Augmenting prehospital care.

INTRODUCTION: The challenging environment of prehospital casualty care demands providers to make prompt decisions and to engage in lifesaving interventions, occasionally without them being adequately experienced. Telementoring based on augmented reality (AR) devices has the potential to decrease the decision time and minimise the distance gap between an experienced consultant and the first responder. The purpose of this study was to determine whether telementoring with AR glasses would affect chest thoracotomy performance and self-confidence of inexperienced trainees. METHODS: Two groups of inexperienced medical students performed a chest thoracotomy in an ex vivo pig model. While one group was mentored remotely using HoloLens AR glasses, the second performed the procedure independently. An observer assessed the trainees' performance. In addition, trainees and mentors evaluated their own performance. RESULTS: Quality of performance was found to be superior with remote guidance, without significant prolongation of the procedure (492 s vs 496 s, p=0.943). Moreover, sense of self-confidence among participant was substantially improved in the telementoring group in which 100% of the participants believed the procedure was successful compared with 40% in the control group (p=0.035). CONCLUSION: AR devices may have a role in future prehospital telementoring systems, to provide accessible consultation for first responders, and could thus positively affect the provider's confidence in decision-making, enhance procedure performance and ultimately improve patient prognosis. That being said, future studies are required to estimate full potential of this technology and additional adjustments are necessary for maximal optimisation and implementation in the field of prehospital care.

Thyroid hormone action at the cellular level.

A large body of circumstantial evidence suggests that the basic unit of thyroid hormone action is the triiodothyronine nuclear receptor complex. This complex stimulates the formation, directly or indirectly, of a diversity of messenger RNA (mRNA) sequences. A generalized increase in mRNA as well as a disproportionate increase in a limited number of RNA sequences have been demonstrated. Regulation of thyroid hormone effects may be carried out largely at a local cellular level. Highly selective alterations in sensitivity to the triiodothyronine nuclear receptor complex may occur at specific target genes. Metabolic factors and hormones participate in such regulation. In a given tissue, alterations in the total number of receptor sites has not been shown to be useful as an index of thyroid hormone response, and local modulation of the response to the triiodothyronine receptor complex by a variety of factors other than triiodothyronine may be carried out at a postreceptor level.

Thyroid hormone receptor-containing fragment released from chromatin by deoxyribonuclease I and micrococcal nuclease.

Limited deoxyribonuclease I and micrococcal nuclease digestion of hepatic nuclei from euthyroid rats injected with 125I-labeled triiodothyronine ([125I]T3) releases a discrete [125I]T3-labeled chromatin fragment (5.8S) which is larger than the T3 receptor (3.5S). These results suggest the T3 receptor is associated with a restricted fraction of hepatic chromatin that has a nuclease sensitivity characteristic of transcriptionally active chromatin.

Cebus monkeys: effect on branching of gustavia trees.

One 50-tree site with monkeys and one without were similar with respect to tree height and tree diameter, but the site with monkeys had trees with significantly more branches than those on the site without monkeys. Possibly removal of terminal buds during feeding by the monkeys releases the lateral buds from apical dominance and allows increased branching.

Specific triiodothyronine binding sites in the anterior pituitary of the rat.

Studies with L-[(125)l] triiodothyronine and L-[(125)l] thyroxine, and with equilibrium dialysis of plasma proteins indicate that rat pituitary binds L-triiodothyronine 9.8 times as strongly as it does L-thyroxine. Injection of even small doses of nonradioactive L-triiodothyronine reduces the pituitary/ plasma ratio of radioactive L-triiodothyronine, an indication of the existence of pituitary binding sites with a limited capacity for L-triiodothyronine. Limited capacity binding sites for L-thyroxine could not be demonstrated.

Stereospecific transport of triiodothyronine from plasma to cytosol and from cytosol to nucleus in rat liver, kidney, brain, and heart.

