We used in vivo perfusion of the lower intestine (colon plus coprodeum) to examine the osmoregulatory role of that organ in glaucous-winged gulls (Larus glaucescens Naum.) drinking sea water and fresh water. The area of the lower intestine was small, 0.59 cm2 100 g−1 body mass. Its electrolyte and water transport rates were unaffected by salt water acclimation. Sodium and chloride were always reabsorbed from the intestinal lumen to the plasma. The maximum sodium transport rate we measured was 37 μequiv cm−2h−1, from a sodium concentration of 200 mequiv l−1. Potassium transport from the different perfusion fluids differed, and was generally negatively correlated with sodium transport, suggesting some form of sodium-potassium exchange. Solute-linked water was reabsorbed at a rate of 3.5 μl μequiv−1 Na+, permitting the lower intestinal epithelium to maintain a luminal fluid up to 130 mosmol kg−1 hyperosmotic to plasma. The serosal-tomucosal osmotic permeability coefficient of the lower intestinal epithelium was 0.21 μl cm−2h−1 (mosmol kg−1)−1. Ureteral urine from birds in both groups was usually strongly (up to 250 mosmol kg−1) hypertonic to plasma, but sodium, potassium and chloride concentrations were low and constituted less than 25 % of total osmolytes. We suggest that the lower intestine of L. glaucescens does not play a major role in reclaiming urinary water and electrolytes for recycling to the salt glands.

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