Effects of mono- and multi-valent cations on the inward-rectifying potassium channel in isolated protoplasts from maize roots

Transport properties mediated by ionic channels were studied by the patch-clamp technique in protoplasts from cortical parenchyma cells of maize roots (CPMR). While outward currents could be seen only occasionally, macroscopic voltage-and time-dependent potassium-selective inward currents (I-Kin(+)) were frequently observed in the whole-cell configuration. These currents increased continuously as a function of K+ concentration (in the range 3-200 mM) and the slow-saturating macroscopic chord-conductance was fitted by a Michaelis-Menten function with K-m = 195 +/- 39 mM. Other ions, like sodium and lithium, did not permeate at all through the maize root inward-channel, or like ammonium (P-NH4(+)/ PK+ = 0.16 divided by 0.25) and rubidium (PRb+/PK+ approximate to 0.10) displayed a very low permeability ratio. Up to 5 mM Rb+ did not induce any inhibition of the K+ inward current, whereas submillimolar concentrations of Cs+ were sufficient to block, in a voltage-dependent manner, the inward currents. A decrease of the external potassium concentration favoured Cs+ inhibition (K-m = 89 +/- 6 mu M and 26 +/- 2 mu M in 200 and 100 mM KCl, respectively). The potassium inward-currents were reversibly and consistently inhibited by submillimolar external concentrations of the metal ions Ni2+, Zn2+ and Co2+, while 1 mM La3+ only slightly decreased (approximate to 10%) both the single channel conductance (9.2 +/- 1.2 pS in 100 mM potassium) and the macroscopic current. In contrast to the case with Cs+, inhibition induced by other metal ions did not show any voltage dependence. These results suggest that, as with animal potassium channels, the inward channel of maize-root cortical cells has a narrow pore of permeation and metal ions decrease the K+ current, possibly by acting on binding sites located outside the pore.