TRANSPORT OF CA2-ALBUM L SUSPENSION CELLS - ATP-DEPENDENT IMPORT AND INOSITOL-1,4,5-TRISPHOSPHATE-INDUCED RELEASE( ACROSS THE TONOPLAST OF INTACT VACUOLES FROM CHENOPODIUM)

The removal of Ca2+ from the medium by intact vacuoles and microsomes of Chenopodium album was investigated by measuring INDO-1 fluorescence emission at 400 and 480 nm and the response of Ca2+ -selective mini-e lectrodes, The removal of Ca2+ depended on the presence of MgATP, disp laying an apparent K-mATP of about 50 mu M, a K-mCa of 400-500 nM, and a nucleotide specificity (%) of ATP (100) > CTP (49) > GTP (28) > UTP (20) > ADP = AMP (0). In the presence of saturating MgATP, the vacuol es reduced the [Ca2+] of the medium below 30 nM. Part of the Ca2+ remo ved from the medium was released again after adding micromolar concent rations of inositol-1,4,5-trisphosphate. This release of Ca2+ was inhi bited by heparin. Since digitonin caused the release of the entire amo unt of Ca2+ removed from the medium in the presence of MgATP, we argue that the Ca2+ is not bound to membranes or sequestered otherwise, but is transported into the vacuoles (or vesicles) and remains freely mob ile there. In accordance with the current literature, we conclude that the plant vacuole is an important store for mobile Ca2+ to be release d for purposes of signal transduction. Since changes in the trans-tono plast Delta pH and inhibition of the H+-translocating pumps had no sig nificant influence on the ATP-dependent removal of Ca2+ from the cytop lasmic side, we argue that in C. album ATP-driven Ca2+ transport is th e predominant form of Ca2+ translocation into the vacuole.