Involvement of water channels in the hydration-induced calcium release in Nitella flexilis

An internodal cell of Nitella flexilis treated with 10 mM KCI was vacuole-perfused with an isotonic solution containing ethyleneglycol-bis-(beta-aminoethylether)N,N,N',N'-tetraacetic acid (EGTA) and its content including cytoplasm was squeezed out into a vessel and covered with silicone oil. When the hypotonic solution was added into the cytoplasmic drop which had been mixed with aequorin, a significant increase in the light emission from aequorin was detected with the photomultiplier, indicating a release of Ca2+ from some cell organelles storing Ca2+. This increase in the light emission was strongly inhibited by treating cells with 0.1 mM HgCl2 which is known to inhibit water channels in the plasma membrane. The inhibition was completely recovered by washing HgCl2 with 2-mercaptoethanol. This suggests that water channels may exist in the membrane of Ca2+ stores and play an essential role in the hydration-induced Ca2+ release.