| Summary: | The retention of high concentrations of urea in the tissues of marine elasmobranchs is the key to their osmoregulatory strategy. The mechanisms responsible for the low urea permeability of the gill epithelium from the spiny dogfish ('Squalus acanthias') were investigated using enriched basolateral membrane vesicles (BLMV). Urea uptake (Km = 10 mM) was sodium dependent and inhibited by phloretin (I50 = 0.08 mM) and urea analogs (e.g. thiourea and methylurea). Much of the impermeability of the BLMV may be due to the very high cholesterol content apparent from the high cholesterol to phospholipid ratio (3.68). The high phosphatidylcholine and low polyunsaturated fatty acid levels are also likely to confer increased order to the bilayer membrane, making it less fluid and less permeable. Taken together, these findings indicate that low urea permeability in the dogfish gill is primarily due to an active urea transporter that returns urea to the blood against the concentration gradient and a unique lipid composition that minimizes diffusion urea across the basolateral membrane.
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