HYPOOSMOTIC SHOCK OF TOBACCO CELLS STIMULATES CA2+ FLUXES DERIVING FIRST FROM EXTERNAL AND THEN INTERNAL CA2+ STORES

Hypo-osmotic shock of aequorin-transformed tobacco cells induces a bip hasic cytosolic Ca2+ influx. Because both phases of Ca2+ entry are rea dily blocked by Ca2+ channel inhibitors, we conclude that the Ca2+ tra nsients are mediated by Ca2+ channels. Evidence that the first but not second Ca2+ transient derives from external Ca2+ stores is that the f irst but not second influx is (i) eliminated by membrane-impermeable C a2+ chelators, (ii) enlarged by supplementation of the medium with exc ess Ca2+, and (iii) reduced by the addition of competitive cations suc h as Mg2+ and Mn2+. Furthermore, entry of Ca-45 during osmotic shock i s prevented by inhibitors of the first but not second phase of Ca2+ en try. Evidence that the second wave of Ca2+ influx stems from release o f intracellular Ca2+ is based on the above data plus observations that probable modulators of intracellular Ca2+ channels selectively block this phase of Ca2+ influx. Finally, a mechanism of communication betwe en the two Ca2+ release pathways has become apparent, since perturbati ons that elevate or reduce the first Ca2+ transient lead to a compensa ting diminution/elevation of the second and vice versa. These data thu s suggest that osmotic shock leads to the sequential opening of extrac ellular followed by intracellular Ca2+ stores and that these Ca2+ rele ase pathways are internally compensated.