Ivan W. F. Davidson

Consideration of the Target Organ Toxicity of Trichloroethylene in Terms of Metabolite Toxicity and Pharmacokinetics

Trichloroethylene (TRI) is readily absorbed into the body through the lungs and gastrointestinal mucosa. Exposure to TRI can occur from contamination of air, water, and food; and this contamination may be sufficient to produce adverse effects in the exposed populations. Elimination of TRI involves two major processes: pulmonary excretion of unchanged TRI and relatively rapid hepatic biotransformation to urinary metabolites. The principal site of metabolism of TRI is the liver, but the lung and possibly other tissues also metabolize TRI, and dichlorovinyl-cysteine (DCVC) is formed in the kidney. Humans appear to metabolize TRI extensively. Both rats and mice also have a considerable capacity to metabolize TRI, and the maximal capacities of the rat versus the mouse appear to be more closely related to relative body surface areas than to body weights. Metabolism is almost linearly related to dose at lower doses, becoming dose dependent at higher doses, and is probably best described overall by Michaelis-Menten kinetics. Major end metabolites are trichloroethanol (TCE), trichloroethanol-glucuronide, and trichloroacetic acid (TCA). Metabolism also produces several possibly reactive intermediate metabolites, including chloral, TRI-epoxide, dichlorovinyl-cysteine (DCVC), dichloroacetyl chloride, dichloroacetic acid (DCA), and chloroform, which is further metabolized to phosgene that may covalently bind extensively to cellular lipids and proteins, and, to a much lesser degree, to DNA. The toxicities associated with TRI exposure are considered to reside in its reactive metabolites. The mutagenic and carcinogenic potential of TRI is also generally thought to be due to reactive intermediate biotransformation products rather than the parent molecule itself, although the biological mechanisms by which specific TRI metabolites exert their toxic activity observed in experimental animals and, in some cases, humans are not known. The binding intensity of TRI metabolites is greater in the liver than in the kidney. Comparative studies of biotransformation of TRI in rats and mice failed to detect any major species or strain differences in metabolism. Quantitative differences in metabolism across species probably result from differences in metabolic rate and enterohepatic recirculation of metabolites. Aging rats have less capacity for microsomal metabolism, as reflected by covalent binding of TRI, than either adult or young rats. This is likely to be the same in other species, including humans. The experimental evidence is consistent with the metabolic pathways for TRI being qualitatively similar in mice, rats, and humans. The formation of the major metabolites--TCE, TCE-glucuronide, and TCA--may be explained by the production of chloral as an intermediate after the initial oxidation of TRI to TRI-epoxide.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in carbohydrate metabolism of squirrel monkeys with chromium dietary supplementation.

Summary Adult squirrel monkeys maintained on a commercial chow demonstrated an impaired carbohydrate metabolism as evaluated by diagnostic tolbutamide and glucose tolerance tests. The possibility that the diet was deficient in trivalent chromium was investigated. The diet of normal and impaired monkeys was supplemented by the addition of trivalent and divalent chromium to the drinking water. The trivalent chromium supplementation produced an improvement in the tolbutamide and glucose tolerance responses of impaired monkeys when the drinking water was maintained at neutral pH but not at mildly acid pH. Normal monkeys supplemented with divalent chromium resulted in the appearance of impaired responses to tolbutamide and glucose tolerance tests.

