Biomedical subjects

G J Huxham

Publications and source records attributed to G J Huxham.

At least 19 recordsLinked to original sources

PubMed1996

Sodium channel heterogeneity in the apical membrane of porcine thyroid epithelial cells.

Porcine thyroid epithelial cells cultured as a monolayer with their apical membranes facing the medium are known to absorb Na+ and secrete Cl-. Two types of Na+ channels were found in cell-attached patches of apical membrane. A low conductance Na+ channel (conductance g = 4 picosiemens (pS)) remained open for seconds and showed a high selectivity for Na+ compared with K+. In contrast, a high conductance Na+ channel (g = 10 pS) flickered rapidly and had reduced selectivity. Both types of Na+ channel became more prevalent when the cells were exposed to Na(+)-free medium, though only the high conductance channel increased in prevalence on addition of prostaglandin E2, a stimulator of adenylate cyclase which increases Na+ absorption in this cultured epithelium. Two minority types of channel were also found: a non-selective small conductance cation channel which had been reported previously, and an intermediate conductance channel found only in Na(+)-free medium. It was concluded that passage of Na+ across the apical membrane of thyroid cells is mediated by typical epithelial Na+ channels, but that the two types of channel are differentially regulated.

Animals↗

PubMed1995

Chloride channels in the apical membrane of thyroid epithelial cells are regulated by cyclic AMP.

Porcine thyroid epithelial cells cultured as a monolayer with their apical membranes facing the medium are known to absorb Na+ and to secrete the anions Cl- and HCO3-. Chloride channels were found in the apical membrane, and displayed a reversal potential close to the resting membrane potential, linear current-voltage relationships, a conductance at physiological temperature of 6.5 pS, and a small but significant permeability to HCO3-. Stimulation of ion transport with prostaglandin E2 or 8-(4-chlorophenylthio) adenosine 3':5'-cyclic monophosphate promoted activation of Cl- channels in cell-attached patches, and excised patches were reactivated by exposure of their cytoplasmic surface to protein kinase A and ATP. Physiological temperatures were necessary for activation of Cl- channels in cell-attached patches. The channels exhibited sub-states with a conductance exactly half that of the full unit conductance, suggesting a dual-barrelled channel structure. It is concluded that the apical membrane of thyroid epithelial cells contains cyclic AMP-activated Cl- channels controlling anion transport.

Animals↗

PubMed1993

Activation of sodium transport mediates regulation of thyroid follicle volume in response to hypotonic media.

The thyroid epithelium possesses a bidirectional fluid transport system capable of absorbing Na+ and secreting Cl-. In the present studies we have examined its possible role in the regulation of thyroid follicular size. When exposed to hypotonic media (200 mosM) cultured porcine thyroid follicles first swelled and then displayed a regulatory volume decrease (RVD) over 60 min. This was associated with a transient depolarization of the transepithelial potential difference (TEP) and subsequent hyperpolarization with a time course similar to RVD. Phenamil (1 microM), an antagonist of epithelial Na+ channels, did not affect initial swelling but prevented the subsequent follicular RVD. Phenamil abolished hyperpolarization of TEP, but the loop diuretic bumetanide, which inhibits Cl- secretion in thyroid cells, did not prevent it. Exposure to hypotonic medium produced a slow hyperpolarization of the intracellular potential (basolateral membrane potential) consistent with an increase in basolateral membrane K+ conductance. Ba2+ and quinidine, which are known to inhibit K+ channels in epithelia, prevented RVD. Addition of the K+ ionophore valinomycin (1 microM) caused follicle shrinkage that was prevented by phenamil (1 microM). We conclude that cultured follicles respond to hypotonically induced stretch by activating outwardly directed Na+ transport through a mechanism which involves change in the basolateral K+ conductance. This response would be characteristic of a system that controlled follicle volume. However, it is not clear from these studies whether the cells responded primarily to the increase in follicle volume or to the change in cell volume that is expected to accompany hypotonic challenge.

Amiloride↗

PubMed1992

Chloride conductance of apical membrane in cultured porcine thyroid cells activated by cyclic AMP.

