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Search for other papers by J. R. E. Davis in
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ABSTRACT
We have studied the effects of cyclic AMP (cAMP) on TSH secretion by cultured rat pituitary cells, using forskolin and dibutyryl cAMP (dbcAMP) to raise the cellular cAMP content by different mechanisms. Forskolin (10 μmol/l), a stimulator of adenylate cyclase, raised the cAMP content within 10 min, but had a more delayed effect on TSH release, with no significant stimulation for at least 6 h, but a clear dose-dependent effect at 24 h. Incubation with dbcAMP likewise increased TSH release after 6–24 h. By contrast, high cellular cAMP levels induced by either forskolin or dbcAMP augmented the TSH response to TRH at an early stage, before any detectable change in unstimulated TSH release. Pretreatment of cells with forskolin led to a parallel upward shift in the subsequent TRH dose-response curve, without a significant change in median effective dose or any change in cellular TSH content.
These findings suggest that cAMP acts to increase the availability of TSH for acute release by TRH by modulation of an intracellular releasable hormone pool, and indicate synergistic interactions between the adenylate cyclase system and the phospholipid-calcium stimulus-release coupling mechanism of TRH.
J. Endocr. (1986) 109, 365–369
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Search for other papers by M A Greer in
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Abstract
The standard method of studying hormone secretion in vitro is to make instantaneous changes in the concentration of stimulators in the medium. However, in vivo the extracellular concentration of such substances changes more gradually; secretion does not occur in square-wave bursts and agonists or antagonists transmitted through the bloodstream are diluted and diffused by plasma or tissue fluid to further decelerate the rate of change in concentration at the cell surface. We have therefore compared in GH4C1 cells the dynamics of changes in cytosolic Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion in response to two very different secretagogues, thyrotrophin-releasing hormone (TRH) and depolarizing K+, using a square-wave or ramp exposure for 5 min. The dynamics of hormone secretion were analysed by column perifusion (2 × 106 cells/column). Ca2+ dynamics were monitored by dual excitation microfluorimetry from 20–30 optically isolated cells using the Ca2+ indicator, fura-2. With square-wave exposure, both TRH (0·1–100 nm) and K+ (10–50 mm) induced dose-dependent increases in [Ca2+]i and PRL secretion. Concentrations of TRH >1 nm caused a two-phase increase in [Ca2+]i with an initial high-amplitude first phase and a low-amplitude second phase. Depolarizing K+ induced a sharp increase in [Ca2+]i which peaked within 15 seconds, then declined gradually on a sloping plateau. Both TRH and K+ induced an acute dose-dependent PRL secretory burst peaking within 2·5 min with a subsequent rapid decline. With ramp exposure, high doses of TRH (final concentration 10–100 nm) triggered an acute rise in [Ca2+]i; however, the peaks were clearly lower than those induced by the maximum concentration reached given as a square-wave. TRH (0·1–100 nm) induced PRL secretion in a dose-dependent manner. Ramp depolarizing K+ induced dose-dependent parallel 'ramp' increases in [Ca2+]i concentration and PRL secretion without a 'burst' rise in either. These data suggest that a rise in [Ca2+]i plays a more critical role in K+-induced than in TRH-induced PRL secretion; intracellular transduction systems which do not involve [Ca2+]i appear more important for the latter.
Journal of Endocrinology (1994) 142, 145–152
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ABSTRACT
In view of recent reports suggesting that thyroid hormone control of TRH degradation occurs outside the central nervous system in animals, the effect of thyroid status on serum and tissue degradation of TRH in man was investigated. In six patients with hyperthyroidism and six patients with hypothyroidism, constant TRH infusions were carried out for determination of plasma clearance rate (PCR) and half-life of disappearance (t ½) of TRH, with simultaneous determination of half-life of disappearance in serum in vitro (t ½p). Using a kinetic model, this enabled the calculation of the half-life of disappearance in the extravascular tissue compartment (t ½t). All patients were reinvestigated after they had become euthyroid. PCR, t ½ and t ½p were 22·1 ±3·4 ml/kg per min, 6·8±1·1 min and 17·3±6·7 min (means ± s.d.) respectively in the euthyroid patients. The t ½p was slightly but significantly prolonged during hyperthyroidism. The t ½ was 5·6 min in the hyperthyroid patients compared with 9·4 min in the hypothyroid patients. The calculated t ½t was 6·5 min in the euthyroid patients. In the patients with untreated hyperthyroidism, t ½t was significantly reduced (22·7±10·7%; mean ± s.d.), while it was considerably prolonged (41·1±24·6%) in patients with untreated hypothyroidism. The percentage reduction or prolongation of t ½t was negatively correlated with the logarithm of the serum concentrations of thyroxine (r = 0·92) and tri-iodothyronine (r= 0·91) in the untreated patients. Thus, thyroid hormones induce alterations in the pharmacokinetics of TRH. This may partly be due to induction by thyroid hormones of membrane-bound pyroglutamyl aminopeptidase.
