Isosmotic secretion in the avian nasal salt gland: X-ray microanalysis of luminal and intracellular ion distributions

Vertebrate salt glands: short- and long-term regulation of function

Excess salt loads in most non-mammalian vertebrates are dealt with by a variety of extra-renal salt-secreting structures collectively described as salt glands. The best studied of these are the supra-orbital nasal salt glands of birds. Two distinct types of response to osmoregulatory disturbances are shown by this structure: a progressive adaptive response on initial exposure to a salt load that results in the induction and enhancement of the secretory performance or capabilities of the gland; and the rapid activation of existing osmoregulatory mechanisms in the adapted gland in response to immediate osmoregulatory imbalance. Not only is the time-frame of these two types of response very different, but the responses usually involve fundamentally different processes: e.g., the growth and differentiation of osmoregulatory structures and their components in the former case, compared with the rapid activation of ion channels, pumps etc. in the latter. Despite marked differences in the nature and time-frame of these responses, they both are apparently triggered by neuronally released acetylcholine, which acts at muscarinic receptors on the secretory cells to induce an inositol phosphate-dependent increase in cytosolic-free calcium concentrations ([Ca2+]i). Therefore, the question arises as to how the cells produce the appropriate distinct response using a single common signal (i.e., an increase in [Ca2+]i). Examination of the features of this signaling pathway in the two conditions described, reveals that they each are uniquely tuned to generate a response with the characteristics appropriate for the cells' requirements. This tuning of the signal involves often rather subtle changes in the overall signaling pathway that are part of the adaptive differentiation process.

Progress in quantitative X-ray microanalysis of frozen-hydrated bulk biological samples

The analysis of bulk frozen-hydrated biological samples has developed now to a level where practical application of the technique is possible. Provided the sample is carefully coated with a conductive metal, the development of a space charge capable of causing a significant distortion of the electron diffusion volume does not seem to occur, and analytical resolution can be conveniently held to approximately 2 micron (both depth and lateral resolution). Two valid quantitative methods are available, and two methods of determining dry weight fractions are also available. An area where further research could lead to improvement in analysis of frozen-hydrated bulk samples is in the investigation of fracturing methods. If fracture planes that were flat and reproducible could be easily obtained, some of the difficulties of analysing frozen-hydrated bulk samples would be considerably reduced.

Reevaluation of the effect of substance P on nasal salt gland secretion in the duck (Anas platyrhynchos)

Adrenergic and osmoregulatory responses to substance P (SP) were studied in salt-loaded ducks. SP inhibited nasal salt secretion by decreasing the salt concentration of the nasal fluid, but reversed the inhibitory effect of angiotensin II (ANG II) on nasal water and K secretion. SP also stimulated salivation, urinary water excretion, and urinary salt excretion, but diminished the diuretic and saluretic responses to ANG II. The osmoregulatory effects of SP were accompanied by elevated plasma concentrations of norepinephrine and epinephrine. The data suggest that SP may contribute to adrenergic and osmotic regulation in ducks.

The duct system of the avian salt gland as a transporting epithelium: structure and morphometry in the duck Anas platyrhynchos

The duct system of the nasal salt gland of the duck comprises central central canals, secondary ducts and main ducts. The secondary and main ducts consist of a layer of columnar cells overlying a layer of small cuboidal cells. The columnar cells have complex intercellular spaces showing evidence of Na+K+-ATPase at the apical regions. Approximately 70% of surface area of the duct system is external to the gland. During adaptation to salt water the duct system increases in size as does the gland. Although the components of the gland of adapted ducks, including the duct system within the gland, increase in size compared with normal ducks, the percentage volume densities of the components remain similar in both categories of ducks, i.e. the duct system increases in size in proportion to the glandular tissue. The volume of the duct system external to the gland is six to seven times larger than the volume within the gland. Thus, if ductal modification of secreted fluid occurs, it will be most likely to take place in the ducts external to the gland. Total surface areas of the duct system were measured from serial sections of glands and ducts from one normal and one adapted duck. These were used to calculate possible flux rates of water and sodium across the duct epithelium, assuming the occurrence of either water reabsorption of sodium secretion. Although these flux rates are high it is shown that they are similar to calculated flux rates across the luminal surface of the secretory tubules.