Active urea transport and an unusual basolateral membrane composition in the gills of a marine elasmobranch

In elasmobranch fishes, urea occurs at high concentrations (350-600 mM) in the body fluids and tissues, where it plays an important role in osmoregula tion. Retention of urea by the gill against this huge blood-to-water diffus ion gradient requires specialized adaptations to the epithelial cell membra nes. Experiments were performed to determine the mechanisms and structural features that facilitate urea retention by the gill of the spiny dogfish Sq ualus acanthias. Analysis of urea uptake by gill basolateral membrane vesic les revealed the presence of a phloretin-sensitive (half inhibition 0.09 mM ), sodium-coupled, secondary active urea transporter (Michaelis constant = 10.1 mM, maximal velocity = 0.34 mu mol.h(-1).mg protein(-1)). We propose t hat this system actively transports urea out of the gill epithelial cells b ack into the blood against the urea concentration gradient. Lipid analyses of the basolateral membrane revealed high levels of cholesterol contributin g to the highest reported cholesterol-to-phospholipid molar ratio (3.68). T his unique combination of active urea transport and modification of the pho spholipid bilayer membrane is responsible for decreasing the gill permeabil ity to urea and facilitating urea retention by the gill of Squalus acanthia s.