Inositol 1,4,5‐trisphosphate‐ and guanosine 5′‐O‐(3‐thio triphosphate)‐induced Ca2+ release in cultured airway smooth muscle

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Abstract
The interaction between inositol 1,4,5‐trisphosphate (InsP3) and guanosine 5′‐O‐(3‐thio triphosphate) (GTPγS) releasable calcium (Ca2+) pools was examined using 45Ca effluxes in permeabilized cultured airway smooth muscle cells from rabbit trachea. Addition of InsP3 or GTPγS caused a concentration‐dependent release of intracellular Ca2+. The release of Ca2+ by InsP3 was much greater than with GTPγS. Pretreatment with maximally effective InsP3 (10 μm) abolished the GTPγS‐induced Ca2+ release, whereas pretreatment with 100 μm GTPγS reduced the InsP3‐induced Ca2+ release by 25%. Ryanodine (100 μm), also gave a large release of intracellular Ca2+. After pretreatment with 100 μm ryanodine, GTPγS did not induce Ca2+ release, and InsP3‐induced Ca2+ release was reduced by 76%. Caffeine (50 mm), produced a slow release of intracellular Ca2+. Pre‐exposure to 50 mm caffeine had no effect on the GTPγS‐induced Ca2+ release but reduced the InsP3 releasable Ca2+ by 58%. Pretreatment with ryanodine abolished the caffeine‐induced Ca2+ release, and addition of caffeine before ryanodine reduced the ryanodine‐induced Ca2+ release by 64.4%. These results suggest that there are at least three pools of Ca2+ present within airway smooth muscle cells. The largest pool is released by InsP3 or ryanodine, another is released either by a high concentration of InsP3 or on application of GTPγS, and the third by InsP3 alone. Ca2+ may be able to move from the GTPγS‐sensitive pool into the InsP3‐ and ryanodine‐sensitive pool when this becomes depleted. In contrast, the opposite movement of Ca2+ cannot occur.

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