Effects of oral zinc loading on zinc metabolism in humans II: in vivo kinetics

Deletion of metal transporter Zip14 (Slc39a14) produces skeletal muscle wasting, endotoxemia, Mef2c activation and induction of miR-675 and Hspb7

Skeletal muscle represents the largest pool of body zinc, however, little is known about muscle zinc homeostasis or muscle-specific zinc functions. Zip14 (Slc39a14) was the most highly expressed zinc transporter in skeletal muscle of mice in response to LPS-induced inflammation. We compared metabolic parameters of skeletal muscle from global Zip14 knockout (KO) and wild-type mice (WT). At basal steady state Zip14 KO mice exhibited a phenotype that included muscle wasting and metabolic endotoxemia. Microarray and qPCR analysis of gastrocnemius muscle RNA revealed that ablation of Zip14 produced increased muscle p-Mef2c, Hspb7 and miR-675-5p expression and increased p38 activation. ChIP assays showed enhanced binding of NF-[Formula: see text] to the Mef2c promoter. In contrast, LPS-induced systemic inflammation enhanced Zip14-dependent zinc uptake by muscle, increased expression of Atrogin1 and MuRF1 and markedly reduced MyoD. These signatures of muscle atrophy and cachexia were not influenced by Zip14 ablation, however. LPS-induced miR-675-3p and -5p expression was Zip14-dependent. Collectively, these results with an integrative model are consistent with a Zip14 function in skeletal muscle at steady state that supports myogenesis through suppression of metabolic endotoxemia and that Zip14 ablation coincides with sustained activity of phosphorylated components of signaling pathways including p-Mef2c, which causes Hspb7-dependent muscle wasting.

Predictors of the Size of the Exchangeable Zinc Pool Differ between Children and Adults

BACKGROUND

The size of the rapidly exchanging pool of body zinc has been suggested as having potential utility as a biomarker of zinc status. Knowledge of the relations of exchangeable zinc pool (EZP) size to relevant variables is necessary to adequately evaluate its use as a biomarker.

OBJECTIVE

We used regression analysis to investigate associations of EZP with age, sex, body size, and zinc nutrition variables.

METHODS

Data were compiled from 18 isotope tracer studies of zinc absorption in 247 children and adults (248 observations) in which EZP and relevant variables (e.g., weight, age, absorbed zinc) were measured. Linear regression analyses were performed separately on data from adults and children.

RESULTS

In children, EZP was most strongly associated with weight (r = 0.78). The best-fitting regression models of EZP (R² ≥ 0.68) had weight or age and weight-for-age z score as predictors. Other variables had little effect on EZP when controlling for weight. Absorbed zinc was observed to be a predictor of EZP only in zinc intervention trials of infants. The mean EZP/wt was 4 mg/kg between 8 and 120 mo of age. In adults, EZP was observed to vary in a complex manner with (in order of importance) age, absorbed zinc, weight, sex, and plasma zinc concentration. EZP data from zinc-deprived subjects were lower than the 95% prediction interval of a model of normative data.

CONCLUSIONS

EZP was observed to maintain a constant size relative to weight and was influenced only slightly by other factors in children. In contrast, EZP in adults varied with several factors, including absorbed zinc, and was statistically smaller in zinc-deprived individuals. The findings suggest that EZP may have utility as an indicator of zinc status in adults, but there is less evidence for this in children. Additional data are needed to reach a definitive conclusion.

Zinc: physiology, deficiency, and parenteral nutrition

The essential trace element zinc (Zn) has a large number of physiologic roles, in particular being required for growth and functioning of the immune system. Adaptive mechanisms enable the body to maintain normal total body Zn status over a wide range of intakes, but deficiency can occur because of reduced absorption or increased gastrointestinal losses. Deficiency impairs physiologic processes, leading to clinical consequences that include failure to thrive, skin rash, and impaired wound healing. Mild deficiency that is not clinically overt may still cause nonspecific consequences, such as susceptibility to infection and poor growth. The plasma Zn concentration has poor sensitivity and specificity as a test of deficiency. Consequently, diagnosis of deficiency requires a combination of clinical assessment and biochemical tests. Patients receiving parenteral nutrition (PN) are susceptible to Zn deficiency and its consequences. Nutrition support teams should have a strategy for assessing Zn status and optimizing this by appropriate supplementation. Nutrition guidelines recommend generous Zn provision from the start of PN. This review covers the physiology of Zn, the consequences of its deficiency, and the assessment of its status, before discussing its role in PN.

