Identifiability and interval identifiability of mammillary and catenary compartmental models with some known rate constants.

The identifiability problem is addressed for n-compartment linear mammillary and catenary models, for the common case of input and output in the first compartment and prior information about one or more model rate constants. We first define the concept of independent constraints and show that n-compartment linear mammillary or catenary models are uniquely identifiable under n-1 independent constraints. Closed-form algorithms for bounding the constrained parameter space are then developed algebraically, and their validity is confirmed using an independent approach, namely joint estimation of the parameters of all uniquely identifiable submodels of the original multicompartmental model. For the noise-free (deterministic) case, the major effects of additional parameter knowledge are to narrow the bounds of rate constants that remain unidentifiable, as well as to possibly render others identifiable. When noisy data are considered, the means of the bounds of rate constants that remain unidentifiable are also narrowed, but the variances of some of these bound estimates increase. This unexpected result was verified by Monte Carlo simulation of several different models, using both normally and lognormally distributed data assumptions. Extensions and some consequences of this analysis useful for model discrimination and experiment design applications are also noted.

Changes in metabolism of TRH in euthyroid sick syndrome.

OBJECTIVE: The aim of this study was to examine the metabolism of a simple dose, intravenously administered TRH bolus of 200 microg, in patients with euthyroid sick syndrome (ESS). PATIENTS AND METHODS: A TRH test was performed on ten ESS patients and ten controls upon admission (d1) and after recovery (d2). Blood samples were collected at 0, 10, 20 and 30min after TRH injection. We analyzed the volume of distribution (V(d)), the plasma clearance rate (PCR), the fractional clearance rate (FCR), the half-life (t(1/2)) and the TSH response to the injection of TRH. RESULTS: All patients had lower tri-iodothyronine (T(3)) levels compared with controls (0.9 +/- 0. 1nmol/l vs 1.9 +/- 0.1 nmol/l; P < 0.0001; mean +/- S.D.; paired t-test). In addition, the V(d) (16.7 +/- 5.9/l vs 30.6 +/- 0.6/l; P < 0.0005) and PCR (2.0 +/- 0.80 l/min vs 3.3 +/- 0.25 l/min; P <0. 0005) were found statistically lowered in patients than in controls, whereas FCR (0.119 +/- 0.01 permin vs 0.110 +/- 0.01 per min; P < 0. 025) was found increased in patients as opposed to controls. The t(1/2) of exogenously administered TRH was increased in ESS compared with controls (7.2 +/- 0.7 min vs 6.3 +/- 0.6 min; P <0.005). TSH response to TRH was found significantly repressed at 10, 20 and 30 min after TRH injection. On d2, these findings had reverted to normal and no changes regarding the kinetics of TRH and the response of TSH could be detected between patients and controls. CONCLUSIONS: The results demonstrate an impairment of TRH metabolism in ESS. The findings may suggest altered enzymatic activity, responsible for TRH degradation in states of acute ESS. These changes might be involved in the pathogenesis of ESS and represent part of an adaptive mechanism to this syndrome.

Direct measurement of the contributions of type I and type II 5'-deiodinases to whole body steady state 3,5,3'-triiodothyronine production from thyroxine in the rat.

