Pulses Of TRH Research Articles

Thyrotropin-releasing hormone (protirelin) inhibits potassium-stimulated glutamate and aspartate release from hippocampal slices in vitro

Excess excitatory amino acid release is involved in pathways associated with seizures and neurodegeneration. Thyrotropin-releasing hormone (TRH; protirelin), a brain-derived tripeptide, has shown efficacy in the treatment of such disorders, yet its mechanism of neuroprotection is poorly understood. Using superfused hippocampal slices, we tested the hypothesis that TRH could inhibit evoked glutamate/aspartate release in vitro. Rat hippocampal slices were first equilibrated in oxygenated Krebs buffer (KRB) (120 min) then superfused for 10 min with KRB (control), or KRB containing 0.1, 1, or 10 μM TRH respectively, prior to and during 5 min depolarization with high potassium KRB (50 mM [K +] ± TRH). Fractions (1 min) were collected during the 5 min stimulation and for an additional 10 min thereafter and analyzed for glutamate and aspartate by HPLC. TRH had no effect on baseline glutamate/aspartate release, while all three TRH doses significantly ( P < 0.05) inhibited peak 50 mM [K +]-stimulated glutamate/aspartate release, and glutamate remained below control ( P < 0.05) at 15 min post stimulation. A 5 min pulse of TRH (10 μM) had no affect on basal glutamate/aspartate release, whereas the TRH pre-pulsed slices failed to release glutamate/aspartate by [K +]-stimulation given 15 min later. These results are the first to show a potent and prolonged inhibitory effect of TRH on evoked glutamate/aspartate release in vitro. These initial studies suggest that exogenous and/or endogenous TRH may function, in part, to modulate excess glutamate release in specific CNS loci. Additional studies are in progress to fully understand the mechanism of this potent effect of TRH and its implication in various CNS disorders.

Calcium/calmodulin-dependent protein kinase-II activation in rat pituitary cells in the presence of thyrotropin-releasing hormone and dopamine.

PRL release from rat lactotrophs in response to TRH is Ca2+ dependent. TRH-induced PRL release is inhibited either after repeated pulses of TRH or in the presence of dopamine. TRH, however, generates increases in intracellular Ca2+ concentrations ([Ca2+]i) in both conditions. Calcium/calmodulin-dependent protein kinase-II (CaM kinase-II) is a ubiquitous enzyme implicated in secretion. To determine whether down-regulation of CaM kinase-II activity caused the lack of responsiveness to increases in [Ca2+]i, we measured the generation of calcium/calmodulin-independent kinase activity. Anterior pituitary cells contain a 50-kilodalton form of CaM kinase-II, determined by immunoblot, and the enzyme is in lactotrophs, determined by immunocytochemistry. TRH rapidly and transiently increased calcium/calmodulin-independent kinase activity; the increase was maximal by 15 sec and returned to basal by 2 min. When TRH pulses (1 microM, 15 sec) were applied every 10 min, each pulse caused an increase in calcium/calmodulin-independent kinase activity of similar magnitude, and the activity returned to basal values between pulses. Pretreatment of cells with dopamine (1 microM; 30 min) inhibited PRL release, but did not prevent the increase in calcium/calmodulin-independent kinase activity. These results indicate that TRH still activates CaM kinase-II when PRL release is inhibited. Dopamine and repeated pulses of TRH must inhibit PRL release at a site after the TRH-induced increase in [Ca2+]i and at a site other than CaM kinase-II.

Neuropeptides of Anterior Pituitary Origin: Autocrine or Paracrine Functions?

