Nystatin affects zinc uptake in human fibroblasts

The mechanism(s) by which zinc is transported into cells has not been identified. Since zinc uptake is inhibited by reducing the temperature, zinc uptake may depend on the movement of plasma membrane micoenvironments, such as endocytosis or potocytosis. We investigated the potential role of potocytosis in cellular zinc uptake by incubating normal and acrodermatitis enteropathica fibroblasts with nystatin, a sterol-binding drug previously shown to inhibit potocytosis. Zinc uptake was determined during initial rates of uptake (10 min) following incubation of the fibroblasts in 50 μg nystatin/mL or 0.1% dimethyl-sulfoxide for 10 min at 37°C. The cells were then incubated with 1 to 30 μM ⁶⁵zinc. Michaelis-Menten kinetics were observed for zinc uptake. Nystatin inhibited zinc uptake in both the normal and AE fibroblasts. Reduced cellular uptake of zinc was associated with its internalization, not its external binding. In normal fibroblasts, nystatin significantly reduced theK _m 56% and theV _max 69%. In the AE fibroblasts, nystatin treatment significantly reduced theV _max 59%, but did not significantly affect theK _m. The AE mutation alone affected theV _max for cellular zinc uptake. The control AE fibroblasts exhibited a 40% reduction inV _max compared to control normal fibroblasts. We conclude that nystatin exerts its effect on zinc uptake by reducing the velocity at which zinc traverses the cell membrane, possibly through potocytosis. Furthermore, the AE mutation also effects zinc transport by reducing zinc transport.     

Differences in the cellular zinc content and 5′-nucleotidase activity of normal and acrodermatitis enteropathica (AE) fibroblasts

The acrodermatitis enteropathica (AE) mutation affects zinc (Zn) metabolism in human fibroblasts. We hypothesize that the mutation affects the cell Zn content, which subsequently affects the activity of various zinc-dependent enzymes, such as 5′-nucleotidase. Therefore, normal and AE fibroblasts were grown in normal medium containing physiological levels of Zn (16 Μmol/L) for ∼24 h. The medium was replaced by normal medium (16 Μmol/L Zn), Zn-depleted medium (1.5 Μmol/L Zn), or Zn-supplemented medium (200 Μmol/L Zn) for another 24 h. Regardless of the Zn concentration of the growth medium, the AE fibroblasts contained significantly less Zn than normal fibroblasts grown in comparable medium. Nevertheless, growth of the fibroblasts in 200 Μmol/L Zn medium significantly increased the cell Zn content fourfold of both normal and AE fibroblasts. The activity of 5′-nucleotidase in the AE fibroblasts grown in 16 Μmol/L Zn or 1.5 Μmol/L Zn medium was also significantly lower than in normal fibroblasts. Changing the growth medium from 16 Μmol/L Zn to 1.5 Μmol/L Zn medium did not affect the activity of the enzyme in either genotype. Cells grown in 200 Μmol/L Zn medium exhibited threefold greater 5′-nucleotidase activity in AE fibroblasts, but had no affect on enzyme activity in normal cells. In summary, altering the cell Zn content of normal fibroblasts did not result in a significant change in their 5′ -nucleotidase activity. However, AE fibroblasts grown in 200 Μmol/L Zn medium exhibited recovery of their 5′-nucleotidase activity to normal levels. These results support the hypothesis that the AE mutation affects the cellular Zn content. The lower cell Zn content subsequently affects the activity of 5′-nucleotidase.     

Effects of cadmium and copper on zinc transport kinetics by isolated renal proximal cells

Zinc, cadmium, and copper are known to interact in many transport processes, but the mechanism of inhibition is widely debated, being either competitive or noncompetitive according to the experimental model employed. We investigated the mechanisms of inhibition of zinc transport by cadmium and copper using renal proximal cells isolated from rabbit kidney. Initial rates of⁶⁵Zn uptake were assessed after 0.5 min of incubation. The kinetics parameters of zinc uptake obtained at 20°C were a J_max of 208.0±8.4 pmol· min⁻¹·(mg protein)⁻¹, aK _m of 15.0±1.5 μM and an unsaturable constant of 0.259±0.104 (n=8). Cadmium at 15 μM competitively inhibited zinc uptake. In the presence of 50 μM cadmium, or copper at both 15 and 50 μM, there was evidence of noncompetitive inhibition. These data suggest that zinc and cadmium enter renal proximal cells via a common, saturable, carrier-mediated process. The mechanisms of the noncompetitive inhibition observed at higher concentrations of cadmium or with copper require further investigation, but may involve a toxic effect on the cytoskeleton.     