The thyroid epithelium transports fluid bidirectionally using active transport of Na+ ions from apical to basal poles and active transport of Cl- in the reverse direction. In these studies we sought evidence for cyclic AMP activated Cl- channels on the apical membranes of thyroid cells in monolayer culture. A Cl(-)-dependent basal-positive short-circuit current (ISC) was demonstrated in bicameral chambers after blocking Na+ transport with phenamil, and responded to prostaglandin (PG) E2 with a spike of 5-10 min duration followed by a plateau. The onset of the spike coincided with an increase in the conductance of the epithelium. Application of an external Cl- concentration gradient, by replacing the medium in the apical compartment with Cl(-)-free medium, resulted in an increase in ISC after, but not before, addition of PGE2. Forskolin and thyroid-stimulating hormone (TSH), but not A23187, also stimulated Cl- transport. In conjunction with previous observations that Cl- transport was mediated by a bumetanide-sensitive NaKCl2 symporter on the basal membrane, these observations indicated the presence of a cyclic AMP activated Cl- conductance in the apical membrane of thyroid cells.

Animals↗

PubMed1992

Bidirectional ion transport in thyroid: secretion of anions by monolayer cultures that absorb sodium.

Cultured porcine thyroid cell monolayers transport Na+ in an apical-to-basal direction, resulting in the development of a basal-positive transepithelial potential difference (TEP) and the formation of domes (fluid-filled elevations of the cell layer above the culture dish substrate). Stimulation by prostaglandin E2 (PGE2) increases the magnitude of the TEP, the short-circuit current (Isc) measured in Transwell Ussing chambers, and the height of domes in cultures grown on impermeable substrates. A phenamil-resistant, PGE2-stimulated component of the Isc in Transwells and of the TEP in monolayers in conventional culture dishes was inhibitable by bumetanide, a diuretic drug that blocks NaKCl2 symporters, mediating active transport of Cl-. The rate of decrease in height of domes in cultures after addition of phenamil, presumably indicative of transport of fluid in a basal-to-apical direction, was also reduced by bumetanide. Studies with Transwells in Cl(-)-free, HCO(3-)-free or Cl(-)- and HCO(3-)-free media indicated that thyroid cells transported HCO3- as well as Cl- in a basal-to-apical direction. It was concluded that the thyroid epithelium is both sodium absorbing and anion secreting.

Absorption↗

PubMed1991

Regulation of thyroid follicular volume by bidirectional transepithelial ion transport.

Previous studies have shown that thyroid cells in monolayer culture exhibit bidirectional ion transport comprising apical-to-basal amiloride-sensitive Na+ transport and oppositely directed bumetanide-sensitive Cl- transport. We have now investigated the role of ion transport in the regulation of thyroid follicular size using follicular primary porcine thyroid cell cultures. Bumetanide (10 microM) added at the beginning of culture inhibited the formation of follicular lumina and caused a fall in follicle height when added to 3-day-old cultures. In contrast, phenamil (1 microM; an amiloride analog) increased follicle size both in freshly isolated and 3-day-old cultures. The effect of bumetanide was prevented by the prior addition of phenamil. Micropuncture studies showed that follicles had a lumen-negative, basal-positive transepithelial potential difference which was progressively reduced in magnitude by the serial addition of bumetanide (10 microM) and phenamil (1 microM). We conclude that thyroid follicles possess a bidirectional ion transport system which transports Na+ in an apical-to-basal direction and Cl- in the opposite direction. The balance between these two processes determines net solute flux and hence follicular size. A physiological role of ion transport in the thyroid may be to regulate follicular volume suggesting that abnormalities of ion transport may be responsible for disorders of follicular size.

Animals↗

PubMed1991

Epidermal growth factor (EGF) inhibits the secretomotor response of the thyroid: effects of EGF on radioiodine turnover and fluid transport in cultured porcine thyroid cells.

Thyrotrophin (4-256 microU/ml) promoted an increase in the rate of release of radioiodine from the organic iodine pool of cultured porcine thyroid cells in follicular formations. This action of TSH was antagonized by low concentrations of epidermal growth factor (EGF; 0.1-5 nmol/l). The maximal effect of EGF was reached by 0.5 nmol/l. EGF (0.5-5 nmol/l) also inhibited the stimulatory effect of 8-chloro cyclic AMP (0.06-1.0 nmol/l) on radioiodine turnover. Exposure of thyroid cultures to media with a calcium concentration of 17.7 mumol/l (1% of normal) resulted in a very marked increase in the rate of release of radioiodine. The effect of TSH in low-calcium media was to inhibit the increased release of radioiodine, and EGF (0.5 nmol/l) antagonized this inhibitory effect of TSH. The calcium ionophore, A23187, stimulated radioiodine release in a dose-dependent fashion, and EGF (1.7 nmol/l) inhibited this response. Fluid transport in thyroid monolayers was stimulated by prostaglandin E2 (PGE2; 1 mumol/l). EGF (5 nmol/l) also stimulated fluid transport, but antagonized the effect of PGE2 added subsequently. It was concluded that EGF exerted acute antagonistic effects on thyroid cell responses in vitro to cyclic AMP and agents promoting accumulation of cyclic AMP in time-frames too short for these inhibitory effects to be attributable to the dedifferentiative effect of the growth factor.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1990

Control of ion transport in the thyroid: prostaglandin E2 activates cation transport on the basal membrane of cultured porcine thyroid cell monolayers.

