Properties of three different ion channels in the plasma membrane of the slime mold Dictyostelium discoideum

Abstract

‘Patch-clamp’ experiments in the cell-attached configuration have shown the existence of three distinct types of ion channels in the plasma membrane of Dictyostelium discoideum. Channels DI (slope conductance 11 pS) and DII (slope conductance 6 pS) promote an outward current at depolarizing voltages. A third ion channel (HI, slope conductance 3 pS) opens preferentially at hyperpolarization and promotes inward current flow. It is suggested that under physiological conditions current through the DI and DII channels is carried by K +, whereas Ca 2+ may be the current carrier in the HI channel. The density of these ion channels in the membrane of D. discoideum is low: approx. 0.1/μm 2 for the DI and HI channel and 0.02/μm 2 for the DII channel. The gating properties of the ion channels appear to be complicated because openings are grouped into bursts of activity. The probability of the DI channel being in the open state increases with depolarization. The mean channel life-time is about 20 ms and voltage-independent. The burst duration increases with depolarization whereas the interburst time decreases. The minimal kinetic model accounting for the behaviour of the DI channel is a three-state model with two closed and one open state. A detailed analysis of the gating of the DII and the HI channel was prevented by their low rate of occurrence (DII) or fast inactivation (HI). The formation of a seal resistance ≥ 1 GΩ depends critically on the composition of the pipette solution. Examination of a series of monovalent and divalent cations as well as different organic and inorganic anions has shown that ‘gigaseals’ are formed only in the presence of at least 1 mM Ca 2+ or Sr 2+, whereas Ba 2+, Mg 2+ and monovalent cations (Li +, Na +, K +, Rb +, Cs +) do not support the formation of high seal resistances. Anions seem not to affect the seal formation.