Effects of zinc on the mineralization of bone nodules from human osteoblast-like cells

Zinc is an important mineral that is required for normal bone development. However, the direct effects of zinc on the mineralization of bone cells of human origin are not clear. The objective of this study was to determine the effects of zinc on the differentiation of SaOS-2 human osteoblast-like cells and the formation of mineralized bone nodules. Cells were cultured for 8 d and then transferred to zinc-free medium and treated with varying concentrations (0–50 μM) of zinc. Alkaline phosphatase (ALP) activity was used as a measure of osteoblast differentiation, and bone nodules were detected by von Kossa staining. After 4, 6, and 8 d of treatment, zinc increased ALP activity at 1 and 10 μM, but decreased activity at 50 μM. After 9 d of treatment, zinc increased both the number and area of mineralized bone nodules at low concentrations (1 and 10 μM), but decreased both at higher concentrations (25 and 50 μM). These findings demonstrate that zinc has biphasic effects on the differentiation and mineralization of human osteoblast-like cells.     

Zinc transport by fibroblasts from patients with acrodermatitis enteropathica

Acrodermatitis enteropathica (AE) is a zinc deficiency disease. To date, the only defect has been demonstrated in the gut. We have investigated zinc uptake in fibroblasts established from four unrelated patients with AE using normal skin fibroblasts as controls. Zinc content of AE and control cells was similar (0.3 fmol/cell). Zinc accumulation over 24 h from a complete culture medium was similar in both normal controls and mutant cells. The fraction of zinc removed by Pronase treatment remained constant at 50 pmol/micrograms DNA, whereas the zinc remaining after Pronase treatment accumulated rapidly for 8 h, then more slowly. Analysis of binding data showed no significant difference between AE and control cells, with apparent Ka values of 4-6 X 10(6) M-1 and between 1 and 2 X 10(8) receptors/cell. Analysis of Pronase resistant data showed no difference between the control and the mutant cells with apparent Km values of 0.2-0.3 microM and Vmax values of 17-19 pmol/micrograms DNA/h. No difference in zinc efflux rates was detected. We conclude that the defect that underlies acrodermatitis enteropathica is either not expressed in fibroblasts or cannot be detected under these experimental conditions.     

Effect of different zinc sources and levels on inhibition of the apoptosis induced by glucocorticoid of thymocytes in vitro

This experiment was conducted to investigate the effects of zinc sulfate and zinc methionine (Zn-Met) and their levels on apoptosis induced by glucocorticoid of thymocytes and the possible mechanism. Dexamethasone was used to make the apoptosis model of thymocytes; zinc sulfate and zinc methionine were supplemented to the medium at levels of 0,50, 100, 500, and 1000 μM. The activity of cells,Cu,Zn superoxide dismutase (Cu,Zn-SOD), DNA ladder pattern, intracellular calcium concentration, and the percentage of apoptosis nuclei were determined. Both ZnSO₄ and Zn-Met could modulate apoptosis; they inhibited apoptosis and decreased DNA fragmentation. The regulation was concentration dependent. At levels of 50 and 100 μM, the effect of Zn-Met on inhibiting apoptosis was less efficient than that of ZnSO₄ (p<0.05), but the activity of the cells cultured with Zn-Met was higher than those cultured with ZnSO₄; they showed no difference in modulating apoptosis when added at levels of 500 and 1000 μM to the medium (p>0.05). Intracellular calcium concentrations of cells cultured with Zn-Met were higher than those cultured with ZnSO₄ at the same levels. Zinc supplementation decreased the concentration of intracellular calcium significantly (p<0.05) and increased the activity of Cu,Zn-SOD in the extract of the cells (p<0.05). Both zinc sulfate and Zn-Met could modulate apoptosis of thymocytes induced by glucocorticoid; the mechanism might involve the exchange of intracellular calcium, the redox of cells, and the two forms of zinc might go different ways in the regulations.     

