Intracellular Ca 2+ release by Ins P 3 in plants and effect of buffers on Ca 2+ diffusion

Abstract

Several evidences favour the idea that Ins P 3 , a well known second messenger in anim al 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 Ca 2+ by Ins P 3 . We also report patch-clamp experiments on isolated red beet vacuoles. In some of the patches, the Ins P 3 mediated Ca 2+ channel exhibited a 50 pS conductance state beside the 30 pS state mentioned in a previous work. The Ins P 3 -mediated vacuolar current was smaller with Ba 2+ or Sr 2+ ions replacing Ca 2+ in both the pipette and external solutions. Heparin (1 mg ml -1 ) in the extravacuolar medium decreased by 60% only, the Ins P 3 mediated Ca 2+ 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 Ca 2+ ions between the Ins P 3 mediated Ca 2+ channel and the Ca 2+ -sensitive channel in the tonoplast has been studied. The coupling was dependent on the Ca 2+ 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.