Despite biochemical evidence for the existence of high-affinity phenylalkylamine receptors in higher plants, their effects on channel activity have only been demonstrated at relatively high concentrations. We have performed a quantitative single-channel analysis of the changes induced by extracellular verapamil in the rca channel [a wheat root plasma membrane Ca2+-selective channel (Piñeros & Tester, 1995. Planta 195:478–488)]. Concentrations as low as 0.5 μm verapamil induced a blockade of the inward current, with no evident reduction of the single-channel current amplitude. Blockade by verapamil was concentration and voltage dependent. Preliminary analysis suggested the blockade was due to a reduction in the maximum open state probability rather than a change in V0.5. Further analysis of the association and dissociation rate constants revealed a binding site located 56 to 59% down the voltage drop from the extracellular face of the channel, with a K d (0) of 24 to 26 μm. This results in a K d at −100 mV of 2 μm. Methoxyverapamil had qualitatively the same effects. This intra-pore binding site can be accessed directly from the extracellular side of the rca channel, but apparently not from the cytosolic side.
The Journal of Membrane Biology – Springer Journals
Published: May 15, 1997
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