Kinetic characterization of Zinc transport process and its inhibition by Cadmium in isolated rat renal basolateral membrane vesicles: In vitro and In vivo studies

We firstly characterized zinc uptake phenomenon across basolateral membrane vesicles (BLMVs) isolated from normal rat kidney. The process was found to be time, temperature, and substrate concentration dependent, and displayed saturability. Zn²⁺ uptake was competitively inhibited in the presence of 2 mM Cd with Ki of 3.9 mM. Zinc uptake was also inhibited in the presence of sulfhydryl reacting compound suggesting involvement of {–}SH groups in the transport process. Further, to elucidate the effect of in vivo Cd on zinc transport in BLMVs, Cd nephrotoxicity was induced by subcutaneous administration of CdCl₂ at dose of 0.6 mg/kg/d for 5 days in a week for 12 weeks. An indolent renal failure developed in Cd exposed rats was accompanied with a significantly high urinary excretion of Cd²⁺, Zn²⁺ and proteins. The histopathology and electron microscopy of kidneys of Cd exposed rats documented changes of proximal tubular degeneration. Notably, Cd content in renal cortex of Cd exposed rats was 215 μg/g tissue that was higher than the critical concentration of Cd in kidneys which was associated with significantly higher Zn and metallothionein (MT) contents. Zinc uptake in BLMVs isolated from kidneys of Cd exposed rats was significantly reduced. Further, kinetic studies revealed that decrease in zinc uptake synchronized with decrease in maximal velocity (V_max) and increase in affinity constant which is suggestive of decreased number of active zinc transporters. Furthermore, conformational modulation of Zn transporter in BLM was further supported by observed variation in transition temperature for zinc transport in BLMVs isolated from Cd-exposed kidney.     

Zinc deficiency does not enhance LDL uptake by P 388 D1 macrophages in vitro

The aim of the study was to investigate the effect of zinc depletion on the susceptibility of Wistar rat low-density lipoproteins (LDL) to peroxidation and their uptake by macrophages, before and after in vitro oxidation. The rats were fed for 7 wk a Zn-adequate diet (100 ppm) ad libitum (AL), a Zn-deficient diet (0.2 ppm) ad libitum (ZD), or a Zn-adequate diet according to the pair-feeding method (PF). Zinc status was determined and, for each group, blood was pooled, and LDL were isolated and labeled with¹²⁵Iodine. An aliquot of each LDL sample was oxidized using Fe^II 10 μM/ascorbate 250 μM. Oxidized and nonoxidized (native) LDL were incubated with P 388 D1 macrophages, and their rates of uptake and degradation by macrophages were measured. Before oxidation, LDL uptake and degradation were not modified by the diet, suggesting that Zn deficiency did not modify rat LDL in vivo. After oxidation, both LDL uptake and degradation were significantly enhanced in the three groups. Nevertheless, we did not observe a significant effect of Zn deficiency. This observation suggests that, in our experimental conditions, Zn deficiency did not modify LDL catabolism.     

Change in apparent and true absorption and retention of dietary zinc with age in rats

Two groups of 16 rats each were fed the same diet with 12.9 ppm Zn. Nine days after each animal was injected with⁶⁵Zn for assessing fecal zinc of endogenous origin, zinc intake and excretion were determined for a six-day period at the age of about five (group I) and nine (II) weeks. At mean growth rates of 5.1 and 5.2 g/day, food consumption per gram of gain was 2.01 g in group I vs 2.86 g in II. Overall, zinc retention amounted to 21 vs 25 μg Zn/g of gain. Apparent absorption averaged 92 vs 74% of Zn intake (132 vs 189 μg/day), while true absorption averaged 98 vs 92%. It was concluded that endogenous fecal zinc excretion was limited to the indispensable loss (F _em) in group I (7 μg/day), while it exceeded this minimum loss in group II (33 μg/day). True retention, which reflected total zinc utilization (true absorption times metabolic efficiency), was derived from apparent absorption plusF _em (11 μg/day for group II according to the greater metabolic body size of the rats). It averaged 98% of Zn intake in group I vs 80% in group II. The mean metabolic efficiency was 100% vs 87%. The conclusion was that these marked differences between age groups in utilizing the dietary zinc reflected the efficient homeostatic adjustments in absorption and endogenous excretion of zinc to the respective zinc supply status.     

