All living things depend on the presence and movement of water within and between cells to sustain life-giving functions. Respiration, digestion, cognition, circulation, and other continuous processes depend on the steady movement and exchange of molecules across membranes; water is the medium that allows this movement to happen. Left to its own devices, water seeks to find a balance between both sides of a membrane to achieve equal concentrations of dissolved minerals and organic compounds. But the inner workings of living things often rely on differences across membranes to fuel various functions.
One way of achieving this is to leverage water’s innate balance-seeking driving force. This dance relies on compounds attached to or imbedded in cell membranes that pull or push water or dissolved constituents across the membrane so that the concentration is greater on one side than the other. At the opportune moment, the cell then uses water’s balance-seeking force to do work.
In other situations, living things need to counter water’s innate balancing act in order to keep a steady concentration of dissolved constituents inside the cell, regardless of outside conditions. Such is the challenge for spotted green pufferfish that encounter varying degrees of salinity in their environment. To maintain a steady level of chloride ions inside their cells, pufferfish have two membrane bound proteins (CFTR and NKCC) in specialized epithelial cells of their gills that form ion channels across the membrane. The channels function to secrete chloride ions from inside the cell in saltier environments and absorb chloride ions in freshwater environments.
Click to view a diagram of a NKCC1 cotransporter in a new window.Edit Summary
“Anion channels are expressed in all biological membranes. Because chloride is the most abundant inorganic anion in the intracellular media and in body fluid, anion channels are commonly referred to as chloride channels. Chloride channels are involved in a broad range of physiological functions, including the transepithelial transport, cell volume regulation, stabilization of membrane potential, synaptic inhibition, extracellular and vesicular acidification, and endocytotic trafficking that may be tissue-, cell type-specific or housekeeping distribution in various tissues.” (Tang et al. 2010:688)
“Cl− is the most abundant anion in animal tissue and plasma. Moreover, chloride channels play essential roles in the regulation of cellular excitability, transepithelial transport, cell volume regulation and acidification of intracellular organelles…Osmoregulation and ion balance in teleosts is predominantly performed by the transportation of relevant ions (Na+ and Cl–) through epithelial transporter systems across the gill, the major osmoregulatory tissue. In order to maintain homeostasis of plasma osmolality and ion components, marine or freshwater (FW) teleosts excrete excess salts of the blood to hyperosmotic environments or actively absorb salts from external hyposmotic environments, respectively, via the gill epithelial transport systems. The systems used by teleosts to adapt to seawater (SW) or FW differ not only in the direction of ion and water movements but also in the molecular components of the transporters. Most teleosts are stenohaline fishes, living entirely in either SW or FW. Because euryhaline teleosts adapt to either SW or FW by efficiently switching epithelial transporter systems, they exhibit great ability to maintain plasma osmolality within narrow physiological ranges in different salinity environments.” (Tang et al. 2010:683)
Chloride channel ClC-3 in gills of the euryhaline teleost, Tetraodon nigroviridis: expression, localization and the possible role of chloride absorptionJournal of Experimental BiologyFebruary 12, 2010
“The current ion-transporting system depicted in the working model for NaCl secretion of gill epithelial ionocytes in SW teleosts includes (Fig. 1) the ouabain-inhibited Na+-K+-ATPase (NKA) and bumetanide-sensitive Na+-K+-2Cl- cotransporter 1 (NKCC1) located in the basolateral membrane; the anion channel cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane; and the tight-junction proteins, claudins, and occludins, located between adjacent epithelial cells. The basolateral NKCC1 mediates the entry of Na+ and Cl- into the cellular compartment down the electrochemical gradient provided by NKA, followed by passive exit of Cl- and Na+, respectively, through the apical channel CFTR and paracellular tight-junction pathway…Functional NKA consists of two protein subunits, the catalytic alpha-subunit and glycoprotein beta-subunit, assembled in a 1:1 ratio to create an alphabeta-heterodimer. NKA provides an electrochemical gradient to drive operations of other transport pathways and exhibits four alpha- (alpha1 ~ 4) and three beta-isoforms (beta1 ~ 3) in mammals…NKCC1 is thought to be the secretory isoform of NKCC, a member of the SLC12A family…In gills of these species, the main expressed isoform is the NKCC1a gene. Furthermore, using whole-mount in situ hybridization, the NKCC1a gene was demonstrated to be localized in ionocytes of medaka gills, which supports immunocytochemical data from extensive previous studies. When euryhaline teleosts were exposed to SW, increased mRNA levels of branchial NKCC1a and elevated protein levels of branchial NKCC1 were reported. The observations strongly suggest a role of NKCC1a in the NaCl secretion function in gill ionocytes of euryhaline teleosts during acclimation to SW. The CFTR, a cAMP-activated Cl- channel (ClC), mainly appears in branchial ionocytes of SW fish, indicating its role in NaCl secretion upon salinity challenge…This accumulated evidence supports the crucial role of CFTR, similar to that of NKCC, in NaCl secretion function in SW fish gill ionocytes.” (Hwang et al. 2011:R28-R30)