Izuo Tsutsui

Role of calcium ion in the excitability and electrogenic pump activity of theChara corallina membrane: I. Effects of La3+, verapamil, EGTA, W-7, and TFP on the action potential

SummaryThe cytoplasmic streaming of the normal internodal cell of giant algaChara stops transiently at about the peak of action potential. Application of La3+ or verapamil (a calcium channel blocker) or removal of external Ca2+ by EGTA caused a partial depolarization of the resting potential, partial decrease of the membrane conductance and a marked decrease of the amplitude of action potential. Under these conditions, the conductance at the peak of action potential reduced markedly and the streaming of cytoplasm did not cease during action potential (excitation-cessation (EC) uncoupling). The effects of Ca2+ channel blockers could not be removed by addition of CaCl2 to the external medium. In contrast, the effect of EGTA on the excitability could be removed to a greater extent and the cytoplasmic streaming ceased at about the peak of action potential by the addition of Ca2+ externally. Application of calmodulin antagonists W-7 or TFP caused similar effects on the action potential and on the cytoplasmic streaming.

Role of calcium ion in the excitability and electrogenic pump activity of theChara corallina membrane: II. Effects of La3+, EGTA, and calmodulin antagonists on the current-voltage relation

SummaryThe steady N shapeI/V curves were obtained by applying slow ramp hyper- and depolarization pulses toChara cells under the voltage-clamp condition. Application of calcium channel blocker, 20 μm La3+, to theChara membrane caused, in about 30 min, a marked reduction of the transient inward current and later almost complete blocking of the pump current, while the steady outward current remained almost unaffected. Removal of external Ca2+ with 0.5mm EGTA caused similar results. Application of calmodulin antagonists, 10 μm TFP or 20 μm W-7, also gave very similar results, i.e., the decrease of the transient inward current and of H+-pump activity. These results suggest that not only the excitatory mechanisms but also the H+-pump activity ofChara membrane are regulated by calmodulin within a comparatively narrow range of internal Ca2+ level.

Electrical tolerance (breakdown) of theChara corallina plasmalemma: I. Necessity of Ca2+

SummaryThe relationship between the external Ca2+ concentrations [Ca2+]0 and the electrical tolerance (breakdown) in theChara plasmalemma was investigated. When the membrane potential was negative beyond −350∼−400 mV (breakdown potential, BP), a marked inward current was observed, which corresponds to the so-called “punch-through” (H.G.L. Coster,Biophys. J.5:669–686, 1965). The electrical tolerance of theChara plasmalemma depended highly on [Ca2+]0. Increasing [Ca2+]0 caused a more negative and decreasing it caused a more positive shift of BP. BP was at about −700 mV in 200 μM La3+ solution. [Mg2+]0 depressed the membrane electrical tolerance which was supposed to be due to competition with Ca2+ at the Ca2+ binding site of the membrane. Such a depressive effect of Mg2+ was almost masked when the [Ca2+]0/[Mg2+]0 ratio was roughly beyond 2.

Electrical tolerance (breakdown) of the Chara corallina plasmalemma: II. Inductive property of membrane and effects of pHo and impermeable monovalent cations on breakdown phenomenon.

SummaryChanges in the chord conductanceG and the membrane electromotive forceEm in the so-called breakdown region of large negative potential of theChara plasmalemma were analyzed in more detail. In addition to the increase inG, the voltage sensitivity of the change inG increased, which was the cause of marked inductive current in the breakdown region. The breakdown potential, defined as a critical potential at which both low and high slope conductances of theI–Vm relationship cross, almost coincided with the potential at which an inductive current began to appear. This breakdown potential level changed with pHo in a range between 5 and 9. TheChara plasmalemma was electrically most tolerant around pHo 7.In some cellsEm shifted to a positive level as large as +50∼+70 mV during the breakdown phenomenon. Such a large positive shift ofEm is caused mainly by the increase in conductance of Cl− and partly Ca2+ and K+.