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Research Article

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

^{1 2}Health Sciences University of Hokkaido ³Fukuoka Dental College

Submitted 1 May 2009 ; revision received 28 August 2009 ; accepted in final form 21 September 2009

ABSTRACT

Using multiphoton microscopy, we established that rat parotid ductal cells exhibit spontaneous oscillations in intracellular Ca²⁺ concentration ([Ca²⁺]_i). These oscillatory Ca²⁺ responses were observed during continuous perfusion with a physiological salt solutions at 37ºC in the absence of calcium mobilizing agonist stimulation. The timing and patterns of these spontaneous Ca²⁺ oscillations varied between individual ductal cells, and the average number of Ca²⁺ responses in a single responding ductal cells was 2.1 in a 10-min recording period. High-speed scanning (0.6 s/image) revealed that most spontaneous elevations in [Ca²⁺]_i were initiated at the luminal side of ductal cells, and spread toward the basal side within 2 s. Electron microscopic analysis after Ca²⁺ imaging indicated that spontaneously oscillating ducts contained numerous granules at the luminal side, which is characteristic of granular ducts. These Ca²⁺ oscillations were completely blocked by the purinergic receptor inhibitors, PPADS and suramin, but were not blocked by the muscarinic antagonist, atropine, or the -adrenergic antagonist, phentramine. Simultaneous observation of fura-2 fluorescence and differential interference contrast (DIC) images showed that spontaneous elevations of [Ca²⁺]_i were well correlated with changes in shape of ductal cells. Using a plasma membrane fluorescence probe, synaptogreen C4, we found that the changes in DIC images reflected spontaneous cell swelling of ductal cells. Our findings present the possibility that purinergic receptors mediate spontaneous Ca²⁺ oscillations in parotid ductal cells and regulate electrolyte reabsorption from the primary saliva in the resting state.

Salivary duct; Spontaneous Ca2+ oscillation; Purinergic receptors; Cell swelling; multiphoton microscopy