Serum extracellular superoxide dismutase activity as an indicator of zinc status in humans

The present study focused on whether serum extracellular superoxide dimutase (EC-SOD) activity can be used as a functional indicator of marginal zinc deficiency in humans. Subjects in this study were 444 healthy adults over 30 yr of age living a normal rural life in Kyunggi province, Korea. The mean dietary zinc intake of subjects obtained from one 24-h recall was 6.41 ± 4.35 mg and the average serum zinc concentration of the subjects was 11.06 ± 2.44 (μmol/L. Subjects were divided into three groups by serum zinc concentrations: adequate (serum zinc >10.7 (μmol/L), low (serum zinc 9.0–10.7 μmol/L), and very low (serum zinc <9.0 μmol/L) groups. A total of 50 subjects were selected from the three groups for analysis of EC-SOD activities. The EC-SOD activity of subjects increased with increasing serum zinc concentrations, and the activities of the three groups were significantly different as indicated by the Kruskal-Wallis test (p = 0.0239). Also, serum EC-SOD activities were significantly correlated with serum zinc concentrations (r = 0.289,p = 0.04). Serum EC-SOD activities, however, were not significantly correlated to the dietary zinc intakes. In conclusion, these results show that EC-SOD activities are decreased in subjects with low serum zinc concentrations and suggest that EC-SOD activity may be a functional indicator of zinc nutritional status in humans.     

Dynamics of zinc uptake and accumulation in the hyperaccumulating and non-hyperaccumulating ecotypes of Sedum alfredii Hance

Sedum alfredii Hance has been identified as a Zn-hyperaccumulating plant species native to China. The characteristics of Zn uptake and accumulation in the hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) of S. alfredii were investigated under nutrient solution and soil culture conditions. The growth of HE was normal up to 1000 μM Zn in nutrient solution, and 1600 mg Zn kg⁻¹ soil in a Zn-amended soil. Growth of the NHE was inhibited at Zn levels ≥250 μM in nutrient solution. Zinc concentrations in the leaves and stems increased with increasing Zn supply levels, peaking at 500 and 250 μM Zn in nutrient solution for the HE and the NHE, respectively, and then gradually decreased or leveled off with further increase in solution Zn. Minimal increases in root Zn were noted at Zn levels up to 50 μM; root Zn sharply increased at higher Zn supply. The maximum Zn concentration in the shoots of the HE reached 20,000 and 29,000 mg kg⁻¹ in the nutrient solution and soil experiments, respectively, approximately 20 times greater than those of the NHE. Root Zn concentrations were higher in the NHE than in the HE when plants were grown at Zn levels ≥50 μM. The time-course of Zn uptake and accumulation exhibited a hyperbolic saturation curve: a rapid linear increase during the first 6 days in the long-term and 60 min in the short-term studies; followed by a slower increase or leveling off with time. More than 80% of Zn accumulated in the shoots of the HE at half time (day 16) of the long-term uptake in 500 μM Zn, and also at half time (120 min) of the short-term uptake in 10 μM ⁶⁵Zn²⁺. These results indicate that Zn uptake and accumulation in the shoots of S. alfredii exhibited a down-regulation by internal Zn accumulated in roots or leaves under both nutrient solution and soil conditions. An altered Zn transport system and increased metal sequestration capacity in the shoot tissues, especially in the stems, may be the factors that allow increased Zn accumulation in the hyperaccumulating ecotype of S. alfredii. Section Editor: F. J. Zhao     

Shoot-bud regeneration in subcultured callus of engelmann spruce

Summary Callus cultures ofPicea engelmannii (Parry, Engelmann spruce) were initiated and established from mature embryos cultured on von Arnold and Eriksson’s medium (AE) supplemented with N⁶-benzyladenine (10μM) and naphthalene acetic acid (10 μM). Cultures were maintained by subculture at 3-to-4-wk intervals. After three subcultures, callus was transferred to AE medium with only N⁶-benzyladenine (25 μM). Adventitious buds appeared on the surface of the callus after 2-to 4-wk and grew to adventitious shoots on AE medium without growth hormones or on AE medium with kinetin (0.1 μM). Shoot-forming capacity was maintained through 7 further subcultures. This study was supported by the Natural Sciences and Engineering Research Council of Canada grant G1438 to T. A. Thorpe and D. I. Dunstan.     

The reproducibility of the plasma response to a physiological dose of zinc in healthy subjects

Zinc absorption may be estimated by measuring the area under the plasma zinc curve following the ingestion of a zinc supplement. The aim of this study is to determine the reproducibility of such a response when a small dose of zinc is administered to healthy volunteers. Five female subjects were asked to consume 4.5 mg elemental zinc, and blood samples were obtained at 30 min intervals for 5 h. The experiment was repeated in the same volunteers 12–16 d later. The area under the plasma zinc curve was 30% lower after the second zinc tolerance test compared with the first (11.0 vs 15.8 μmol/1 h). This difference could not be explained by differences in the fasting plasma zinc levels (12.9 μmol/L Experiment one, 15.1 μmol/L Experiment 2) nor was it related to technical or clinical parameters. The area under the curve after 5 h was strongly correlated with the response after 4 h. Hence we conclude that a small dose of zinc can be used to determine zinc absorption and a shorter experimental period may be used. However, trials must be designed to take into account the sequence variability in the response.     