Physiologic changes in plasma chromium of normal and pregnant women: Effect of a glucose load

Plasma chromium (Cr) concentrations were determined for healthy nonpregnant women and also for women in late pregnancy of similar ages. Plasma Cr was found to be significantly lower in the fasting fireanant subjects (2.97 ± 0.11 standard error of the mean [S.E.M]) than in normal subjects (4.70 ± 0.15 S.E.M.). An intravenous glucose load (25 Gm.) produced a prompt and sustained fall in plasma Cr in normal women but it failed to change the plasma Cr level of pregnant subjects. A similar fall in plasma chromium was also observed in nonpregnant subjects after an oral glucose load. The plasma Cr response was specific in that plasma copper and iron were unaltered by the glucose load. The mean rate of glucose assimilation after intravenous glucose loading was nearly identical for the pregnant and nonpregnant subjects. Plasma inorganic phosphate decreased in both pregnant and nonpregnant subjects but the fall was diminished in pregnancy. Fasting plasma immunoreactive insulin (IRI) was not significantly different for pregnant and nonpregnant subjects, but the plasma IRI response to a glucose load was greatly increased in pregnancy. The significance of the plasma Cr changes and the other experimental results are discussed in relation to the known altered carbohydrate metabolism of pregnancy.

Comparative vasodilator effects of nitroglycerin, pentaerythritol trinitrate and biometabolites, and other organic nitrates

Previous studies in man have shown pentaerythritol (PE) trinitrate, given either sublingually or orally, produces a prolonged hypotensive effect. The coronary vasodilator and systemic vasodepressor activities of PE trinitrate and its metabolites, PE dinitrate, PE mononitrate and PE, were evaluated in dogs to determine whether the metabolites were active and contributory. Coronary vasodilator activity was estimated with a flow transducer placed on the left anterior descending artery, and reduction of arterial pressure was determined directly via the femoral artery. Quantitative comparisons were made from dose-response curves established for nitroglycerin (ng), PE nitrates, and other common organic nitrates after intrajugular administration. Increase of coronary blood flow and reduction of arterial pressure were proportionally related, and the proportionality was the same for all drugs. Relative to NG, the potency of PE trinitrate was about 20 percent, erythrityl tetranitrate 12 percent, and isosorbide dinitrate 3.5 percent. The ratios of vasodilator activity of PE trinitrate and its metabolities were: PE trinitrate 100; PE dinitrate 1.5; PE mononitrate 0.5; and PE O. Tachyphylaxis was observed after close-order injections of NG or PE trinitrate. In addition, there was cross tolerance between NG and PE trinitrate and also between PE trinitrate and its less active metabolites.

Renal excretion of trace elements: chromium and copper.

Summary The ingestion by normal adults of a standard oral glucose load (75 g) resulted in a significant decrease of the fasting rate of urinary excretion of the trace metal chromium. This effect was not accompanied by any significant parallel change in GFR. Copper excretion rate was not significantly changed by the glucose load. In contrast with the effect of a glucose load, diuresis, induced by a water load, increased the excretion rate of both chromium and copper by more than 100%. The increases in metal excretion with diuresis were not attributable to increased GFR. Observations on the renal excretion of chromium and copper in the dog showed that tubular reabsorption of chromium and copper was greater than 98% in normal fasting conditions. In dogs receiving intravenous infusions of hypotonic saline and of vasopressin, a marked correlation was obtained between urine flow rate, the chromium and copper excretion rates, and also the fraction of the filtered loads of the metals excreted. However, tubular reabsorption of chromium and copper was greater than 97% even at maximal urine flow rates. These observations suggest that the kidney of both man and dog may possess an active tubular reabsorptive capacity for chromium and copper. We are grateful to Professor J. Maxwell Little for helpful discussions and suggestions, and we thank W. L. Secrest for expert technical assistance. These studies were supported in part by NIH Grant No. RR-5404.