Several neuropeptides classically associated with the hypothalamus have been found in the anterior pituitary. The question arises whether they are locally synthesized and if they play a paracrine or autocrine role on pituitary cell functions. Using normal and tumoral human pituitaries we found neuropeptides (TRH, SRIH, GHRH) and dopamine in variable quantities according to the nature of the tissue. They were all present in normal pituitaries, while stimulatory hormones (TRH and GHRH) were predominantly found in tumoral tissue, implying an imbalance of pathophysiological importance between the stimulatory and inhibitory control of hypophyseal hormones (PRL and GH) in pituitary adenomas. Both normal and tumoral pituitaries released TRH, SRIH and GHRH in large amounts suggesting their local synthesis. The in situ synthesis was demonstrated for SRIH by the evidence of SRIH mRNA, the detection of SRIH immunoreactivity in peculiar cells and the presence of SRIH precursor. The possible role of these pituitary neuropeptides was suggested for instance by the negative correlation found in vitro between SRIH and GH secretions. Moreover, neuropeptides could interact with each other. Indeed DA stimulated TRH release while PRL secretion decrease at the same time. Pulses of TRH had differential effects on SRIH release according to the nature of the tissue as TRH inhibited SRIH release from adenoma while it stimulated SRIH release from normal pituitary. Concerning the effects of SRIH and GHRH on GH secretion, there was an endogenous regulatory pattern comparable to that observed in rat portal blood vessels. Pulses of GHRH induced GH secretion only when endogenous SRIH release was not stimulated.(ABSTRACT TRUNCATED AT 250 WORDS)

A prepro-TRH connecting peptide (prepro-TRH 160-169) potentiates TRH-induced TSH release from rat perifused pituitaries by stimulating dihydropyridine- and omega-conotoxin-sensitive Ca2+ channels.

The stimulation of TSH secretion by TRH involves the phosphatidylinositol second messenger pathway via activation of phospholipase C. This effect is mediated by a GTP-binding protein and leads to a mobilization of intracellular Ca2+ stores and an activation of protein kinase C. However, TRH stimulation also results in an influx of extracellular Ca2+. Since we have previously demonstrated that a non-TRH fragment of the prepro-TRH molecule, the connecting peptide PS4 (prepro-TRH 160-169), was able to potentiate the TRH-induced TSH release in a dose-dependent manner, we attempted to determine whether this potentiation might be due to a Ca(2+)-dependent phenomenon and whether a specific class of voltage-dependent Ca2+ channels, the L type Ca2+ channels, might be involved in the effect of PS4. This was studied by perifusing normal pituitary fragments with medium containing either the Ca2+ ionophore, ionomycin, and Co2+ ions, or organic compounds well known to block L-type Ca2+ channels, and by measuring the TSH response to a pulse of TRH (10 nM) in the presence or absence of PS4 (100 nM).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcitonin Inhibits Basal and Thyrotropin-Releasing Hormone-Induced Release of Prolactin from Anterior Pituitary Cells: Evidence for a Selective Action Exerted Proximal To Secretagogue-Induced Increases in Cytosolic Ca2+*

Salmon calcitonin (sCT)-like peptide is present in the central nervous system and pituitary gland of the rat, and this peptide inhibits basal and TRH-stimulated PRL release from cultured rat anterior pituitary (AP) cells. The present studies were designed to examine further the inhibitory actions of sCT on basal and TRH-stimulated PRL release and investigated 1) the temporal dynamics of the responses, 2) the effects of sCT on PRL release induced by other secretogogues, and particularly those acting via elevations of cytosolic Ca2+, and 3) the selectivity of sCT action on basal and stimulated AP hormone release. The inhibition of basal PRL release by sCT (0.1-10 nM) was dose-dependent and was characterized by a rapid onset with a gradual recovery to normal rates of release after the period of sCT inhibition. The inhibitory effect of sCT on basal PRL release was reversed by treatment with either the Ca2+ ionophore A23187 or with the phorbol ester, phorbol myristate acetate (PMA). sCT infusion did not affect the basal release of GH, TSH, FSH, or LH by perifused AP cells. When administered in short pulses, TRH, at concentrations from 1-100 nM, elicited a dose-dependent increase in PRL release. When coadministered with short 10 nM TRH, sCT (1-100 nM) inhibited TRH-induced PRL release in a dose-dependent manner, with a maximal inhibition of 78% at a concentration of 10 nM, and an ED50 concentration of approximately 3 nM. During longer (30 min) pulses of TRH (100 nM), PRL release increased sharply over 4-fold within 2 min, followed within 12 min by a rapid decline to a level 1.5-2-fold higher than basal, and this level was maintained for the remainder of the stimulation period. sCT pretreatment inhibited the overall PRL response to TRH. In contrast to its inhibition of TRH-induced PRL release, sCT failed to prevent the stimulation of PRL release by either ionophore A23187, PMA, vasoactive intestinal peptide, or forskolin. In addition, sCT failed to block TRH-induced TSH release or GnRH-induced LH release.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcitonin Inhibits Thyrotropin-Releasing Hormone Induced Increases in Cytosolic Ca2+in Isolated Rat Anterior Pituitary Cells*