We have investigated the transport of L- and D-triiodothyronine (T3) from plasma to cellular cytoplasm and from cytoplasm to nucleus by estimating the concentration of free hormone in these compartments in rat liver, kidney, brain, and heart. We assessed the distribution of T3 in various tissues and its metabolism by standard isotopic techniques and measured plasma and cytosolic tissue T3 by radioimmunoassay. In addition, we determined the fraction of radiosensitive T3 associated with the cytosol in individual tissues and estimated the cytosolic volume per gram of tissue. Equilibrium dialysis allowed us to determine the binding power of cytosols and plasma, and in vitro saturation techniques provided values for the affinity (ka) for L- and D-T3 of isolated nuclei in aqueous solution at 37 degrees C. We calculated the free cytosolic hormone from the product of cytosolic T3 and the binding power of cytosol for T3, and the free intranuclear T3 from the ka and previously determined ratio of occupied-to-unoccupied binding sites under steady state conditions in euthyroid animals. Our results showed that the free cytosolic/free plasma concentrations for L-T3 and D-T3, respectively, were: liver 2.8, 21.6; kidney 1.17, 63.3; heart 1.31, 1.58; brain 0.86, 0.24. The free nuclear/free cytosolic ratios for L-T3 and D-T3, respectively, were: liver 58.2, 3.70; kidney 55.9, 1.54; heart 80.6, 24.9; and brain 251, 108.6. Our findings suggest that stereospecific transport occurs both from plasma to cytosol and from cytosol to nucleus. The high gradients from cytosol to nucleus imply that there is an energy-dependent process and appear to account for the differences in the nuclear association constant determined in vivo and in vitro.

Formation of iodoprotein during the peripheral metabolism of 3,5,3'-triiodo-L-thyronine-125I in the euthyroid man and rat.

3,5,3'-Triiodo-L-thyronine-(125)I (T3-(125)I) metabolism was studied in nine euthyroid human subjects on blocking doses of nonradioactive iodide. After the intravenous injection of T3-(125)I, the fractional disappearance rate of plasma radioactivity progressively disappearance rate of plasma radioactivity progressively decreased with time. Analysis of individual plasma samples by dialysis, electrophoretic, and extraction techniques revealed three radioactive components: T3-(125)I, iodide-(125)I, and an unidentified material which was nonextractable in acid butanol (NE(125)I). Ne(125)I rose to maximal levels 24-36 hr after injection of T3-(125)I and then decreased with a fractional rate which approached, after 12-14 days, approximately 0.05 day(-1) (t(1/2) = 14 days). The plasma T3-(125)I concentration, obtained by subtraction of iodide-(125)I and NE(125)I from the plasma total (125)I, declined at a constant fractional rate with time with a t(1/2) of 1.5 days. Qualitatively similar results were obtained in rats. After 72 hr, 57% of the plasma and 40% of the liver radioactivity was NE(125)I. Chromatographic purification of the T3-(125)I before injection did not alter these results. The extrathyroidal origin of NE(125)I was further demonstrated by similar results in thyroidectomized rats maintained on thyroxine. NE(125)I from human sera separated from the other radioiodinated substances by ion-exchange chromatography was quantitatively precipitated by trichloracetic acid, not dialyzable, insoluble in CHCl(3):CH(2)OH, and migrated with albumin during starch-gel electrophoresis. Based on these properties, NE(125)I was tentatively identified as iodoalbumin. Observations in rats equilibrated with (125)I, as well as nonradioactive iodine determinations in human sera before and after acid butanol extraction, indicate that 10-20% of the serum organic iodine is in the form of iodoprotein. Our studies suggest that this moiety may be derived at least in part from the peripheral metabolism of the thyroid hormones.

Concentration of L-thyroxine and L-triiodothyronine specifically bound to nuclear receptors in rat liver and kidney. Quantitative evidence favoring a major role of T3 in thyroid hormone action.

