Effects of thyroid disease on glucose oxidative metabolism in man. A compartmental model analysis

Using mass measurements in tracer studies--a systematic approach to efficient modeling

Tracer enrichment data are fitted by multicompartmental models to estimate rate constants and fluxes or transport rates. In apolipoprotein turnover studies, mass measurements are also available, for example, apolipoprotein B levels in very low-density lipoprotein, intermediate-density lipoprotein, and low-density lipoprotein, and are often essential to calculate some of the rate constants. The usual method to use mass measurements is to estimate pool masses along with rate constants. A systematic alternative approach is developed to use flux balances around pools to express some rate constants in terms of the other rate constants and the measured masses. The resulting reduction in the number of parameters to be estimated makes the modeling more efficient. In models that would be unidentifiable without mass measurements, the usual approach and the proposed approach yield identical results. In a simple two-pool model, the number of unknown parameters is reduced from 4 to 2. In a published five-pool model for apolipoprotein B kinetics with three mass measurements, the number of parameters is reduced from 12 to 9. With m mass measurements, the number of responses to be fitted and the number of parameters to be estimated are each reduced by m, a simplification by 1/4 to 1/3 in a typical pool model. Besides a proportionate reduction in computational effort, there is a further benefit because the dimensionality of the problem is also decreased significantly, which means ease of convergence and a smaller likelihood of suboptimal solutions. Although our approach is conceptually straightforward, the dependencies get considerably more complex with increasing model size. To generate dependency definitions automatically, a Web-accessible program is available at http://biomath.info/poolfit/constraints.

Thyroid hormone action on intermediary metabolism. Part I: respiration, thermogenesis and carbohydrate metabolism

The effect of thyroid hormones on mitochondrial respiration are summarized: T3 directly stimulates mitochondrial respiration and the synthesis of adenosine 5'-triphosphate (ATP). Cytosolic ATP availability is increased by a thyroid hormone-induced increase in adenine nucleotide translocation across the mitochondrial membrane; the steady state ATP concentration and the cytosolic ATP/adenosine 5'-diphosphate (ADP) ratio is even decreased in hyperthyroid tissues because of the simultaneous stimulation of the synthesis and consumption of ATP. With regard to the thyroid hormone-induced energy wasting processes, heart work, intra- and interorgan futile cycling and Na+/K+-ATPase are involved to varying degrees. As a consequence of the thyroid hormone-induced hydrolysis of ATP, thermogenesis is increased in hyper- and decreased in hypothyroidism. Despite an increased rate of glucose utilization, clinical and experimental hyperthyroidism is often characterized by an abnormal oral glucose tolerance test. This finding is due to the thyroid hormone-induced increase in intestinal glucose absorption as well as the still enhanced endogenous glucose production in the liver. Hypothyroid patients show a reduced glucose tolerance test because of a decrease in intestinal glucose absorption and a sometimes reduced glucose turnover. The thyroid hormone-induced alterations in glucose metabolism are most probably not due to alterations in serum insulin levels and/or to a peripheral insulin resistance at the receptor level.

Diurnal hormone-metabolite profiles in hypothyroidism

To investigate the influence of thyroid hormones on intermediary metabolism in man, hormone and metabolite profiles were obtained over a 12-h period of normal meals and activity in eight hypothyroid subjects before and during thyroxine replacement therapy, and in sixteen matched controls. The fasting blood glucose concentration and the mean 12-h blood glucose concentration were normal in hypothyroid subjects but the blood glucose response to breakfast was exaggerated. Fasting blood lactate and pyruvate levels were normal but post-prandial hyperlactataemia and hyperpyruvicaemia were found and mean 12 h values for lactate (hypothyroid 1.80 +/- 0.06 v. control 0.77 +/- 0.03 mmol/l, P less than 0.01) and pyruvate (0.10 +/- 0.01 v. 0.08 +/- 0.003 mmol/l, P less than 0.01) were elevated. Blood alanine concentrations were elevated only in the evening. Although plasma non-esterified fatty acid levels were normal, fasting blood glycerol levels were decreased (0.06 +/- 0.01 v 0.08 +/- 0.01 mmol/l, P less than 0.001) and this decrease persisted throughout the 12-h period. Blood total ketone body concentrations did not differ from controls, but, as for plasma NEFA and blood glycerol, the normal preprandial rise in concentration was absent. Serum insulin, glucagon and growth hormone concentrations did not differ from control values at any time. Six months of thyroxine (T4) treatment produced a rise in blood glycerol concentration (mean 12 h value during T4 therapy, 0.06 +/- 0.01; before T4 therapy, 0.04 +/- 0.005 mmol/l; P less than 0.01) but not to control values (0.08 +/- 0.01 mmol/l). Concentrations of glucose and other gluconeogenic precursors were unaltered by therapy but the insulin response to meals and the mean 12 h serum insulin concentration were increased.