Calcitonin (CT) and related peptides, such as CT gene-related peptide and salmon CT (sCT)-like peptide, are present in the rat nervous system and the pituitary gland, and sCT markedly inhibits basal and TRH-stimulated PRL release from anterior pituitary (AP) cells. Because TRH-induced PRL release is known to involve increases in cytosolic free Ca2+ derived from both extracellular and intracellular sources, the objective of the present study was to test whether sCT interferes with this effect. Secretogogue-induced elevations of cytosolic free Ca2+ ([Ca2+]i) in acutely dispersed AP cells were monitored using the fluorescent Ca2+ indicator Indo-1 AM and flow cytometry. AP cells were enzymatically dispersed to single cell suspensions and loaded with 20 microM Indo-1 AM for 30 min. Indo-1-loaded AP cells were scanned at a rate of approximately 500 cells/sec for 200-300 sec in a flow cytometer, and the ratio of fluorescence due to Ca2+ bound to Indo-1 to free Indo-1 (Indo-1 ratio), which is an index of [Ca2+]i, was determined for each cell. Under basal conditions, AP cells showed stable Indo-1 ratios during the scans, and 100% of the cells responded to the Ca2+ ionophore ionomycin with increases in the Indo-1 ratio. Approximately 25-30% of the AP cells responded to a 1 microM pulse of TRH with marked increases in the Indo-1 ratio, indicative of increases in [Ca2+]i, with the response consisting of two phases, an initial rapid rise that was unaffected by the presence of EGTA in the extracellular environment, followed by a decrease to a sustained secondary phase that was completely eliminated by EGTA. In a normal extracellular Ca2+ environment, pretreatment with 100 nM sCT almost totally inhibited the response to 1 microM TRH. In EGTA-pretreated AP cells, the initial EGTA-insensitive phase of the TRH-induced [Ca2+]i increase was also abolished by prior exposure to sCT. These results suggest that sCT inhibits TRH-stimulated PRL release in AP cells by attenuating the TRH-induced increase in [Ca2+]i, an effect that probably occurs as a consequence of inhibition of the stimulatory effect of TRH on the Ca2+/phospholipid messenger system.

TRH stimulates the release of POMC-derived peptides from goldfish melanotropes

The release of immunoreactive (ir) α-MSH and ir ACTH from goldfish ( Carassius auratus) melanotropes was investigated using superfused isolated dispersed neurointermediate lobe cell columns. Stimulation of neurointermediate lobe cell columns with pulses of TRH evoked dose-dependent increases in the concomitant release of ir α-MSH and ir ACTH. Reversed-phase high performance liquid chromatography (RP-HPLC) was used to characterize the α-MSH and ACTH immunoreactivities released from a neurointermediate cell column under spontaneous release conditions. Six peaks of ir α-MSH were revealed. Three of these peaks were identified as des-acetyl α-MSH, mono-acetyl α-MSH and di-acetyl α-MSH. Seven peaks of ir ACTH were revealed. Four of these peaks were tentatively identified as ACTH variants. These studies suggest that TRH stimulates the release of peptide hormones from teleost melanotropes and that the goldfish neurointermediate lobe in vitro releases numerous peptides derived from POMC.

