Effects of copper on a reconstructed freshwater rainbow trout gill epithelium: Paracellular and intracellular aspects

Trace metals in the teleost fish gill: biological roles, uptake regulation, and detoxification mechanisms

In fish, the gill plays a vital role in regulating the absorption of trace metals and is also highly susceptible to metal toxicity. Trace metals such as iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn) are involved in various catalytic activities and molecular binding within the gill, thereby supporting a range of physiological processes in this organ. While beneficial at normal levels, these metals can become toxic when present in excess. Conversely, nonessential metals like cadmium (Cd) and lead (Pb) can gain entry into gill cells through similar metal transport pathways, potentially interfering with various cellular processes. The transepithelial transport of these metals across the gill epithelium is governed by a variety of metal transport and metal binding proteins. These include the Cu transporter 1 (CTR1), divalent metal transporter 1 (DMT1), and members of the Zrt-/Irt-like protein (ZIP) and zinc transport (ZnT) families. Additionally, some of these metals can compete with major ions (e.g., calcium, sodium) for absorption sites in the gill. This complex crosstalk suggests an interdependent mechanism that balances metal uptake to meet physiological needs while preventing excessive accumulation. In this article, I review the roles of trace metals in proteins/enzymes that support the different functions in the gill of teleost fish. I also discuss current understanding of the pathways involved in regulating the branchial uptake of metals and their influence on ionic regulation, and the potential detoxification mechanisms in the gill. Finally, I summarize knowledge gaps and potential areas for further investigation.

Tissue Reaction to Low-Density Polyacrylamide Gel as a Carrier for Microimplants in the Adipose Fin of Rainbow Trout

The implantation of optical sensors is a promising method for monitoring physiological parameters of organisms in vivo. For this, suitable hydrogels are required that can provide a biocompatible interface with the organism's tissues. Amorphous hydrogel is advantageous for administration in animal organs due to its ease of injection compared to resilient analogs. In this study, we investigated the applicability of a semi-liquid 2.5% polyacrylamide hydrogel (PAAH) as a scaffold for fluorescent polyelectrolyte microcapsules (PMs) in rainbow trout. The hydrogel was injected subcutaneously into the adipose fin, which is a small, highly translucent fold of skin in salmonids that is convenient for implanting optical sensors. Using histological methods, we compared tissue organization and in vivo stability of the applied hydrogel at the injection site after administration of uncoated PMs or PMs coated with 2.5% PAAH (PMs-PAAH) for a period of 3 to 14 days. Our results showed that the introduction of PMs into the gel did not have a masking effect, as they were recognized, engulfed, and carried away by phagocytes from the injection site. However, both PMs and PMs-PAAH were found to provoke chronic inflammation at the injection site, although according to cytokine expression in the fish spleen, the irritating effect was local and did not affect the systemic immunity of the fish. Therefore, our study suggests low applicability of 2.5% polyacrylamide as a scaffold for injectable sensors within a timeframe of days.

Shifted mineral ions transport in the mollusk Unio pictorum exposed to environmental concentrations of diclofenac

Previous studies showed that diclofenac (DCF), when released in the environment, can be toxic to aquatic animals (fish and mollusks), affecting gills, which are the main organ of ionic regulation. This study focuses on detecting the effects of relevant environmental concentrations of DCF (0.1-1 μg L^-1) on the transport of main mineral cations, i.e. sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg), by widely distributed freshwater bivalve mollusks Unio pictorum. After 96-h exposure to river aerated water at 25 °C with DCF concentrations of 0 (control), 0.1 (treatment I), and 1 μg L^-1 (treatment II), the mollusks were transferred to deionized water, and daily (for 7 days) concentrations of these cations in the medium have been measured. Animals exposed to 1 μg L^-1 DCF maintained the ionic balance between the organism and the diluted medium at a significantly higher level of Na, K, and Mg ions in water compared to the control and animals exposed to 0.1 μg L^-1 DCF. At 0.1 μg L^-1 DCF, the greater loss concerning the control (p < 0.05) was found only for Na ion. There were no differences in the dynamics of Ca ions between control and both treatments. This study showed that detectable environmental concentrations of DCF in natural waters can influence the transport of main cations required by freshwater animals to maintain their ionic balance, and the observed effect (elevated ion loss) is ion-specific and also dose-dependent.