Voltage--Current Characteristics of Chara australis During Changes of pH and Exchange of Ca-Mg in the External Medium

Chara australis was the subject of voltage-clamp experimentsin which external calcium, magnesium, and hydrogen were varied.The voltage clamp was applied across the combined plasmalemma,cytoplasm, and tonoplast of internodal cells. Calcium is necessaryin the batching medium for transient currents to occur duringa depolarizing voltage clamp. Magnesium cannot replace calciumas mediator in this function. Lowering the medium pH appearsto affect more than one variable in the membrane system. Weargue that more than one ion is responsible for the transientcurrent; large increases in membrane current of sodium, potassium,and chloride occur.     

The Effects of pH on the Ionic and Electrical Properties of the Internodal Cells of Chara australis

The effects of pH on the resting and action potentials and onthe fluxes of potassium, sodium, and chloride across the membranesof internodal cells of Chara australis have been investigated. Experiments were carried out in an artificial pond water (A.P.W.)of standard composition: CaCl₂, 01 mM; KCl, 0.1 mM; NaCl, 1.0mM. The resting potential decreased as the pH was lowered from6.5, being depolarized by about 75 mV at pH 4.5. Measurementsof the ion fluxes as a function of pH suggested that this depolarizationwas caused by an increase in the permeability to sodium anda decrease in permeability to potassium at pH 4.5. Action potentialsof constant peak value can be elicited for some time at pH 4.5,but after 20 min or so the cell becomes refractory. All theseeffects on resting and action potentials are fully reversible.We briefly speculate about the mechanism of these pH effects.     

Changes of Ionic and Electrical Properties of Chara australis when the External Ca is replaced by Mg

The calcium in the normal artificial pond water (A.P.W.) forChara australis was replaced by magnesium and the resultingchanges in resting potential, action potential, and ion permeabilitieswere measured. The effect of lowering the pH of the A.P.W. from6.5 to 4.5 in the presence of either Ca or Mg was also observed.It was concluded that ionic permeability changes induced byvarying the pH are Ca or Mg mediated and the ionic permeabilitychanges occuring during an action potential can be mediatedby Ca but not by Mg.