Confluent monolayers of cultured porcine thyroid cells transport fluid from the apical to the basal surface, forming circumscribed zones of detachment (domes) from the culture dish substrate. Stimulation of fluid transport by prostaglandin E2 (PGE2; 1 mumol/l) was associated with an increase in transepithelial potential (TEP). Intracellular potentials (equal to the potential difference across the apical membrane of the cell, Eapical) and the TEP were measured in individual domes so that the potential difference across the basal membrane of the cell (Ebasal) could be calculated from the relationship TEP = Eapical-Ebasal. The PGE2-induced increase in TEP was associated with hyperpolarization of the basal membrane, accompanied by a slight depolarization of the apical membrane. Lines of best fit by least-squares regression showed Eapical = -20.3 mV +0.219 TEP (correlation coefficient r = 0.627; P less than 0.001) and Ebasal = -20.3 mV -0.781 TEP (r = 0.944; P less than 0.001). Phenamil (1 mumol/l), a Na+ channel selective amiloride analogue, reduced the TEP from 13.25 +/- 0.58 (S.E.M.; n = 56) to 2.39 +/- 0.16 mV (n = 51; P less than 0.001) and hyperpolarized the apical membrane potential from -20.7 +/- 0.68 (n = 60) to -32.2 +/- 0.83 mV (n = 105; P less than 0.001). The response of the TEP to phenamil was immediate, and was promptly reversed on washing; in contrast, addition of 5-(N-ethyl-N-isopropyl)amiloride (20 mumol/l; selective for Na+/H+ antiporters) resulted in a slow depolarization over 30 min with a slow recovery after washout.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

What 'makes' a good doctor?

The relation between gender, personality, school scores, grades at medical school and eventual achievement as a medical practitioner 8 years after qualifying has been explored by path analysis in a cohort of medical students. Factor analysis of data derived from a questionnaire identified a significant factor accounting for 75% of the common variance of the professional achievement scores. Standardized path coefficients were computed to indicate the relative importance of the causal factors to postgraduate achievement. Gender played an important role at many levels. For example it was apparent that many of our women graduates were seriously disadvantaged in their professional careers. Of the school subjects, chemistry was a surprising long-term predictor of postgraduate achievement. Academic achievement during medical school training, particularly in the final year, was a significant predictor, while personality attributes made their contribution to one or other aspect of achievement at earlier stages in training but made little additional direct contribution to postgraduate performance.

Achievement↗

PubMed1989

Inhibitory effects of amiloride and its analogues on prostaglandin E2-stimulated fluid transport by cultured porcine thyroid cells: evidence for apical membrane Na+ channels.

Confluent monolayers of cultured porcine thyroid cells transport fluid from the apical to the basal surface, forming circumscribed zones of detachment (domes) from the culture dish substrate. Fluid transport, as measured by increase in dome height, was stimulated by prostaglandin E2 (PGE2; 1 mumol/l) and inhibited by amiloride (0.1-100 mumol/l). Values of the inhibition constant (Ki) with 95% confidence limits for each of a series of amiloride analogues were: 3',4'-dichlorobenzamil (DCB), 0.090 (0.045-0.18) mumol/l; 2',4'-dimethylbenzamil (DMB), 0.14 (0.074-0.27) mumol/l; amiloride, 0.72 (0.33-1.8) mumol/l; 5-(N,N-hexamethylene)amiloride (HMA), 17 (5.9-43) mumol/l; 5-(N-ethyl-N-isopropyl)amiloride (EIPA), 33 (15-71) mumol/l; and 2-guanidinobenzimidazole, 243 (110-570) mumol/l. Triaminopyrimidine was ineffective at concentrations up to 1 mmol/l. Since DCB and DMB are known to have a higher affinity for Na+/H+ channels, while HMA and EIPA show higher affinity for Na+/H+ antiports, it was concluded that PGE2-stimulated fluid transport involved an apical membrane Na+ channel.

Amiloride↗

PubMed1988

Role of calcium in the secretomotor response of the thyroid: effects of calcium ionophore A23187 on radioiodine turnover, membrane potential and fluid transport in cultured porcine thyroid cells.

