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Biomedical subjects

M C Sheppard

Publications and source records attributed to M C Sheppard.

At least 199 records · Page 11Linked to original sources
PubMed1986

3,5-Diiodotyrosine and thyronine in the urine of patients with chronic renal disease.

Urinary 3,5-diiodotyrosine (DIT) and thyronine (T0) excretion was investigated in 18 patients with chronic renal disease. In accord with previous findings serum T4 and thyroid hormone binding proteins measured in 17 patients were in the low or normal range. Urinary albumin excretion was elevated in all 18 and T4 binding prealbumin (TBPA) in 15 of the 18. Urinary T0 excretion measured in 12 patients was also significantly lower than normal (mean +/- SD 4.4 +/- 2.6 vs 15.8 +/- 5.8 nmol/24 h renal vs normal 2 P less than 0.001). In contrast urinary DIT excretion was significantly elevated in renal patients compared with normal subjects (2.0 +/- 1.5 vs 0.75 +/- 0.41 nmol/24 h, respectively). Possible sources of the increased DIT are discussed.

Adult↗
PubMed1986

Effect of hypothyroidism on pituitary cytoplasmic concentrations of messenger RNA encoding thyrotrophin beta and alpha subunits, prolactin and growth hormone.

Thyroid hormones may directly regulate gene expression in the anterior pituitary. In order to examine this possibility we have studied the effect of hypothyroidism in the rat on pituitary cytoplasmic concentrations of messenger RNA (mRNA) encoding thyrotrophin (TSH) beta and alpha subunits, prolactin and GH. We demonstrated a marked increase in TSH beta and alpha subunit mRNA, accompanied by a decrease in GH mRNA, in the hypothyroid state, changes largely reversed by thyroid hormone replacement. We have thus shown a direct influence of thyroid status on the pretranslational events occurring in pituitary hormone synthesis. The simultaneous rise in cytoplasmic TSH beta and alpha mRNA levels and fall in GH mRNA in hypothyroidism suggests that thyroid status exerts a differential effect on the expression of these genes.

Animals↗
PubMed1986

Modulation of thyrotrophin release from an intracellular pool by pre-exposure to thyrotrophin-releasing hormone and dibutyryl cyclic AMP.

Amplification of desensitization of TSH response to thyrotrophin-releasing hormone (TRH) may be important mechanisms in the regulation of its secretion. We have investigated this possibility in vitro, using monolayer culture of rat anterior pituitary cells. Cells (1-1.5 X 10(5)/250 microliters per well) were cultured for 72 h, exposed to TRH or dibutyryl cyclic AMP (dbcAMP) for 6 or 8 h, washed, and then treated for 4 h with various doses of TRH, or with K+ (55 mmol/l) as a non-specific secretagogue. Pretreatment with TRH (20 nmol/l) for 8 h reduced subsequent TSH release: basal release fell to 64% of the control value (1.01 +/- 0.10 micrograms/l pretreated, 1.58 +/- 0.16 control) and release in response to TRH (100 nmol/l) to 69% of the control (2.7 +/- 0.19 micrograms/l vs 3.98 +/- 0.22); K+ response was reduced to 86% of the control (3.77 +/- 0.21 micrograms/l vs 4.39 +/- 0.20), significantly less than the other reductions. The extent of the parallel downward shift of the TRH dose-response curve was proportional to dose and duration of prior TRH exposure. There was no significant change in the dose of TRH required to cause half-maximal TSH release (ED50: pretreated 4.8, control 2.8 nmol TRH/l) suggesting depletion of an intracellular pool of TSH rather than 'desensitization'. After 6-h pretreatment with dbcAMP, subsequent TSH responses were augmented: basal release was 130% of the control, response to TRH (100 nmol/l) was 137% and to K+ it was 132% of the control, with a parallel upward shift of the TRH dose-response curve but no change in cellular TSH content.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Tri-iodothyronine and phenytoin reduce prolactin messenger RNA levels in cultured rat pituitary cells.

