PH-DEPENDENCE OF INOSITOL 1,4,5-TRISPHOSPHATE-INDUCED CA2-MUSCLE CELLS OF THE GUINEA-PIG( RELEASE IN PERMEABILIZED SMOOTH)

1. The dependence on pH of inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release was studied in saponin-skinned smooth muscle cells from g uinea-pig portal vein, using the indicator fura-2 to monitor Ca2+ rele ase. 2. Increasing pH between 6.7 and 7.3 enhanced the rate of IP3-ind uced Ca2+ release at all the Ca2+ concentrations above 30 nM without c hanging the bell-shaped dependence of the Ca2+ release rate on Ca2+ co ncentration with a peak near 300 nM. 3. The ascending limb of the biph asic Ca2+ dependence was shifted slightly toward the lower Ca2+ concen tration at pH 7.3, suggesting an increase in the Ca2+ sensitivity of I P3-induced Ca2+ release at the higher pH. 4. With the elevation in pH from 6.7 to 7.3 at 100 nM Ca2+, about 7-fold higher IP3 concentration was required to release half of the Ca2+ in the store within 15 s. Thi s pH-dependent change in the IP3 sensitivity was smaller at 1 muM Ca2 and was indiscernible in the absence of Ca2+. 5. These results sugges t that H+ may inhibit binding of IP3 and Ca2+ to the modulator sites o f the Ca2+ release mechanism. However, these effects on the binding si tes may not fully explain the complex effect of pH, and there may be p H-dependent step(s) involved in the gating mechanism of IP3 channels. The present study demonstrates the importance of pH as a modulator of IP3-induced Ca2+ release.