Time dependent K+ currents through plasmalemma of laticifer protoplasts from Hevea brasiliensis

The purpose of our work was to investigate the functioning of K+ channels in protoplasts of laticifers of Hevea brasiliensis Muell. Arg., anastomosed into a network devoid of large central vacuoles, after tapping stress. Physiological functions such as proton pump activity and uptake of sucrose (a rubber precursor) were maintained, when the voltage‐clamp method was used in vivo to record the whole‐cell K+ current during the stress response. A time‐dependent inward current was induced in 50 mM KCl and rapidly inactivated (about 100 ms). The activation potential of this inward K+ channel was not closely dependent on Ek. This would be coherent with the ‘valve model’ of Schroeder and Fang (1991, Proc. Natl. Acad. Sci. USA 88: 11583–11587) involving the activation of a H+‐pump accounting for the K+ uptake observed in laticiferous cells under stress. The activation half‐time of outward currents was clearly voltage dependent: from about 350 to 60 ms for 125 and 155 mV, respectively. Time‐dependent outward current sensitivity to 5 mM BaCl2 or CaCl2 or to 5 μM Erythrosin B showed that the K+ channels could be Ca2+‐dependent. Because of the positive values of the activation potential of the outward current, the possibility opens that an action potential exists, these cells being specialized for stress response.