Journal of Endocrinology (1991) 128, 153–159
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Search for other papers by A. M. Gurney in
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ABSTRACT
The activity of ornithine decarboxylase (ODC) in the rat anterior pituitary gland varies during the oestrous cycle, with a rise in activity seen at pro-oestrus. This enzyme, which is rate-limiting for the synthesis of the polyamines, can be specifically and irreversibly blocked by α-difluoromethylornithine (DFMO). A previous study showed that when this drug was administered to rats in vivo on the afternoon of pro-oestrus, it suppressed the normal surge in plasma prolactin levels that occurred later that day. The effect of DFMO was associated with reduced levels of putrescine in the anterior pituitary gland, suggesting that ODC activity in the lactotroph might be involved in the prolactin surge. We have examined the effects of DFMO on the secretion of prolactin from anterior pituitary cells, isolated either from male rats or from females at different stages of the oestrous cycle. The drug was found to reduce prolactin secretion stimulated by thyrotrophin-releasing hormone (TRH), but only in cells isolated from pro-oestrous animals and only for 2 days after cell isolation. Basal secretion was unaffected by DFMO. The results imply that ODC is important for TRH-stimulated prolactin secretion at pro-oestrus, and it is specific for pro-oestrus. The prolactin surge could therefore be influenced by this ODC-dependent effect of TRH. The pro-oestrous-specific response to TRH may be a consequence of the increased ODC activity seen at this time. Alternatively, the increased ODC activity could be a consequence of coupling to TRH receptors, which are known to increase in number at pro-oestrus.
Journal of Endocrinology (1993) 137, 133–139
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ABSTRACT
Anterior pituitary glands from broiler fowl were incubated by themselves, with hypothalamic tissue or with thyrotrophin releasing hormone (TRH) in medium containing dopamine and its antagonist pimozide. The presence of hypothalamic tissue or TRH resulted in a stimulation of release of prolactin. Neither dopamine nor pimozide affected prolactin release directly from the pituitary gland. Dopamine inhibited the release of prolactin stimulated by hypothalamic tissue or TRH, in a concentration-dependent fashion. Pimozide diminished the response to dopamine. After pituitary glands were preincubated for 20 h in medium containing oestradiol-17β, the basal release of prolactin was enhanced as was the response to TRH. Both basal and TRH-stimulated release of prolactin from the oestrogen-primed pituitary glands was inhibited by dopamine, an effect blocked by pimozide. Hypothalami from broiler fowl were incubated for up to 8 h in medium containing dopaminergic drugs and pituitary glands were incubated in this medium, alone or with pimozide. As indicated by the prolactin released by the pituitary glands, the hypothalami appeared to secrete prolactin-releasing activity in a time-related fashion. Dopaminergic activity was also present in the hypothalami, since pimozide enhanced the prolactin-releasing activity of the medium. Dopamine apparently inhibited and pimozide stimulated the secretion of releasing activity from the hypothalamus. These results suggest that dopamine inhibits release of prolactin directly from the pituitary gland only when prolactin secretion is high. The hypothalamus secretes at least two factors regulating prolactin secretion, a prolactin-releasing factor and a dopaminergic prolactin-inhibiting factor. Dopamine may also play an inhibitory role in the regulation of secretion of the prolactin-releasing factor.