Ca2+Transients Induced by Thyrotropin-Releasing Hormone Rapidly Lose Their Ability to Cause Release of Prolactin

To investigate the relationship of changes in cytosolic free calcium concentrations [( Ca2+]c) caused by TRH to changes in PRL secretion, we simultaneously monitored PRL release and [Ca2+]c, using the fluorescent Ca2+ indicator indo-1, in freshly isolated perifused cells from rat anterior pituitary glands. We found that a 30-sec pulse of 100 nM TRH triggered a transient spike of [Ca2+]c, but prolonged PRL release for up to 30 min; continuous administration of TRH caused a sustained elevation in [Ca2+]c, but the same pattern and amount of PRL release as that caused by the pulse of TRH. PRL secretion was refractory to further pulses of TRH given at 10-min intervals for 40 min, but did respond to a second pulse of TRH given 40 min after the first pulse with no intervening pulses. Pulses of TRH given every 10 min still triggered spikes of [Ca2+]c of the same magnitude as the first pulse, indicating that the cause of the refractory state must occur at a post-receptor step that is after the mobilization of [Ca2+]c. A 30-sec pulse of a high concentration of KCl caused a transient spike of [Ca2+]c and transient, not prolonged, release. Additional pulses of KCl cause progressively less PRL release, although the magnitude of the spikes in [Ca2+]c did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine has no effect on thyrotropin-releasing hormone mobilization of calcium from intracellular stores in rat anterior pituitary cells.

To investigate whether dopaminergic reduction of PRL secretion is caused by an inhibition of release of Ca2+ from intracellular stores, we used a perifusion system to monitor simultaneously changes in intracellular Ca2+ concentrations ([Ca2+]c) and changes in PRL secretion from rat anterior pituitary cells, and from enriched populations of lactotrophs. We eliminated influx of extracellular Ca2+ by using medium with no added Ca2+ and 0.2 mM EGTA, conditions which abolished the increase in [Ca2+]c caused by 56 mM KCl. In this low Ca2+-containing medium, 100 nM TRH induced a burst of [Ca2+]c; the magnitude of the peak was the same whether the cells were perifused with low Ca2+-containing medium for 2 or 20 min before adding TRH, indicating the source of intracellular Ca2+ was stable under these conditions. After 2 min in this medium, TRH was still able to stimulate almost as much PRL release as in medium containing 1.8 mM Ca2+, and 1 microM dopamine inhibited this release, but did not affect the magnitude of the TRH-induced increase in [Ca2+]c. Preincubation for 5 min with dopamine did not affect the ability of a 30-sec incubation with TRH to stimulate the accumulation of inositol phosphate, inositol bisphosphate, and inositol trisphosphate, either in medium containing 1.8 mM Ca2+, or in low Ca2+-containing medium. Preincubation with dopamine for 5 min had no effect on TRH-induced mobilization of intracellular calcium. A source of Ca2+ is needed to refill internal Ca2+-stores discharged by TRH, and as dopamine lowered [Ca2+]c which might fill these stores, we tested to see if dopamine prevented.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a dual effect of gamma-aminobutyric acid on thyrotropin (TSH)-releasing hormone-induced TSH release from perifused rat pituitaries.

