Guanylin activates Cl− secretion into the lumen of seawater eel intestine via apical Cl− channel under simulated in vivo conditions

Guanylin (GN) action on seawater eel intestine was examined under simulated in vivo conditions, where isotonic luminal fluid has low NaCl and high MgSO4 (MgSO4 Ringer). In Ussing chamber, MgSO4 Ringer induced serosa-negative potential difference (PD) even after bumetanide treatment, which is due to the higher paracellular Na+ permeability over Cl−, as confirmed by the replacement by MgCl2 (no Cl− gradient) or Na2SO4 Ringer (no Na+ gradient). Luminal GN reversed serosa-negative PD, probably by enhancing Cl− secretion into the lumen, as the GN effect was blocked by apical Cl− channel blockers [diphenylamine-2-carboxylic acid (DPC), 5-nitro-2-(3-phenylpropylamino) benzoic acid, glibenclamide but not cystic fibrosis transmembrane regulator (CFTR)inh-172] or replacement of luminal fluid by MgCl2 Ringer. The blockers' effect was undetectable when normal Ringer was on both sides. In the sac preparation, NaCl secretion occurred into the lumen (Na+ > Cl−), and GN further enhanced Cl− secretion (Cl− > Na+), resulting in water secretion. These GN effects were also blocked by DPC. Quantitative analyses showed that isotonic NaCl is absorbed when luminal fluid is normal Ringer, but, when luminal fluid is MgSO4 Ringer, hypertonic NaCl, almost equivalent to seawater, is secreted into the lumen after GN. These results indicate that GN stimulates the secretion of hypertonic NaCl into the lumen of seawater eel intestine, like rectal gland of marine elasmobranchs, to get rid of excess NaCl although marine teleost intestine is thought to have only absorptive-type cells with a unique Na-K-Cl cotransport system. The secreted NaCl may activate the cotransport system and further help absorb water in the final segment of seawater eel intestine.

Ion regulation in fish gills: recent progress in the cellular and molecular mechanisms

Fish encounter harsh ionic/osmotic gradients on their aquatic environments, and the mechanisms through which they maintain internal homeostasis are more challenging compared with those of terrestrial vertebrates. Gills are one of the major organs conducting the internal ionic and acid-base regulation, with specialized ionocytes as the major cells carrying out active transport of ions. Exploring the iono/osmoregulatory mechanisms in fish gills, extensive literature proposed several models, with many conflicting or unsolved issues. Recent studies emerged, shedding light on these issues with new opened windows on other aspects, on account of available advanced molecular/cellular physiological approaches and animal models. Respective types of ionocytes and ion transporters, and the relevant regulators for the mechanisms of NaCl secretion, Na+ uptake/acid secretion/NH4+ excretion, Ca2+ uptake, and Cl− uptake/base secretion, were identified and functionally characterized. These new ideas broadened our understanding of the molecular/cellular mechanisms behind the functional modification/regulation of fish gill ion transport during acute and long-term acclimation to environmental challenges. Moreover, a model for the systematic and local carbohydrate energy supply to gill ionocytes during these acclimation processes was also proposed. These provide powerful platforms to precisely study transport pathways and functional regulation of specific ions, transporters, and ionocytes; however, very few model species were established so far, whereas more efforts are needed in other species.

Teleost fish osmoregulation: what have we learned since August Krogh, Homer Smith, and Ancel Keys

In the 1930s, August Krogh, Homer Smith, and Ancel Keys knew that teleost fishes were hyperosmotic to fresh water and hyposmotic to seawater, and, therefore, they were potentially salt depleted and dehydrated, respectively. Their seminal studies demonstrated that freshwater teleosts extract NaCl from the environment, while marine teleosts ingest seawater, absorb intestinal water by absorbing NaCl, and excrete the excess salt via gill transport mechanisms. During the past 70 years, their research descendents have used chemical, radioisotopic, pharmacological, cellular, and molecular techniques to further characterize the gill transport mechanisms and begin to study the signaling molecules that modulate these processes. The cellular site for these transport pathways was first described by Keys and is now known as the mitochondrion-rich cell (MRC). The model for NaCl secretion by the marine MRC is well supported, but the model for NaCl uptake by freshwater MRC is more unsettled. Importantly, these ionic uptake mechanisms also appear to be expressed in the marine gill MRC, for acid-base regulation. A large suite of potential endocrine control mechanisms have been identified, and recent evidence suggests that paracrines such as endothelin, nitric oxide, and prostaglandins might also control MRC function.

Contribution of the Na+-K+-2Cl− Cotransporter (NKCC1) to Transepithelial Transport of H+, NH4+, K+, and Na+ in Rat Outer Medullary Collecting Duct

ABSTRACT. In rat kidney, the “secretory” isoform of the Na-K-Cl cotransporter, NKCC1 (BSC-2), localizes to the basolateral membrane of the α intercalated cell, the acid secreting cell of the outer medullary collecting duct (OMCD). This laboratory has reported that NKCC1 mediates Cl− uptake across the basolateral membrane in series with Cl− secretion across the apical membrane in rat OMCD. NKCC1 transports NH4+, K+, and Na+ as well as Cl−; therefore, a role for the cotransporter in the process of HCl, NH4Cl, KCl, and NaCl secretion has been suggested. Thus, it was determined if bumetanide, an inhibitor of NKCC1, alters transepithelial cation transport in rat OMCD. OMCD tubules from deoxycorticosterone pivalate (DOCP)–treated rats were perfused in vitro. Hydration of CO2, rather than NH4+, provides the principle source of H+ for net acid secretion. In HCO3−/CO2-buffered solutions, no effect of bumetanide on net K+ flux was detected. Under some conditions, bumetanide addition resulted in a small reduction in secretion of net H+ equivalents. Transepithelial Na+ flux, JNa, was −1.5 ± 1.7 pmol/mm per min, values not different from zero. However, with the application of bumetanide to the bath, JNa was +5.2 ± 1.3 pmol/mm per min (P < 0.05), which indicates net Na+ absorption. In conclusion, inhibition of NKCC1 in rat OMCD changes transepithelial movement of Na+ and Cl−. The role of NKCC1 in the secretion of net H+ equivalents is small.