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

Excess salt loads in most non-mammalian vertebrates are dealt with by a var iety of extra-renal salt-secreting structures collectively described as sal t glands. The best studied of these are the supra-orbital nasal salt glands of birds. Two distinct types of response tee osmoregulatory disturbances a re shown by this structure: a progressive adaptive response on initial expo sure to a salt load that results in the induction and enhancement of the se cretory performance or capabilities of the gland; and the rapid activation of existing osmoregulatory mechanisms in the adapted gland in response to i mmediate osmoregulatory imbalance. Not only is the time-frame of these two types of response very different, but the responses usually involve fundame ntally different processes: e.g., the growth and differentiation of osmoreg ulatory structures and their components in the former case, compared with t he rapid activation of ion channels, pumps etc, in the latter. Despite mark ed differences in the nature and time-frame of these responses, they both a re apparently triggered by neuronally released acetylcholine, which acts at muscarinic receptors on the secretory cells to induce an inositol phosphat e-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 cond itions described, reveals that they each are uniquely tuned to generate a r esponse with the characteristics appropriate for the cells' requirements. T his tuning of the signal involves often rather subtle changes in the overal l signaling pathway that are part of the adaptive differentiation process. (C) 1999 Wiley-Liss, Inc.