The effects of gamma-aminobutyric acid (GABA) on the spontaneous and TRH-induced TSH release were investigated in vitro on perifused rat pituitaries. The dynamic pattern of TSH release was measured in response to a 6-min pulse of TRH (10 nM) with or without GABA addition. GABA had no effect on spontaneous TSH release but exhibited a dual effect on TSH-stimulated release according to the dose (as calculated by the induced-basal ratio): a potentiation of the TSH response to TRH at the lowest concentrations tested (less than or equal to 10 nM) and an inhibition for GABA concentrations equal or higher than 100 nM. The GABA potentiation was mimicked by muscimol (10 microM) and isoguvacine (10 nM) but not by baclofen (1 microM). Bicucullin (1 microM) or picrotoxin (1 microM) added 15 min before GABA was unable to reverse the GABA potentiation of the TSH response, although SR 95103 (1 and 10 microM), a specific GABA A antagonist, partially or totally antagonized this response. Diazepam (7 nM) was able to potentiate the TSH response by 216% when GABA was added to the system at a concentration (60 nM) which does not modify by itself the TSH response. The inhibitory effect of GABA (100 nM) was completely abolished by bicucullin (1 microM), by picrotoxin (1 microM), and by SR 95103 (1 microM). Picrotoxin not only blocked the inhibitory action of GABA but significantly (P less than 0.05) potentiated the TSH response to TRH. Our data suggest a dual GABA-ergic control of TRH-stimulated TSH release directly on the pituitary, probably mediated by two different kinds of GABA receptors: a GABA A receptor site mediating the inhibitory effect and a nonclassical GABA A receptor site of higher affinity for its stimulatory action.

Biphasic effect of thyrotropin-releasing factor (TRH) on α-melanotropin secretion from frog intermediate lobe in vitro

The kinetics of a-MSH secretion induced by prolonged TRH infusion were studied using perfused frog neurointermediate lobe (NIL). During a 2 h administration of TRH (10 −8 M), the secretion rate of α-MSH displayed two phases. During the first phase, secretion of α-MSH increased rapidly reaching a maximum within 20 min and then, despite continued TRH infusion, this secretion slowly declined. The second phase was characterized as plateau of elevated release (relative to basal secretion); within this second phase there was often a small peak of released α-MSH occurring at about 100 min. Exposure of NIL to another TRH (10 −8 M) pulse 90 min later induced a normal stimulation of α-MSH secretion, thus demonstrating the viability of tissue in perifusion. Continuous infusion of cycloheximide (10 −5 M) during a 5 h period totally inhibited the biosynthetic activity of NIL but did not influence TRH-induced α-MSH secretion. In particular, cycloheximide had no effect on the second phase of the response to prolonged infusion of TRH. Similarly, during continuous infusion of the monovalent carboxylic ionophore monensin (10 −6 M), the biphasic response to prolonged infusion of TRH (10 −8 M) was still observed. Administration of a short pulse of TRH (10 −7 M) during the declining part of the first phase or during the second phase of prolonged TRH (10 −8 M) infusion induced a significant enhancement of α-MSH stimulation. From these results we conclude that (1) prolonged TRH infusion causes α-MSH release in a biphasic manner; (2) attenuation of the secretory response to continuous TRH administration does not result from exhaustion of the releasable pool of α-MSH; (3) the biphasic response depends neither on de novo biosynthesis nor on intracellular transit of the prohormone from the endoplasmic reticulum to the Golgi complex. Thus, we propose that prolonged exposure of frog NIL to TRH may cause transconformational changes of membrane TRH receptors from high to low affinity states.

Benzodiazepines inhibit thyrotropin (TSH)-releasing hormone-induced TSH and growth hormone release from perifused rat pituitaries.

