Effects of cytoplasmic Mg2+ on slowly activating channels in isolated vacuoles of Beta vulgaris

The slow vacuolar (SV) channel can mediate a large part of the ionic curren t in plant tonoplasts, but its actual physiological role is still unclear. We demonstrate that in vacuoles from the taproots of sugar beet (Beta vulga ris L.), besides Ca2+, cytoplasmic Mg2+ also plays an important role in pro moting the activation of the SV channel. An increase in Mg2+ concentration decreases the time constants of channel activation and deactivation, and de termines a consistent shift, towards negative voltages, of the conductance characteristic; as an example, when the free concentration of Mg2+ was incr eased from the micromolar range up to 10 mM the activation shifted by about -60 mV. The experimental results obtained, which are based on a fast perfu sion procedure allowing us to change the solution bathing the vacuole in a few milliseconds, suggest that magnesium-binding is a faster process than t he voltage-activation gating of the channel, which constitutes the rate-lim iting step controlling channel opening. Interestingly, the activation of th e channel mediated by Mg2+ depends on the cooperative binding of at least t hree magnesium ions. We verified that cytoplasmic magnesium favours the act ivation of SV channels in the presence of nanomolar cytoplasmic calcium con centrations. A critical discussion on the Calcium Induced Calcium Release ( CICR) mechanism proposed for the SV channel is presented.