Editorial Focus: Using phenotypic plasticity: focus on "Identification of renal transporters involved in sulfate excretion in marine teleost fish".

FOR OSMOREGULATING vertebrates, diffusive water loss is an unavoidable consequence of living in seawater. Marine teleost fish drink seawater to offset this diffusive water loss dictated by the steep osmotic gradient between blood and the surrounding seawater, a water loss occurring mainly across the gills. Imbibed seawater is processed along the gastrointestinal tract by desalinizing the medium in the esophagus to allow for solute-coupled water absorption in the intestine (5). Fluid absorption across the intestinal epithelium is intimately linked to NaCl absorption via Na -K -2Cl and Na Cl cotransporters as well as Cl /HCO3 exchange and renders the divalent ions, Mg and SO4 2 , highly concentrated in the intestinal fluids (6). The NaCl gain associated with intestinal water absorption combines with diffusive NaCl gain across the respiratory epithelium to a need for active NaCl secretion across the gill. It has long been recognized that this active NaCl secretion occurs via basolateral NKCC and apical CFTR-like Cl channels facilitating transcellular Cl secretion to which paracellular Na secretion is coupled (2). Thus, many aspects of NaCl and osmotic homeostasis in marine teleosts are understood, although details of regulation of these processes remain to be examined. In contrast, little is known about the transporters involved in the homeostasis on the major divalent ions in seawater, Mg and SO4 2 . Sulfate concentrations in marine teleost fish blood rarely exceed 1 mM, despite ambient SO4 2 concentrations of 30 mM in seawater and levels as high as 150 mM in intestinal fluids (9). In light of these substantial gradients, especially across the intestinal epithelium, intestinal uptake of Mg and SO4 2 is relatively modest (3) but nevertheless must be compensated for. It has long been recognized that Mg and SO4 2 homeostasis is achieved by renal elimination with urine SO4 2