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In modern piglet nutrition, diets are often supplemented with high doses of zinc oxide to decrease the incidence of weaning-associated diseases such as post-weaning diarrhea. However, the underlying mechanisms by which high zinc concentrations exert their effects within the organism are not sufficiently elucidated and concerns exist regarding the environmental pollution by zinc-loaded liquid manure.
The present thesis investigated the effect of zinc on transport properties in the small intestine of piglets initially in a feeding trial. Zinc effects seemed to depend on its acute application at the intestinal epithelium and furthermore on the side of application in vitro. In association with previous observations of Lodemann et al. (2013) showing a distinct influence of high zinc levels on viability and barrier properties of intestinal epithelial cells, the present thesis further aimed at elucidating the homeostatic mechanisms that may prevent toxic effects of high zinc concentrations in the intestine of piglets.
The first study involved a feeding trial with piglets being fed marginal (50 ppm), normal (150 ppm), or pharmacological concentrations (2500 ppm) of zinc oxide and an investigation of its impact on jejunal transport properties. Ussing chamber experiments with jejunal isolated epithelia were performed with several substances stimulating either absorptive (L-glutamine, Dglucose) or secretory (prostaglandin E2, carbachol, E. coli heat-stable enterotoxin) transport. In addition to chronic dietary zinc supplementation, the effect of acute zinc application was tested in vitro. The dietary zinc supplementation had no significant influence on absorptive and secretory responses. However, with an exception for carbachol, the acute zinc treatment in vitro led to small but significant decreases in both absorptive and secretory capacities. In conclusion, chronic zinc supplementation in the post-weaning phase sustainably affected neither the absorptive nor the secretory transport properties within the jejunum. However, as the jejunal transport was influenced by acute zinc addition in vitro, it is proposed that the potential epithelial effects of zinc depend on the acute presence of this ion at the intestinal epithelium.
In the second part of this thesis, cell culture experiments with the porcine cell line IPEC-J2 and the human cancer cell line Caco-2 were performed. On the assumption that the tight regulation of intracellular zinc homeostasis in enterocytes of weaned piglets is a crucial necessity, the aim of this study was to elucidate the way that porcine intestinal epithelial cells regulate intracellular zinc homeostasis and maintain it during a challenge with increasing extracellular zinc concentrations. A further question was whether the differentiation status of the cells (preconfluent/ postconfluent) or the duration of zinc exposure (6 or 24 h) affected the response to increasing zinc concentrations.
The intracellular zinc content rose dose-dependently in both cell lines with increasing extracellular zinc concentrations. Correspondingly, the expression of the zinc efflux transporter 1 (ZnT1) and of metallothionein (MT1A) were upregulated, whereas the zinc influx transporter 4 (ZIP4) was downregulated. The mRNA expression of the metal response element-binding transcription factor 1 (MTF-1) remained largely unchanged. However, a higher MTF-1 abundance was detected in IPEC-J2 cells after incubation with the highest zinc concentration (200 μM).
The effects of increasing zinc concentrations were partly different between cell lines and maturation status. The time of exposure to zinc also evoked an impact.
In conclusion, the second study showed that an adaptation of the tested target genes was responsible for regulating zinc homeostasis in response to increasing zinc concentrations. However, despite the induction of compensatory mechanisms, an increase in intracellular zinc levels was observed after high extracellular zinc levels had been applied, indicating that high zinc levels and a longer incubation time might exceed the capacity of homeostatic regulation. Toxic effects, as previously assessed by Lodemann et al. (2013), can occur because of an imbalance of zinc homeostasis. Therefore, long-term supplementation with high levels of dietary zinc cannot be recommended, as it may induce homeostatic imbalances with negative effects on the animal.