J. Endocr. (1984) 103, 63–69
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SUMMARY
The experiment was designed to determine whether chronic administration of gonadotrophin releasing hormone (GnRH) plus thyrotrophin releasing hormone (TRH) to pubertal bulls would hasten sexual maturation. Nine bulls, three each at 4, 5 and 6 months of age were given i.m. injections of GnRH (0·4 μg/kg) plus TRH (2 μg/kg) three times daily (09.00, 16.00 and 23.00 h) for 30 days. Equivalent animals were given 0·9% saline vehicle as controls. At the outset of injections the 6-, 5- and 4-month-old bulls weighed 201, 157 and 169 kg respectively; equivalent weights at the final injection were 222, 186 and 198 kg. Plasma levels of LH, prolactin and testosterone were determined weekly around the injection given at 09.00 h. Twenty-four-hour plasma testosterone and LH profiles were obtained immediately after the last injection. The animals were unilaterally castrated at the end of this 24 h period and the second testis was removed 60 days later to assess testicular weights and epididymal sperm numbers. Surges of LH, testosterone and prolactin occurred throughout the experimental period in response to GnRH/TRH injection. The LH surge in response to the first injection was nearly fivefold larger than those measured at subsequent weekly intervals but prolactin and testosterone responses did not decline during the 28-day experimental period. Induced LH and prolactin peaks were larger than those occurring in the control bulls, but control animals produced an equivalent amount of testosterone. These bulls produced quasi-adult levels of testosterone even at 4 months of age. Chronic GnRH/TRH administration resulted in a depression of plasma testosterone, and a disruption of the normal episodic release pattern during the 24 h after the final treatment, but normal patterns returned within 60 days. Testicular weights and epididymal sperm numbers were not increased by chronic GnRH/TRH treatment.
Search for other papers by T. J. Lauterio in
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Search for other papers by C. G. Scanes in
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ABSTRACT
The possible role of thyroid hormones in the rise in plasma GH observed in protein-restricted chicks was examined. Increased sensitivity of protein-restricted chicks to secretagogue challenge (TRH or GH-releasing factor) appears to account, at least in part, for increased GH concentrations in protein-restricted chicks. Thyroid hormones administered acutely were able to suppress plasma GH concentrations in protein-restricted chicks. Further, chronic thyroid hormone supplementation to low protein diets normalized circulating thyroid hormone concentrations and also normalized the response to GH secretagogue challenge. This decreased sensitivity to TRH provocation occurred without an accompanying change in plasma concentrations of insulin-like growth factor-I, a reputed inhibitor of GH secretion in the chicken.
J. Endocr. (1988) 117, 223–228
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Search for other papers by B. L. Brown in
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ABSTRACT
The possible role of calcium as a primary mediator in the control of prolactin secretion from normal pituitary cells was examined. Basal prolactin secretion, and secretion stimulated by thyrotrophin releasing hormone (TRH), raised K + or the calcium ionophore, A23187, were all dependent on the presence of extracellular Ca2+. The calcium channel antagonists, methoxyverapamil, cobalt and manganese, inhibited basal, TRH- and K+-stimulated prolactin secretion. In addition, prolactin secretion stimulated by a phosphodiesterase inhibitor, isobutylmethylxanthine, which increases cellular cyclic AMP, was inhibited by these Ca2+ antagonists. These observations indicate that Ca2+ may be the primary intracellular mediator in the control of prolactin secretion, with cyclic AMP having a secondary modulatory role on Ca2+ influx, probably on voltage-dependent Ca2+ channels.
J. Endocr. (1984) 101, 319–325
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Search for other papers by J. C. Meijer in
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ABSTRACT
The effects of i.v. administration of thyrotrophin-releasing hormone (TRH) and of somatostatin on circulating plasma levels of porcine GH in the chronically catheterized pig fetus have been examined. Growth hormone levels increased markedly (P<0·01) following TRH administration, but there was no change in thyroxine levels by 1 h after treatment. Administration of somatostatin caused a significant (P<0·05) decrease in mean GH levels, but the response was variable between pigs. Saline administration had no significant effect on GH levels. These results suggest that the mechanisms regulating postnatal GH release are present in the fetal pig, but may not be fully developed 8–12 days before delivery.
J. Endocr. (1985) 106, 121–124
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Search for other papers by N. WHITE in
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The ability to assay small amounts of the peptide releasing hormones in biological fluids would aid greatly in the assessment of hypothalamic function. We have recently described a specific radioimmunoassay for luteinizing hormone releasing hormone (LH-RH) (Jeffcoate, Fraser, Gunn & Holland, 1973a, b) and in this study we report a radioimmunoassay for the tripeptide, thyrotrophin releasing hormone (TRH).
Thyrotrophin releasing hormone (2 mg) was conjugated to 10 mg bovine serum albumin in borate buffer pH 9·0 by the bis-diazotized benzidene method (Bassiri & Utiger, 1972 a). After 2 h at 5 °C the mixture was dialysed against distilled water for 48 h and against 0·15 m-NaCl for 24 h. This technique conjugates TRH by the imidazole ring of histidine to the protein. A sample of conjugate (2·5 mg) in saline was homogenized with Freund's complete adjuvant and injected into 20 intradermal sites in a White New Zealand rabbit.
