Transient swelling of salivary acinus induced by acetylcholine stimulation: water secretion pathway in rat submandibular gland.

The volume changes of isolated acini and acinar cells from rat submandibular glands were measured from digitized images recorded upon stimulation of acetylcholine (ACh) or reduction of the perfusate osmolarity and water secretion pathway in salivary gland was studied. When acinus is exposed to a hyposmotic solution, water flows into the acinar cells and into the lumen via acinar epithelia. If the water enters the lumen chiefly via the cells, the swelling of the lumen would follow the same time course as the cell swelling or slower. The results show that reduction of the perfusate osmolarity evoked a transient increase followed by a gradual increase in the volume of unstimulated acinus, while it evoked only a gradual increase in the volumes of unstimulated acinar cells. Thus, the time course of the acinar swelling is faster than that of the acinar cell swelling. Reduction of the perfusate osmolarity also evoked a transient swelling in ACh stimulated acini. When acinus is stimulated by ACh, water also flows into the lumen via acinar epithelia according to the osmotic gradient which was generated by the active electrolyte transport of acinar cells. If the water enters the lumen chiefly from the cells, there would be no overall change in acinar volume. The results show that stimulation of ACh (5 microM) evoked a transient increase followed by a gradual decrease in the volume of the acinus, while it evoked only a decrease in the volume of acinar cells. Video-enhanced optical microscopy exhibited that ACh stimulation caused transient swelling of the luminal space, prior to causing the volume of acinar cells to decrease and the transient swelling of the lumen followed the same time course as that of acinus. Thus, the transient acinar swelling is explained by the transient swelling of luminar volume. These results suggest that water is probably drawn into the lumen from interstitial space directly in the salivary acinus.