Ca2+ dependence of the inositol 1,4,5-trisphosphate (IP3)-induced Ca release was studied in saponin-skinned smooth muscle fiber bundles of the guinea pig taenia caeci at 20-22 degrees C. Ca release from the skinned fiber bundles was monitored by microfluorometry of fura-2. Fiber bundles were first treated with 30 microM ryanodine for 120 s in the presence of 45 mM caffeine to lock open the Ca-induced Ca release channels which are present in approximately 40% of the Ca store of the smooth muscle cells of the taenia. The Ca store with the Ca-induced Ca release mechanism was functionally removed by this treatment, but the rest of the store, which was devoid of the ryanodine-sensitive Ca release mechanism, remained intact. The Ca2+ dependence of the IP3-induced Ca release mechanism was, therefore, studied independently of the Ca-induced Ca release. The rate of IP3-induced Ca release was enhanced by Ca2+ between 0 and 300 nM, but further increase in the Ca2+ concentration also exerted an inhibitory effect. Thus, the rate of IP3-induced Ca release was about the same in the absence of Ca2+ and at 3 microM Ca2+, and was about six times faster at 300 nM Ca2+. Hydrolysis of IP3 within the skinned fiber bundles was not responsible for these effects, because essentially the same effects were observed with or without Mg2+, an absolute requirement of the IP3 phosphatase activity. Ca2+, therefore, is likely to affect the gating mechanism and/or affinity for the ligand of the IP3-induced Ca release mechanism. The biphasic effect of Ca2+ on the IP3-induced Ca release is expected to form a positive feedback loop in the IP3-induced Ca mobilization below 300 nM Ca2+, and a negative feedback loop above 300 nM Ca2+.

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