Zinc metabolism in the winter flounder (Pseudopleuronectes americanus)

Shears, Margaret Ann (1983) Zinc metabolism in the winter flounder (Pseudopleuronectes americanus). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The gastrointestinal uptake of Zn²⁺ was studied in the winter flounder using an in situ technique, an equilibrium dialysis technique and a non-absorbed marker technique. -- The entire digestive tract of the flounder was capable of absorbing Zn²⁺, with the uppermost portion of the intestine having the highest and the stomach the lowest capacity. A seasonal study revealed that the capacity to absorb Zn²⁺ was greatest during the summer feeding period. Zn²⁺ absorption appeared to involve at least two steps, the first a rapid accumulation of Zn²⁺ by the tissue and the second, a slower transfer of Zn²⁺ into the body. The total amount of Zn²⁺ absorbed increased with increasing loads of Zn²⁺ in the lumen; the transfer mechanism did not appear to be saturated at the highest Zn²⁺ loads tested. However, Zn²⁺ uptake was inhibited by several other metals and by the amino acid, histidine. -- The capacity of the digestive tract to absorb Zn²⁺ was not affected by feeding the flounder a high-Zn²⁺ diet or by increasing the body Zn²⁺ load by parenteral injections. In the event of exposure to elevated levels of Zn²⁺ in the diet, it is suggested that elimination mechanisms may play a greater role in Zn²⁺ homeostasis in the winter flounder than limitation of gastrointestinal uptake. -- The dynamics of Zn²⁺ turnover in the flounder was investigated by examining the distribution of ⁶⁵Zn in the tissues following single intramuscular injections of the radiotracer. The tissues exhibited different rates of accumulation and loss of ⁶⁵Zn, the most rapid being in tissues such as the kidney, liver, gill and gastrointestinal tract. -- Retention of ⁶⁵Zn was examined in live flounder using a whole-body detector. The loss of ⁶⁵Zn appeared to vary seasonally; when flounder were monitored during the summer feeding period the rate of ⁶⁵Zn loss (monitored in the area of the peritoneal cavity) increased over that seen in the winter non-feeding period (i.e. TB₁/₂=223 and 1510 days, respectively). Whole-body ⁶⁵Zn retention patterns were similar in flounder injected with saline or a load of stable Zn²⁺. Under these experimental conditions the rate of Zn²⁺ loss was not affected by an excess of stable Zn²⁺. -- Experiments conducted to determine the possible site(s) of Zn²⁺ excretion into the digestive tract following i.v. injections of ⁶⁵Zn indicated that Zn²⁺ could be secreted into the lumen contents all along the tract. Other possible sites of Zn²⁺ elimination include the gills, kidney and body surface. -- Chromatographic techniques were its ed. to examine the Zn²⁺-binding proteins in the cytosols of several tissues of the winter flounder. A low molecular weight ⁶⁵Zn (Zn²⁺)-binding protein, with properties characterizing it as metallothionein, was isolated from the mucosal and liver cytosols of Zn²⁺-injected flounder. Metallothionein did not appear to be serving the same function(s) in the intestine of the flounder as commonly hypothesized for mammals. The presence of the protein in the mucosal cytosol did not appear to be associated with any enhancement or depression of Zn²⁺ absorption. The role of metallothionein in the normal metabolism of Zn²⁺ in the flounder remains to be resolved. That it could play a role in homeostasis is suggested by the presence of a low molecular weight Zn²⁺-binding protein, with elution characteristics similar to metallothionein, in the tissues of normal flounder.

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