Introduction
Salt secreting supraorbital glands are present in at least 10 orders of birds, especially marine species. Some terrestrial birds, like ostriches, also have salt glands but their function is limited (Gray & Brown, 1994). Salt secretion is inhibited by acetazolamide, a carbonic anhydrase (CA) inhibitor (Fänge et al., 1958), and the presence of CA was shown biochemically by Nechay et al. (1960). Although the salt secreting mechanisms have been thoroughly studied the localization of CA has, to our knowledge, not been performed.
Methods
All birds were raised on commercial feed with fresh water provided ad libitum. Salt glands collected at slaughter from 4 adult domestic ducks and 2 adult ostriches were fixed in 2.5% buffered glutaraldehyde, embedded in water-soluble resin (Historesin) and 2 mm thin sections were processed for histochemistry according to Ridderstråle (1976), resulting in a black precipitate at sites of CA-activity. Controls run with acetazolamide (10-5 M) in the medium were devoid of precipitate.
Results
The salt gland consists of blind ended secretory tubules which drain into the duct system. The tubule is composed of principal secretory cells and, towards the blind end, undifferentiated peripheral cells.
Duck: In the principal cells the baso-lateral cell membrane infoldings show staining for CA, which increases in intensity towards the central duct. The peripheral cells are unstained. Columnar and cuboidal cells of both the central and larger ducts present strong staining of the lateral cell membranes.
Ostrich: Principal cells show both apical and baso-lateral membrane staining. Peripheral cells have weakly stained lateral and occasionally basal membranes. Only the cuboidal cells of the central ducts show some membrane bound CA activity.
Conclusions
Earlier studies have implied that duct cells do not participate in the formation of salt gland secretions. Marshall et al. (1985), using x-ray microanalysis, suggests that a primary iso-osmotic fluid is produced by the principal cells and that the duct cells are responsible for concentrating this fluid. According to Shuttleworth and Wood (1992) activation of secretion results in a rapid intracellular acidification, which is restored to resting levels by unknown mechanisms. We suggest that the membrane bound CA found in our study could be involved in this pH recovery. Especially since secretion is impaired by the CA inhibitor acetazolamide (Fänge et al., 1958). The strong ductular staining in ducks supports the idea of a secretory function for the ducts. The small amount of CA in the ostrich gland may be correlated to their lower secretory ability (Gray & Brown, 1995).
References
Fänge, R., Schmidt-Nielsen, K. & Robinson, M. 1958. Am J Physiol 195(2), 321-326
Gray, D.A. & Brown, C.R. 1995. Phys Zool 68(1), 164-175
Marshall, A.T. Hyatt, A.D., Phillips, J.G. & Condron, R.J. 1985. J Comp Physiol B 156, 213-227
Nechay, B.R., Larimer, J.L. & Maren, T.H. 1960. J Pharmacol Exp 130, 401-410
Ridderstråle, Y. 1976. Acta Physiol Scand 98, 465-469
Shuttleworth, T.J. & Wood, C.M. 1992. Am J Physiol 262(31), C221-C228