Measuring intestinal fluid transport in vitro: Gravimetric method versus non-absorbable marker

Demonstrating the translocation of nanoplastics across the fish intestine using palladium-doped polystyrene in a salmon gut-sac

Fish are widely reported to ingest microplastics with low levels accumulating in the tissues, but owing to analytical constraints, much less is known about the potential accumulation of nanoplastics via the gut. Recently, the labelling of plastics with inorganic metals (e.g., palladium) has allowed measurements of nanoplastic uptake. The aim of the current study was to quantitatively assess the uptake of nanoplastics by the fish gut using palladium-doped nanoplastics (with a mean hydrodynamic radius of 202 ± 7 nm). By using an ex vivo gut sac exposure system, we show that in 4 h between 200 and 700 million nanoplastics (representing 2.5-9.4% of the administered nanoplastics dose) can enter the mucosa and muscularis layers of the intestine of salmon. Of the particles taken up, up to 700,000 (representing 0.6% of that taken into the tissue) of the nanoplastics passed across the gut epithelium of the anterior intestine and exit into the serosal saline. These data, generated in highly controlled conditions provide a proof-of-concept study, suggesting the potential for nanoplastics to distribute throughout the body, indicating the potential for systemic exposure in fish.

Synthesis and Evaluation of PEG-PR for Water Flux Correction in an In Situ Rat Perfusion Model

Phenol red (PR) is a widely used marker for water flux correction in studies of in situ perfusion, in which intestinal absorption usually leads to the underestimation of results. In this paper, we propose a novel marker polyethylene glycol (PEG)-PR (i.e., PR modified by PEGylation) with less permeability and evaluate its application in an in situ perfusion model in rats. PEG-PR was synthesized by the chemical conjunction of polyethylene glycol-4k/5k (PEG-4k/5k) and PR. The synthesized PEG-PR was then characterized using ¹H-NMR, ¹³C-NMR, ultraviolet (UV), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analyses. The low permeability of PEG-PR was assessed using everted gut sac (EGS) methods. The apparent permeability coefficients (P_app, 3–8 × 10⁻⁷ cm/s) of PEG4k/5k-PR exhibited a nearly 15-fold reduction compared to that of PR. The different concentrations of PEG4k/5k-PR did not contribute to the P_app value or cumulative permeable percentage (about 0.02–0.06%). Furthermore, the larger molecular weight due to PEGylation (PEG5k-PR) enhanced the nonabsorbable effect. To evaluate the potential application of the novel marker, atenolol, ketoprofen, and metoprolol, which represent various biopharmaceutics classification system (BCS) classes, were selected as model drugs for the recirculation perfusion method. The water flux corrected by PEG4k/5k-PR reflected the accuracy due to the nonabsorbable effect, while the effective intestinal membrane permeability (P_eff) of atenolol corrected by PEG4k/5k-PR showed a statistically significant increase (p < 0.05) in different intestinal segments. In conclusion, PEG-PR is a promising marker for the permeability estimation when using the in situ perfusion model in rats.

Cold tolerance is linked to osmoregulatory function of the hindgut in Locusta migratoria

There is growing evidence that maintenance of ion and water balance determine cold tolerance in many insects. The hindgut of terrestrial insects is critical for maintaining organismal homeostasis as it regulates solute- and water-balance of the hemolymph. Here we used ex vivo everted gut sacs of L. migratoria to examine the effects of temperature (0 - 30°C), thermal-acclimation, hypoxia, and ionic and osmotic forces on bulk water and ion (Na+, K+ and Cl−) movement across the rectal epithelium. These findings were related to simultaneous in vivo measurements of water and ion balance in locusts exposed to similar temperatures. As predicted, we observed a critical inhibition of net water and ion reabsorption at low temperature that is proportional to the in vivo loss of water and ion homeostasis. Further, cold-acclimated locust, known to defend ion and water balance at low temperature, were characterised by improved reabsorptive capacity at low temperature. These findings strongly support the hypothesis that transport mechanisms in the hindgut at low temperature are essential for cold tolerance. The loss of osmoregulatory capacity at low temperature was primarily caused by reduced active transport while rectal paracellular permeability to fluorescein isothiocyanate dextran was unchanged at 0 and 30°C. During cold exposure, water reabsorption was independent of major cation gradients across the epithelia while reduction in mucosal Cl− availability and increase in mucosal osmolality markedly depressed water reabsorption. These findings are discussed in perspective of existing knowledge and with suggestions for future physiological studies on cold acclimation and adaptation in insects.