The calcium ionophore A23187 (0.1-1 mumol/l) inhibited membrane electrical polarization, uptake of 125I, fluid transport and TSH-stimulated release of radioiodine from the organic pool in follicular cultures of porcine thyroid cells. At higher concentrations (1-30 mumol/l), A23187 promoted release of radioiodine from the organic pool. Stimulation of release of radioiodine from the organic pool by veratridine (a sodium channel agonist, 0.4-1 mmol/l) and A23187 was dependent on the calcium concentration of the medium, while TSH action was independent. Incubation in medium of very low calcium concentration (0.0177 mmol/l) resulted in enhanced release from the organic pool, which was inhibited by TSH (256 microU/ml), A23187 (25 mumol/l) or veratridine (0.5 mmol/l). These data therefore do not support the hypothesis that calcium acts as a mediator of the secretomotor action of TSH, but suggest the possibility of a TSH-induced increase in intracellular calcium as a regulatory negative-feedback mechanism.

Animals↗

PubMed1988

Electrophysiological correlates of fluid transport in cultured porcine thyroid cells.

Confluent monolayers of cultured porcine thyroid cells transport fluid from the apical to the basal surface, forming circumscribed zones of detachment from the culture dish substrate (domes). The transepithelial potential (TEP), positive on the basal side, was 12.9 +/- 0.4 (S.E.M.) mV (n = 93) under control conditions, increasing to 38.9 +/- 0.3 mV (n = 281) when fluid transport was stimulated by prostaglandin E2 (PGE2; 1 mumol/l). Forskolin (1 mumol/l) and 8-(4-chlorophenylthio) adenosine 3',5'-cyclic monophosphate (0.5 mmol/l) were also effective in increasing TEP. Addition of amiloride in concentrations sufficient to block fluid transport (100 mumol/l) reduced the TEP to 5.8 +/- 0.3 mV (n = 76). Substitution of N-methyl-D-glucamine for sodium in the medium reduced the PGE2-stimulated TEP to 13.4 +/- 0.8 mV (n = 32). Substitution of gluconate for chloride increased the TEP to 40.3 +/- 0.4 mV (n = 160). Removal of bicarbonate or potassium from the medium, or addition of ouabain (200 mumol/l) were also effective in reducing the TEP. In media of low bicarbonate concentration (1 mmol NaHCO3/l), acetazolamide (1 mmol/l) reduced the TEP. Fluid transport by the monolayer as measured by the change in height of domes was increased by PGE2 (1 mumol/l). PGE2-stimulated fluid transport was inhibited by sodium or chloride ion substitution, bicarbonate removal or the addition of ouabain (200 mumol/l) or amiloride (100 mumol/l). It was concluded that fluid transport in thyroid monolayers is mediated by rheogenic sodium transport with chloride transport being passive, electrogenically coupled to sodium transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1987

Ionic mechanisms regulating thyroidal secretion: effects of ouabain and medium sodium concentration on radioiodine release from cultured porcine thyroid cells.

Thyrotrophin stimulated release of radioiodine from the organic iodine pool of cultured porcine thyroid cells. The response was well developed within 2-4 h of incubation. Inhibition of the Na+/K+ pump with ouabain or incubation in sodium-free medium inhibited the response to TSH. The magnesium content of cultures was reduced by ouabain, and increasing the magnesium concentration of the medium to 10 mmol/l reversed the inhibition of the TSH response by ouabain. After prolonged incubation (4-6 h), ouabain in magnesium-enriched medium stimulated release of radioiodine. Its effects were not additive with those of TSH. Incubation for 4-6 h in media of reduced sodium concentration (34 mmol/l) also stimulated release. Sodium-free medium alone did not alter basal release rates, but magnesium enrichment of sodium-free medium promoted release after 4 h of incubation. It was concluded that the previously reported inhibition of the TSH response in thyroid tissue by ouabain or sodium-free medium was due to secondary derangements of cellular function rather than to a specific blockade of the secretomotor signal. The data are consistent with the hypothesis that a reduction in the sodium ion electrochemical gradient across the cell membrane mediates the secretomotor effect of TSH.

Animals↗

PubMed1987

Cyclic AMP-stimulated fluid transport in the thyroid: influence of thyroid stimulators, amiloride and acetazolamide on the dynamics of domes in monolayer cultures of porcine thyroid cells.

