INTRACELLULAR CA2+ RELEASE BY INSP(3) IN PLANTS AND EFFECT OF BUFFERS ON CA2+ DIFFUSION

Several evidences favour the idea that InsP3, a well known second messenger in animal cells, can also be used by plants. The studies supporting this hypothesis are reviewed, with special emphasis on the experiments concerning the release of vacuolar Ca2+ by InsP3. We also report patch-clamp experiments on isolated red beet vacuoles. In some of the patches, the InsP3-mediated Ca2+ channel exhibited a 50 pS conductance state beside the 30 pS state mentioned in a previous work. The InsP3-mediated vacuolar current was smaller with Ba2+ or Sr2+ ions replacing Ca2+ in both the pipette and external solutions. Heparin (1 mg ml-1) in the extravacuolar medium decreased by 60% only, the InsP3-mediated Ca2+ current. The inhibitory effect of heparin was not specific: with the same concentration used, another current in the tonoplast was reduced by 46%. The coupling by diffusion of Ca2+ ions between the InsP3-mediated Ca2+ channel and the Ca2+-sensitive channel in the tonoplast has been studied. The coupling was dependent on the Ca2+ buffers (BAPTA or EGTA) in the extravacuolar medium. At least 20 mm BAPTA were necessary to prevent coupling between channels. In the range of concentrations tested (less than 30 mm), EGTA could not prevent coupling. A review of the theoretical models describing diffusion in a buffered medium is presented, and a simplified version from the models is proposed to discuss our results.