ATP, pH and Mg2+ modulate a cation current in Beta vulgaris vacuoles: a possible shunt conductance for the vacuolar H(+)-ATPase.

Macroscopic instantaneous and time-dependent currents have been measured in the vacuolar membrane of Beta vulgaris using a patch clamp configuration analogous to whole cell mode. At low cytosolic Ca2+ and in the absence of Mg2+, only an instantaneous current was observed. This current is carried predominantly by cations (PK:PCl 7:1, PNa:PCl 4:1 and arginine is also conducted). The instantaneous current can be activated by ATP4- (e.g., ATP-activated mean K+ current density was -20 mA.m-2 at a membrane voltage of -20 mV) and by increasing cytosolic pH and Mg2+ (raising Mg2+ from 0 to 0.4 mM induced a mean current density increase of -7 mA.m-2 at -20 mV). Such current can be activated by simultaneous addition of putative in vivo concentrations of ATP4-/MgATP/Mg2+free (in the presence of bafilomycin to inhibit the vacuolar ATPase) and further modulated by cytosolic pH. With vacuolar K+ concentration greater than that of the cytosol, activation of the instantaneous current would mediate vacuolar K+ release over the range of physiological membrane voltage. It is argued that the ATP(4-)-activated current, in addition to acting as a K+ mobilization pathway, could provide a counter-ion (shunt) conductance, allowing the two electrogenic H+ pumps which reside in the vacuolar membrane to acidify the vacuolar lumen. A separate time-dependent current, which was not observed at low Ca2+ concentrations (less than 500 nM) could also be elicited by addition of Mg2+ at the cytoplasmic membrane face. This current was stimulated by increasing cytoplasmic pH.