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Biomedical subjects

A P Hillier

Publications and source records attributed to A P Hillier.

At least 19 recordsLinked to original sources
PubMed1975

Thyroxine dissociation in human plasma: measurement of its rate by a continuous-flow dialysis method.

A method is described for measuring the rate at which thyroxine dissociates from binding proteins in human plasma. The method depends upon the fact that free thyroxine is able to diffuse through Visking dialysis membrane whereas bound thyroxine cannot. At 20 degrees C half of the plasma thyroxine dissociates within about 200 seconds and 75% within 10 min. At 37 degrees C half of the hormone dissociates in less than 20 seconds and the reaction is virtually complete in 3-4 min. These values agree well with those previously obtained by a resin-uptake method. In the Discussion it is calculated that the pool of free thyroxine in the plasma is replinished about 5 times 10-6 times each day and that an individual thyroxine molecule spends only about 15 mseconds in free solution during each transit.

Dialysis↗
PubMed1975

The exchange of thyroxine between plasma and liver: the role of free thyroxine.

An investigation has been made into the exchange of thyroxine between human plasma and the perfused rat liver. A stop-flow method was used to hold plasma within the liver sinusoids for periods up to 20 min. At 20 degrees C it was found that thyroxine present in the sinusoids transferred to the liver at a rate of 5-6%/min; and that thyroxine transferred from the liver to the plasma at a rate of 1%/min. In a second series of experiments, trace amounts of free thyroxine were injected into the stream of plasma just as it entered the tissue; the liver captured 18.5% of this injected hormone. In the discussion it is noted that 29% of the plasma thyroxine dissociates into free solution each minute. If the liver is able to capture 18.5% of this hormone (as the above experiment suggests) then the plasma-to-liver transfer rate would be 5-6%/min, which is the rate actually observed. These observations strongly suggest that the transfer of thyroxine from plasma to liver involves its initial release into the free state followed by trapping by the tissue binding sites.

Animals↗
PubMed1975

The rate of triiodothyronine dissociation from binding sites in human plasma.

A method is described for estimating the rate of triiodothyronine (T3) dissociation from binding sites in human plasma. At trace concentration about 25% of the hormone dissociates rapidly, the reaction being more than half complete within 1 second. The remaining 75% dissociates exponentially with a t1/2 of 4.2 seconds at 37-38 degrees C. When the plasma is loaded with thyroxine to displace the T3 from thyroxine-binding globulin, the slow exponential reaction is lost and all of the hormone dissociates rapidly. These results indicate that the free and bound T3 in plasma are in a state of rapid exchange.

Binding Sites↗
PubMed1972

Active secretion of thyroxine into bile: the role of tissue thyroxine-binding sites.

1. An investigation has been made into factors regulating the secretion of thyroid hormones into bile. The preparation employed was the isolated rat liver perfused with a modified Tyrode solution.2. Tracer amounts of radioactive thyroxine were injected into the Tyrode solution just before its entry into the liver. This allowed the thyroxine-binding sites in the tissue to capture most of the injected hormone. It was found that this tissue-bound hormone was then gradually secreted into the bile and that the process could be resolved into two components. There was a steady, continuous secretion of hormone, which maintained a fairly constant bile:liver ratio in thyroxine concentration of about 2.3. There was also, superimposed upon this, an extra transient secretion of thyroxine occurring just after the injection. This transient effect was abolished by injecting the hormone at 22 degrees C and it was probably due to the sudden flood of free thyroxine into the tissue during the 15 sec injection period.3. Very similar results were obtained with tri-iodothyronine except that the secretion process maintained a higher bile:liver ratio of about 3.7.4. The secretion mechanism was extremely sensitive to changes in temperature and it was able to generate very high concentrations of free thyroxine in the bile. It was concluded that the over-all process was probably an active one.5. The presence of 30% bovine serum in the perfusion fluid did not reduce the bile:liver thyroxine ratio even though it caused a massive fall in the amount of thyroxine in the tissue.6. These results show that the rate of thyroxine secretion into bile is determined by the total amount of thyroxine trapped in the tissue. It is not directly related to the concentration of free thyroxine in the perfusing medium.7. It is suggested that the secretion mechanism works from a small intracellular pool of free thyroxine which is in rapid exchange with the large pool of tissue-bound hormone, and that thyroxine exchange between this small intracellular pool and the free thyroxine pool in the plasma is relatively slow.