A biokinetic model for zinc for use in radiation protection

The physiology of the essential trace element zinc has been studied extensively in human subjects using kinetic analysis of time-dependent measurements of administered zinc tracers. A number of biokinetic models describing zinc exchange between plasma and tissues and endogenous excretion of zinc have been derived as fits to data for specific study groups. More rudimentary biokinetic models for zinc have been developed to estimate radiation doses from internally deposited radioisotopes of zinc. The latter models are designed to provide broadly accurate estimates of cumulative decays of zinc radioisotopes in tissues and are not intended as realistic descriptions of the directions of movement of zinc in the body. This paper reviews biokinetic data for zinc and proposes a physiologically meaningful biokinetic model for systemic zinc for use in radiation protection. The proposed model bears some resemblance to zinc models developed in physiological studies but depicts a finer division of systemic zinc and is based on a broader spectrum of data than previous models. The proposed model and the model for zinc currently recommended by the International Commission on Radiological Protection yield reasonably similar estimates of total-body retention and effective dose for internally deposited radioisotopes of zinc but much different systemic distributions of activity and much different dose estimates for some individual tissues, particularly the liver.

Carbonic anhydrase I, II, and VI, blood plasma, erythrocyte and saliva zinc and copper increase after repetitive transcranial magnetic stimulation

INTRODUCTION

Repetitive transcranial magnetic stimulation (rTMS) has been used to treat symptoms from many disorders; biochemical changes occurred with this treatment. Preliminary studies with rTMS in patients with taste and smell dysfunction improved sensory function and increased salivary carbonic anhydrase (CA) VI and erythrocyte CA I, II. To obtain more information about these changes after rTMS, we measured changes in several CA enzymes, proteins, and trace metals in their blood plasma, erythrocytes, and saliva.

METHODS

Ninety-three patients with taste and smell dysfunction were studied before and after rTMS in an open clinical trial. Before and after rTMS, we measured erythrocyte CA I, II and salivary CA VI, zinc and copper in parotid saliva, blood plasma, and erythrocytes, and appearance of novel salivary proteins by using mass spectrometry.

RESULTS

After rTMS, CA I, II and CA VI activity and zinc and copper in saliva, plasma, and erythrocytes increased with significant sensory benefit. Novel salivary proteins were induced at an m/z value of 21.5K with a repetitive pattern at intervals of 5K m/z.

CONCLUSIONS

rTMS induced biochemical changes in specific enzymatic activities, trace metal concentrations, and induction of novel salivary proteins, with sensory improvement in patients with taste and smell dysfunction. Because patients with several neurologic disorders exhibit taste and smell dysfunction, including Parkinson disease, Alzheimer disease, and multiple sclerosis, and because rTMS improved their clinical symptoms, the biochemical changes we observed may be relevant not only in our patients with taste and smell dysfunction but also in patients with neurologic disorders with these sensory abnormalities.

Overview of health risk assessments for zinc

Zinc is an essential trace mineral nutrient required for growth and reproduction in man and other living organisms. Zinc deficiency has been identified as global public health issue that significantly impacts developing countries; as a result, zinc essentiality and the impacts of deficiency have been extensively studied. Zinc is also widely used in commercial products and is a high-production-volume industrial metal. This has provided opportunities for human exposure and prompted multiple assessments of adverse health impacts that might result from exposure excess. Zinc thus provides an illustrative case study of the methodological contrasts between nutritional and toxicological evaluations and highlights the need for risk assessment guidelines that may be tailored to accommodate the properties of essential trace elements.

Dietary phytase increases the true absorption and endogenous fecal excretion of zinc in growing pigs given a corn-soybean meal based diet

We investigated the effect of dietary phytase on the true absorption and endogenous fecal excretion of zinc (Zn) using (67)Zn in growing pigs given a corn-soybean meal based diet. Ten crossbred barrows were fed the control diet containing 90-mg/kg Zn, 2.3-g/kg phytate-phosphorus and 3.7-g/kg non-phytate-phosphorus or the phytase diet containing similar amounts of Zn and phytate-phosphorus, and 1.4-g/kg non-phytate-phosphorus with 750-PU/kg phytase for 12 h/day. On day 6, the pigs were given 200 g of the corresponding diet labeled by (67)Zn for 2 h. We measured feed intake, fecal Zn concentration and (67)Zn abundance for the determination of apparent absorption, true absorption and endogenous fecal excretion of Zn. Although the apparent absorption of Zn did not significantly differ between the dietary groups, the phytase group had significantly more (P < 0.05) true absorption of Zn than the control group. The endogenous fecal excretion of Zn tended to be more (P = 0.07) in the phytase group than in the control group. These results suggest that dietary phytase improves Zn bioavailability through increasing the true absorption of Zn in growing pigs, which results in stimulating the endogenous fecal excretion of Zn when dietary Zn satisfies its requirement.

