INOSITOL 1,4,5-TRISPHOSPHATE- AND GUANOSINE 5'-O-(3-THIOTRIPHOSPHATE)-INDUCED CA2+ RELEASE IN CULTURED AIRWAY SMOOTH-MUSCLE

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 Ca-45 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-mu-M) abolished the GTP-gamma-S-induced Ca2+ release, whereas pretreatment with 100-mu-M GTP-gamma-S reduced the InsP3-induced Ca2+ release by 25%. 3 Ryanodine (100-mu-M), also gave a large release of intracellular Ca2+. After pretreatment with 100-mu-M 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.