Thyroid hormones may regulate prolactin gene transcription. We have previously found that phenytoin inhibits tri-iodothyronine (T3) nuclear binding, and have suggested that phenytoin may act as a partial T3 agonist. We have therefore investigated the effects of phenytoin and T3 on prolactin release and gene transcription, using the technique of cytoplasmic dot hybridization with complementary DNA probes to estimate prolactin messenger (m) RNA concentrations in cytoplasm from cultured rat pituitary cells. Tri-iodothyronine treatment led to a small but significant fall in prolactin release by 72 h, but caused marked dose- and time-dependent reductions in prolactin mRNA levels at 48-72 h. Phenytoin, however, caused more rapid falls in both prolactin release and mRNA concentrations. Neither T3 nor phenytoin significantly altered GH mRNA levels. These studies suggest effects of phenytoin similar, but not identical, to those of T3 in the lactotroph.

Animals↗
PubMed1986

A modulatory role for cyclic AMP in the control of thyrotrophin release: studies with forskolin and dibutyryl cyclic AMP.

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 mumol/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.

Animals↗
PubMed1986

Demonstration of thyrotrophin beta-subunit messenger RNA in rat pituitary cells in primary culture--evidence for regulation by thyrotrophin-releasing hormone and forskolin.

We have reported previously the effect of thyroid status in vivo on pituitary cytoplasmic concentrations of messenger RNA (mRNA) encoding the thyrotrophin (TSH) beta-subunit (Franklyn, Lynam, Docherty et al, 1985). Studies in vitro of the regulation of TSH beta gene transcription have been confined to thyrotrophic tumour cells. We now report the demonstration of TSH beta-subunit mRNA in non-tumorous rat pituitary cells in primary culture. Treatment of cells with thyrotrophin-releasing hormone (TRH) and with forskolin resulted in a marked increase in cellular concentration of TSH beta-mRNA. These results suggest that TRH exerts a direct effect on the pretranslational events involved in TSH synthesis and further that the adenylate cyclase system may be involved in the regulation of synthesis. We have thus described a novel system for the study of TSH beta-subunit gene expression in normal rat pituitary cells in vitro.

Animals↗
PubMed1985

Down-regulation of nuclear T3 receptors by thyroid hormones in the rat anterior pituitary.

The concentration of T3 nuclear receptors may be modulated by thyroid hormones. We have examined the effect of thyroid hormone excess and deficiency in the rat on the in vitro binding of T3 to anterior pituitary nuclear receptors. Nuclei prepared from hypothyroid, hyperthyroid and control rats by homogenisation and centrifugation were incubated with [125I]T3 and unlabelled T3 at increasing concentrations. After incubation for 60 min at 37 degrees C, nuclei were separated by filtration and bound radioactivity counted. Scatchard analysis of binding data revealed no significant difference in binding affinity or capacity for T3 between nuclei from hypothyroid and control rats. Results from hyperthyroid and control animals revealed no change in affinity, but a marked reduction in binding capacity was observed, suggesting depletion of receptor number. This finding of down-regulation may represent a protective mechanism against the effects of high circulating thyroid hormone levels on the tissues.

Animals↗
PubMed1985

Amiodarone and thyroid hormone action.

In clinically euthyroid subjects on long-term amiodarone therapy free thyroxine (T4) concentrations were increased and free triiodothyronine (T3) levels reduced. There was also a marked increase in reverse T3 in the treated group. These changes are consistent with inhibition of peripheral deiodination of T4 and reverse T3. Despite the rise in T4 serum thyrotrophin (TSH) levels were increased, suggesting an effect of amiodarone on the anterior pituitary. To investigate the interaction of amiodarone with the cellular actions of thyroid hormones we examined the influence of the drug in vitro on the binding of T3 to isolated nuclei prepared from rat anterior pituitary tissue. Amiodarone inhibited the nuclear binding of T3 in a dose dependent fashion. Addition of amiodarone in vitro also stimulated TSH release from cultured rat anterior pituitary cells, consistent with a T3 antagonistic effect. These studies provide evidence for a direct influence of amiodarone on the thyrotroph, mediated via nuclear T3 receptor binding.

