Intracellular Ca²⁺ shift and signal transduction from the tubulovesicular portion of gastric parietal cells during gastrin stimulation or Ca²⁺ ionophore treatment: comparison between luminescent and fluorescent probes, and electron probe X-ray microanalyzer

<jats:p> In gastrin-stimulated, aequorin-loaded parietal cells from guinea pig gastric mucosa, a rapid but transient increase in the cytosolic free Ca<jats:sup>2+</jats:sup> concentration ([Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub>), owing to Ca<jats:sup>2+</jats:sup> released from the store(s), and a more prolonged Ca<jats:sup>2+</jats:sup> entry from outside the cells were observed. However, there was a little increase in [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> when similar measurements were assessed by quin 2 or fura-2 in physiological saline. However, depletion or elimination of Na<jats:sup>+</jats:sup> from the incubation medium caused a significant increase in the [Ca<jats:sup>2+</jats:sup>]; response to gastrin as measured by quin 2. These findings suggest that aequorin and quin 2 (or fura-2) provide information about different aspects of Ca<jats:sup>2+</jats:sup> homeostasis and that there is an inhomogeneity of [Ca<jats:sup>2+</jats:sup>]<jats:sub>i</jats:sub> in the cytoplasm during gastrin stimulation. By the gastrin stimulation, the intracellular Ca<jats:sup>2+</jats:sup> gradients were shifted from the unidentified portion(s) to the restricted apical cytoplasm, as determined by electron probe X-ray microanalysis. Therefore, localization and identification of the source of intracellular Ca<jats:sup>2+</jats:sup> as a pool were determined by an X-ray microanalyzer. In the resting state, the tubulovesicle had high Ca<jats:sup>2+</jats:sup> concentration compared with the level in the apical cytoplasm. Cells treated with the Ca<jats:sup>2+</jats:sup> ionophore ionomycin had a decreased tubulovesicular Ca<jats:sup>2+</jats:sup> level, followed by a reciprocal increase in area of the canalicular membrane. The secretory canaliculus in stimulated cells had lower Ca<jats:sup>2+</jats:sup> or higher K<jats:sup>+</jats:sup> and Cl<jats:sup>−</jats:sup> concentrations than that of tubulovesicles or cytoplasm in the resting state, respectively. These findings suggest that the Ca<jats:sup>2+</jats:sup> pool of the parietal cell is in the tubulovesicles and (or) luminal cell membrane and that the Ca<jats:sup>2+</jats:sup> released from the store(s) may mediate a flow of K<jats:sup>+</jats:sup> or Cl<jats:sup>−</jats:sup> into the secretory canaliculus. </jats:p>