The perifusion technique was used to investigate the action of diazepam (DZ), a benzodiazepine molecule known to compete for TRH receptor binding in rat pituitary, on TRH-induced TSH and GH release. The release kinetics for the two hormones from quartered pituitaries were measured in response to a 6-min pulse of TRH (10 nM), without or with DZ addition for a period of 30 min before and during the TRH pulse, plus an additional 15-min period. The dynamic patterns of TSH and GH release in response to TRH were characterized by a rapid increase in hormone release, declining slowly over the next 20 min. The rate of release represented 2.98 +/- 0.02 (+/- SE) and 1.75 +/- 0.06 times the corresponding basal level for TSH and GH, respectively, when evaluated over the first 15 min of the response to TRH. Addition of increasing doses of DZ suppressed the stimulatory effect of TRH in a dose-related manner, with an ID50 of 3 nM for both TSH and GH. The maximal effect of DZ was obtained with a concentration of 10 nM for both hormones. Ro 15-1788 (100 nM), a selective antagonist of the central type of benzodiazepine-binding sites (added to the perifusion system 30 min before DZ and then during the whole period of DZ perifusion), completely abolished (P less than 0.01) the inhibitory effect of DZ (10 nM) on the TRH-induced TSH and GH responses. When added alone before the TRH pulse, Ro 15-1788 had no effect on the TSH response to TRH. In contrast, PK 11,195 (100 nM), a selective antagonist of the nonneuronal benzodiazepine-binding sites, was unable to abolish the inhibitory action of DZ on TRH-stimulated TSH release. In addition, the effects of four other types of benzodiazepine (flurazepam, chlordiazepoxide, midazolam, and medazepam), all tested at a 10-nM concentration, corroborated these findings. Furthermore, DZ inhibition of the TSH response was nullified by picrotoxin (1 microM), but not by bicuculline (1 microM), two gamma-aminobutyric acid antagonists that had no effect, by themselves, on this response. For comparison, the effect of DZ (10 nM) was also tested on the release of GH in response to human GH-releasing factor-(1-44)-NH2 (10 nM) and was found to be ineffective.(ABSTRACT TRUNCATED AT 400 WORDS)

Human pancreatic growth hormone-releasing factor-40 (hpGRF-40) allows stimulation of GH release by TRH.

The effects of synthetic hpGRF-40 on GH release from continuously perifused male rat anterior pituitary cells were studied. Pulses (2.5 min) of hpGRF-40 stimulated GH release in a log-linear dose response relationship: concentrations of 0.03, 0.1, 0.3, 1, 3, 10, 30 and 100 nM given in a random order elicited a GH response above baseline of 1.2 +/- 0.3, 2.4 +/- 0.4, 2.8 +/- 0.2, 4.3 +/- 0.2, 6.2 +/- 0.7, 7.0 +/- 1.0, 8.7 +/- 1.7, and 10.8 +/- 0.8 micrograms/10(7) cells (mean +/- SEM; n = 3; r = 0.93), respectively. During a 5-h hpGRF-40 infusion, GH stimulation peaked within 5 min and waned to near baseline by the end of the fifth h. The integrated GH responses to 0.03, 0.1 and 0.3 nM hpGRF-40 were 37.6 +/- 7.4, 52.9 +/- 8.5, and 66.15 +/- 8.2 micrograms/10(7) (mean +/- SEM; n = 3; r = 0.72), respectively. The interaction of TRH and hpGRF-40 in the control of GH secretion was studied to investigate the mechanism of the "paradoxical" TRH stimulation of GH release associated with GH excess states in humans. Dispersed cells were perifused with either 100 nM TRH for 0.5 h, 5 nM hpGRF-40 for 4 h, or 5 nM hpGRF-40 for 4 h, to which a 0.5 h pulse of TRH was added at 2 h. GH levels did not change significantly in the presence of TRH alone. When TRH was added to the ongoing hpGRF-40 perifusion, GH release increased from 1.4 +/- 0.06 to 4.0 +/- 1.0 micrograms/min.10(7) cells (n = 4; P = 0.03). Thus, dispersed pituitary cells are highly sensitive to very low concentrations of hpGRF-40 administered as both an acute pulse and as a tonic infusion. When the cells are exposed to a maximal concentration of hpGRF-40 (i.e. 5 nM), TRH becomes a secretagogue at the pituitary level, thus suggesting the site and mechanism of the "paradoxical" GH response to TRH observed in some acromegalics.