Metabolic aspects of the calorigenic effect of thyroid hormone in mammals

The increased BMR in hyperthyroidism may be accounted for by the use of chemical energy for metabolic processes and work. Major contributors are the heart work and futile cycling of FFA into triglyceride in adipose tissue, whereas gluconeogenesis in liver and the maintenance of sodium and potassium concentration gradients across the plasma membranes are unlikely to play any significant role. Information on the use of energy for protein turnover and urea production is lacking. The rate of oxygen uptake is not increased in the brain, spleen and testis. The main metabolic fuel seems to be free fatty acid. The mechanism which enables the hyperthyroid tissue to maintain normal concentrations of ATP, ADP and inorganic phosphate, (Pi) despite an increased turnover of energy-rich phosphates, is not fully elucidated. However, ultimately the increased rate of oxygen uptake in hyperthyroidism seems to rely upon an increased capacity for the transport of cytosolic ADP and Pi into mitochondria. The transport capacity is increased by an increased area of the mitochondrial membrane per g tissue and by a change in the kinetics of translocation of substrates for oxidative phosphorylation. Other transport processes across the mitochondrial membrane are also changed by hyperthyroidism, e.g. long chain fatty acid transport via carnitine acyl transferase, and oxaloacetate transport via substrate shuttles.

In vivo glucose turnover in hypo- and hyperthyroid starved rat

The effect of hypo- and hyperthyroidism on glucose turnover in vivo was determined in unanesthetized rats starved for 48 h. Glucose pool and decay rate of specific radioactivity of blood glucose was measured after bolus injection of a mixture of 3H-(2)- and 14C-(U)-glucose under steady state conditions. Compared with euthyroid controls (= 100%), hypothyroidism resulted in a decrease of blood glucose concentration (81%), glucose pool (52%), glucose disappearance rate (39%), and total glucose recycling (12%). In contrast, hyperthyroidism led to an increase of blood glucose concentration (148%), glucose pool (121%), glucose disappearance rate (185%), and total glucose recycling (163%). T 1/2 for glucose was calculated to be 46 min in the hypo-, 34 min in the eu-, and 22 min in the hyperthyroid state. The concentration of circulating glucoregulatory hormones, corticosterone and glucagon were elevated in hyperthyroid rats, while glucagon was diminished in hypothyroid animals. No difference in the level of insulin was found. These data demonstrate that glucose turnover in vivo is a function of the thyroid state being reduced in hypo- and considerably increased in hyperthyroidism.

Glucose and free fatty acid turnover in thyrotoxicosis and hypothyroidism, before and after treatment

Glucose and free fatty acid (FFA) turnover (RT) were measured by isotopic methods in groups of patients with thyrotoxicosis and hypothyroidism and compared with normal subjects. Patients with thyrotoxicosis were then studied after treatment with oral propranolol and again after treatment with carbimazole. Patients with hypothyroidism were studied again after treatment with thyroxine. Glucose RT was increased by about a third in thyrotoxicosis (12.7 mumol. min.-1 kg-1 +/- 1.0 SEM compared to 9.5 +/- 0.5 in normal subjects) while FFA RT was doubled (12.5 mumol. min.-1 kg-1 +/- 1.2 compared to 6.3 +/- 0.9 in normals). Propranolol was without effect, but carbimazole normalized these variables. In hypothyroidism both glucose and FFA RT were normal (9.1 +/- 0.7 and 6.7 +/- 0.9 respectively). Mean glucose and FFA RT both rose following thyroxine treatment, but not significantly; the post-treatment values remaining within the normal range (10.3 +/- 0.7 and 7.0 +/- 1.3 respectively). Oxidation of FFA, as estimated by 14CO2 excretion, was increased in thyrotoxicosis (42.8% +/- 2.9 SEM compared to 34.8 +/- 2.3 in normal subjects) and fell with carbimazole treatment (34.7 +/- 4.1). In hypothyroid subjects FFA oxidation increased following thyroxine treatment (from 34.4 +/- 4.1). In hypothyroid subjects FFA oxidation increased following thyroxine treatment (from 34.4 +/- 3.3 to 46.2 +/- 3.6).

Carbohydrate metabolism in hypothyroid myopathy

The carbohydrate metabolism in hypothyroid patients was investigated. After an overnight fast, the blood glucose level was 24% lower and the blood lactate level was 35% lower in the untreated hypothyroid patients than that observed in the treated hypothyroid patients or in the normal subjects. There was no difference in the blood alanine or plasma free fatty acid values between the subject groups. Skeletal muscle biopsied from two hypothyroid patients with marked myopathy showed normal glycogen content, 0.83%-0.86% (normal 1.06%), but reduced activity of acid maltase, 32-50 nmoles/min/g (normal 97). Forearm ischemic stimulation applied to hypothyroid patients failed to elevate the level of lactate. The results are compatible with impaired glycogenolysis from the skeletal muscle, which may be a contributory factor in the myopathy in hypothyroidism.

