Salt Gland Function in the Green Sea Turtle Chelonia Mydas

When salt-gland-cannulated green sea turtles were stimulated by the intravenous injection of various salt loads the secretion osmolality rose quickly from a basal value of 300–400 mosmol kg-1 to a plateau of 1680–2000. The height of the plateau and the duration of the response were dependent on the amount of salt given. An increase in flow rate accompanied the initial rise in concentration, but the flow rate thereafter declined while the plateau concentration was held. The fluid was protein-free, and was mainly composed of Na+ and Cl-, in similar relative concentrations to those in sea water, but had substantial amounts of K+, Mg2+ and HCO3- and negligible amounts of glucose. Urea concentrations did not change with osmolarity, and were similar to that of plasma, suggesting that the walls of the salt gland ducts are permeable to urea. The composition of the tears was uninfluenced by the concentration of the injected solutions, and its ionic content did not directly depend upon the identity of the added salts. The two eyes of individual turtles frequently did not function in synchrony. The average amount of the salt load remaining in a turtle at the end of salt gland activity, 4·5 mmol ion kg body mass-1, was very similar to the median load required to stimulate the salt gland (4·0mmolion kg body mass-1). This suggests that there is a threshold salt load level of around 4·5 mmol ion kg body mass-1 that determines salt gland on/off activity. The flow/concentration relationships of the salt gland fluid are consistent with water reabsorption from a primary isosmotic secretion being the major mechanism for concentrating the tears. A high maximal secretory rate is indicated for the primary fluid, 0·32 ml g salt gland-1 min-1, equivalent to that of the mammalian kidney. Calculations suggest that 83% of the initial fluid is recovered, Na+ is passively concentrated, Cl- and Mg2+ concentrations are enhanced, and that the duct walls are either impermeable to glucose or that glucose is actively taken up.