Protective Effects of Zinc on Oxidative Stress Enzymes in Liver of Protein-Deficient Rats

Persons afflicted with protein malnutrition are generally deficient in a variety of essential micronutrients like zinc, copper, iron, and selenium, which in turn affects number of metabolic processes in the body. To evaluate the protective effects of zinc on the enzymes involved in oxidative stress induced in liver of protein-deficient rats, the current study was designed. Zinc sulfate at a dose level of 227 mg/L zinc in drinking water was administered to female Sprague-Dawley normal control as well as protein-deficient rats for a total duration of 8 weeks. The effects of zinc treatment in conditions of protein deficiency were studied on rat liver antioxidant enzymes, which included catalase, glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD), glutathione reduced (GSH), and glutathione-S-transferase (GST). Protein deficiency in normal rats resulted in a significant increase in hepatic activities of catalase, glutathione peroxidase, glutathione reductase, and glutathione-S-transferase and the levels of lipid peroxidation. A significant inhibition in the levels of reduced glutathione and the enzyme activity of superoxide dismutase has been observed after protein deficiency in normal rats. Interestingly, Zn treatment to protein-deficient animals lowered already raised activity catalase, glutathione peroxidase, and glutathione-S-transferase and levels of lipid peroxidation to significant levels when compared to protein-deficient animals. Also, Zn treatment to the protein-deficient animals resulted in a significant elevation in the levels of GSH and SOD activity as compared to their respective controls, thereby indicating its effectiveness in regulating their levels in adverse conditions. It has also been observed that concentrations of zinc, copper, iron, and selenium were found to be decreased significantly in protein-deficient animals. However, the levels of these elements came back to within normal limits when zinc was administrated to protein-deficient rats. This study concludes that zinc has the potential to regulate the activities of oxidative stress enzymes as well as essential hepatic elements.

Effects of oral zinc supplementation on serum leptin levels in Ache males of eastern Paraguay

Significant population variation has been demonstrated in serum leptin levels, independent of adiposity. Included within these populations, Ache males have significantly lower leptin levels compared to American subjects with comparable adiposity. The underlying causes of these differences are not understood but zinc has been shown to be an important regulator of leptin in humans and rodents. Zinc deficiency is associated with somatic wasting and significant declines in circulating leptin levels. Zinc deficiency is common in underdeveloped regions and may therefore contribute to low leptin levels among Ache men. To ascertain the potential role of zinc on Ache male leptin profiles, zinc supplementation was conducted to observe leptin responsiveness. Ache males were given daily oral zinc supplementation (50mg zinc gluconate) (n = 8) or placebo (n = 6) for 10 days. Serum leptin levels were measured on the first and last days of the treatment/placebo period. Despite supplementation exceeding triple the USDA recommended daily requirements, serum leptin levels did not change in the zinc-supplemented group (P = 0.61) and were not significantly different from males receiving placebo (P = 0.72). It is concluded that zinc is not likely to be a contributing factor in male Ache leptin profiles and that other sources of variation such as chronic energetic stresses affecting leptin production and perhaps receptor number or sensitivity should be considered.

Fractional zinc absorption using a single isotope tracer

BACKGROUND

Fractional absorption of zinc (Zn) has been measured using dual isotopes of Zn given simultaneously. An oral test dose and an intravenous (i.v.) reference dose are administered, followed by the measurement of the double isotopic enrichment (E) in urine 48 h after administration. We postulated that an estimate of the %E in urine for a given i.v. dose of Zn may be used to eliminate the need for venipuncture and the second Zn isotope.

OBJECTIVES

To determine a constant (k) for the Zn enrichment of urine after i.v. administration of a dose of labeled Zn in Zn-replete subjects. To use 'k' to calculate fractional absorption of Zn, and to compare these values to values obtained using the standard dual isotope method.

DESIGN

Single-arm cohort.

SETTING

The Hospital for Sick Children, Toronto, Canada.

SUBJECTS

Twenty-three healthy adults were recruited from the Metropolitan Toronto area. Seventeen subjects completed the study.

INTERVENTIONS

A 2.29 mg i.v. dose of (67)Zn followed immediately by a 2.50 mg oral dose of (70)Zn.

RESULTS

Population mean percentage enrichment (%E) of (67)Zn in urine was 1.43 (95% CI 1.26, 1.60). The ratio of the i.v. dose to mean %E in urine (k) was estimated to be 1.60 mg (95% CI 1.43, 1.82). There was no difference in the mean fractional absorption of Zn calculated using the single compared to the dual isotope method: 12.58% (95% CI 2.22, 22.94) vs 12.68% (95% CI 4.52, 20.85), respectively (P=0.89). The correlation coefficient between the two methods was 0.81 (P<0.0001).

CONCLUSIONS

The dual isotope method may be replaced by using a constant (k) and a single oral dose of isotopic-enriched Zn to estimate fractional absorption of Zn within a population.