The use of isotope turnover techniques in the study of carbohydrate metabolism in man

It now appears that the bulk of methodological, analytical and interpretative problems associated with the use of isotope turnover techniques for the study of carbohydrate metabolism in man are resolved. As illustrated by a number of examples of the use of these techniques for the assessment of carbohydrate metabolism they seem, to the author, to have been more critically useful in the resolution of questions of (a) mechanism of hormone and drug action and (b) of interactions between metabolites, than they have been in defining pathological states, although the volume of information that is being accumulated is sure to prove useful for future research. Although it is this author's opinion that the employment of the radioactive isotopes at the low levels allowed by todays technology does not impose an unreasonable risk to the research subject, the promise of increased sensitivity for the detection of stable isotopes and the promise of their increased availability in a wide variety of compounds are factors that are sure to provide impetus for the wider use of these most valuable techniques in medical research.

Interrelations in the oxidative metabolism of free fatty acids, glucose, and glycerol in normal and hyperlipemic patients. A compartmental model

Palmitate, glucose, and glycerol oxidation to CO(2) have been investigated in the fasted state in ten normal subjects and nine patients (six hyperlipoproteinemias, one xanthomatosis, and two glycogenosis) after intravenous injection of [1-(14)C]palmitate, [1-(14)C]glucose, or [1-(14)C]glycerol in tracer amounts. The specific activities and concentrations of plasma palmitate, glycerol, or glucose and expired CO(2) were measured at various intervals after the injection for a period of 24 h. All the studies were analyzed in terms of a multicompartment model describing the structure for each of the subsystems, the transfer of carbon label between subsystems, and the oxidation to CO(2). A bicarbonate subsystem was also included in the model to account for its role in shaping the CO(2) curves. All the CO(2) activity following a palmitate injection could be accounted for by a direct oxidative pathway from plasm FFA with the addition of a 20-min delay compartment. The same also applied to glucose, except that the delay compartment had a mean time of about 150 min. Only about a third of the injected glycerol was directly oxidized to CO(2) from plasma; the delay time was about 4 min. Most of the remainder was converted to glucose. In normals about 45% of the FFA is oxidized to CO(2) directly. This constitutes about 30% of the total CO(2) output. In hyperlipemia the CO(2) output is nearly unchanged and the contribution from FFA is nearly the same. There is a considerable increase (factor of 2), however, in FFA mobilization, most of which is probably diverted to triglyceride synthesis. The glucose and glycerol subsystems are roughly the same in normals and hyperlipemics. About 50% of glucose is oxidized by the direct pathways which accounts for about 35% of the CO(2) output. Glycerol accounts for only 1.5% of the CO(2) produced. Major changes occurred in the glycerol and glucose subsystems in glycogenosis. The changes are consistent with the known deficiency in glucose-6-phosphatase in this disorder. There is a considerable reduction (factor of 2 or more) in the release of glucose to plasma (gluconeogenesis) and in the conversion of glycerol to glucose. Despite the integration of the kinetics of the glucose, glycerol, and FFA subsystems over a 24-h period, 36% of the CO(2) production was still unaccounted for in normals and 50% in hyperlipemics. Thus, some of the carbon must wind up in very slowly turning-over pools which supply CO(2) through subsystems not covered in these studies (triglycerides, glycogen, amino acids, etc.). All the modeling was carried out with the aid of the SAAM25 computer program.

Glucose turnover and disposal in maturity-onset diabetes

The glucose turnover rate in maturity-onset diabetes in man has been variously reported as increased, normal, and decreased. The present experiments suggest that these discrepancies may have been due to methodology, and to nonrecognition of a circadian cycle in the glucose turnover rate that is present in health, and marked in diabetes. During the early morning hours the glucose turnover rate in maturity-onset diabetes is increased in proportion to the fasting blood glucose level. It may reach three to four times the rate found in health. During the evening hours the increments are about one-half as great. The glucose outflow rate constant, k, lower in diabetes than in health, is also lower in both groups in the evening than in the morning. An analysis of the relative contributions of glucose overproduction and underutilization to the development of hyperglycemia in maturity-onset diabetes indicates that overproduction is the greater factor. The relative role of underutilization appears to increase as the fasting blood glucose level increases. The circulating glucose oxidation rate in maturity-onset diabetes is only slightly lower than in health, but the fraction oxidized is markedly lower, and only a small fraction is excreted. The principal conclusion is that in maturity-onset diabetes there is a hypertrophied flux of endogenous glucose, most of which is neither oxidized nor excreted. The precursors and the qualitative and quantitative metabolic fates of this excess glucose are unknown.