To estimate the relative contribution of l-triiodothyronine (T(3)) and l-thyroxine (T(4)) to thyroidal effects, we have measured the concentration of iodothyronine bound to specific hepatic nuclear receptor sites by three different techniques: (a) specific radioimmunoassay after separation of T(3) and T(4) by preparative paper chromatography; (b) in vivo kinetic approaches as reported previously; and (c) isotopic equilibration. By these three methods, receptor concentration of T(3) and T(4) in liver was 0.51+/-0.19 (SD) and 0.08+/-0.06; 0.52+/-0.12 and 0.08+/-0.02; and 0.50+/-0.13 and 0.10+/-0.03 pmol/mg DNA, respectively. The percentage contribution of T(3) and T(4) to total receptor iodothyronine was thus 86.8+/-9.0 and 13.2+/-9.4; 86.3+/-3.5 and 13.7+/-3.5; and 83.7+/-5.6 and 16.3+/-5.6%, respectively. In kidney, specifically bound nuclear T(3) and T(4) were estimated both by isotopic equilibration and by in vivo kinetic techniques to be 0.28+/-0.11 and 0.03+/-0.01 pmol/mg DNA, respectively. Thus, T(3) constituted 89.4+/-3.2% of total receptor iodothyronine in this tissue. No other iodothyronines or analogs were bound to the nuclear sites in either tissue. Kidney and liver nuclear T(3) concentrations also were identical to values previously reported with in vivo kinetic techniques. Other studies from this laboratory have suggested that thyroid effect is related to the molar concentration of iodothyronine bound to specific nuclear sites, that the sites are similar in various tissues, and that iodothyronine in plasma is in equilibrium with nuclear T(3). If these relationships are assumed, T(3) contributes between 85 and 90% of thyroidal effects in the euthyroid rat. The remaining 10-15% of thyroidal effect appears to result from the intrinsic activity of T(4).

Estimation of rapidly exchangeable cellular thyroxine from the plasma disappearance curves of simultaneously administered thyroxine-131-I and albumin-125-I.

A mathematical analysis of the plasma disappearance curves of simultaneously injected thyroxine-(131)I and albumin-(125)I allows the development of simple formulas for estimating the pool size and transfer kinetics of rapidly exchangeable intracellular thyroxine in man. Evidence is presented that the early distribution kinetics of albumin-(125)I can be used to represent the expansion of the thyroxine-(131)I-plasma protein complex into the extracellular compartment. Calculations indicate that approximately 37% of total body extrathyroidal thyroxine is within such exchangeable tissue stores. The average cellular clearance of thyroxine is 42.7 ml per minute, a value far in excess of the metabolic clearance of this hormone. Results of external measurements over the hepatic area and studies involving hepatic biopsies indicate that the liver is an important but probably not the exclusive component of the intracellular compartment. The partition of thyroxine between cellular and extracellular compartments is determined by the balance of tissue and plasma protein binding factors. The fractional transfer constants are inversely related to the strength of binding of each compartment and directly proportional to the permeability characteristic of the hypothetical membrane separating compartments. Appropriate numerical values for these factors are assigned. An increased fractional entrance of thyroxine-(131)I into the cellular compartment was noted in a patient with congenital decrease in the maximal binding capacity of thyroxine-binding globulin and in three patients after the infusion of 5,5-diphenylhydantoin. Decreased intracellular space and impaired permeability characteristics were observed in five patients with hepatic disease. Studies of the rate of entrance of thyroxine-(131)I and albumin-(125)I into the pleural effusion of a patient with congestive heart failure suggested that transcapillary passage of thyroxine independent of its binding protein is not a predominant factor in the total distribution kinetics of thyroxine-(131)I. The thesis is advanced that the distribution of thyroxine, both within the extracellular compartment and between the extracellular and intracellular compartments, is accomplished largely by the carrier protein and the direct transfer of thyroxine from one binding site to another. The concept of free thyroxine is reassessed in terms of this formulation.

Increased thyroxine turnover and thyroidal function after stimulation of hepatocellular binding of thyroxine by phenobarbital.

Administration of phenobarbital to rats in a dosage schedule previously demonstrated to increase hepatocellular binding of thyroxine results in increased hormonal turnover, due both to increased deiodination and to fecal disposition of thyroxine iodine. The rate of biliary excretion of thyroxine iodine is roughly proportional to the hepatic content of exchangeable thyroxine. The enhanced peripheral disposition of thyroxine appears to lead to increased thyroidal function, as measured by isotopic iodine studies, and the maintenance of a normal nonradioactive serum PBI. On the other hand, thyroidectomized animals maintained on a constant replacement dose of L-thyroxine and treated with phenobarbital exhibit a marked fall in serum PBI. These findings suggest that increased thyroxine flux in phenobarbital-treated animals is secondary to primary stimulation of hepatocellular binding. Exchangeable intracellular thyroxine may thus be an important determinant of hormone turnover and, possibly, of hormonal action.

Relationship between the accumulation of pituitary growth hormone and nuclear occupancy by triiodothyronine in the rat.