SPONSORSHIP

Gerber Products Company, Fremont, MI.

Effect of acute zinc depletion on zinc homeostasis and plasma zinc kinetics in men

BACKGROUND

Zinc homeostasis and normal plasma zinc concentrations are maintained over a wide range of intakes.

OBJECTIVE

The objective was to identify the homeostatic response to severe zinc depletion by using compartmental analysis.

DESIGN

Stable zinc isotope tracers were administered intravenously to 5 men at baseline (12.2 mg dietary Zn/d) and after 5 wk of acute zinc depletion (0.23 mg/d). Compartmental modeling of zinc metabolism was performed by using tracer and mass data in plasma, urine, and feces collected over 6-14 d.

RESULTS

The plasma zinc concentration fell 65% on average after 5 wk of zinc depletion. The model predicted that fractional zinc absorption increased from 26% to essentially 100%. The rate constants for zinc excretion in the urine and gastrointestinal tract decreased 96% and 74%, respectively. The rate constants describing the distribution kinetics of plasma zinc did not change significantly. When zinc depletion was simulated by using an average mass model of zinc metabolism at baseline, the only change that accounted for the observed fall in plasma zinc concentration was a 60% reduction in the rate constant for zinc release from the most slowly turning over zinc pool. The large changes in zinc intake, excretion, and absorption-even when considered together-only explained modest reductions in plasma zinc mass.

CONCLUSION

The kinetic analysis with a compartmental model suggests that the profound decrease in plasma zinc concentrations after 5 wk of severe zinc depletion was mainly due to a decrease in the rate of zinc release from the most slowly turning over body zinc pool.

Development of a compartmental model of human zinc metabolism: identifiability and multiple studies analyses

A compartmental model of zinc metabolism has been developed from stable isotope tracer studies of five healthy adults. Multiple isotope tracers were administered orally and intravenously, and the resulting enrichment was measured in plasma, erythrocytes, urine, and feces for as long as 3 wk. Data from total zinc measurements and model-independent calculations of various steady-state parameters were also modeled with the kinetic data. A structure comprised of 14 compartments and as many as 25 unknown kinetic parameters was developed to adequately model the data from each of the individual studies. The structural identifiability of the model was established using the GLOBI2 identifiability analysis software. Numerical identifiability of parameter estimates was evaluated using statistical data provided by SAAM. A majority of the model parameters was estimated with sufficient statistical certainty to be considered well determined. After the fitting of the model and data from the individual studies using SAAM/CONSAM, results were submitted to SAAM extended multiple studies analysis for aggregation into a single set of population parameters and statistics. The model was judged to be valid based on criteria described elsewhere.

Kinetics of zinc metabolism: variation with diet, genetics and disease

Kinetic studies are used to investigate metabolic processes. By adding an isotope to a system and measuring its movement in the system over time, pool sizes and transport rates can be determined by mathematically modeling the data. This approach enables rate differences to be determined in conditions that have been modified by diet, environment, genetics or disease. Kinetic studies in humans have shown that there are multiple pools of zinc that turnover from minutes to years and that processes, including zinc absorption and excretion, are regulated to maintain tissue levels when zinc intake varies. Animal studies allow for greater understanding of kinetics because more tissues can be sampled and environmental and genetic factors can be controlled. Kinetic studies in animals will provide information on the overexpression or the deletion of genes coding for specific proteins involved in zinc transport and metabolism. The advances that have been made in our understanding of the role of zinc in metabolism have been aided by the development of techniques for measuring isotopes in biological materials. In the future, the kinetics of zinc bound to different compounds will be measured. Modeling will enable this information, at the molecular level, to be integrated with knowledge of zinc metabolism at the cellular, organ and whole body level. To understand more fully the role of zinc in human health, kinetic studies are needed in healthy and disease states to identify differences in metabolic processes. This knowledge can be used as a basis for dietary and therapeutic recommendations.

Comparison of estimates of zinc absorption in humans by using 4 stable isotopic tracer methods and compartmental analysis

BACKGROUND

Adjustment of gastrointestinal absorption is the primary means of maintaining zinc homeostasis; however, a precise, accurate method for measuring zinc absorption in humans has not been identified.

OBJECTIVE

The purpose of this study was to compare the estimates of the fraction of dietary zinc absorbed (FZA) by using 4 stable isotopic tracer methods: mass balance (MB) corrected for endogenous secretion, fecal monitoring (FM), deconvolution analysis (DA), and the double isotopic tracer ratio (DITR) method.

DESIGN

All 4 methods were applied to a single data set for each of 6 women. FZA was also determined for each subject by using a detailed compartmental model of zinc metabolism, and that value was used as the reference with which the simpler methods were compared.