Animals↗
PubMed1972

Autoregulation of thyroxine secretion into bile.

1. An investigation has been made into the effect on thyroxine secretion into bile of suddenly raising the thyroxine concentration in the liver.2. Isolated rat livers, perfused with Tyrode solution, were injected with trace amounts of radioactive thyroxine. This injected hormone was captured by the binding sites in the tissue and then gradually secreted into the bile. A second injection of 100 mug stable (non-radioactive) thyroxine caused an enormous increase in the secretion rate. At its peak, the concentration of thyroxine in the bile was about 24 x its concentration in the liver (this was 10 x higher than the concentration ratio at tracer thyroxine levels). The effect extended well into the physiological range and injections of only 0.1 mug stable thyroxine caused a transient doubling of the secretion rate. Tri-iodothyronine also accelerated thyroxine secretion but it was 30 x less potent than thyroxine.3. In contrast, the secretion of tri-iodothyronine into bile was only very slightly affected by injections of stable thyroxine or tri-iodothyronine, the maximum response being a 20-30% increase in the secretion rate.4. This autoregulating mechanism would help to stabilize the concentration of thyroxine in the tissues but not the concentration of tri-iodothyronine.

Animals↗
PubMed1972

Deiodination of thyroid hormones by the perfused rat liver.

1. An investigation has been made into the deiodination of thyroid hormones by the perfused rat liver. The hormones were labelled with (125)I in the phenolic ring and the rate of deiodination was estimated by measuring the release of radio-iodide into the perfusate.2. At tracer concentrations, 0.98% of the liver thyroxine is deiodinated/5 min. The deiodination of tri-iodothyronine is considerably faster, 3.3%/5 min.3. Deiodination is very sensitive to changes in temperature.4. The reaction shows saturation kinetics typical of many enzymes, the reciprocal of the rate of deiodination being proportional to the reciprocal of the hormone concentration in the tissue. The maximum rate of deiodination of each hormone is about 1.5 mug/min for a whole liver preparation weighing 16 g.5. Tri-iodothyronine inhibits thyroxine deiodination and vice versa, suggesting that a single enzyme is responsible for both reactions.6. Propyl thiouracil (PTU) at high concentrations inhibits the deiodination of both hormones.7. An abnormally high rate of deiodination is associated with the actual injection of hormone into the preparation. This suggests that only the free (unbound) hormone in the tissue is directly available to the deiodinating enzyme.8. About half of the whole body deiodination of thyroxine is relatively insensitive to PTU. It is suggested that most of this type of deiodination is performed in the liver and that the process is one of inactivation.

Animals↗
PubMed1972

Transport of thyroxine glucuronide into bile.

1. The secretion of thyroxine glucuronide into bile has been investigated in the isolated perfused rat liver and also in the intact animal. Radioactive material for the study was prepared from the bile of rats treated with [(125)I]thyroxine.2. The isolated liver actively transports thyroxine glucuronide from the perfusion fluid (Tyrode solution) into the bile. At equilibrium, its concentration in the bile is about 1800 x its concentration in the perfusion fluid.3. Experiments with pulse infusions of glucuronide indicate that the substance enters the bile almost as soon as it is captured by the tissue.4. Secretion of thyroxine glucuronide is strongly inhibited by bromosulphophthalein, indicating a common carrier mechanism. It is suggested that secretion probably involves two stages - transfer from the perfusion fluid into the cell and then from the cell into the bile. The second stage is certainly active.5. When thyroxine glucuronide is injected intravenously into anaesthetized rats, some 77% is recovered from the bile, half of this being excreted within the first 14 min.

Animals↗
PubMed1971

Human thyroxine-binding globulin and thyroxine-binding pre-albumin: dissociation rates.

