Inositol 1,4,5-trisphosphate- and guanosine 5'-O-(3-thiotriphosphate)-induced Ca2+ release in cultured airway smooth...

1. The interaction between inositol 1,4,5-trisphosphate (InsP3) and guanosine 5'-O-(3-thio triphosphate) (GTP gamma S) releasable calcium (Ca2+) pools was examined using 45Ca effluxes in permeabilized cultured airway smooth muscle cells from rabbit trachea. 2. Addition of InsP3 or GTP gamma S caused a concentration-dependent release of intracellular Ca2+. The release of Ca2+ by InsP3 was much greater than with GTP gamma S. Pretreatment with maximally effective InsP3 (10 microM) abolished the GTP gamma S-induced Ca2+ release, whereas pretreatment with 100 microM GTP gamma S reduced the InsP3-induced Ca2+ release by 25%. 3. Ryanodine (100 microM), also gave a large release of intracellular Ca2+. After pretreatment with 100 microM ryanodine, GTP gamma S did not induce Ca2+ release, and InsP3-induced Ca2+ release was reduced by 76%. 4. Caffeine (50 mM), produced a slow release of intracellular Ca2+. Pre-exposure to 50 mM caffeine had no effect on the GTP gamma S-induced Ca2+ release but reduced the InsP3 releasable Ca2+ by 58%. 5. Pretreatment with ryanodine abolished the caffeine-induced Ca2+ release, and addition of caffeine before ryanodine reduced the ryanodine-induced Ca2+ release by 64.4%. 6. 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 GTP gamma S, and the third by InsP3 alone. Ca2+ may be able to move from the GTP gamma S-sensitive pool into the InsP3- and ryanodine-sensitive pool when this becomes depleted. In contrast, the opposite movement of Ca2 + cannot occur.