RESULTS

The estimates of FZA (&xmacr; +/- SD) determined by DA (0.27 +/- 0. 08) and the DITR technique in plasma (0.30 +/- 0.10), 24-h urine samples (0.29 +/- 0.09), and spot urine samples (0.291 +/- 0.089) all compared well with the FZA reference value from the compartmental model (0.30 +/- 0.10). The MB and FM methods tended to overestimate FZA compared with the reference value.

CONCLUSIONS

The determination of FZA by MB or FM is laborious, is sensitive to subject compliance, and may result in an overestimate. DA, although relatively accurate, has the disadvantage of requiring multiple blood drawings over several days. In contrast, the DITR technique applied to a spot urine specimen obtained >/=3 d after tracer administration provides an accurate measure of FZA and is easy to implement; therefore, it is the recommended method for determination of FZA.

Decreased parotid saliva gustin/carbonic anhydrase VI secretion: an enzyme disorder manifested by gustatory and olfactory dysfunction

BACKGROUND

Taste and smell dysfunction has been reported to occur in patients with a variety of clinical problems. We wanted to investigate a specific group of patients in whom taste and smell dysfunction occurred putatively related to a specific biochemical abnormality in a salivary growth factor [gustin/carbonic anhydrase (CA) VI] considered responsible for maintenance of taste bud function.

METHODS

Eighteen patients developed loss and/or distortion of taste and smell after an acute influenza-type illness. They were evaluated clinically, by psychophysical tests of taste and smell function, by measurement of parotid salivary gustin/CAVI by a radioimmunoassay and by measurement of serum, urine, and salivary zinc. Biopsies of circumvallate papillae were obtained in 6 patients and examined by transmission electron microscopy. Similar studies were performed in 55 asymptomatic volunteers with biopsies of circumvallate papillae performed in 4.

RESULTS

Taste and smell acuity were impaired in patients compared with healthy volunteers and parotid gustin/CAVI, salivary, and serum zinc concentrations were lower in patients than in healthy volunteers. Taste buds in circumvallate papillae of patients exhibited severe vacuolization, cellular degeneration, and absence of dense extracellular material.

CONCLUSIONS

These results describe a clinical disorder formulated as a syndrome of hyposmia (decreased smell acuity), hypogeusia (decreased taste acuity), dysosmia (distorted smell function), dysgeusia (distorted taste function), and decreased secretion of parotid saliva gustin/CAVI with associated pathological changes in taste bud anatomy. Because gustin/CAVI is found in humans only in parotid saliva and has been associated with taste bud growth and development these results suggest that inhibition of synthesis of gustin/CAVI is associated with development of taste bud abnormalities and thereby loss of taste function.

A two-compartment model for zinc in humans

The oral absorption of zinc, from a test meal of minced beef, mashed potatoes and peas, have been measured in 19 healthy adults using the radiotracer 65Zn. The oral absorption, expressed as a percentage of the administered dose, was 20 +/- 5% (mean +/- 1 SD) in good agreement with previous results. In a subset of 9 subjects, tracer retention in whole body and whole blood was followed out to one year. The data were fitted to a simple two compartment model yielding total body zinc (TBZn), the zinc content in each of the 2 compartments and zinc turnover. The TBZn values ranged from 15.5 to 35.9 mmol while zinc turnover ranged from 0.043 to 0.073 mmol/d in keeping with results reported for significantly more complicated compartmental models applied to more comprehensive 65Zn tracer data sets. Additionally, TBZn correlated well with total body potassium, a measure of lean body mass, measured by whole body counting of the naturally-occurring potassium radioisotope, 40K. The zinc content of the more rapidly turning over compartment ranged from 3.2 to 5.6 mmol in reasonable agreement with exchangeable zinc pool estimations reported for short term studies using stable zinc isotopes. Therefore, the simple dataset and model employed in the present study yielded information on the short- and long-term behaviour of zinc compatible with both more complex radiotracer studies and analytically more demanding stable isotope studies.

Using models to explore whole-body metabolism and accessing models through a model library

A model is a mathematical representation of a system that can be used to explore the system in a number of ways: to determine the system's internal connections, to calculate properties of the system such as flow rates and pool sizes, and to make predictions about the system's behavior under different conditions. The use of modeling to explore whole-body metabolism is demonstrated using a compartmental model of zinc kinetics as an example. Because models are useful tools for exploring systems, a facility called a "model library" is being established on the Internet to provide access to working versions of published models.

