Ion secretion by salt glands of desert iguanas (Dipsosaurus dorsalis)

Abstract

Unlike the NaCl-secreting salt glands of many birds and reptiles, the nasal salt glands of lizards can secrete potassium as well as sodium, with either chloride or bicarbonate as the accompanying anion. The factors responsible for initiating secretion by the gland and the rates of cation and anion secretion were studied in the desert iguana, Dipsosaurus dorsalis. Lizards were given combinations of ions for several days, and secreted salt was collected daily and analyzed for sodium, potassium, chloride, and bicarbonate. Maximum total cation secretion rate was 4.4+/-0.38 micromol/g/d. Cation secretion ranged from 24% to 100% potassium; even high NaCl loads did not abolish potassium secretion. Maximum bicarbonate secretion was about 0.5 micromol/g/d; chloride was the predominant anion. Secretion rate increased only in response to those treatments that included potassium and/or chloride; sodium ions and other osmotic loads (e.g., sucrose) did not increase secretion. This is in contrast to birds and some other reptiles with salt glands, which initiate NaCl secretion in response to any osmotic load. The specificity of the response of the salt gland of Dipsosaurus may be related to the ecological importance of dietary potassium and chloride for herbivorous desert lizards.