Beschreibung:
<jats:p><jats:list list-type="explicit-label">
<jats:list-item><jats:p>The interaction between inositol 1,4,5‐trisphosphate (InsP<jats:sub>3</jats:sub>) and guanosine 5′‐O‐(3‐thio triphosphate) (GTPγS) releasable calcium (Ca<jats:sup>2+</jats:sup>) pools was examined using <jats:sup>45</jats:sup>Ca effluxes in permeabilized cultured airway smooth muscle cells from rabbit trachea.</jats:p></jats:list-item>
<jats:list-item><jats:p>Addition of InsP<jats:sub>3</jats:sub> or GTPγS caused a concentration‐dependent release of intracellular Ca<jats:sup>2+</jats:sup>. The release of Ca<jats:sup>2+</jats:sup> by InsP<jats:sub>3</jats:sub> was much greater than with GTPγS. Pretreatment with maximally effective InsP<jats:sub>3</jats:sub> (10 μ<jats:sc>m</jats:sc>) abolished the GTPγS‐induced Ca<jats:sup>2+</jats:sup> release, whereas pretreatment with 100 μ<jats:sc>m</jats:sc> GTPγS reduced the InsP<jats:sub>3</jats:sub>‐induced Ca<jats:sup>2+</jats:sup> release by 25%.</jats:p></jats:list-item>
<jats:list-item><jats:p>Ryanodine (100 μ<jats:sc>m</jats:sc>), also gave a large release of intracellular Ca<jats:sup>2+</jats:sup>. After pretreatment with 100 μ<jats:sc>m</jats:sc> ryanodine, GTPγS did not induce Ca<jats:sup>2+</jats:sup> release, and InsP<jats:sub>3</jats:sub>‐induced Ca<jats:sup>2+</jats:sup> release was reduced by 76%.</jats:p></jats:list-item>
<jats:list-item><jats:p>Caffeine (50 m<jats:sc>m</jats:sc>), produced a slow release of intracellular Ca<jats:sup>2+</jats:sup>. Pre‐exposure to 50 m<jats:sc>m</jats:sc> caffeine had no effect on the GTPγS‐induced Ca<jats:sup>2+</jats:sup> release but reduced the InsP<jats:sub>3</jats:sub> releasable Ca<jats:sup>2+</jats:sup> by 58%.</jats:p></jats:list-item>
<jats:list-item><jats:p>Pretreatment with ryanodine abolished the caffeine‐induced Ca<jats:sup>2+</jats:sup> release, and addition of caffeine before ryanodine reduced the ryanodine‐induced Ca<jats:sup>2+</jats:sup> release by 64.4%.</jats:p></jats:list-item>
<jats:list-item><jats:p>These results suggest that there are at least three pools of Ca<jats:sup>2+</jats:sup> present within airway smooth muscle cells. The largest pool is released by InsP<jats:sub>3</jats:sub> or ryanodine, another is released either by a high concentration of InsP<jats:sub>3</jats:sub> or on application of GTPγS, and the third by InsP<jats:sub>3</jats:sub> alone. Ca<jats:sup>2+</jats:sup> may be able to move from the GTPγS‐sensitive pool into the InsP<jats:sub>3</jats:sub>‐ and ryanodine‐sensitive pool when this becomes depleted. In contrast, the opposite movement of Ca<jats:sup>2+</jats:sup> cannot occur.</jats:p></jats:list-item>
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