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J. Biol. Chem., Vol. 276, Issue 25, 22296-22306, June 22, 2001
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From the Department of Cell Biology, University of Virginia Health
Sciences Center, Charlottesville, Virginia 22908
Resting secretion of salivary proteins by the
parotid gland is sustained in situ between periods of
eating by parasympathetic stimulation and has been assumed to involve
low level granule exocytosis. By using parotid lobules from ad
libitum fed rats stimulated with low doses of carbachol as an
in vitro analog of resting secretion, we deduce from the
composition of discharged proteins that secretion does not involve
granule exocytosis. Rather, it derives from two other acinar export
routes, the constitutive-like (stimulus-independent) pathway and the
minor regulated pathway, which responds to low doses of cholinergic or
Resting (Basal) Secretion of Proteins Is Provided by the Minor
Regulated and Constitutive-like Pathways and Not Granule Exocytosis in
Parotid Acinar Cells*
,,
-adrenergic agonists (Castle, J. D., and Castle, A. M. (1996) J. Cell Sci. 109, 2591-2599). The protein
composition collected in vitro mimics that collected from
cannulated ducts of glands given low level stimulation in situ. Analysis of secretory trafficking along the two pathways of
resting secretion has indicated that the constitutive-like pathway may
pass through endosomes after diverging from the minor regulated pathway
at a brefeldin A-sensitive branch point. The branch point is deduced to
be distal to a common vesicular budding event by which both pathways
originate from immature granules. Detectable perturbation of neither
pathway in lobules was observed by wortmannin addition, and neither
serves as a significant export route for lysosomal procathepsin B. These findings show that parotid acinar cells use low capacity, high
sensitivity secretory pathways for resting secretion and reserve
granule exocytosis, a high capacity, low sensitivity pathway, for
massive salivary protein export during meals. An analogous strategy may
be employed in other secretory cell types.
*
This work was supported by Grant DE08941 from the National
Institutes of Health.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
Both authors contributed equally to this work.
§
To whom correspondence should be addressed: Dept. of Cell Biology,
University of Virginia Health System, School of Medicine, Charlottesville, VA 22908. Tel.: 804-924-1786; Fax: 804-982-3912; E-mail: jdc4r@virginia.edu.
Copyright © 2001 by The American Society for Biochemistry and Molecular Biology, Inc.
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