Abstract
In salivary acinar cells, the pattern of the Ca2+ signals that regulates fluid and enzyme secretion has yet to be resolved, as there are conflicting reports in the literature. We have used a two-photon technique to directly visualize the acinar cell lumen in living fragments of exocrine tissue and simultaneously recorded agonist-induced changes in intracellular Ca2+. We show near-synchronous global Ca2+ responses in submandibular acinar cells, distinct from the typical apical to basal Ca2+ wave usually seen in rodent pancreatic acinar cells. In an effort to explain the basis of these near-synchronous global Ca2+ responses we used immunocytochemical experiments to localize luminal proteins and inositol trisphosphate receptors (InsP3Rs) in tissue fragments. Zona occludens 1 (ZO-1), a tight junction protein, shows that individual submandibular acinar cells are often nearly completely encircled by a narrow luminal structure. By contrast, in pancreatic fragments, ZO-1 staining shows short luminal branches terminating abruptly at the apical pole of single acinar cells. Co-immunostaining of InsP3Rs type 2 and type 3 showed them in the same region as ZO-1 in both exocrine tissues. Functional experiments showed that the near-synchronous global Ca2+ responses were still observed in the absence of extracellular Ca2+ and also in the presence of ryanodine. We conclude that the elaborate luminal region of submandibular cells leads to a hitherto unrecognized extensive distribution of InsP3Rs in a band around the cell and that this underlies the near-synchronous global Ca2+ response to agonists. We suggest that this may be a structural adaptation in submandibular cells to support the copious amounts of fluid secreted.
Highlights
The importance of intracellular Ca2+ in stimulus-secretion coupling in secretory epithelial cells has been long recognized (Matthews et al, 1973; Hunter et al, 1983; Petersen, 1992)
Ca2+ oscillations in epithelia were first shown in parotid acinar cells (Gray, 1988) and since Kasai and Augustine’s seminal paper on pancreatic acinar cells (Kasai and Augustine, 1990) an apical initiation and basal propagation of Ca2+ waves has become dogma in the field (Thorn et al, 1993; Toescu et al, 1992; Elliot et al, 1992; Lee et al, 1997)
We postulated that a reduction in cell volume, and subsequent concentration of dye, might be a major problem in imaging submandibular acinar cells
Summary
The importance of intracellular Ca2+ in stimulus-secretion coupling in secretory epithelial cells has been long recognized (Matthews et al, 1973; Hunter et al, 1983; Petersen, 1992). Ca2+ oscillations in epithelia were first shown in parotid acinar cells (Gray, 1988) and since Kasai and Augustine’s seminal paper on pancreatic acinar cells (Kasai and Augustine, 1990) an apical initiation and basal propagation of Ca2+ waves has become dogma in the field (Thorn et al, 1993; Toescu et al, 1992; Elliot et al, 1992; Lee et al, 1997). It is not clear if this is the case for salivary acinar cells where the patterns of Ca2+ signals in are still not resolved.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have