Inactivation of plasmalemma conductance in alkaline zones of <Emphasis Type="Italic">Chara corallina</Emphasis> after generation of action potential

A microelectrode study with Chara corallina cells has shown that post-excitation changes of membrane potential and plasmalemma resistance, induced by the action potential (AP) generation, differ substantially for cell areas producing zones of high and low external pH. In cell regions producing alkaline zones, the AP generation was followed by post-excitation hyperpolarization by about 50 mV, concomitant with four- to eightfold increase in plasmalemma resistance and a considerable drop of pericellular pH. In the acidic areas the post-excitation hyperpolarization was weak or absent, and the membrane resistance showed no significant increase within 1–2 min after AP. The membrane excitation in the acidic zones was accompanied by a noticeable pH increase near the cell surface, indicative of the inhibition of plasma membrane H⁺ pump. The results suggest that the high local conductance of the plasmalemma is closely related to alkaline zone formation and the depolarized state of illuminated cell under resting conditions. Excitation-induced changes of membrane potential and pH in the cell vicinity were fully reversible, with the recovery period of ∼15 min at a photon flux density of ∼100 μE/(m² s). At shorter intervals between excitatory stimuli, differential membrane properties of nonuniform regions turned smoothed and could be overlooked. It is concluded that the origin of alkaline zones in illuminated Chara cells cannot be ascribed to hypothetical operation of H⁺/HCO₃⁻ symport or OH⁻/HCO₃⁻ antiport.