Production of T3 from T4 in tissues is catalyzed by two 5'-deiodinases, type I (D1) and type II (D2), but the quantitative contribution of each pathway to whole body T3 production is not well established. In the presence of propylthiouracil (PTU), D1, but not D2, can be effectively blocked, providing an experimental probe for addressing this problem. Decades ago, this approach provided indirect estimates ranging from 23-44% contribution by D2, based on plasma T3 appearance rate comparisons (PAR3 = PCR3 [T3]p) in periodically T4-injected athyreotic rats vs. controls. Two, more recent studies, using constant infusions of T4 for replacement, achieved 22% and 65% estimates, respectively, from PAR3 comparisons. We have revisited this problem more directly and precisely, with two major differences in experiment design. We used direct whole body steady state measurements of T3 production, instead of indirect plasma-only data (PAR3). We also used (euthyroid) physiological doses of both T4 (0.9 microg/day x 100 g BW) and T3 (0.15 microg/day x 100 g BW) for replacement in two thyroidectomized rat groups, instead of T4 only, in a 7-day constant steady state, dual tracer infusion protocol. The first group also had chronically implanted 150-mg PTU pellets (TXR-PTU); the other had implanted 0.1 N NaOH placebo pellets (TXR-EU); each delivered their product at constant rates. A third euthyroid intact group was used as the controls. The completeness of D1 inhibition was ascertained in a fourth group, identically treated with 150-mg PTU pellets, in which negligible D1 activity was found in liver and kidney using labeled rT3 as substrate for the 5'-D assays and minimal (1 mM) dithiothreitol as cofactor. In the TXR-PTU group, the percentage of T4 converted to T3 was 11.8%, compared with 23.4% (P < 0.0005) in the TXR-EU group, and 22.7% (P = NS) in controls. Thus, in euthyroid steady state, D2 contributes about half of the T3 produced from T4.

3,5,3'-Triiodothyronine (T3) clearance and T3-glucuronide (T3G) appearance kinetics in plasma of freshwater-reared male tilapia, Oreochromis mossambicus.

Distribution and metabolism of the thyroid hormone 3,5, 3'-l-triiodothyronine (T3) were studied in several ways to gain insights into these processes in the warm water fish tilapia Oreochromis mossambicus. Trace doses of 125I-labeled T3 (T*3)1 were injected intraarterially, extraarterially, or intraperitoneally in freshwater-reared male tilapia to explore plasma clearance kinetic responses to these different input modalities. Multicompartmental analysis of the plasma clearance data indicated a kinetic distribution of T*3 much like that reported for the rat and human, with about 2% of total body T*3 in plasma, 5% in rapidly exchanging tissues such as kidney and liver, and 93% in slowly exchanging tissues such as muscle. However, plasma clearance rates (PCR, 5.37 mL/h . 100 g body wt) and plasma appearance rates (PAR3 = PCR x [T3] plasma = 36.3 ng/h . 100 g body wt) were quite different than these indices in rat and human and 5 to 50 times larger than values reported for rainbow trout. On a whole-body basis, normalized for body weight, the tilapia we studied produced and accumulated much more T3 than rat, human, or rainbow trout. Enzymatic and chromatographic analyses of the plasma clearance data samples indicated substantial production of labeled glucuronide, but not sulfate, conjugates of iodothyronines (TiG) of unknown origin appearing in plasma. The TiG appeared beginning a few hours postinjection, peaked at 6 hours, and yielded a predicted steady-state TiG level of 8.3% of the T3 level in plasma. In contrast, in published studies, no conjugates were detected in rainbow trout plasma from 2 to 24 h after iv injection of T*3, T*4, or reverse-T*3, although conjugates of all were present in bile. To our knowledge, although T3 and T4 sulfate conjugates are present in the sera of several mammals, this is the first quantification of iodothyronine glucuronides reported in blood of any species under normal conditions. This might have physiological significance for the tilapia, with T3G providing a reversible storage form of T3 in blood, as has been suggested for sulfate conjugates of T3 and T4 in blood of several mammals.

Effects of 5,5'-diphenylhydantoin on the metabolic pathway of thyroid hormone in rats.

Treatment of rats with phenytoin (DPH), an anti-epileptic drug, results in lower tissue thyroid hormone (TH) levels and interferes with the metabolic pathway of TH. To test the hypothesis that DPH affects the enterohepatic cycle of TH and, thus, the kinetics of TH turnover, we performed a kinetic experiment (three-compartment analysis) and a steady-state, double-isotope equilibrium experiment in rats treated for 3 weeks with DPH (50 mg/kg body weight per day) and in untreated controls. This included measurements of TH and TH metabolite levels, as well as the activities of enzymes involved in the TH metabolic pathway. DPH treatment resulted in a decrease in the production of thyroxine (T4) (by 25%) and tri-iodothyronine (T3) (by 37%), a decrease in the T3 concentration in all three pools, and a redistribution of T4 from the fast to the slow pool. The amount of T4 increased in intestinal contents and feces by 66% and 71% respectively. Expressed as a fraction of daily TH disposal, fecal loss of T4 was enhanced from 10 to 23% and that of T3 from 16 to 21%. An increase in T4 and T3 UDP-glucuronyltransferase activities was observed, suggesting that the increased fecal loss of T4 and T3 is secondary to an increased biliary output of their glucuronides. The reduced secretion of TH and increased fecal clearance during DPH treatment can lead in the long run to depletion of TH stores.