Adult↗
PubMed1985

Abnormalities of thyroid function tests in hospital inpatients.

Results of thyroid function tests were analysed in 199 clinically euthyroid inpatients with normal serum thyroid stimulating hormone values. Serum total triiodothyronine was less than 1.25 nmol/l in 61.8% of samples, free triiodothyronine less than 3.9 pmol/l in 57.8%, total thyroxine less than 63 nmol/l in 21.1% and free thyroxine less than 9.5 pmol/l in 17.6%. In contrast, thyroxine binding globulin ratio was below normal (less than 5) in only 5 samples. A significant positive correlation (P less than 0.001) of serum free thyroxine with total thyroxine, thyroxine/thyroxine binding globulin ratio and free triiodothyronine was present as well as a significant negative correlation (P less than 0.001) with serum thyroid stimulating hormone. There was no correlation of free thyroxine measurements with serum albumin or non-esterified fatty acid concentrations. Although serum free thyroxine is low in a number of patients with non-thyroidal illnesses, this does not appear to be due to a rise in non-esterified fatty acids or a fall in albumin as has been proposed. Serum thyroid stimulating hormone measurements are essential to confirm the diagnosis of hypothyroidism in such subjects.

Humans↗
PubMed1985

The influence of age and sex on tests of thyroid function.

Serum concentrations of total thyroid hormones, thyroid hormone binding proteins, free thyroid hormones and thyroid stimulating hormone (TSH) were measured in a large group of healthy normal subjects (n = 218) to determine the influence of age and sex on tests of thyroid function. No effect of age on measurements of total thyroxine (T4), total triiodothyronine (T3) or reverse T3 was evident. Concentrations of free T4, free T3 and TSH were similarly unaffected, suggesting that increasing age alone does not significantly influence thyroid hormone measurements. There was, however, a rise in thyroxine-binding globulin (TBG) concentration with increasing age and a fall in thyroxine-binding prealbumin (TBPA). An influence of sex on tests of thyroid function was clearly evident. Levels of total T4, total T3, free T4 and the T4: TBG ratio were lower in women between 16 and 49 years than in men, despite an increase in TBG. Lower levels of free T3 were confined to the group of women aged 16-29 years. TSH values were unaffected by sex. These results suggest that sex hormone status in the pre-menopausal female subject may result in an increase in TBG concentration, and in addition may have an opposing effect on circulating thyroid hormone concentrations.

Adolescent↗
PubMed1985

The effect of propranolol on circulating thyroid hormone measurements in thyrotoxic and euthyroid subjects.

Circulating concentrations of total and free thyroid hormones and thyroid hormone binding proteins were measured in thyrotoxic and euthyroid subjects treated with propranolol. In the thyrotoxic group, total triiodothyronine (T3) concentration fell after propranolol therapy, suggesting an effect of the drug on the peripheral conversion of thyroxine (T4) to T3. In euthyroid subjects, a rise in circulating concentrations of free T4 and reverse T3 (rT3) was observed, while only a small decrease in free T3 was evident. Thyroxine binding globulin (TBG) concentration fell during propranolol treatment while thyroxine binding prealbumin (TBPA) concentration rose. The changes observed in the euthyroid state are consistent with inhibition of peripheral deiodination of T4 and rT3; an additional effect of propranolol on binding protein metabolism was evident.

Humans↗
PubMed1985

Immunoreactive neurotensin in spontaneous syndromes of obesity and diabetes in mice.