Studies were undertaken in hypothyroid rats in an effort to define the kinetics of growth hormone (GH) accumulation in response to i.v. pulse injections of triiodothyronine (T(3)) and to calculate the relationship between nuclear occupancy by T(3) and the instantaneous rate of accumulation of pituitary GH. Results were contrasted to the findings in previous studies of the induction of hepatic mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) and malic enzyme (ME) by T(3). The dose of T(3) required to achieve half-maximal accumulation of GH in 24 h was 0.6 mug/100 g body wt, a value 15-fold less than the half-maximal dose for alpha-GPD and ME induction at a comparable time after injection. Although significant increase in pituitary GH were evident as early as 3 h after injection of maximally effective doses of T(3), the rate of increase became linear only 12 h after injection. After achievement of peak values, the pituitary content of GH decayed with a similar terminal t((1/2)) of 3.9 days and 4.1 days in two groups of animals injected with a single dose of 1.0 and 50 mug T(3)/100 g body wt, respectively. In vivo isotopic displacement studies carried out at the equilibrium time point indicated that the pituitary nuclear binding capacity was 5.5 ng T(3)/g tissue and that the plasma concentration at which one-half of the nuclear sites are occupied is 1.0 ng/ml. Nuclear occupancy as a function of time was calculated from the estimated plasma T(3) concentration after injection of the dose and the half-occupancy plasma concentration. These data were then analyzed by application of the mathematical model previously developed to ascertain the relationship between nuclear occupancy and the rate of hepatic enzyme induction. Results indicated that the pituitary nuclear occupancy-response relationship was generally linear, in marked contrast to the highly amplified relationship between nuclear occupancy and the response of ME and alpha-GPD to T(3) in the liver. In supplementary experiments, euthyroid rats received daily injections of 200 mug of T(3) for 7 days to keep nuclear sites nearly saturated for the duration of the experiment. No significant increase in the pituitary GH content above euthyroid base-line levels was noted. This also contrasts with the marked increase above euthyroid levels in alpha-GPD and ME observed in previous studies. Our findings suggest the existence of major differences between the specific mechanisms which lead to the induction of pituitary GH and the hepatic enzymes by T(3).

Decreased serum triiodothyronine in starving rats is due primarily to diminished thyroidal secretion of thyroxine.

Although thyroxine (T4) 5'-deiodinase activity is diminished in liver homogenates of starved rats, no information is available regarding the effect of starvation on net T4 to triiodothyronine (T3) conversion in the intact rat. It appeared important to clarify this relationship since rat liver homogenates are widely used as a model for the study of the factors responsible for reduced circulating T3 in chronically ill and calorically deprived patients. In contrast to the expected selective decrease in circulating T3 levels in calorically restricted humans due to diminished T4 to T3 conversion, 5 d of starvation of two groups of male Sprague-Dawley rats resulted, paradoxically, in a greater decrease in serum T4 than in serum T3 levels. Kinetic studies show that starvation is associated with no change in the metabolic clearance rate (MCR) of T3, a 20% increase in the MCR of T4, a 67% reduction in turnover rate of T4, but only a 58% reduction in the turnover rate of T3. Moreover, in the first group of rats studied, direct chromatographic analysis of the isotopic composition of total body homogenates after the injection of 125I-T4 showed that 21.8% of T4 is converted to T3 in control rats and 28.8% in starved rats, suggesting that virtually all extrathyroidal T3 in starved and control rats is derived from the peripheral conversion of T4, and that there is little or no direct thyroidal secretion of T3. Our findings strongly point to a reduced thyroidal secretion of T4 as the primary cause of the observed reduction in circulating T3. Since the mechanisms leading to reduced levels of plasma T3 differ in humans and rats, it may be important to reexamine the use of liver homogenate preparations as models for study of the pathogenesis of the "low T3 syndrome" in humans.

In vitro binding of L-triiodothyronine to receptors in rat liver nuclei. Kinectics of binding, extraction properties, and lack of requirement for cytosol proteins.

