase was strongly reduced by the lesion without modification of the affinity. Exposure to 72 h food deprivation decreased GH pulses and TSH levels in control rats but did not modify GH secretory profiles or TSH levels of lesioned rats. Plasma glucose and insulin levels were equally decreased after fasting in control and lesioned rats. Altogether, our results demonstrate that starvation-induced modifications of GH and TSH secretions are of central origin while glucose and insulin changes are peripherally triggered. They suggest that the hypothalamus is the only source of SRIF implicated in this effect.     

Effect of hypophysectomy and growth hormone administration on somatostatin and gastrin content in the stomach of rats

The effect of hypophysectomy and bovine growth hormone (GH) administration on somatostatin (SRIF) content as well as gastrin content in the rat stomach was investigated. SRIF content was determined by a specific radioimmunoassay. The total SRIF content in the stomach had decreased 4 weeks after hypophysectomy but was restored significantly in those rats which were subjected to bovine GH administration for 7 days after hypophysectomy. Furthermore, in control rats, an increase in SRIF content in the stomach was observed after 7 days of GH administration. Similar changes in total content of gastrin were observed after hypophysectomy and bovine GH administration, although these changes were not significant. These results indicate that GH may influence gastric function through changes in SRIF and gastrin content in the stomach.     

Calcium modifies the effects of thyroliberin and somatostatin on hormone release from cell cultures of the rat anterior pituitary

Role of calcium (Ca2+) in the effects of thyroliberin (TRH) and somatostatin (SRIF) on the release of growth hormone (GH), prolactin (PRL) and thyroid stimulating hormone (TSH) from the rat adenohypophyseal cells in primary monolayer cultures has been studied. Decrease of extracellular Ca2+ diminished the stimulatory effects of TRH on TSH and PRL release. Ca2+ is also an important factor in the mechanism of SRIF action. Data obtained in the experiments with high Ca2+ levels in the medium indicate that some antagonistic interrelationship exists between Ca2+ and SRIF. These results suggest that the participation of cAMP alone is not sufficient for stimulus-secretion coupling. Another messenger, namely Ca2+, is necessary for the effects of hypothalamic hormones. On the other hand, the contribution of Ca2+ to the secretory process in mammotrophs, somatotrophs and thyrotrophs is not equal. PRL and TSH secretion is more dependent on the presence of extracellular Ca2+ than the release of GH.     

The effect of some brain/gut peptides on plasma cholesterol levels in the rat

The effect of substance P (SP), bombesin (BBS), somatostatin (SRIF) and neurotensin (NT) on the plasma cholesterol levels was studied in the rat. SP and BBS had no effect, but SRIF produced a significant transient hypocholesterolemia and NT a significant transient hypercholesterolemia 15 minutes after the intravenous injections. Neither SRIF nor NT influenced the rate of the 7-hydroxylation of cholesterol. The data suggest a relation of SRIF and NT to the maintenance of the plasma cholesterol concentration.     

The effect of some brain/gut peptides on plasma cholesterol levels in the rat

The effect of substance P (SP), bombesin (BBS), somatostatin (SRIF) and neurotensin (NT) on the plasma cholesterol levels was studied in the rat. SP and BBS had no effect, but SRIF produced a significant transient hypocholesterolemia and NT a significant transient hypercholesterolemia 15 minutes after the intravenous injections. Neither SRIF nor NT influenced the rate of the 7α-hydroxylation of cholesterol. The data suggest a relation of SRIF and NT to the maintenance of the plasma cholesterol concentration.     

Neostriatal administration of somatostatin:differential effect of small and large doses on behavior and motor control

The administration of small doses of somatostatin (SRIF) (0.01 and 0.1 microgram) into the neostriatal complex of unrestrained, freely moving rats induced general behavioral excitation associated with a variety of stereotyped movements, tremors, and a reduction of rapid eye movements (REM) and deep slow wave sleep (SWS). In contrast, the higher doses of SRIF (1.0 and 10.0 microgram) caused movements to be uncoordinated and frequently induced more severe difficulties in motor control such as contralateral hemiplegia-in-extension which restricted or completely prevented the expression of normal behavioral patterns. As a result, the animals appeared drowsy and inhibited. Analysis of the sleep-waking cycle revealed prolonged periods of a shallow SWS while REM sleep and deep SWS were markedly reduced; electroencephalogram recordings revealed periods of dissociation from behavior. The administration of endocrinologically inactive as well as the active analogues of SRIF failed to induce effects comparable with those observed after the administration of the same dose of the native hormone (10.0 microgram).     

Effects of somatostatin on human thyroid folliculi in vitro

Somatostatin (SRIF) inhibits calcitonin and T3-T4 secretion in thyroid. We have investigated the in vitro effect of SRIF on the basal and TSH induced [3H]thy incorporation, thyroglobulin (tgb) RNA and cAMP level in follicular cells, isolated from normal and adenomatous human thyroids. [3H]thy uptake has been evaluated as TCA-precipitable material in 2, 4, 8, 24 h incubated follicles and 24 h incubated adherent cells. Tgb RNA has been quantified with cytoplasmic dot blot hybridization and cAMP level with RIA method. SRIF reduces basal and TSH-induced [3H]thy in both suspension follicles and epithelial adherent cells. However it does not modify tgb RNA nor cAMP levels in incubated follicles. These data suggest a direct antiproliferative effect of SRIF on human thyroid.     

Modulation of somatostatin receptors,somatostatin content and Gi proteins by substance P in the rat frontoparietal cortex and hippocampus

Substance P (SP) and somatostatin (SRIF) are widely spread throughout the CNS where they play a role as neurotransmitters and/or neuromodulators. A colocalization of both neuropeptides has been demonstrated in several rat brain areas and SP receptors have been detected in rat cortical and hippocampal somatostatinergic cells. The present study was thus undertaken to determine whether SP could modulate SRIF signaling pathways in the rat frontoparietal cortex and hippocampus. A single intraperitoneal injection of SP (50, 250 or 500 micro g/kg) induced an increase in the density of SRIF receptors in membranes from the rat frontoparietal cortex at 24 h of its administration, with no change in the hippocampus. The functionality of the SRIF receptors was next investigated. Western blot analysis of Gi proteins demonstrated a significant decrease in Gialpha1 levels in frontoparietal cortical membranes from rats treated acutely (24 h) with 250 micro g/kg of SP, which correlated with a decrease in functional Gi activity, as assessed by use of the non-hydrolyzable GTP analog 5'-guanylylimidodiphosphate. SRIF-mediated inhibition of basal or forskolin-stimulated adenylyl cyclase activity was also significantly lower in the frontoparietal cortex of the SP-treated group, with no alterations in the catalytic subunit of the enzyme. SRIF-like immunoreactivity content was increased in the frontoparietal cortex after acute (24 h) SP administration (250 or 500 micro g/kg) as well as in the hippocampus in response to 7 days of SP (250 micro g/kg) administration. All these SP-mediated effects were prevented by pretreatment with the NK1 receptor antagonist RP-67580. Although the physiologic significance of these results are unknown, the increase in SRIF receptor density together with the desensitization of the SRIF inhibitory signaling pathway might be a mechanism to potentiate the stimulatory pathway of SRIF, inducing a preferential coupling of the receptors to PLC.