Steady-state regulation of whole-body thyroid hormone pool sizes and interconversion rates in hypothyroid and moderately T3-stimulated rats.

Steady-state regulation of whole-body T3 and T4 distribution and metabolism were directly evaluated and compared in hypothyroid, euthyroid, and in euthyroid rats moderately T3-stimulated by continuous infusion of 0.15 microg/day L-T3 per 100 g BW, thereby supplementing euthyroid T3 sources by two thirds. Our goal was to develop deeper insights into the hierarchy, quantitative adequacy, and sensitivity of this regulatory system, in response to these hormone production challenges in constant steady state. We used a novel whole-body steady-state experiment design model and data analysis approach, which entails nonexclusive whole-body homogenate extracts and blood collected after 7-day infusions of tracer T3 (T3*) or T4*, quantitatively analyzed chromatographically for T3*, T4* and metabolite* concentrations. Hormone regulation implications across the 3 groups were assessed by comparing (per 100g BW) total body T4 to T3 and T3 from T4 conversion rates (CR(4-3) and CR(3-4)), total body pool sizes (Qtot) and distribution volumes (V(D)), total body production (PR), or plasma appearance rates (PAR), plasma clearance rates (PCR), and elimination rates (k). In the hypothyroid rats, absolute production of T3 from T4 was only a fourth of that in euthyroids: CR(3-4) = 1.55 vs. 6.77 ng/h, but the percent (efficiency) of whole-body T4 converted to T3 was more than double that in euthyroids: %CR(4-3) = 45.4% vs. 21.0%, reflecting an effective doubling of type I and/or type II 5'-deiodinase activity on a whole-body basis in response to severe curtailment of thyroidal production. Whole-body T3 pools and T3 production and clearance rates were all about 2 to 3 times lower in hypo- than in euthyroids: minimum Qtot = 36.8 vs. 100 ng, V(D3) = 148 vs. 236 ml, PAR3 = 3.44 vs. 9.09 ng/h, PCR3 = 13.8 vs. 21.3 ml/h; and nearly all T4 pool size, production, clearance and elimination rates also were very substantially reduced: PCR4 = 0.540 vs. 0.941 ml/h, PR4 = 4.11 vs. 38.3 ng/h, Qtot4 = 128 vs. 702 ng, k4 = 0.0322 vs. 0.0530 h(-1). In moderately T3-stimulated rats, presumed central feedback effects of the added T3 on T4 production and total body pool size also were quite pronounced: PR4 = 21.4 ng/h and Qtot4 = 346 ng were reduced to about half that in euthyroids, but T4 elimination indices were virtually unchanged, and T3 production and elimination were minimally affected. Thus, overall, stabilizing negative feedback regulation of TH functioning at different hierarchical levels is quite bidirectionally sensitive. We found very tight (inhibitory) control over thyroidal T4 secretion, possibly also T3 secretion, and probably also absolute T3 production from T4, in response to moderate (+68%) supplements in T3 production; and the efficiency of total body T3 production from available T4 was amplified substantially in the severe primary hypothyroid state, although not nearly enough to compensate for the malady. Finally, the blood to total body pool fractions (Qb/Qtot) of both T3 and T4, but not the plasma or blood hormone levels, remained remarkably constant in response to these oppositely directed hormone production challenges, suggesting this ratio as an actively regulated, homeostatically-maintained entity.

Kinetic analysis of thyroid hormone secretion and interconversion in the 5-day-fasted rainbow trout, Oncorhynchus mykiss.

