Inhibition of vacuolar ion channels by polyamines

In this work, direct effects of cytosolic polyamines on the two principle v acuolar ion channels were studied by means of patch-clamp technique. Fast a nd slow activating vacuolar channels were analyzed on membrane patches isol ated from vacuoles of the red beet taproot. The potency of the fast and of the slow vacuolar channel blockage by polyamines decreased with a decrease of the polycation charge, spermine(4+) > spermidine(3+) > putrescine(2+). I n contrast to the inhibition of the fast vacuolar channel, the blockage of the slow vacuolar channel by polyamines displayed a pronounced voltage-depe ndence. Hence, in the presence of high concentration of polyamines the slow vacuolar channel was converted into a strong inward rectifier as evidenced by its unitary current-voltage characteristic. The blockage of the slow va cuolar channel by polyamines was relieved at a large depolarization, in lin e with the permeation of polyamines through this channel. The voltage-depen dence of blockage was analyzed in terms of the conventional model, assuming a single binding site for polyamines within the channel pore. Taking advan tage of a simple linear structure of naturally occurring polyamines, conclu sions on a possible architecture of the slow vacuolar channel pore were dra wn. The role of common polyamines in regulation of vacuolar ion transport w as discussed.