The dissociation of the exaggerated prolactin and thyrotropin responses in seminiferous tubule failure following the administration of a double-pulse of thyrotropin-releaslng hormone

PRL and TSH secretion has been evaluated in 11 patients with seminiferous tubule failure and 9 controls. When compared to the controls, the patients had increased basal FSH, TSH and PRL levels. However, LH, E2, T and thyroid hormone levels were similar to the controls. Both groups were given two pulses of TRH (200 micrograms) at 30 min intervals. Following the initial pulse of TRH, the patients demonstrated exaggerated TSH and PRL responses. The administration of a second pulse of TRH led to a further increment of TSH secretion in the patients. There was, however, no PRL response to the second TRH pulse in either patients or controls although mean PRL levels remained significantly greater in the patients.

Dynamics and regulation of TSH secretion by superfused anterior pituitary cells.

Superfused dispersed cells respond rapidly to 2- to 10-min pulses of TRH (10(-10) to 10(-7) M) in a dose-dependent manner. The effects of decreasing the stimulus duration can be overcome by a proportional increase in concentration of TRH. A TRH stimulus of 10 min or greater duration results in a sharp peak in TSH secretion followed by a lower plateau. Somatostatin (10(-8) M inhibits the response to TRH (t X 10(-9) M). T3 (2.0 microgram/dl) inhibits TRH-induced TSH secretion by superfused pituitary fragments, but not by dispersed cells. Corticosterone (50 microgram/dl), however, inhibits crude CRF-induced ACTH secretion by such cells.

Studies on the role of calcium and cyclic nucleotides in the control of TSH secretion

The role of putative mediators in the control of thyrotropin (TSH) secretion has been investigated by monitoring hormone release from isolated anterior cells in response to agents and conditions which modify cyclic nucleotide concentrations or calcium fluxes. The secretory response to 5-min pulses of TRH, raised K + concentration and A-23187 was diminished within 5 min when Ca 2+-free perifusion was begun 80 sec prior to the pulse. In contrast, the response to theophylline and IBMX was unaffected under these conditions. Both IBMX and dibutyryl cyclic AMP potentiated the effects of TRH and raised K +concentration but not that of A-23187. Methoxyverapamil inhibited TSH secretion stimulated by TRH, raised K + concentration and IBMX but not that induced by A-23187. Calcium efflux showed a similar temporal profile to hormone secretion in response to TRH, IBMX and raised K + concentration. It is proposed that one interpretation of these findings is that there is an interrelationship between calcium ions and the cyclic nucleotides in the control of TSH secretion and that cyclic nucleotides may act at the level of the Ca 2+ channel to modulate Ca 2+ entry.

Studies on the control and dynamics of thyrotropin secretion from isolated adenohypophyseal cells

Thyrotropin (TSH) secretion from isolated anterior pituitary cells has been studied using the technique of cell column perifusion. The consistency in secretory rate and temporal profiles of TSH output in response to stimulation illustrated that the system is suitable for studying the kinetics of stimulation and inhibition of secretion. During perifusion TSH release was stimulated in response to a variety of secretogogues, namely TRH, raised potassium concentrations and phosphodiesterase inhibitors. The onset and termination of the secretory responses were rapid and displayed a temporally biphasic pattern of secretion. Dose-related increases in TSH output in response to TRH and consistent responses to repetitive pulses of TRH (5.5 × 10 −10 M) during a 4 h period were demonstrated. Studies on the dynamics of thyroid hormone feedback on TRH-stimulated TSH secretion indicated that inhibition was manifest within l h and reached a maximum after 2 1 2 h during continual exposure to thyroid hormones. Isobutylmethylxanthine (IBMX) potentiated the effect of raised K + as well as that of TRH on TSH secretion, suggesting an as yet unidentified relationship between Ca 2+ and cyclic AMP.