Calcium retrieval from vacuolar pools: Characterization of a vacuolar calcium channel

Voltage patch-clamp experiments at the whole-vacuole and single-channel levels were employed to study the retrieval of Ca²⁺ from vacuoles into the cytoplasm in sugar beet cell (Beta vulgaris L.) suspension cultures. Channels allowing the movement of Ca²⁺ out of the vacuole were identified at physiological conditions of pH, vacuolar membrane potential, and vacuole/cytoplasm Ca²⁺ concentrations. The operation of the channel was voltage dependent and inositol-1,4,5-triphosphate insensitive and displayed high selectivity for Ca²⁺ ions. These channels bear similarities to the dihydropyridine-sensitive L-type Ca²⁺ channels from animal cells. Bay K-8644, an agonist, increased the frequency of channel openings, whereas nifedipine, an antagonist, reduced the channel activity. Both effects were elicited only from the vacuolar side of the channel. Channel activities were also inhibited by verapamil, La³⁺, and cytoplasmic Ca²⁺ concentrations higher than 1 × 10^-6 M. The modulation of the channel currents by cytoplasmic Ca²⁺ would suggest the role of these channels in triggering the initiation of signal transduction processes in plant cells.