Spontaneous Ca2+ oscillations via purinergic receptors elicit transient cell swelling in rat parotid ducts

Rat parotid ductal cells were found to exhibit spontaneous Ca2+ oscillations. These oscillatory Ca2+ responses were observed during continuous perfusion with physiological salt solution at 37℃ in the absence of calcium mobilizing agonist stimulation. These Ca2+ oscillations were completely blocked by the purinergic receptor inhibitors, pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid) (PPADS) and suramin, but were not blocked by the muscarinic antagonist, atropine, nor the α-adrenergic antagonist, phentolamine. Simultaneous observation with fura-2 fluorescence and differential interference contrast (DIC) images showed that the spontaneous elevations of [Ca2+]i were well correlated with the shape changes of the ductal cells. Using a plasma membrane fluorescence probe, we found that the changes in DIC images reflected spontaneous cell swelling of ductal cells. Electron microscopic analysis after Ca2+ imaging indicated that the spontaneously oscillating duct cells contained numerous granules at the luminal side, which is characteristic of the granular duct cells. These results indicate that the spontaneous [Ca2+]i increase occurs through purinergic receptors, and activates Ca2+-dependent ion transporters and/or channels. Our findings present the possibility that spontaneous Ca2+ oscillations via purinergic receptors are involved in the regulation of the electrolyte composition of saliva in resting states.