Effect of D-myo-inositol 1,4,5-trisphosphate on the electrical properties of the red beet vacuole membrane

The effect of channel opening in the tonoplast by D-myo-inositol 1,4,5-trisphosphate [ins(1,4,5)P3] has been examined on red beet (Beta vulgaris) vacuoles. Patch-clamp measurements of the vacuolar potential and current were performed on vacuoles isolated in 0.1 micromolar free Ca2+ medium. With vacuoles clamped at +30 millivolts, the ins(1,4,5)P3 induced changes in current were depending on the Ca2+ buffer strength in the external medium. The spontaneous depolarization of vacuoles in which H+-pumps were activated by 5 millimolar MgATP was increased from +6 to +18 millivolts by 1 micromolar Ins(1,4,5)P3. We have interpreted our data by assuming that even with 2 millimolar EGTA to buffer Ca2+ at 0.1 micromolar in the external medium, ins(1,4,5)P3 released enough Ca2+ from the vacuole to produce an accumulation of this ion near the tonoplast. Apart from their dependency with free Ca2+ in the cytoplasm, the electrical properties of the tonoplast could be depending on the Ins(1,4,5)P3 and Ca2+ buffer values in the cytoplasm