The effect of toxic doses of cholecalciferol (Vitamin D3) on the serum zinc levels in rats

Zinc is a trace element important to bone mineralization as well as, in general, nutrition. It is known that cholecalciferol (vitamin D₃) affects bone metabolism. In this study toxic doses of vitamin D₃ were injected subcutaneously (25 μg/d) to rats for 5 wk. It caused a significant increase in serum zinc levels (p<0.02). On the other hand, no significant increase was detected in the other groups. Excessive amounts of vitamin D₃ caused bone breakdown and increased the levels of zinc in blood.     

Effects of zinc ex vivo and intracellular zinc chelator in vivo on taurine uptake in goldfish retina

Taurine and zinc exert neurotrophic effects. Zinc modulates Na⁺/Cl⁻-dependent transporters. This study examined the effect of zinc (ZnSO₄) ex vivo and zinc chelator N,N,N′,N′-tetrakis-(2-pyridylmethyl) ethylenediamine (TPEN) in vivo on [³H]taurine transport in goldfish retina. The effect of TPEN in vivo on taurine and zinc levels was determined. Isolated cells were incubated in Ringer with zinc (0.1–100 μM). Taurine transport was done with taurine (0.001–1 mM) and 50 nM [³H]taurine. Zinc (100 μM) noncompetitively inhibited taurine transport. TPEN was administered intraocularly and retinas extracted 3, 5 and 10 days later. Taurine was determined by HPLC (nmol/mg protein) and zinc by spectrophotometry ICP (mg/mg protein). Taurine and zinc levels decreased at 3 days and increased at 10 days after TPEN administration. At 10 days after intraocular TPEN, taurine transport affinity increased (K _s = 0.018 ± 0.006 vs. 0.028 ± 0.008 mM). Apparently, zinc deficiency affects the taurine–zinc complex and taurine availability. The increased taurine uptake affinity by TPEN was possibly associated with a response to maximize retinal taurine content at low zinc concentration.     

Electrocardiographic alterations induced by zinc ions on isolated guinea pig heart preparations

On guinea pig isolated hearts perfused according to the Langendorff technique, by Chenoweth's solution, and by various concentration of ZnCl₂, surface E.C.G. was recorded for 65 min, after the equilibration period. Concentrations of zinc corresponding to plasma free and exchangable fractions (1.5 μM) exerted only a nonstatistically significant sinus bradycardia. Bradycardia was prominent between the 25th and 35th min of perfusion, with higher concentrations of zinc (7.5 and 15 μM). In even higher (toxic) concentrations of zinc (30 μM), various types of arrhythmias were recorded, such as first degree AV block, idioventricular rhythms, atrial, and ventricular extrasystoles, and asystolia, evoked since the 15th min of perfusion. The above-mentioned findings in relation to low plasma zinc concentrations found in coronary patients and experimental myocardial infarction as well as myocardial injury related to metal fume fever, brought our attention to the involvement of this divalent cation to cardiac physiology and pathophysiology.     

Effects of trichophytosis on serum zinc levels in calves

The purpose of this study was to determine the zinc levels in calves with trichophytosis and to research the importance of zinc for fungi. The sera of 20 calves with trichophytosis and 10 healthy calves were used in this study. Zinc levels of the sera were measured by the atomic absorption spectrophotometer method. Serum zinc levels of diseased and healthy animals were found to be 42.0±16.6 μg/dL and 75.8±5.9 μg/dL, respectively. Serum zinc levels of diseased calves were lower than healthy ones and this difference were found to be important statistically (p<0.001), whereas there is no statistical difference on the levels of lymphocyte, monocyte, granulocyte, erythrocyte, hemoglobin, hematocrit, and mean corpuscular volume between groups. These parameters were not influenced by low zinc levels.     

