The glomerular filtration rate (GFR) of teleost fishes is highly variable and is influenced by glomerular intermittency, environmental salinity, renal perfusion pressure, and some hormones. In freshwater (FW) teleosts, the primary function of the kidney is to excrete excess water while retaining most of the filtered solutes, and GFR is a major determinant of urine flow. Low permeability to water prevails in the distal nephron of FW teleosts, and the majority of the filtered Na and Cl is reabsorbed without osmotic accompaniment of water. Prolactin appears to regulate osmotic permeability to water. Isolated and perfused distal tubules from FW teleosts revealed a transepithelial voltage (Vt) that was positive in the lumen. Both Na and Cl participate in generating lumen-positive Vt. Marine teleosts, which are exposed to Na loading and dehydration, ingest seawater to compensate for their osmotic water loss, and secrete divalent ions, mainly Mg and SO4, from the kidney. The urine flow of marine teleosts is primarily determined by fluid secretion accompanied by divalent ions and subsequent isosmotic reabsorption with NaCl. Interdependence of Na and Cl transport has been noted in the urinary bladder of marine teleosts. There is presently no known humoral substance that regulates NaCl and divalent ion transport in the teleost kidney.