Confluent monolayer cultures of porcine thyroid cells form dome-shaped elevations by local separation from the plastic culture dish. Formation of domes by epithelial cells in culture is generally considered to be evidence of fluid transport. A computer-controlled data acquisition system was developed to quantitate fluid transport in thyroid cultures by serial measurements of dome elevation. Thyrotrophin (10 mU/ml), prostaglandin E2 (PGE2; 0.01-1 mumol/l), forskolin (1 mumol/l), 8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate (0.5 mmol/l) and 3-isobutyl-1-methyl-xanthine (0.5 mmol/l) promoted increases in dome height over 5-120 min. Dome growth in the presence of PGE2 (1 mumol/l) was inhibited by amiloride (0.1-100 mumol/l), ouabain (200 mumol/l), or by removal of bicarbonate and glucose from the medium. In media of reduced bicarbonate concentration (1 mmol/l compared with the control concentration of 10 mmol/l), dome growth was inhibited by acetazolamide (0.01-1 mmol/l). These data are consistent with cyclic AMP-stimulated transport of fluid from apical to basal pole of the cells, dependent on sodium entry through the apical pole by an Na+/H+ exchanger.

Acetazolamide↗

PubMed1986

Sodium dependence of the thyrotrophin-induced depolarization in cultured porcine thyroid cells.

Cultured porcine thyroid cells exhibit a resting membrane potential of about -73 mV and depolarize to about -54 mV on exposure to TSH. The depolarizing response to TSH was preserved in a medium consisting only of inorganic salts and buffers, but was abolished in sodium-free medium, demonstrating dependence on an inward sodium current. Increasing the potassium concentration of the medium resulted in a reduction in the resting membrane potential of 60 mV per tenfold change in potassium concentration, and a diminished TSH response. A hyperpolarizing TSH response was observed in a sodium- and bicarbonate-free medium, indicating that a hyperpolarizing ion current (probably carried by potassium) was also enhanced in the presence of TSH. Tetrodotoxin blocked the TSH response. We conclude that the response of the thyroid cell membrane to TSH involves increases in permeability to sodium and potassium, and that the thyroid membrane ion channels bear some similarity to the voltage-dependent sodium channels of excitable tissues, despite the absence of action potentials in the thyroid.

Animals↗

PubMed1986

Effect of epidermal growth factor on the membrane potential of cultured porcine thyroid cells.

Cultured porcine thyroid cells maintained in media containing TSH exhibited a membrane potential of -50 mV, and hyperpolarized by about 10 mV within 1 h of the addition of epidermal growth factor (EGF; 10 ng/ml). Follicle cells had depolarized to -45 mV after 4 h of exposure to EGF. Cells maintained in dibutyryl cyclic AMP (dbcAMP) did not alter their membrane potential when exposed to EGF for up to 4 h. Cultures washed to remove the TSH or dbcAMP hyperpolarized to -75 mV within 30 min, and a reversible depolarization to -60 mV was observed on addition of EGF. It was concluded that EGF acts as a physiological antagonist of TSH and also exerts a separate depolarizing influence on cultured thyroid cells.

Animals↗

PubMed1986

Role of sodium influx in thyrotrophin action: effects of the sodium channel agonist veratridine and thyrotrophin on radioiodine turnover and membrane potential in cultured porcine thyroid cells.

Veratridine, a sodium channel agonist, depolarized cultured thyroid cells and increased the secretion of radioiodine from the organically bound pool. These effects were similar to those of TSH. Depolarization of the cells by increasing the potassium concentration of the medium failed to promote secretion, indicating that the sodium influx, rather than the depolarization itself, mediated the response. Veratridine, like TSH, also acutely reduced the cells' iodide uptake and inhibited the iodide transport pump. Unlike TSH, however, veratridine reduced, rather than increased, the fractional exit rate of iodide anion from the unbound pool. The data are consistent with the hypothesis that a sodium influx mediates some but not all of the actions of TSH on the thyroid gland, including the stimulation of secretion of thyroid hormones.

Animals↗

PubMed1985

Does medical training affect personality?

We have compared results obtained for psychometric tests (Cattell's 16 PF and Eysenck's personality inventory) on a cohort of medical students who sat the tests in their second medical school year and repeated them in their sixth and final year. Most of the changes were in the direction expected of students maturing through the 19-24 year age period, but there were some significant differences from those obtained by other workers using the same tests on general student populations. Thus, in our study male students became 'brighter', more 'emotionally stable', less 'timid', more 'tough-minded', and more 'self-controlled', all of these to a significantly greater extent than expected with the general student population. Similar changes were observed from females, although except for an increase in emotional stability, these were not significantly different from the values expected in the general population.

Adult↗