Hydrostatic and osmotic pressure activated channel in plant vacuole

The vacuolar membrane of red beet vacuoles contains a channel which was not gated by voltage or Ca²⁺ ions. Its unit conductance was 20 pS in 200 mM symmetrical KCl solutions. It was stretch activated: the conductance remained constant but the probability of opening was increased by suction or pressure applied to a membrane patch. A 1.5-kNm^-2 suction applied to isolated patches or a 0.08-kNm^-2 pressure applied to a 45-μm diameter vacuole induced an e-fold change in the mean current. A 75% inhibition of the channel current was obtained with 10 μM Gd³⁺ on the cytoplasmic side. The channel was more permeable for K⁺ than for Cl^- (P_K/P_Cl ~ 3). A possible clustering for this channel was suggested by the recordings of the patch current. The channel properties were not significantly affected by a change in sorbitol osmolality in the solutions under isoosmotic conditions, between 0.6 and 1 mol/kg sorbitol. However, the channel was very sensitive to an osmotic gradient. A 0.2-mol/kg sorbitol gradient induced a two-fold increase in unit conductance and a thirty-fold increase in the mean patch current of the channel. A current was measured, when the osmotic gradient was the only driving force applied to the vacuolar membrane. The hydrostatic and osmotic pressure (HOP) activated channel described in this paper could be gated in vivo condition by a change in osmolality, without the need of a change in the turgor pressure in the cell. The HOP channel represents a possible example of an osmoreceptor for plant cells.