1. A simple method is described for estimating the rate of thyroxine dissociation from binding sites in plasma.2. At tracer concentrations in human plasma thyroxine dissociates at two distinct exponential rates. These are due to the two thyroxine-binding proteins present in human plasma.3. The t((1/2)) for thyroxine dissociation from thyroxine-binding globulin is 8.1 +/- 0.7 (S.E.) min at room temperature and 38.6 +/- 2.1 sec at 37 degrees C.4. The t((1/2)) for thyroxine dissociation from thyroxine-binding pre-albumin is 53 +/- 5.0 sec at room temperature and 7.9 +/- 0.3 sec at 37 degrees C.5. Dissociation of thyroxine from both proteins is a single exponential process, suggesting that each protein has only one type of specific binding site.6. These results indicate that the minute pool of free thyroxine in plasma is turning over about 100 times each second, with about two thirds of this flux being due to release and binding of the hormone by pre-albumin.

Binding Sites↗
PubMed1971

The mechanism of thyroxine transfer from plasma to tissue binding sites.

1. An examination has been made of the uptake of thyroxine by rat livers perfused with very dilute solutions of thyroxine-binding globulin (TBG) and thyroxine-binding pre-albumin (TBPA).2. Under these circumstances the rate of transfer of thyroxine to the liver is very similar to the rate of thyroxine dissociation for both TBG and TBPA.3. It is concluded that thyroxine transfer from plasma to tissue-binding sites involves its initial release into the free state and not, as has been suggested, directed exchange.

Animals↗
PubMed1970

The rates of release and binding of thyroxine by bovine serum.

1. Estimates have been made of the rates of release and binding of thyroxine by bovine serum proteins. The method involves measuring the rates at which columns of Amberlite resin take up thyroxine from serum.2. At 37 degrees C the dissociation rate (the rate of release of bound thyroxine into free solution) is about 2.2% sec(-1).3. The binding of thyroxine by serum is a complex process, the majority of the hormone being ;loosely' bound within 0.1 sec but the over-all process taking some 20 sec to completion.4. A mechanism is proposed for the uptake of thyroxine by the liver. It involves a simple competition between the plasma and the liver binding sites for the free thyroxine released into solution by the dissociation reaction.

Animals↗
PubMed1970

The binding of thyroid hormones to phospholipid membranes.

1. Thyroxine and tri-iodothyronine are bound by liposomes (bimolecular phospholipid membranes produced by dispersing egg-yolk lecithin in water). At pH 7.4 the ;partition coefficient' between water and the phospholipid is 1.2 x 10(4) for thyroxine and 2.2 x 10(4) for tri-iodothyronine. The partition coefficient falls off rapidly in more alkaline solutions as the phenolic group on the hormones becomes ionized.2. The binding of thyroxine by membranes is very rapid, occurring at rates equivalent to those found in the binding of thyroxine by serum.3. Lecithin membranes are readily permeable to thyroxine (the oil:water partition coefficient is 1.7).4. The binding of thyroid hormones by perfused rat hearts and the binding of thyroid hormones by liposomes are similar in their sensitivity to pH. Simple alcoholic extracts of tissues have strong thyroxine-binding activity.5. These results are discussed in relation to the nature and function of the tissue thyroxine-binding reaction.

Animals↗
PubMed1969

The release of thyroxine from serum protein in the vessels of the liver.

1. An examination has been made of the uptake of [(131)I]L-thyroxine and L-3,5,3'-tri-iodothyronine by the perfused rat liver.2. With no serum in the perfusion fluid and at flow rates less than 1.5 ml./g.min the liver removes about 85-90% of the hormone passing through in the perfusion fluid. At higher flow rates, the proportion of the hormone taken up is reduced and the results suggest that half of the hormone would be removed if the perfusion fluid spent about 1 sec in the liver.3. The percentage of the hormone extracted by the liver is not affected by variations in temperature (between 22 and 37 degrees C) or by variations in thyroxine concentration (between 1 x 10(-4) and 1 x 10(-1) mug/ml.).4. Accumulated radioactive thyroxine is released only slowly into the perfusion fluid. The rate is doubled by perfusion with solutions containing 10% serum.5. With serum present in the perfusion fluid the uptake of thyroxine can be resolved into two distinct components. A small proportion is taken up very rapidly and a large proportion very slowly. It is suggested that the rapid process represents uptake of free thyroxine and the slow process uptake of thyroxine released from the serum protein; the half-time of the slow process being a measure of the rate of release.6. The proportion of thyroxine taken up by the fast process is about 9% in solutions containing 1% serum and falls progressively at higher serum concentrations.7. With tri-iodothyronine the proportion of the hormone in the free state and the rate of uptake by the slow process are much greater than with thyroxine.8. In the presence of high concentrations of thyroxine the proportion of the hormone in the free state and the rate of uptake by the slow process are both increased.9. The rate of release of thyroxine from its binding protein is proportional to the percentage of the hormone in the free state.

