In vivo and in vitro effects of high-K(+) stress on branchial expression of ROMKa in seawater-acclimated Mozambique tilapia

Stable potassium isotopes (41K/39K) track transcellular and paracellular potassium transport in biological systems

As the most abundant cation in archaeal, bacterial, and eukaryotic cells, potassium (K⁺) is an essential element for life. While much is known about the machinery of transcellular and paracellular K transport–channels, pumps, co-transporters, and tight-junction proteins—many quantitative aspects of K homeostasis in biological systems remain poorly constrained. Here we present measurements of the stable isotope ratios of potassium (⁴¹K/³⁹K) in three biological systems (algae, fish, and mammals). When considered in the context of our current understanding of plausible mechanisms of K isotope fractionation and K⁺ transport in these biological systems, our results provide evidence that the fractionation of K isotopes depends on transport pathway and transmembrane transport machinery. Specifically, we find that passive transport of K⁺ down its electrochemical potential through channels and pores in tight-junctions at favors ³⁹K, a result which we attribute to a kinetic isotope effect associated with dehydration and/or size selectivity at the channel/pore entrance. In contrast, we find that transport of K⁺ against its electrochemical gradient via pumps and co-transporters is associated with less/no isotopic fractionation, a result that we attribute to small equilibrium isotope effects that are expressed in pumps/co-transporters due to their slower turnover rate and the relatively long residence time of K⁺ in the ion pocket. These results indicate that stable K isotopes may be able to provide quantitative constraints on transporter-specific K⁺ fluxes (e.g., the fraction of K efflux from a tissue by channels vs. co-transporters) and how these fluxes change in different physiological states. In addition, precise determination of K isotope effects associated with K⁺ transport via channels, pumps, and co-transporters may provide unique constraints on the mechanisms of K transport that could be tested with steered molecular dynamic simulations.

Systemic versus tissue-level prolactin signaling in a teleost during a tidal cycle

Euryhaline Mozambique tilapia (Oreochromis mossambicus) are native to estuaries where they encounter tidal fluctuations in environmental salinity. These fluctuations can be dramatic, subjecting individuals to salinities characteristic of fresh water (FW < 0.5‰) and seawater (SW 35‰) within a single tidal cycle. In the current study, we reared tilapia under a tidal regimen that simulated the dynamic conditions of their native habitat. Tilapia were sampled every 3 h over a 24 h period to temporally resolve how prolactin (PRL) signaling is modulated in parallel with genes encoding branchial effectors of osmoregulation. The following parameters were measured: plasma osmolality, plasma PRL₁₇₇ and PRL₁₈₈ concentrations, pituitary prl₁₇₇ and prl₁₈₈ gene expression, and branchial prl receptor (prlr1 and prlr2), Na⁺/Cl^--cotransporter (ncc2), Na⁺/K⁺/2Cl^--cotransporter (nkcc1a), Na⁺/K⁺-ATPase (nkaα1a and nkaα1b), cystic fibrosis transmembrane regulator (cftr), and aquaporin 3 (aqp3) gene expression. Throughout the 24 h sampling period, plasma osmolality reflected whether tilapia were sampled during the FW or SW phases of the tidal cycle, whereas pituitary prl gene expression and plasma PRL levels remained stable. Branchial patterns of ncc2, nkcc1a, nkaα1a, nkaα1b, cftr, and aqp3 gene expression indicated that fish exposed to tidally changing salinities regulate the expression of these gene transcripts in a similar fashion as fish held under static SW conditions. By contrast, branchial prlr1 and prlr2 levels were highly labile throughout the tidal cycle. We conclude that local (branchial) regulation of endocrine signaling underlies the capacity of euryhaline fishes, such as Mozambique tilapia, to thrive under dynamic salinity conditions.