Voltage-Dependence of Extracellular Ca²⁺-Induced Modification in Properties of the Inward Rectifying K⁺ Channels in the Plasma Membrane of Mesophyll Protoplasts of Avena sativa

Abstract

Data obtained using the whole-celi configuration of the patch-clamp technique reveal that characteristics of the inward rectifying K⁺ current across the plasma membrane of protoplasts isolated from mesophyll cells of leaves of oat (Avena sativa) are modified by increasing concentrations or removing the extracellular Ca²⁺.

The whole-cell membrane current reveals two components. The first component an initial current I_I* which is the sum of two currents: (a) a linear ohmic leak current passing through non-gated channels, l_iNGC, and (b) a rectifying inward K⁺ current passing through inward rectifying gated K⁺ channels, I_Ki, that are instantaneously open. The second component of the membrane current at the steady state Iss is a time-dependent K⁺ current IKss defined as Iss-IiNGC and passes through inward rectifying gated K⁺ channels. The tail K⁺ current, IKT, is also defined as IT-IiNGC.

Raising external calcium concentration, [Ca²⁺]o, from 0.1 mM to 10 mM blocked the inward rectifying currents IKi, IKss and IKT. The voltage-dependence of the activation time constant (τa) for time-dependent KC current IKss was not altered significantly by increasing [Ca2+]o whereas the deactivation time constant (τd) of the IKT increased from 16 ms to 30 ms at a Vm of -100 mV.

Removal of [Ca²⁺]o increased the amplitude and altered the characteristics of the inward rectifying K⁺ current. Ten minutes after the removal of [Ca2+]o the increase in IKi was 3.5-fold larger than the increase in IKss. Furthermore, removing [Ca²⁺]o hastened the activation of IKss and the deactivation of IKT. However, the deactivation time constant (Td) remained dependent on membrane voltage (Vm).

Extracellular Ca²⁺ may modulate the function of mesophyll cells by regulating K⁺ transport through the inward rectifying K⁺ channels and this may have significant implications for photosynthesis and cell expansion.