Biphasic kinetics of inositol 1,4,5‐trisphosphate accumulation, in gastrin‐stimulated parietal cells Effects of pertussis toxin and extracellular calcium

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Abstract
The effect of Pertussis toxin (PTx) and extracellular Ca2+ were investigated on gastrin‐induced Ins(1,4,5)P3 mass level in isolated gastric parietal cells. Basal Ins(1,4,5)P3 content was 5.48±0.49 pmol/500 000 cells. Gastrin (10 nM) induced a rapid increase in Ins(1,4,5)P3 content which was maximal after 15 s and corresponded to 2–2.5‐fold basal level; this Ins(1,4,5)P3 content then decreased within 30 s. After a longer time of gastrin exposure (> 1 min), a sustained and unexpected increased in Ins(1,4,5)P3 accumulation was observed which was maximal at 7.5 min (corresponding to 2.3–2.8‐fold basal value) and slightly decreased thereafter. PTx treatment of cells (200 ng/ml) for 3 h or removal extracellular Ca2+ did not affect the rapid rise, but drastically reduced the sustained increase in Ins(1,4,5)P3 content (60–100% inhibition); this inhibition was not evident after 10 min of hormone stimulation. Furthermore, diltiazem, a Ca2+ channel blocker, led to a similar inhibition of the sustained increase. We concluded that: (i) gastrin induced rapid increase in Ins(1,4,5)P3 content via a mechanism insensitive to PTx and to extracellular Ca2+, and (ii) gastrin induced a sustained increase in Ins(1,4,5)P3 level via a mechanism sensitive to PTx and to extracellular Ca2+. Even though the rapid rise in Ins(1,4,5)P3 content may be involved in the intracellular Ca2+ mobilization occurring after the first seconds of hormone stimulation, the physiological role of the sustained Ins(1,4,5)P3 increased level remains to be elucidated.

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