The pharmacodynamics and biotransformation of pentaerythritol trinitrate in man

Pentaerythritol trinitrate (PE‐trinitrate), a metabolite in man of pentaerythritol tetranitrate (PETN), has been found to possess considerably greater vasodilator activity than its parent drug. Pharmacokinetics and metabolism of PE‐trinitrate were studied after single sublingual and oral doses of 14C‐PE‐trinitrate given in clinical dosage form. Drug absorption rate constants obtained from blood concentrations of drug radioactivity showed that absorption was rapid with both modes of administration. Metabolism and elimination after both dosage modes were nearly identical. The drug was not found in urine or in blood except within a few minutes of administration. Urinary elimination of metabolites accounted for 92 per cent of the dose, with less than 2 per cent recovered in fecal excretion as pentaerythritol (PE). The principal urinary metabolites were PE (30 per cent) and PE‐mononitrate (69 per cent). Excretion of these metabolites was first order with half‐times of elimination of 10.5 and 7.3 hours, respectively. From a comparison of the pharmacokinetics and biotransformation of PE‐trinitrate with those previously observed for PETN in similar experimental conditions, it is suggested that the rates of absorption and intracellular accumulation at tissue sites of action are primary determinants of the pharmacological activity and relative potencies of the organic nitrates.

Comparative effects of nitroprusside and nitroglycerin; actions on phasic and tonic components of arterial smooth muscle contraction

Abstract Nitroprusside shares many of the pharmacological actions of nitroglycerin. We investigated the characteristics of the relaxant action of these two vasodilators on vascular smooth muscle. Both drugs completely reduced, in a dose-related fashion, the maximal tension developed in rabbit aortic strips isometrically contracted with angiotensin-II or a submaximal epinephrine contraction (70% of maximal). The ED 50 values for the two drugs were similar, but their dose-response curves were not parallel. For aortic strips maximally contracted with epinephrine, only 70% relaxation was achieved with single maximal effective doses of nitroprusside or nitroglycerin, and the ED 50 was increased 25- and 630-fold respectively. Complete relaxation could not be induced by combined effects of single doses of the two drugs although cumulative exposure to nitroprusside, but not to nitroglycerin, caused complete relaxation. Pretreatment of aortic strips with nitroprusside or nitroglycerin decreased their reactivity to epinephrine (decrease of potency and intrinsic activity) and completely blocked angiotensin contractions. Contraction velocity analysis revealed that nitroprusside and nitroglycerin reduced the tonic contribution to total epinephrine contraction by 64 and 47% respectively; the phasic contribution was reduced by approximately 50% by either vasodilator. Angiotensin-induced contraction was reduced also by nitroprusside and nitroglycerin. It is postulated that the limitation of both phasic and tonic modalities of contraction by nitroprusside and nitroglycerin is due to a membranal action of the vasodilators to promote not only less uptake but also a loss of Ca 2+ which could explain the attenuation of phasic (angiotensin) contraction and in addition the decrease observed for the tonic component of epinephrine-induced contraction.

Contraction velocity analysis of norepinephrine and angiotensin-II activation of vascular smooth muscle.

The contractile response of vascular smooth muscle is known to consist of fast and slow components of contraction. We investigated the effect of norepinephrine and angiotensin-II on these functional properties of vascular smooth muscle in a quantitative fashion by analyzing the velocity of isometric tension development in rabbit aortic strips as a function of time. Basic premises for contraction velocity analysis are: (1) tension development is proportional to agonist concentration and relates to the amount of calcium fixed by the contractile proteins; (2) the rate of tension development (Qt) reflects the rate calcium is mobilized; (3) the rate at which calcium is made available relates to agonist concentration; (4) calcium is available from internal and external sites; (5) the calcium source)s) activated are characteristic of the mechanism of action of a particular agonist, and (6) the source of calcium activated is identifiable. Aortic strips were contracted maximally with norepinephrine and the velocity of contraction found to decrease with time according to the empirical expression Qt = phi 1e-phi 2t + theta 1e-the 2t. Experiments in calcium-free medium served to identify the tonic component of norepinephrine-induced contraction as the second term while angiotensin demonstrated only one term (phasic) which was independent of extracellular calcium. Strips contracted with different concentrations of norepinephrine or angiotensin showed dose-dependent actions on these functional properties of vascular smooth muscle as revealed by their effects on the contraction velocity parameters. An expression describing the contribution of each component to total tension developed was derived by integrating the above expression to give Qtot = phi 1 (1--e-phi 2tmax)/phi 2 + theta 1 (1--e-theta 2tmax)/theta 2. The contribution of each component to total tension development was donse-dependent and their sum gave the total tension observed experimentally with norepinephrine. For angiotensin, the first term of the preceeding expression gave the observed tension response. It is concluded that contraction velocity analysis affords a new approach for studying the effects of vasoactive agents on the functional properties of vascular smooth muscle.