Animals↗
PubMed1969

The uptake of thyroxine and tri-iodothyronine by perfused hearts.

1. An examination has been made of the uptake of radioactive thyroxine and tri-iodothyronine by the perfused hearts of the rat, rabbit, chicken and frog.2. In all species the uptake of both hormones reaches a steady state at which the uptake of tri-iodothyronine is 2-4 times that of thyroxine.3. The uptake of thyroid hormones by perfused hearts can be resolved into two distinct components-first, an initial transient process which takes up hormone and then releases it with a peak uptake at about 20 min from the beginning of perfusion and second, a basic, slow-uptake process which takes several hours to reach a steady state and which has a greater affinity for tri-iodothyronine. Analysis of the ;initial uptake-and-release effect' suggests that it is due to the presence of plasma thyroxine-binding protein in the interstitial fluid of the heart.4. These two uptake processes provide the basis for the biphasic release of thyroid hormones previously described for the rat heart.

Animals↗
PubMed1968

The biliary-faecal excretion of thyroxine during cold exposure in the rat.

1. The biliary-faecal route of thyroxine excretion has been examined by means of thyroxine labelled with (131)I in the thyroidectomized, thyroxine-maintained rat during cold exposure.2. The rate of excretion of thyroxine into the faeces is raised in rats exposed to cold for 2 weeks and fed ad libitum on a high roughage diet.3. When the faecal volumes of warm- and cold-exposed animals are made similar by controlling the roughage content of the diet the difference in their faecal thyroxine excretion rates is much reduced.4. Warm- and cold-exposed rats eating the same amount of food excrete thyroxine into their faeces at similar rates.5. Even when warm- and cold-exposed rats excrete thyroxine into their faeces at the same rate the cold-exposed animals still show a shorter half-life for blood thyroxine.6. An acceleration of the rate of loss of thyroxine from the blood is demonstrable in fasting animals within 10 hr of exposure to cold.7. Ligation of the bile-duct does not affect this acute response to cold.8. The amount of thyroxine lost into the bile within the first 8 hr after intravenous injection is similar in warm- and cold-exposed animals. The clearance of thyroxine into the bile is however greater in the cold-exposed animal since the liver is working against a lower level of blood thyroxine.9. An acceleration of the biliary-faecal route of thyroxine excretion is not the only process at work tending to reduce the biological half-life of thyroxine during cold exposure.

Animals↗
PubMed1968

Thyroxine deiodination during cold exposure in the rat.

1. The urinary radio-iodide excretion following injection of [(131)I]thyroxine and tri-iodothyronine is increased in acutely cold-exposed, thyroidectomized rats.2. This cold response is demonstrable in animals which are fed or fasted or deprived of both food and water. Cold exposure does not increase the rate of urinary excretion of a tracer dose of radio-iodide. It is concluded that cold exposure accelerates the rate of deiodination of both thyroxine and tri-iodothyronine.3. It is confirmed that both adrenaline and noradrenaline administered in vivo can enhance the rate of thyroxide deiodination.4. The sympathetic blocking agents guanethidine and bethanidine reduce the extra deiodination of thyroxine induced by cold exposure. Bethanidine was also found to reduce the deiodination of tri-iodothyronine during cold.5. There is a positive correlation between the deiodination of thyroxine and the urinary catecholamine excretion in the cold-exposed but not the warm-exposed rat.6. It is suggested that the enhanced deiodination during cold exposure is mediated by the release of noradrenaline from the sympathetic nervous system.

Animals↗