Effect of age and dietary protein level on tissue mineral levels in female rats

Mineral (phosphorus, sulfur, potassium, calcium, magnesium, iron, zinc, copper, and manganese) concentrations were measured in plasma, and several tissues from female Wistar rats (young: 3-wk-old; mature: 6-mo-old) were fed on a dietary regimen designed to study the combined or singular effects of age and dietary protein on mineral status. Three diets, respectively, contained 5, 15, and 20% of bovine milk casein. Nephrocalcinosis chemically diagnosed by increased calcium and phosphorus in kidney was prevented in rats fed a 5% protein diet. Renal calcium and phosphorus were more accumulated in young rats than mature rats. A 5% protein diet decreased hemoglobin and blood iron. The hepatic and splenic iron was increased by a 5% protein diet in mature rats but was not altered in young rats. Mature rats had higher iron in brain, lung, heart, liver, spleen, kidney, muscle, and tibia than young rats. A 5% protein diet decreased zinc in plasma and liver. Zinc in tibia was increased with dietary protein level in young rats but was not changed in mature rats A 5% protein diet decreased copper concentration in plasma of young rats but not in mature rats. Mature rats had higher copper in plasma, blood, brain, lung, heart, liver, spleen, and kidney than young rats. With age, manganese concentration was increased in brain but decreased in lung, heart, liver, kidney, and muscle. These results suggest that the response to dietary protein regarding mineral status varies with age.

Drug-induced taste and smell disorders. Incidence, mechanisms and management related primarily to treatment of sensory receptor dysfunction

Drugs in every major pharmacological category can impair both taste and smell function and do so more commonly than presently appreciated. Impairment usually affects sensory function at a molecular level, causing 2 major behavioural changes--loss of acuity (i.e. hypogeusia and hyposmia) and/or distortion of function (i.e. dysgeusia and dysosmia). These changes can impair appetite, food intake, cause significant lifestyle changes and may require discontinuation of drug administration. Loss of acuity occurs primarily by drug inactivation of receptor function through inhibition of tastant/odorant receptor: (i) binding; (ii) Gs protein function; (iii) inositol trisphosphate function; (iv) channel (Ca++,Na++) activity; (v) other receptor inhibiting effects; or (vi) some combination of these effects. Distortions occur primarily by a drug inducing abnormal persistence of receptor activity (i.e. normal receptor inactivation does not occur) or through failure to activate: (i) various receptor kinases; (ii) Gi protein function; (iii) cytochrome P450 enzymes; or other effects which usually (iv) turn off receptor function; (v) inactivate tastant/odorant receptor binding; or (vi) some combination of these effects. Termination of drug therapy is commonly associated with termination of taste/smell dysfunction, but occasionally effects persist and require specific therapy to alleviate symptoms. Treatment primarily requires restoration of normal sensory receptor growth, development and/or function. Treatment which restores sensory acuity requires correction of steps initiating receptor and other pathology and includes zinc, theophylline, magnesium and fluoride. Treatment which inhibits sensory distortions requires reactivation of biochemical inhibition at the receptor or inactivation of inappropriate stimulus receptor binding and/or correction of other steps initiating pathology including dopaminergic antagonists, gamma-aminobutyric acid (GABA)-ergic agonists, calcium channel blockers and some orally active local anaesthetic, antiarrhythmic drugs.

Decreased NaCl sensitivity in zinc-deprived rats

NaCl thresholds and ability to discriminate between NaCl and sucrose were assessed in rats using an operant discrimination conditioning procedure before and during moderate and severe zinc deprivation and during zinc supplementation. NaCl thresholds were approximately 1 mM before dietary zinc manipulation. They increased in all zinc-deprived rats tested 10 and 17 days after initiation of deprivation but did not change in pair-fed controls maintained on supplemental zinc. Threshold changes were greater for those rats severely zinc deprived than for those only moderately deprived and were greater as the period of deprivation lengthened. Plasma zinc concentrations decreased significantly in deprived rats from values obtained at baseline, values in severely deprived rats being significantly lower than in those only moderately deprived. Although zinc-deprived rats discriminated NaCl from sucrose, they made more discrimination errors than controls. Following 24 days of zinc supplementation, previously deprived rats exhibited no significant improvement in gustatory performance, although their body weight increased and plasma zinc concentrations increased; but these later changes were not significant. These results demonstrate that zinc deprivation induces decreased gustatory sensitivity and confirm a role for zinc in taste.

