**9. Muscarinic acetylcholine receptors in the avian salt gland**

Unlike other vertebrate exocrine glands, the avian salt gland secretes an effluent that contains only trace amounts of protein or other macromolecular species, which makes it a unique model for the study of exocrine secretion of electrolytes [89]. This hypertonic secretion principally contains sodium and chloride ions in the rage of 500 to 900 mEq/l concentrations [15]. Administration of acetylcholine or methacholine in herring gulls stimulated salt gland secretion, while secretion in response to either a parenteral salt load or to a direct stimulation of the secretory nerve is blocked by injection of atropine, which suggests the role of muscarinic receptors responsible for its secretory function. In order to demonstrate and characterize these receptors in avian salt gland, radiolabeled muscarinic antagonist [H]quinuclidinyl benzilate ([3 H] QNB) was used in ducklings under fresh water and salt water conditions.

Characterization of these receptors using the radiolabeled antagonist, [3 H]QNB, showed them to be similar to muscarinic receptors from various mammalian sources, *Avian Muscarinic Receptors: An Update DOI: http://dx.doi.org/10.5772/intechopen.111720*

including rabbit iris, rabbit heart, rat parotid gland, and rat brain [90]. Regardless of conditions, since the DNA content of individual cells in the salt gland remains the same in case of salt stress, relating [3 H]QNB binding to DNA allows calculation of average number of receptors per cell. Upon conducting this calculation, it became evident that individual salt gland cells in salt water contains approximately three times as many muscarinic receptors as that of fresh water glands [15]. These results thus provide direct evidence for *de novo* synthesis of muscarinic cholinergic receptors during the plasma membrane hypertrophy that typifies the response of salt gland epithelial cells to chronic salt stress.