Do polyethylene microplastic beads alter the intestinal uptake of Ag in rainbow trout (Oncorhynchus mykiss)? Analysis of the MP vector effect using in vitro gut sacs

Microplastic (MP) vector effects have been well described in the literature but surprisingly little is in known about the impact of MPs on the intestinal uptake of contaminants. The present study aimed to determine whether the intestinal fate of Ag was affected by the presence of polyethylene MP beads. Ag (added as ^110mAg) was introduced into the lumen of rainbow trout (Oncorhynchus mykiss) anterior/mid-intestine gut sac preparations as Ag only, Ag and MPs (co-exposure) and Ag-incubated MPs (where Ag was adsorbed to the MP). Results show that after 3 h exposure the distribution of accumulated Ag between the four intestinal compartments (mucus layer, mucosal epithelium, muscle layer and serosal saline) was not affected by either MP condition when compared to Ag alone (p > 0.05, One way ANOVA). Across all treatment groups mucus layer binding dominated (54.2-72.6%) whereas relatively little Ag was transported to the blood compartment (i.e. combined muscle layer and serosal saline compartments, 8.5-15.0%). Accompanying adsorption/desorption studies were performed in relevant media. Over 24 h, 60.6± 2.9% of the available Ag in artificial freshwater adhered to the surface of the PE MPs. In pH adjusted luminal fluids (pH 2.2, 4.1, 7.4 and 9.8) that span the range of conditions encountered within the rainbow trout digestive tract, there was almost complete dissociation at acidic pHs within 3 h (<2% remaining on MPs at both pH 2.2 and pH 4.1). Such pHs are typical of piscine stomach. Based on our finding we suggest that following the ingestion of MPs with adsorbed pollutants, desorption would occur prior to entering the site of uptake. The MPs themselves have no impact on the trans-epithelial transport of the contaminant, but the net result of the MP vector effect is to potentially introduce labile contaminant forms into the intestine.

High rates of intestinal bicarbonate secretion in seawater tilapia (Oreochromis mossambicus)

Osmoregulation in fish is a complex process that requires the orchestrated cooperation of many tissues. In fish facing hyperosmotic environments, the intestinal absorption of some monovalent ions and the secretion of bicarbonate are key processes to favor water absorption. In the present study, we showed that bicarbonate levels in the intestinal fluid are several fold higher in seawater than in freshwater acclimated tilapia (Oreochromis mossambicus). In addition, we analyzed gene expression of the main molecular mechanisms involved in HCO₃^- movements i.e. slc26a6, slc26a3, slc4a4 and v-type H-ATPase sub C in the intestine of tilapia acclimated to both seawater and freshwater. Our results show an anterior/posterior functional regionalization of the intestine in tilapia in terms of expression patterns, which is affected by environmental salinity mostly in the anterior and mid intestine. Analysis of bicarbonate secretion using pH-Stat in tissues mounted in Ussing chambers reveals high rates of bicarbonate secretion in tilapia acclimated to seawater from anterior intestine to rectum ranging between ~900 and ~1700nmolHCO₃^-cm^-2h^-1. However, a relationship between the expression of slc26a6, slc26a3, slc4a4 and the rate of bicarbonate secretion seems to be compromised in the rectum. In this region, the low expression of the bicarbonate transporters could not explain the high bicarbonate secretion rates here described. However, we postulate that the elevated v-type H-ATPase mRNA expression in the rectum could be involved in this process.