Isolated hepatic nuclei from euthyroid rats were incubated with tracer (125I)L-triiodothyronine (T3) and increasing doses of nonradioactive T3 for 30 min at 37degrees C. The T3 bound specifically to nuclear sites increased with increasing T3 doses to a plateau, which represented the nuclear binding capacity, M. Addition of 1 mM KCN, NaF, dinitrophenol, oriodoacetate did not affect nuclear binding, indicating that active metabolism was not required. Kinetic studies showed that the nuclear sites were equilibrated with T3 within 30 min of incubation (one-half maximal binding at 3 min) and that the rate of release of T3 in vitro (0.058 min-1) was the same for endogenous T3 or for T3 bound to nuclei in vitro. Nuclear T3 resisted extraction with 0.14 M NaC1 buffered at pH 7.5, but both endogenous hormone and T3 bound in vitro were readily extracted by 0.4 M KC1 at pH 8.0. The elution profiles of endogenous and in vitro-bound T3 from Sephadex G-100 columns showed a common protein peak with a molecular weight of 60-65,000, assuming a globular protein. Scatchard analysis of in vitro displacement studies showed a single class of binding sites. Mean M equals 0.23 times 10-9 M or 0.85 ng T3 for nuclei isolated from 1 g of liver. Mean M closely corresponded to that anticipated from reported in vivo studies. The apparent association constant Ka for the nuclear sites, 5.55 times 108 M-1, was lower than in studies in vivo, probably attributable to the different ionic milieu of nuclei in the incubation buffer and in the intact cell. Thus, the identity of the nuclear T3 binding sites studied in vitro to those reported for endogenous hormone is demonstrated by similar binding capacities, release rates, analogue binding affinities (previously reported), and localization to chromatin nonhistone proteins of comparable molecular weight. The role of cytosol protein in nuclear binding was assessed by comparing binding parameters for extensively washed nuclei and nuclei incubated either with contaminating or added cytosol. No difference in Ka or M was found. Moreover, it was unlikely that specific cytosol proteins were already present in nuclei and functioned during incubation as a shuttle for T3, since Ka and M for nuclei obtained from athyreotic rats were similar to Ka and M for nuclei from euthyroid animals. Thus, an initial interaction between T3 and specific cytosol proteins does not appear to be a prerequisite for translocation of T3 to nuclear sites.

Tissue iodoprotein formation during the peripheral metabolism of the thyroid hormones.

The formation of tissue iodoproteins during the peripheral metabolism of the thyroid hormones was examined by determining the concentration of nonethanol-extractable (125)I (NE(125)I) in various tissues after the intravenous injection of 3,5,3'-triiodo-L-thyronine (T3-(125)I) and L-thyroxine-(125)I (T4-(125)I) in groups of rats with iodide-blocked thyroid glands. 3 days after T3-(125)I and 7 days after T4-(125)I injection the concentration of NE(125)I in the liver and kidney was 5-10 times greater than in plasma. Smaller but nonetheless significant concentrations of NE(125)I were demonstrated in skeletal and cardiac muscle. Hepatic subcellular fractionation studies revealed that the major portion of the liver NE(125)I was in the microsomal fraction. Lower concentrations of NE(125)I were present in the nuclear, mitochondrial, and soluble fractions. When similar studies were performed in groups of rats pretreated with phenobarbital, an increase in the metabolic clearance of T3-(125)I (30%) and T4-(125)I (100%) was observed along with a highly significant increase in the NE(125)I concentration of the liver and plasma. The increase in hepatic NE(125)I in these studies was primarily due to the microsomal component. Incubation of hepatic microsomes with T3-(125)I and T4-(125)I showed that NEI formation as well as deiodination appeared to obey simple Michaelis-Menten kinetics. Moreover, the maximal rate of both deiodination and NEI formation was increased when microsomes harvested from phenobarbital-treated rats were employed. These studies indicate that thyroid hormone metabolism results in the formation of structural and soluble tissue iodoproteins in addition to circulating iodoproteins. The rate of formation of these moieties in the liver and plasma appears to be related to the rate of hormone metabolism.

Slow fractional removal of nonextractable iodine from rat tissue after injection of labeled L-thyroxine and 3,5,3'-triiodo-L-thyronine. A possible clue to the mechanism of initiation and persistence of hormonal action.