Estimating the 3,5,3'-triiodothyronine (T3) thyroidal secretion rate and the rate of extrathyroidal conversion of thyroxine (T4) to T3 are two difficult and important quantitative endocrine system problems in vertebrates. To address these questions in fish, two thyroid hormone tracer studies were modeled and analyzed, based on data from two groups of rainbow trout maintained at 12 degrees and fasted for 5 days. The data consisted of three time series: plasma concentrations of radioactive 125I-labeled T3 (T3*) following T3* injection, and both labeled T4 (T4*) and T3* following T4* injection. To facilitate model parameter estimation, plasma volumes were determined independently by injection of labeled bovine serum albumin. The T4* injectate was contaminated by an unknown amount of T3*, and this was considered an additional unknown. A six-compartment model was formulated in terms of 13 uniquely identifiable (quantifiable) parameters, which were estimated simultaneously from the three data sets using a sophisticated optimization algorithm built into a new model-fitting software package called FITMOD. The rates of interest, plus other kinetic indices, were estimated successfully using additional analysis. We found that the thyroid gland secreted 0.835 +/- 0.707 (mean +/- SD) pmol/hr of T3 and 2.44 +/- 2.09 pmol/hr of T4 per 100 g body weight (BW). Also, 8.19 to 11.2% of secreted T4 was monodeiodinated to T3, forming 0.200 to 0.274 pmol/hr of T3 per 100 g BW. This means that 75 to 81% of all T3 produced was secreted by the thyroid in these starved fish--a rather surprising result--while the remaining 19 to 25% resulted from T4 to T3 conversion.

Effects of 5, 5'-diphenylhydantoin on the thyroid status in rats.

Treatment of rats with phenytoin (DPH), an anti-epileptic drug, results in lower tissue thyroid hormone (TH) levels. To investigate if this is accompanied by tissue hypothyroidism, rats were treated for 3 weeks with DPH (50 mg/kg body wt in food). Thyroid hormone-dependent parameters were measured, and the results were compared to those of control rats and to those of athyreotic rats substituted with thyroxine + triiodothyronine (Tx + TH) to reach the same plasma TH levels as DPH-treated rats. These rats were mildly hypothyroid with regard to their TH and TSH levels and TH-dependent parameters. Both DPH and Tx + TH led to a decrease in plasma thyroxine (T4) and triiodothyronine (T3) (+/-70% of the control). The percentage free T4 was unchanged. Plasma thyrotropin (TSH) was increased only in the Tx + TH rats (sixfold). For DPH rats, pituitary hormone content was not different from the control; growth hormone was lower and TSH was higher in Tx + TH rats. In DPH and Tx + TH rats, an increase in hepatic T4 and T3 uridine-diphosphate glucuronyltransferase activity was found, likewise indicating a change in the metabolic pathway of TH. Hepatic iodothyronine deiodinase (ID) type I activity decreased in Tx + TH rats but did not alter in DPH rats. Hepatic alpha-glycerophosphate dehydrogenase (alpha-GPD) decreased in DPH and Tx + TH rats. Malic enzyme in liver was enhanced in DPH rats. In the brains of DPH rats the level of alpha-GPD activity was raised; in Tx + TH it was lowered. The ID type II activity in the brain was reduced in DPH rats, but ID type III did not change for either group. Total body oxygen consumption increased in DPH rats (13%); it decreased in Tx + TH rats (9%). Our results show that DPH causes changes comparable to mild hypothyroidism. The lack of or a diminished hypothyroid response can be explained as the attenuating agonistic effect of DPH, which is supported by O2 consumption, brain ID type II and alpha-GPD activities. The T4 content was reduced by 30% in thyroid digests; this, together with a reduced T4 secretion, can lead to serious hypothyroxinemia during prolonged DPH treatment.

An algorithm for identifiable parameters and parameter bounds for a class of cascaded mammillary models.