Immunoreactive neurotensin was measured in plasma and acid-ethanol extracts of brain, intestine and pancreas of obese hyperglycaemic (ob/ob) mice of the Aston strain, C57BL/KsJ diabetes-obese (db/db) mice, and their respective lean controls. In lean mice, the intestine was the major source of neurotensin (156-194 mg/g wet weight and 169-361 ng/intestine), with smaller amounts in the brain (33-43 ng/g and 13-17 ng/brain), pancreas (0.8-1.1 ng/g and 0.28-0.32 ng/pancreas) and plasma (50-100 pg/ml). Compared with lean controls, ob/ob and db/db mice exhibited 13 and 23% decreases in brain weight, and 37 and 82% increases in intestinal weight. Concentrations of neurotensin in plasma and brain were similar in lean and obese-diabetic mutant mice, but the total content of brain neurotensin was 25% lower in ob/ob mice. Neurotensin was unchanged in the pancreas of db/db mice. However, raised concentrations and total contents of neurotensin were observed in the pancreas of ob/ob mice (72 and 57%, respectively) and the intestine of both ob/ob (56 and 118%) and db/db (35 and 144%) mice. These observations raise the possibility that increased neurotensin concentrations might exert local effects in the intestine and pancreas which contribute to the pathogenesis of obesity-diabetes syndromes in mice.

Animals↗
PubMed1985

Phenytoin and thyroid hormone action.

Circulating free thyroid hormone concentrations are reduced in subjects taking long-term phenytoin, a finding at variance with their euthyroid clinical state and normal serum TSH concentration. It is suggested, therefore, that phenytoin may modify the cellular effects of thyroid hormones. In order to examine the influence of phenytoin on thyroid hormone action in the pituitary gland we studied its effect on the binding of tri-iodothyronine (T3) to isolated nuclei prepared from rat anterior pituitary tissue. Phenytoin inhibited the nuclear binding of T3 in a dose-dependent fashion. Phenytoin also partially inhibited thyrotrophin-releasing hormone-stimulated TSH release from cultured rat anterior pituitary cells. These studies provide evidence for a direct effect of phenytoin on the thyrotroph mediated via nuclear T3 receptor binding.

Animals↗
PubMed1984

Measurement of free thyroid hormones in patients on long-term phenytoin therapy.

Total thyroxine and triiodothyronine levels are often reduced in long-term phenytoin therapy, a finding at variance with the euthyroid clinical state of these subjects. We have measured free (biologically active) thyroid hormone levels in 31 patients on phenytoin therapy and have found a significant reduction in measurements of both free thyroxine and free triiodothyronine. In contrast, addition of phenytoin to normal serum in vitro resulted in an increase in free thyroxine and free triiodothyronine concentrations. The findings in vitro are consistent with displacement of thyroid hormones from thyroxine binding globulin while the reduced free hormone levels in vivo confirm that the effects of phenytoin are not confined to binding inhibition and suggest that phenytoin has induced a change in the cellular metabolism of thyroid hormones.

Humans↗
PubMed1984

Thyroid status in patients after acute myocardial infarction.

Subjects followed serially after acute myocardial infarction demonstrated a rapid and sustained fall in serum total tri-iodothyronine (T3) concentration and a rise in reverse tri-iodothyronine (rT3) concentration. There was a transient fall in total thyroxine (T4) concentration. Thyroxine binding globulin (TBG) levels were unchanged after acute myocardial infarction but prolonged falls were observed in thyroxine binding prealbumin (TBPA) and albumin concentrations. In contrast to the fall in total T4, both measured and calculated free T4 concentrations were unchanged but measured and calculated free T3 concentrations fell as did total T3. Despite the observed fall in T3, basal thyrotrophin (TSH) concentrations did not rise. The reduction in circulating T3 levels after acute myocardial infarction suggests that a hypothyroid state exists. Until tissue thyroid status can be assessed directly, however, this conclusion must remain in doubt.

Acute Disease↗
PubMed1984

Measurement of free thyroxine and free triiodothyronine in thyrotoxicosis and hypothyroidism.

Free T4 and free T3 concentrations in thyrotoxic and hypothyroid patients at presentation differentiated those with thyroid disease from euthyroid control subjects and were closely correlated with T4/TBG ratio. A further group of thyrotoxic patients was followed serially during treatment with carbimazole. Measurements of free T4 and free T3 against closely reflected thyroid status with changes in the hormone measurements being paralleled by changes in T4/TBG ratio.

Carbimazole↗