Fig. 9. Current model for the regulation of granule exocytosis in parotid acinar cells in which the minor regulated pathway provides granule docking/fusion sites. Abbreviations: CL, constitutive-like pathway; CV, condensing vacuole; IG, immature granule; MR, minor regulated pathway; SG, secretory granule. (A) Selective stimulation of the minor regulated pathway (red vesicles) by 40 nM CCh. Analogous selective stimulation occurs in response to <100 nM Iso (Castle and Castle, 1996). Discharge creates docking/fusion sites for secretory granules. (B) Sequential exocytosis of the minor regulated pathway and oldest secretion granules accumulated nearest to the apical surface in response to 2 µM CCh. (C) Sequential exocytosis of the minor regulated pathway and compound exocytosis leading to release of newly synthesized granules deep in the storage pool in response to 1 µM isoproterenol (Iso). (D) Secretory potentiation in response to 40 nM CCh and 1 µM isoproterenol resulting from multiple pathways of compound exocytosis at multiple docking/fusion sites created by the minor regulated pathway.

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