Inhibition of gastric acid secretion reduces zinc absorption in man

Numerous factors seem to affect zinc absorption. Gastric acid secretion has been demonstrated to facilitate iron absorption. The zinc tolerance test (ZTT with ZnSO4 220 mg p.o.) was performed in 11 healthy volunteers to study the effects of administering the acid secretion inhibitor cimetidine (1 g/day p.o. for 3 days) and to evaluate the influence of HCl gastric secretion on zinc absorption in physiological conditions. Zinc absorption was reduced after cimetidine administration (p less than 0.005), suggesting that gastric pH influences zinc absorption. To rule out any direct effect of the drug on zinc absorption in five other healthy adults we further evaluated zinc absorption by using a different H2 antagonist (ranitidine 300 mg/day for 3 days and 300 mg before the test). Cimetidine was also tested in these subjects at half the dosage administered to the first group of subjects. Gastric acidity was monitored at 60-min intervals throughout the test via a nasogastric tube. The areas under the plasma concentration curves for zinc were significantly reduced after ranitidine (p less than 0.01), but not after cimetidine administration. Gastric acid was also reduced after ranitidine, but not after cimetidine (500 mg) administration, suggesting that gastric acid secretion plays a role in the regulation of zinc absorption in man.

Effect of zinc acexamate on gastric lesions induced by aspirin: a morphological study

The morphology of gastric lesions induced by aspirin in the rat and their modification by pretreatment with zinc acexamate (100 mg/kg) were studied by scanning electron microscopy. The influence of mucosal levels of prostaglandin E2 (PGE2) on the development of these lesions was also investigated. High (200 mg/kg) or low (50 mg/kg) doses of aspirin inhibited PGE2 production similarly, but the morphology of these lesions differed considerably. While gross exfoliation of extensive areas of gastric mucosa was observed after 200 mg/kg aspirin, only ultrastructural lesions of surface epithelial cells were present after 50 mg/kg aspirin. Regardless of the dose of aspirin administered, pretreatment with zinc acexamate raised PGE2 levels and increased the presence of mucus. Our results showed that after zinc acexamate, the development of deep erosions appearing with high doses of aspirin was prevented and the ultrastructural lesions induced by low doses of aspirin were not observed. The fact that zinc acexamate did not modify the anti-inflammatory action of aspirin in the carrageenin-induced oedema model suggests that the protective effect of zinc acexamate is exerted locally on the gastric mucosa.

Kinetics of zinc metabolism in the rat: effect of dibutyryl cAMP

N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (Bt2cAMP) administration to rats is shown to reduce serum zinc levels and increase liver zinc and metallothionein significantly. Such redistribution is similar to that seen in the acute phase response to stress. To determine the sites of regulation of zinc metabolism involved in zinc redistribution, we compared the kinetics of intravenously administered 65Zn between control and Bt2cAMP-treated rats. 65Zn disappeared more rapidly from the plasma of Bt2cAMP rats, whereas 65Zn content of liver and liver metallothionein was increased. 65Zn in spleen and bone marrow also increased. No change was seen in muscle or kidney, and 65Zn content decreased in skin, bone, small intestine, and feces. These kinetic data were integrated into two compartmental models describing zinc metabolism in control and Bt2cAMP-treated states. Models were constructed using the SAAM and CONSAM modeling programs. Comparison of models suggest several sites of regulation due to Bt2cAMP. Zinc uptake into liver metallothionein from both plasma and other zinc pools within the liver was increased, whereas the release of metallothionein-bound zinc to plasma decreased. These changes increased liver zinc at the expense of plasma zinc, tissue zinc (principally skin, bone, and small intestine), and fecal zinc excretion. Bone marrow, spleen, and muscle were protected against zinc loss due primarily to decreased rates of tissue zinc loss. It was also postulated that Bt2cAMP increased the transfer of liver zinc to secreted alpha 2-macroglobulin. These results demonstrate that Bt2cAMP-induced zinc redistribution is tissue specific and related in part to metallothionein synthesis.

Theoretical and practical aspects of zinc uptake and absorption

The purpose of this review is to describe our present understanding of the theoretical aspects of zinc uptake and absorption. An attempt has been made to include a selected amount of older literature to illustrate specific points. The zinc acquisition system appears to respond to the dietary zinc supply and the extent of digestive capacity. Up regulation of the uptake process with limited zinc intake seems to increase the pool of zinc that is subsequently exported from mucosal cells to the vascular compartment. Intracellular factors such as metallothionein influence the rate and/or extent of zinc acquisition and are regulated by dietary zinc availability and physiological factors.