Previous studies have shown that a small but significant proportion of radioiodine from labeled L-thyroxine (T(4)) and 3,5,3'-triiodo-L-thyronine (T(3)) is incorporated into plasma and tissue proteins and is not, therefore, extractable with ethanol or other organic solvents. Other studies have shown that the complex consists, at least in part, of the iodothyronine in apparent covalent linkage with protein. In the present series of experiments the disappearance rate of nonextractable iodine (NEI) was determined in plasma, liver, and kidney after the injection of rats with a single dose of T(4) and T(3) labeled with radioiodine in the phenolic ring. The t(1/2) of NEI decay was substantially longer than the t(1/2) of the initial metabolic removal of T(4) (16 hr) and T(3) (4-6 hr). Thus, between days 3 and 11 the average t(1/2) of plasma NEI derived from T(4) was 2.2 days, from T(3), 1.9 days; kidney NEI from T(4), 7.4 days, from T(3), 6.1 days; hepatic NEI from T(4), 4.3 days, from T(3), 5.2 days. The slow disappearance of liver NEI was of special interest in connection with an analysis of previously published data by Tata and associates dealing with the sequential tissue effects after the injection of a single dose of T(3) into thyroidectomized rats. The t(1/2) of decay of the various biological effects measured, primarily in the liver, appeared similar to each other, averaging between 4 and 6 days. These findings are compatible with the existence of a single long-lived intermediate governing the tissue expression of thyroid hormone. The t(1/2) of hepatic NEI in similarly prepared animals (thyroidectomized and injected with 25 mug of T(3)) was found to be 4.5 days. The coincidence in the slow fractional disappearance rates of hepatic NEI and the dissipation of hormonal tissue effects raises the distinct possibility that T(3) interacts with specific cellular receptor sites to form covalent complexes which are slowly removed and serve both to initiate and to perpetuate hormonal action. A mathematical analysis of hormonal reaction mechanisms, based on the assumption of a linearly responsive system, a t(1/2) of T(3) of 4 hr, and a t(1/2) of 4.5 days for the postulated long-lived "messenger" suggests that maximal expression of hormonal activity cannot be attained before 20 hr after the injection of a hormone pulse. This value is broadly consonant with the observed data accumulated by Tata and associates. The existence of a long-lived messenger, possibly a species of NEI, would therefore explain not only the slow dissipation of hormonal effects but also the well-recognized "lag-time" in the expression of hormonal action.

A new radioimmunoassay for plasma L-triiodothyronine: measurements in thyroid disease and in patients maintained on hormonal replacement.

A new procedure for the radioimmunoassay of l-triiodothyronine (T(3)) in human plasma is described in which the iodothyronines are separated from the plasma proteins before incubation with a specific antiserum to T(3). The antibody bound and free T(3) are separated with dextran-coated charcoal. In this system, the mean recovery of T(3) added to plasma was 97.9% and both in vitro conversion of l-thyroxine (T(4)) to T(3) and cross-reaction between T(4) and the anti-T(3) antibody were undetectable (less than 0.1%). The assay procedure allowed the measurement of T(3) in up to 0.5 ml of plasma resulting in improved assay sensitivity (6 ng/100 ml). The mean plasma T(3) in normal subjects was 146+/-24 ng/100 ml (sd). Mean T(3) concentration was increased in hyperthyroidism (665+/-289 ng/100 ml) and decreased in hypothyroidism (44+/-26 ng/100 ml). In patients with severe hypothyroidism, plasma T(3) was between 7 and 30 ng/100 ml. Plasma T(3) concentration was relatively constant throughout the day in three euthyroid subjects. In contrast, in hypothyroid subjects on replacement therapy with T(3), a T(4): T(3) combination or desiccated thyroid plasma T(3) was markedly elevated for several hours after ingestion of the medication. Plasma T(3) was unchanged throughout the day in patients treated with T(4). Thus, insofar as plasma T(3) levels are concerned, replacement therapy with T(4) appears to mimic the euthyroid state more closely than other preparations.

Quantitation of extrathyroidal conversion of L-thyroxine to 3,5,3'-triiodo-L-thyronine in the rat.