A complex structural identifiability problem for a class of unidirectionally interconnected n-compartment linear mammillary models with multiple inputs is discussed. This class is particularly useful in the study of drug/metabolite kinetics and other interconversion kinetic processes. An explicit algorithm is developed for this model class that provides identifiable parameter combinations, parameter bounds, steady-state pool sizes, and production rates, with input forcing and output measurements in central compartments. A six-compartment model of the combined dynamics of the prohormone thyroxine (T4) and hormone triiodothyronine (T3) illustrates how physiological parameter values or their smallest ranges, such as tissue T4 to T3 conversion rates and separate T4 and T3 production rates, can be determined from stimulus-response measurements in plasma alone.

MAMCAT: an expert system for distinguishing between mammillary and catenary compartmental models.

MAMCAT is a user-friendly, interactive, graphics-based PC program for addressing a common compartmental model discrimination problem in the framework of model indistinguishability theory: can mammillary and catenary models of the same order be distinguished from each other? The software is designed to teach the theory and solve specific problems. The user is guided step-by-step through definitions, examples, theory and problem solution. Topological properties are used first to screen out some distinguishable models and thereby reduce the number of candidates for indistinguishability. Transfer function analysis is then used to explore the remaining candidates. Analytical results for up to 5-compartment models are given and a more general algorithm is induced from these results.

Direct measurement of whole body thyroid hormone pool sizes and interconversion rates in fasted rats: hormone regulation implications.

Food deprivation markedly reduces thyroid hormone levels in mammalian plasma, but existing data are incomplete and equivocal regards extrathyroidal hormone production and other indices of overall hormone economy. We have used a novel experiment design and analysis to directly measure the whole-body rate of conversion of T4 into T3 and several other steady-state whole organism parameters, in 4-day fasted and fed control rats. Trace amounts of 125I-labeled T3 (T3) or T4 (T*4) were infused for 7 days from osmotic minipumps implanted sc. On day 7, rats were anesthetized, bled, and killed and carcasses were frozen in liquid N2, pulverized, homogenized, and extracted. Extracts and plasma samples were chromatographed on both Sephadex and HPLC. Tracer infusion rates, whole rat tissue weights, and steady state tissue, blood, and plasma T*3, T*4, and total radioactivity concentrations provided all kinetic parameters of interest from simple steady state computations. T4 secretion (SR4) and whole body pool sizes were reduced 49-55% in fasted rats. But the most notable results were that the percent of available extrathyroidal T4 converted to T3 in fasted [41.6 +/- 7.9% (SD)] was 87% greater than that in the fed (22.3 +/- 7.69%) rats and this, in turn, generated an absolute rate of production of T3 from T4 not significantly different in fasted vs. fed controls (7.17 +/- 2.40 vs. 7.54 +/- 3.10 ng/h.100 g BW). The surprisingly high 42% conversion ratio in fasting is explained in part by larger T3 blood pools (which are not sites of T3 production from T4) relative to tissue T3 pools in fasted rats, not accounted for in earlier whole-body studies. In contrast with this finding of an increased T4 to T3 conversion ratio in fasted rats, based on whole body measurements, T3 plasma concentrations (Cp3), clearance rates (PCR3), appearance rates (PAR3 = PCR3Cp3), and more conventional indirect estimates of the T4 to T3 conversion ratio (100 PAR3/SR4) were all substantially reduced, consistent with reports in fasting humans limited to measurements of T3 and T3 turnover in plasma and interpreted as indicative of reduced whole body T4 or T3 conversion. Directly measured total T3 extrathyroidal distribution volumes, reduced 55% in the fasted group from 241 +/- 19.5 to 109 +/- 8.14 ml/100 g BW, are also of interest because fed rat values are 27-61% greater than virtually all previous estimates of this index of total body T3.(ABSTRACT TRUNCATED AT 400 WORDS)

On plasma volume measurement and the effect of experimental stress in the themale tilapia, Oreochromis mossambicus, maintained in fresh water.

