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1 The Water and Salt Research Center and Institute of Anatomy, University of Aarhus, Aarhus, Denmark
2 Departments of Medicine and Biological Chemistry, The Johns Hopkins University School of Medicine, Baltimore, MD, USA
3 School of Biological Sciences, University of Manchester, Manchester, United Kingdom
* To whom correspondence should be addressed. E-mail: sn{at}ana.au.dk.
In vitro studies of cultured salivary gland cells and gland slices have indicated
that there may be regulated translocation of AQP5 between the apical plasma
membrane and intracellular compartments of the secretory cells. However it
remains unknown whether AQP5 in salivary glands is subject to regulated
trafficking in vivo. In order to examine this possibility, we have investigated the
subcellular localization of AQP5 in rat parotid and submandibular glands fixed
in vivo under conditions of stimulated or inhibited salivary secretion.
Immunofluorescence and immunoelectron microscopy was used to determine
the subcellular distribution of AQP5 in control conditions, following the
stimulation of secretion with pilocarpine (a muscarinic agonist) or epinephrine
(an 
-adrenoceptor agonist) or during inhibition of basal secretion with
atropine (a muscarinic antagonist) or phentolamine (an -adrenoceptor
antagonist). Under control conditions, > 90% of AQP5 was associated with the
apical plasma membrane of acinar and intercalated duct cells, with only rare
gold particles associated with intracellular membrane domains. Pilocarpine
treatment dramatically increased saliva production but had no discernible
effect on AQP5 distribution. However, the increased salivary secretion was
associated with luminal dilation and the appearance of a markedly punctate
AQP5 labeling pattern due to clustering of AQP5 at the microvilli (especially
evident in the parotid gland) after 10 min of drug injection. No changes in the
subcellular localization of AQP5 were seen in response to epinephrine,
atropine or phentolamine treatment as compared to control tissues. Thus
AQP5 is predominantly localized in the apical plasma membrane under
control conditions, and neither the onset nor the cessation of secretion is
associated in vivo with any significant short-term translocation of AQP5
between intracellular structures and the apical plasma membrane.
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