Control of transport by epithelia

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Teleost fish osmoregulate using active ion and water transport mechanisms of the gills, intestine, kidney and urinary bladder. These epithelial structures contain specialized epithelial transport cells with regulated transport proteins. We study regulation of these transporters in situ in the native epithelium, particularly in mitochondria rich chloride cells of the teleost gill and related osmoregulatory organs as our physiological model system. The ion secretion by hardy euryhaline teleosts such as the killifish Fundulus heteroclitus is rapidly regulated by hormones, neurotransmitters and by the osmotic level of the blood. When fish move from seawater to fresh water, their blood becomes diluted; this transient decrease in osmolality rapidly and reversibly inhibits the salt (NaCl) secretion that is normally present in seawater. In turn, the effect helps the animal retain salt while in fresh water. The effect is a direct cell level regulation by ion transportng cells that sense changes in osmolality. Conversely, blood concentration, such as occurs when the fish enter hypersaline environments stimulates saltsecretion. In addition, these cells also undergo compensation to regulate cell volume. The mechanism involves a series of regulatory proteins, protein kinases and phosphatases that activate or deactivate (respectively) the membrane-bound ion transporters. The research is important to comparative physiology, comparative endocrinology, fisheries and aquaculture. Also, because the anion channel "Cystic Fibrosis Transmembrane conductance Regulator" (CFTR) transports salt in this system, studies of CFTR regulation in fish may be important to discovery of CFTR regulation and important to discovering underlying causes of Cystic Fibrosis.