Calmodulin regulates the Ca(2+)-dependent slow-vacuolar ion channel in the tonoplast of Chenopodium rubrum suspension cells.

The patch-clamp technique was applied to vacuoles isolated from a photoautotrophic suspension cell culture of Chenopodium rubrum L. and vacuolar clamp currents, which are predominantly carried by the previously identified Ca(2+)-dependent slow vacuolar (SV) ion channels, were recorded. These currents, which were activated by 1-s voltage pulses of -100 mV (vacuolar interior negative) in the presence of 100 μM Ca(2+) (cytosolic side), could be blocked completely and reversibly by the calmodulin antagonist W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide] and its chlorine-deficient analogue W-5; half-maximum inhibition was found at approx. 6 μM for W-7 and 70 μM for W-5. Inhibition was reversed by addition of 1 μg · ml(-1) calmodulin purified from Chenopodium cell suspensions; reversal by bovine brain calmodulin was scarcely appreciable. We conclude that cytosolic calmodulin mediates the Ca(2+) dependence of the SV-channel in the Chenopodium tonoplast.