Plasma volumes in male tilapia (Oreochromis mossambicus) of different size were estimated following intracardial injection of radioiodinated human serum albumin ((125)I-HSA), coupled with short-term, early sampling transient response analysis of 1251-HSA disappearance from the plasma pool. This approach circumvents vascular marker leakage problems associated with constant steady state indicator dilution methods, minimizes some sampling and mixing problems, and simplifies analysis of the data. Changes in hematological parameters due to experimental stress were also studied, because the fish were not chronically cannulated. Results were used in a novel way to correct estimates of plasma volume upward by 15%, thereby providing a potentially useful alternative approach to vascular volume measurement in species where stress-eliminating or reducing techniques, e.g., cannulation, are impractical or infeasible. Hematrocrits increased 38% at the onset, from 24.9% to 34.4%, and remained essentially constant during the 60 minute kinetic study, and plasma osmolalities increased 7%. Corrected plasma volumes Vp (ml) were a linear function of body weight (BW). The group mean Vp was 2.93% of BW and corresponding blood volumes were 3.9% of BW.

Steady state organ distribution and metabolism of thyroxine and 3,5,3'-triiodothyronine in intestines, liver, kidneys, blood, and residual carcass of the rat in vivo.

We have directly measured the relative distribution and steady state pool sizes of T4 and T3 in blood and extrathyroidal tissues of the whole euthyroid male rat, plus several steady state T3 and T4 metabolism indices in the whole animal, in eight rats infused sc for 7 days with labeled 125I-T3 (T3*) or 125I-T4 (T4*) via implanted minipumps. Liver, kidneys, complete intestine, and residual carcass homogenate extracts and sera were chromatographed on both Sephadex and HPLC. Total tissue T3* and T4* in steady state were assessed and corrected for hormone trapped in residual blood in tissues using radioactive albumin as a vascular marker. Labeled Triac* and/or Tetrac* were prominent among steady state metabolites of T3* or T4* in intestines, feces, and residual carcass, but not in liver or kidneys. Extrathyroidal steady state T3 and T4 pools in residual carcass were 52.8 +/- 5.74 (SD) % of total T3 and 41.2 +/- 4.57% of total T4. The largest extrathyroidal organ pools containing exchangeable T3 and T4 were intestines, with 33.1 +/- 5.46% of total (extrathyroidal) T3 and 18.1 +/- 2.80% of total T4, whereas liver and kidney pools, previously reported as the largest measured, had only 8.0 +/- 1.45% and 2.30 +/- 0.26% of total T3 and 8.72 +/- 1.56% and 1.01 +/- 0.111% of total T4, respectively. Whole blood contained 3.6 +/- 0.385% of total T3 and 31.2 +/- 1.70% of total T4. Also, 21.3 +/- 8.2% of total body T4 was converted to T3 in the whole rat (residual carcass+intestines+liver+kidneys), the production rate of T4 was PR4 = 37.1 +/- 5.28 ng/h.100 g BW and the plasma appearance rate of T3 (PR3min) was 10.6 +/- 2.83 ng/h.100 g BW. T3 production from extrathyroidal T4 was CR3-4 = 6.63 +/- 0.94 ng/h and the minimum T3 secretion rate (SR3min) was 4.02 +/- 0.94 ng/h, each per 100 g BW, indicating that the thyroid secretes more than 38% of whole body T3 in euthyroid steady state. T4 and T3 plasma clearance rates (PCR) were 1.01 +/- 0.29 (T4) and 23.7 +/- 4.36 (T3) ml/h.100 g BW.(ABSTRACT TRUNCATED AT 400 WORDS)

5'- and 5-deiodinase activities in adult rat cecum and large bowel contents inhibited by intestinal microflora.

Enzymatic mechanisms for deiodination of 3,5,3'-triiodothyronine or thyroxine in the phenolic ring (5'-deiodinase) or tyrosyl ring (5-deiodinase) are found in cells of many organs, including the intestinal wall. Deiodinases are highly active in intestinal tissue of developing rat fetuses and relatively inactive in adult intestinal cells, but little is known about these systems in the luminal contents of intestines. We have found both 5- and 5'-deiodinase activities in adult rat intestinal contents and have shown that their expression is inhibited by resident intestinal microflora, which are normally present in the adult but not in the fetus, possibly because they are bound by intestinal bacteria in the adult.