[Carbohydrate metabolism in pregnancy: a possible role of chromium].

RiassuntoLa gravidanza umana normale è caratterizzata da rilevanti modificazioni dell’utilizzazione periferica del glucosio, nonché da alterazioni dell’omeostasi insulino-secretoria e dei meccanismi di risposta. Recenti osservazioni da noi compiute depongono per una relativa carenza di cromo plasmatico durante la gestazione ed il puerperio. Viene suggerita la possibilità che anomalie nella fisiologia del metallo possano essere in rapporto con le caratteristiche cosiddette « diabetogene » della gravidanza.RésuméLa grossesse normale humaine est caractérisée par des remarquables modifications de l’utilisation périphérique du glucose et, en plus, par une altération de l’homéostase concernant la sécrétion insulinique, et des mécanismes de réponse. Un certain nombre d’observations que nous avons effectué récemment déposent en faveur d’un manque relatif de chrome plasmatique pendant la gestation et la puerperalité. On suggère la possibilité qu’une anomalie dans la physiologie du métal puisse avoir un rapport avec les caractéristiques soi-disant « diabétogènes » de la grossesse.ResumenLa gestación humana se caracteriza por las notables modificaciones de la utilización periférica de la glucosa y por las alteraciones de la homeoestasis insulino secretoria y de los mecanismos de respuesta. Las investigaciones que recientemente hemos llevado a cabo abogan por una relativa falta de cromo plasmático durante la gestación y el puerperio. Se sugiere también la posibilidad de que las llamadas características « diabetógenas » de la gestación puedan estar en relación con anomalías de la fisiología del metal.ZusammenfassungDie normale menschliche Schwangerschaft ist durch erhebliche Veränderungen in der peripheren Glukoseverwertung gekennzeichnet, ebenso wie durch Veränderungen der Homöostase der Insulinfreisetzung und der Reaktionsmechanismen. Unsere neueren Beobachtungen sprechen für ein relatives Chromdefizit im Plasma während der Schwangerschaft und des Puerperiums. Es wird die Möglichkeit in Betracht gezogen, dass ein verändertes Verhalten der Chromphysiologie an den sogenannten « diabetogenen » Merkmalen der Schwangerschaft beteiligt ist.SummaryNormal human pregnancy is characterized by substantial changes in peripheral glucose utilization as well as homeostatic alterations in insulin release and response mechanisms. Recent observations we have made indicate a relative deficiency in plasma chromium in pregnancy and the puerperium. The possibility is suggested that changes in chromium physiology may relate to the so-called ‘diabetogenic’ features of pregnancy.

Metabolism of pentaerythritol trinitrate

The absorption, excretion, and biotransformation of 14C‐labeled pentaerythritol (PE) trinitrate was studied in man. The administration of a single sublingual dose was followed by rapid absorption and extensive biotransformation. Six drug metabolites were identified. Final excretion of the drug and its metabolites was almost totally through the kidney. Low levels of unchanged drug were present in plasma and urine. PE mononitrate was the major drug metabolite in plasma and urine. Glucuronides of PE trinitrate, dinitrate, and mononitrate were identified for the first time in man. PE trinitrate glucuronide appeared in plasma rapidly and about 8% of the dose was excreted in urine. Reversible and irreversible pathways are proposed for the formation of the metabolites. The reconversion of PE trinitrate glucuronide to PE trinitrate is postulated to explain the duration of drug activity and excretion.