Effects of diabetes type and treatment on zinc status in diabetes mellitus

Zinc status was assessed in 53 diabetic patients: 18 insulin-dependent diabetic patients (IDDM), 22 noninsulin-dependent diabetic patients (NIDDM) treated with oral antidiabetic agents, and 13 insulin-treated, noninsulin-dependent diabetic patients (IRDM). Plasma zinc concentrations were in the usual range for healthy subjects in these three groups (15.3±0.9 μmol/L). Urinary zinc excretions were elevated in the IDDM group (18.3±4.1 μmol/24 h;p<0.01 vs normal) and in the NIDDM group (17.5±3.5 μmol/24 h;p<0.01 vs normal), but normal in the IRDM group (11.3±2.4 μmol/24 h). In 14 NIDDM patients treated with transient continuous sc insulin injections, urinary zinc decreased from 16.5±2.2 μmol/24 h before insulin treatment to 11.5±0.3 μmol/24 h after insulin treatment without any modification in plasma zinc concentrations.     

The effects of Zn2+ on guinea pig isolated heart preparations

Isolated guinea pig hearts were perfused, by the Langendorff technique, with 30, 15, 7.5, and 1.5 μM Zn²⁺ in Chenoweth solution. Contractile force, coronary flow, and heart rate were recorded by means of Narco IV Physiograph. Calcium inhibitor (Verapamil 1 μM) and anion inhibitor (DIDS: 0.1, 1, and 5 μM) were used subsequently in the perfusing solutions in order to distinguish some of the possible mechanisms that Zn²⁺ uses to exert its action on cardiac myocytes. Isomolar to zinc concentration of Pb (II) and Co (II) were used to elucidate whether zinc effects on heart are specific for this metal. All hearts were used to estimate their zinc and calcium content by means of AAS (Atomic Absorption Spectrometry). Our findings suggest that the higher the Zn²⁺ concentration, the more toxic effects on heart are expressed by rapid reversible contractile force reduction and reversible specific changes of heart rate and flow. Zinc 1.5 μM in the perfusing solution benefits heart performance, but not significantly. Furthermore, the metal exerts specific effects on guinea pig heart, and it is rather possible that these effects on cardiac myocytes are held through cell membrane receptors.     

Taurine and Zinc Modulate Outgrowth from Goldfish Retinal Explants

Taurine and zinc, highly concentrated in the retina, possess similar properties in this structure, such as neuro-protection, membrane stabilization, influencing regeneration, and modulating development, maybe by acting in parallel or as interacting agents. We previously demonstrated that there are some correlations between taurine and zinc levels in hippocampus, dentate gyrus and retina of the developing rat. In the present study we evaluate the possible effects of taurine and zinc on outgrowth from goldfish retinal explants. The optic nerve was crushed 10 days before plating and culturing retinal explants in Leibovitz medium with 10% fetal calf serum and gentamicin. Neurites were measured with SigmaScanPro after 5 days in culture. Taurine (HPLC) and zinc (ICP) concentrations were determined in the retina between 1 and 180 days after crushing the optic nerve. Zinc sulfate (0.01–100 μM), N,N, N′,N′-tetrakis (pyridylmethyl) ethylenediamine (TPEN, 0.1–5 nM) and diethylenetriamine penta-acetic acid (DTPA, 10–300 μM), intracellular and extracellular zinc chelators, respectively, were added to the medium. TPEN was also injected intraocular (0.1 nM). Combinations of them were added with taurine (1–16 mM). Taurine concentrations were elevated in the retina 72 h after the crush, but were normalized by 180 days, those of zinc increased at 24 h, preceding the increase of taurine. The axonal transport of [³H]taurine from the optic tectum to the retina was not affected in fish with or without crush of the optic nerve at early periods after the injection, indicating an increase of it post-lesion. Zinc sulfate produced a bell-shaped concentration dependency on in vitro outgrowth, with stimulation at 0.05 μM, and inhibition at higher levels, also increased the effect of 4 mM taurine at 0.02 μM, but diminished it at higher concentrations in the medium. TPEN decreased outgrowth at 1 nM, but not at 0.5 nM, although the simultaneous presence of 4 mM taurine and 0.5 nM TPEN decreased outgrowth respecting the stimulation by taurine alone. The intraocular administration of TPEN decreased outgrowth in vitro, an effect counteracted by the addition of 4 mM taurine to the culture medium. DTPA decreased outgrowth from 10 μM in the medium. The present results indicate that an optimal zinc concentration is necessary for outgrowth of goldfish retinal explants and that, in zinc deficient retina, taurine could stimulate outgrowth. In addition, the observations of variations in tissue concentrations and of the effects of intraocular administration of TPEN indicate that these effects could occur in vivo. Special issue dedicated to Dr. Simo S. Oja