Binding and degradation of 3,5,3'-triiodothyronine and thyroxine by rat intestinal bacteria.

Intestinal bacteria hydrolyze conjugates of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) secreted in bile, but it is not clear whether they have any other role in metabolism, storage, transport, or action of thyroid hormone in the intestines. We have examined aspects of T3 and T4 binding and degradation processes in fresh feces and cecum contents, obtained from normal control rats and from rats partially decontaminated by treatment with oral antibiotics for 2-3 wk. Samples were homogenized in phosphate buffer, fractionated, and subjected to various test conditions and incubated at 37 degrees C with 125I-labeled T3 (T3*) or T4 (T4*) for 2 or 24 h. Supernatants of high-speed centrifuged incubates were chromatographed to test for degradation products, and percentage binding was measured in the pellets. Substantial binding of T3* and T4* was found in all control rat feces and cecum content samples by 2 h, but binding was absent or significantly reduced in partially decontaminated rat samples. Bacterial binding of T3* and T4* were further shown to be competitive with graded doses of bovine serum albumin. Considerable degradation of T3* and T4* to labeled iodide (I*) only was also observed in feces and cecum content samples and was much greater in control rat than in corresponding partially decontaminated rat samples. Light had no effects in our system and heat reduced I* production. Propylthiouracil and sodium ipodate had little effect or equivocal effects, but dithiothreitol substantially inhibited I* production.(ABSTRACT TRUNCATED AT 250 WORDS)

Enterohepatic regulation and metabolism of 3,5,3'-triiodothyronine in hypothyroid rats.

Several steady state indices of thyroid hormone distribution, metabolism, excretion, and absorption were measured in intact hypothyroid and euthyroid rats, to explore the role of intestines and enterohepatic pathways in the dynamic regulation of whole-body thyroid hormone in these two states. Ten rats were studied, 5 normal control (N) and 5 rendered hypothyroid (3.48 vs. 19.8 ng/ml TSH) by surgical thyroidectomy 3.5 weeks earlier (HYPO). High specific activity 125I-labeled T3 (T3*) was infused at the same constant rate for 7 days from osmotic minipumps implanted sc. Daily urine and feces, and seventh-day cardiac and portal venous blood, bile, and whole intestinal contents were assessed. Bowel and feces were homogenized, extracted, and chromatographed, along with serum, bile, and urine samples. Bile, bowel, and fecal extract samples were also hydrolyzed with aryl-sulfatase and/or beta-glucuronidase and chromatographed to identify conjugates and determine total T3* in all fluid and tissue samples. In the N group, the bowel contained 21.2 +/- 1.22 (SD) times more T3* (mass) than plasma (199 ng vs. 9.39 ng), this ratio falling to 9.03 +/- 1.78 in the HYPO group (30.4 ng vs. 3.37 ng), a shift to relatively more T3* in blood. Urinary T3* was zero in both groups. But fecal excretion was 34 +/- 4.43% of total T3* infused (production) in N and only 20.3 +/- 3.05% in HYPO rats, closely paralleling reduced fecal bulk flow, and thus providing more time for T3* absorption. Endogenous T3 and T4 concentrations measured in portal plasma were 15-31% greater in normals and 69-95% greater in HYPO rats than in corresponding systemic plasma samples, a direct indication of absorption of endogenous T3 and T4 in both groups, with greater absorption in the HYPO group. About 66% total T3* was metabolically degraded in N rats, rising to approximately 80% in HYPO rats. Plasma clearance rates of T3 fell more than 50% in HYPO rats, and total T3 production fell to about 20% of normal. It appears that HYPO rats compensate for low T3 by fecally excreting a much smaller fraction of total T3 production, absorbing more T3 and T4, and leaving a larger fraction for T3 action and degradative metabolism.

Transfer kinetics of 3,5,3'-triiodothyronine and thyroxine from rat blood to large and small intestines, liver, and kidneys in vivo.

