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)P(3)] 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 Ca(2+) medium. With vacuoles clamped at +30 millivolts, the Ins(1,4,5)P(3) induced changes in current were depending on the Ca(2+) 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)P(3). We have interpreted our data by assuming that even with 2 millimolar EGTA to buffer Ca(2+) at 0.1 micromolar in the external medium, Ins(1,4,5)P(3) released enough Ca(2+) from the vacuole to produce an accumulation of this ion near the tonoplast. Apart from their dependency with free Ca(2+) in the cytoplasm, the electrical properties of the tonoplast could be depending on the Ins(1,4,5)P(3) and Ca(2+) buffer values in the cytoplasm.