Alterations in magnesium and zinc metabolism in thyroid disease

Magnesium (Mg) and zinc (Zn) status was assessed in subjects to evaluate the effects of thyroid diseases on Mg and Zn metabolism. Plasma and red blood cell (RBC) concentration and peripheral blood mononuclear cell (MNC) content of Mg and Zn, and 24-hour urinary excretion of Mg, Zn, creatinine (Cr), calcium (Ca), sodium (Na), and potassium (K) were measured in 11 thyrotoxic, 29 hypothyroid, and 25 euthyroid control subjects. Serum albumin, alpha 2-macroglobulin, and the binding of Zn to albumin were also determined. Plasma and RBC Mg concentrations were low in half of the hyperthyroid subjects, but mean values were not significantly different from controls. Urinary excretion and clearance of Mg were lower in hypothyroid subjects, but differences were removed when expressed relative to Cr excretion and clearance. Similar patterns were noted for urinary Ca, Na, and K, suggesting that their reduced excretion reflects alterations in renal hemodynamics. Plasma Zn was lower in hypothyroid subjects and correlated with serum albumin; MNC Zn and urinary Zn were also low. Plasma Zn concentration was normal and serum albumin significantly lower in the hyperthyroid group than in the control group. Further, RBC Zn content was significantly lower in hyperthyroid subjects, and inversely related to plasma thyroxine concentration. The hyperthyroid group also excreted significantly greater amounts of Zn than controls, indicative of a catabolic process. This increased urinary loss may reflect a shift in the distribution of plasma Zn between ultrafilterable and Zn-albumin complexes. In summary, this study provides evidence for marked alterations in Zn homeostasis in persons with thyroid disease. Whether the observations indicate deficiency states and have clinical implications will require further investigation.

Comparative absorption of zinc picolinate, zinc citrate and zinc gluconate in humans

The comparative absorption of zinc after oral administration of three different complexed forms was studied in 15 healthy human volunteers in a double-blind four-period crossover trial. The individuals were randomly divided into four groups. Each group rotated for four week periods through a random sequence of oral supplementation including: zinc picolinate, zinc citrate, and zinc gluconate (equivalent to 50 mg elemental zinc per day) and placebo. Zinc was measured in hair, urine, erythrocyte and serum before and after each period. At the end of four weeks hair, urine and erythrocyte zinc levels rose significantly (p less than 0.005, p less than 0.001, and p less than 0.001) during zinc picolinate administration. There was no significant change in any of these parameters from zinc gluconate, zinc citrate or placebo administration. There was a small, insignificant rise in serum zinc during zinc picolinate, zinc citrate and placebo supplementation. The results of this study suggest that zinc absorption in humans can be improved by complexing zinc with picolinic acid.

Topochemical demonstration of zinc in meningiomas

In agreement with the atomic absorption tests, histochemical demonstration of zinc in meningiomas using the dithizone method revealed a higher level of zinc in fibromatous meningiomas than in its meningiotheliomatous variant. Calcified psammoma bodies and vessels exhibited an intensely positive reaction on their surface. The metabolic function of zinc in the synthesis of fibre proteins, as well as in metallo-enzymes associated with mineralization, explains these findings.

Zinc metabolism in adrenal cortical insufficiency: effects of carbohydrate-active steroids

Detailed studies of zinc kinetics were performed in two patients with adrenal cortical insufficiency to investigate the effects of carbohydrate-active steroids (CAS) on zinc metabolism. Zinc- 69m was administered intravenously to each patient under two conditions: (1) treated with CAS replacement therapy and (2) untreated, ie, without hormone treatment for five to six days. Radioactivity was measured in blood plasma, red blood cells, urine, and stool and by means of external probes placed over liver and thigh. Data were analyzed using a previously developed multicompartmental model, which describes the early phase of zinc metabolism. The results of these studies suggest that CAS promotes the internalization of zinc into red blood cells and liver cells. These results are consistent with previous in vitro and in vivo studies in which CAS was shown to induce the synthesis of metallothionein in liver cells.

Zinc in taste function : A critical review

Zinc has been associated with taste function in humans at several levels of organization-the taste bud, the nerves transmitting taste information, and the brain. Zinc plays specific yet varied roles at each organizational level, although many of these roles have not been clearly identified. They include participation in the structural architecture of the cell, maintenance of cell membrane integrity, and control of activity of several cytoplasmic and membrane enzymes. Early investigators noted that some patients given drugs that altered zinc metabolism or who experienced disease processes associated with abnormalities of zinc metabolism exhibited taste dysfunction. Because of these findings zinc was given to a variety of patients as treatment for taste dysfunction. Initial treatment success was observed, but was quickly tempered by more extensive studies that yielded widely variable results leading to confusion about the role of zinc in both taste function and taste treatment. Further studies revealed that taste disorders were diverse and complex with multiple underlying pathophysiologies that were little understood. Subsequent work by several investigators revealed that patients with zinc deficiency, of any etiology, exhibited taste dysfunction and that treatment of these patients with zinc usually produced improvement of clinical symptoms. These results raised the question of how to define zinc deficiency, for zinc treatment in patients without zinc deficiency was unsuccessful and these patients represent more than three-quarters of all patients with taste dysfunction. New clinical techniques for the definition of human zinc deficiency have been achieved through the use of binding and displacement of(65)Zn on specific sites on erythrocyte membranes; these results offer a guide to the identification of patients (i.e., those with zinc deficiency) who may benefit from zinc treatment.