Studies of the rate of extrathyroidal conversion of thyroxine (T4) to 3,5,3'-triiodo-L-thyronine (T3) were carried out in rats. Total body homogenates were prepared and extracted with ethanol 48, 72, and 96 hr after the intravenous injection of (125)I-T4. (131)I-T3 was added, and the paper chromatographic purification of T3 was effected by serial elution and rechromatography in three paper and one thin-layer cycles. The ratio of (131)I-T3 and (125)I-T3 counting rates in the final chromatograms, which was identical in three different paper chromatography systems, was used to calculate the proportion of (125)I-T3 to (125)I-T4 in the original homogenates. In order to discount the effects of in vitro monodeiodination of T4 during extraction and chromatography, we killed control animals immediately after injection of (125)I-T4 and processed them in a similar fashion to the experimental groups. The average ratio of (125)I-T3 to (125)I-T4 in carcass extracts of animals killed between 48 and 96 hr after isotopic injection was 0.08 whereas the average ratio of (125)I-T3 to (125)I-T4 in chromatograms of control animals was 0.01. On the basis of the proposed model, calculations indicated that about 17% of the secreted T4 was converted to T3. Assuming values cited in the literature for the concentration of nonradioactive T3 in rat plasma, these findings would suggest that about 20% of total body T3 is derived by conversion from T4. Moreover, since previous estimates have suggested that in the rat, T3 has about 3 to 5 times greater biologic activity than T4, these results also raise the possibility that the hormonal activity of T4 may be dependent in large part on its conversion to T3.A necessary assumption in calculating T4 to T3 conversion in this and other studies is that the 3' and 5' positions are randomly labeled with radioiodine in phenolic-ring iodine-labeled T4. Evidence supporting this assumption was obtained in the rat by comparing the amount of labeled T3 produced after injection of phenolic and nonphenolic-ring iodine-labeled T4.

Propylthiouracil inhibits the conversion of L-thyroxine to L-triiodothyronine. An explanation of the antithyroxine effect of propylthiouracil and evidence supporting the concept that triiodothyronine is the active thyroid hormone.

6-n-propylthiouracil (PTU) administered to male Sprague-Dawley rats maintained on 2 and 5 mug L-thyroxine (T(4))/100 g body weight resulted in a marked reduction in the rate of conversion of L-thyroxine to L-triiodothyronine (T(3)). These effects could not be ascribed to induced hypothyroidism since the group maintained on 5 mug T(4)/day had normal levels of liver mitochondrial alpha glycerophosphate dehydrogenase. In confirmation of previous studies, PTU also reduced the fractional rate of deiodination of T(3). These observations provide a possible explanation of the many published observations indicating that PTU antagonizes the tissue effects of T(4) but not of T(3). The data suggest that monodeiodination of T(4) but not of T(3) is essential before hormonal effects can be manifested at the cellular level.

Nonlinear (amplified) relationship between nuclear occupancy by triiodothyronine and the appearance rate of hepatic alpha-glycerophosphate dehydrogenase and malic enzyme in the rat.

Three separate approaches were applied to examine the general relationship between R, the rate of induction of specific enzymes (mitochondrial alpha-glycero-phosphate dehydrogenase and cytosolic malic enzyme) and q, the fractional nuclear occupancy by triiodothyronine in male Sprague-Dawley rats. Daily 200-microgram injections of triiodothyronine per 10u g body wt for 7 days resulted in saturation of the hepatic nuclear sites and the achievement of an apparent new steady state of enzyme levels. The increase achieved over base-line hypothyroid levels was then compared with the increment over hypothyroid base line characteristic of intact euthyroid animals with 47% of nuclear sites occupied. The maximal theoretical reate of steady-state enzyme induction could be protected on the basis of the observed maximal increase in enzyme activity observed 1 day after the injection of graded doses of hormone and lambda, the known fractional rate of enzyme dissipation. The 24-h dose-response studies were used to generate R as a continuous function of q, both in hypothyroid as well as in euthyroid animals. This approach involved the numerical solution of an ordinary differential equation describing the rate of change of enzyme as a function of R, which was assumed to be uniquely related to q. Results of these analyses indicated that the ratio of the maximal rate of induction of enzyme at full occupancy to the rate of induction under euthyroid conditions assumes a value between 9.0 and 19.5, depending on the precise analytic and experimental approach applied. This value is far in excess of the theoretical ratio 2.13 which on would anticipate if R were linearly related to q and 47% of the nuclear sites occupied under physiological conditions. Thus, the signal for enzyme induction appears to undergo progressjive amplification with increasing nuclear occupancy. Moreover, the curve describing the relationship between R and q appears highly nonlinear throughout (concave upwards). Although the molecular mechanism responsible for amplification is unknown, recognition of this phenomenon may be helpful in understanding tissue effects of thyroid hormone excess. Moreover, the analytic technique for determining R as a function of q may be of general applicability in studying hormonal response systems under nonsteady-state conditions.