Control of secretion from the avian salt gland.

F;~NGE, RAGNAR, KNUT SCHMIDT-NIELSEN AND MARYANNE ROBINSON, Control of secretion from the avian salt gland. Am. J. Physiol. 195(2): 32 1-326. 1958.In birds with a marine habitat the nasal glands are modified into salt glands, able to excrete excess sodium chloride. The nervous control of the salt glands was investigated in the herring gull, Lams argentatus. Normal secretion of the salt gland seems to be released by a nerve reflex involving higher nerve centers and central osmoreceptors. The reflex can be evoked experimentally by intravenous injection of a hypertonic NaCl solution or by increasing the osmotic pressure of the blood in other ways. The gland is innervated from a nerve plexus in the anterior part of the orbit of the eye. Secretion is produced by stimulation of a nerve, probably a branch of the VII cranial nerve, which connects with the plexus. The plexus also receives sympathetic fibers, but no secretion was observed after stimulation of the cervical sympathetic. The gland is stimulated to secrete its osmotically highly concentrated fluid (700-800 rnM Na) by Mecholyl and acetylcholine, indicating a parasympathetic innervation as the normal excitatory pathway. The secretion that normally occurs in response to a salt load is blocked by anesthesia. It is also inhibited by atropine, adrenaline or acetazolamide (Diamox). LTHOUGH the nasal gland (glandula latA eralis nasi) was described as early as in 1664 by Nicolaus Steno (I), it has been neglected by physiologists, probably because in man it exists only in the embryological stage (2). Nasal glands are found in birds, reptiles and most mammals. They usually are tubulous and serous, and their function is supposed to be that of moistening and protecting the nasal membranes. However, in birds with a marine habitat (gull, albatross, pelican, penguin, etc.), the nasal gland is modified into a remarkably efficient organ for excretion of excess sodium chloride from the body. This salt gland has an interesting microscopic structure somewhat reminiscent of the medulla of the mammalian kidney, consisting of closely packed parallel secretory tubules surrounded by blood capillaries. A description of the anatomy and microscopic structure of the salt gland of the herring gull is given by FZnge, Schmidt-Nielsen and Osaki (zb). The secretory fluid from the salt gland of marine birds Received for publication May 23, 1958. and reptiles has a salt concentration many times as high as that of the blood, and often nearly twice as high as that of sea water (3-5). Since the avian and reptilian kidneys cannot produce a very concentrated urine, the salt gland is essential in the excretion of salts from sea water ingested with the food or for drinking (6). Normally the salt gland secretes only when there is an osmotic stimulus, i.e. after absorption of sodium chloride from food or water with a high salt content. The secretory response is probably brought about by a nerve reflex, supposedly involving osmoreceptors situated in the brain or in the wall of certain blood vessels. It has been the purpose of the present investigation to study if, and to what extent, the salt gland is controlled by nerve action. A secretory nerve is described, and the effects on the gland of nerve stimulation and of different drugs are reported (see also 7). MATERIAL AND METHODS Material. Young specimens of herring gull (Laws argentatus) w&e trapped and kept in 321 by 10.0.33.2 on M ay 3, 2017 http://ajple.physiology.org/ D ow nladed fom R. F;iNGE, K. SCHMIDT-NIELSEN AND M. ROBINSON FIG. I. Approach for cannulation of the salt gland duct, showing the palate from below. The left half of the drawing shows the location of the incision. In the right half of the drawing a part of the roof of the mouth is removed, exposing the region of the vestibular concha with the two openings of the ducts from the salt gland. S.G. : openings from the supramaxillary glands. captivity. The food consisted of fresh fish, later substituted by canned cat food of a brand containing only ground fish. The body weight of the birds varied from 715 to 1090 gm. Operation Technique. In most cases chloral hydrate was used for anesthesia. It was given intraperitoneally in a solution obtained by dissolving 1 gm chloral hydrate in I ml water. After an initia 1 dose of 0.3 ml, one or two further doses of 0.1 ml were added, if necessary, to obtain the desired degree of anesthesia. Intravenous injection of Nembutal was used in some cases. During the operation the animals were loosely wrapped in cheese cloth and fixed to an operating table. Exposure of the secretory nerve was made by removing the eyelids and pulling the The major eye aside nerves of or entirely removing it. the orbit could then be found without difficulty (see description later in the text). The cervical sympathetic, which in birds runs in the foramen transversum of the cervical vertebrae, is difficult to reach, The operation to expose it was made according to Langley (8). C annulation of the gland ducts was made through incisions made in the palate (fig. I), *by means of polyethylene tubings, 0.8 mm o.d., which were introduced into the lateral duct openings. Attempts to cannulate the median duct openings in a similar way were unsuccessful. Nerve Stimulation. Before stimulating t-he secretory nerve it was cut as far cent.rally as possible. Stimulation was given by silver electrodes from a thyratron stimulator. In most experiments the frequency was zo/sec. at TO volts intensity. Osmotic Load. About IO ml/kg body weight . of IO% NaCl solution were injected into a vein on the median side of the foot. Drugs Used. The drugs used were: acetyl choline chloride, methacholine chloride (Mecholyl), atropine sulfate, Z-adrenaline chloride, histamine phosphate, pilocarpine hydrochloride and acetazolamide (Diamox). The doses given in the text refer to the amount of the salts injected per kilogram bird. Collection of Secreted Fluid. Samples were either collected from the cannulated lateral duct or as drops from the tip of the beak. In the former case the sample came from about one-fourth of the gland, while in the latter case the total secretion from the salt gland was collected. Usually samples were collected in five minute periods. Analyses of sodium, potassium and chloride were performed as in previous work (3).