Enterohepatic circulation of thyroid hormone in the rat involves transfer of hormone to intestines in bile from the liver and absorption of some intestinal hormone, via portal blood to liver. Transfer of hormone to intestines from mesenteric arterial blood has generally been considered minimal, although this alternate pathway has received little attention. We have measured uptake kinetics of iv trace doses of radioactivity labeled T3 (T3*) and T4* from blood to intestines, longitudinally in 14 segments of total intestines in situ, with bowel contents included and bile duct ligated, and also to liver and kidneys, for comparison. Both T3* and T4* entered the entire length of intestines from blood, and into contents as well as tissue. Tissue uptake of T3* and T4* were fairly uniform longitudinally, but uptake into contents and thus into total intestines (tissue + contents) decreased linearly from pylorus to anus. Unidirectional uptake rate constants (hours-1) or clearance rates (milliliters per h) of T3* were 13 times greater than corresponding T4* fractional uptake rates to intestines. In contrast, T4 mass fluxes (congruent to 38 ng/h.100 g body wt) exceed T3 fluxes (congruent to 6.3 ng/h.100 g body wt) about 6-fold, because plasma contains far more T4 than T3. Comparing these results with published biliary and fecal T3 and T4 flux data, it appears that mesenteric arterial mass influxes to intestines are 3-5 times greater than both biliary secretion and fecal excretion of T3 and T4. Intestinal kinetics, in the absence of biliary influx, are characterized by a moderately rapid uptake followed by a very slow washout phase, which fit into neither the fast nor slow compartment paradigms of a 3-compartment mammillary model, in contrast to liver and kidney kinetics, which fit well into a single fast compartment. Intestinal dynamics are consistent with an organ in which storage and exchange of thyroid hormone dominate over metabolic and excretory processes.

Sites and patterns of absorption of 3,5,3'-triiodothyronine and thyroxine along rat small and large intestines.

It is clear that some thyroid hormone is absorbed from mammalian intestines, but numerous aspects of this process remain unresolved, including elucidation of the locations, extent and mechanisms of absorption, and the role of absorption in thyroid hormone economy. Our goal was to identify the sites and patterns and estimate rates of absorption of both tracer T4* and T3* comprehensively along the entire length of rat intestines, with normal contents included, to gain further insight into the role of this organ in whole-body thyroid hormone regulation. We measured absorption directly in situ in fed rats, using a variant of the classic intestinal loop technique used by others demonstrating absorption of T4 or of T3 from portions of rat intestines under various conditions. Rats were anesthetized, bile ducts were ligated, and absorption was measured from pylorus to anus, in 14 loops of intestines previously injected with T3* or with T4*, or with T3* and T4* injected in adjacent loops. Rats were maintained under otherwise approximately normal conditions, with intestines in situ and abdomen closed, until killing at 2 h. Excised loop radioactivity was measured and loops were homogenized, extracted, and chromatographed quantitatively to evaluate remaining and absorbed T3* and T4*. Absorption of both T3* and T4* were clearly present and were approximately uniform from all small and large intestinal sections, all containing normal intestinal contents, indicating that the entire organ is involved in whole-body thyroid hormone regulation. Furthermore, T3* and T4* were absorbed at approximately the same rate, adding to evidence reported by others for a simple diffusion absorption mechanism.

Decomposition-based qualitative experiment design algorithms for a class of compartmental models.

Qualitative experiment design, to determine experimental input/output configurations that provide identifiability for specific parameters of interest, can be extremely difficult if the number of unknown parameters and the number of compartments are relatively large. However, the problem can be considerably simplified if the parameters can be divided into several groups for separate identification and the model can be decomposed into smaller submodels for separate experiment design. Model decomposition-based experiment design algorithms are proposed for a practical class of large-scale compartmental models representative of biosystems characterized by multiple input sources and unidirectional interconnectivity among subsystems. The model parameters are divided into three types, each of which is identified consecutively, in three stages, using simpler submodel experiment designs. Several practical examples are presented. Necessary and sufficient conditions for identifiability using the algorithm are also discussed.