Hyperpolarization-activated Ca²⁺-permeable channels in the plasma membrane of tomato cells

The hyperpolarization of the electrical plasma membrane potential difference has been identified as an early response of plant cells to various signals including fungal elicitors. The hyperpolarization-activated influx of Ca²⁺ into tomato cells was examined by the application of conventional patch clamp techniques. In both whole cell and single-channel recordings, clamped membrane voltages more negative than -120 mV resulted in time- and voltage-dependent current activation. Single-channel currents saturated with increasing activities of Ca²⁺ and Ba²⁺ from 3 to 26 mM and the single channel conductance increased from 4 pS to 11 pS in the presence of 20 mM Ca²⁺ or Ba²⁺, respectively. These channels were 20-25 and 10-13 times more permeable to Ca²⁺ than to K⁺ and to Cl^-, respectively. Channel currents were strongly inhibited by 10 μM lanthanum and 50% inhibited by 100 μM nifedipine. This evidence suggests that hyperpolarization-activated Ca²⁺-permeable channels provide a mechanism for the influx of Ca²⁺ into tomato cells.