Studies of excitable membrane formed on the surface of protoplasmic drops isolated from Nitella II. Tension at the surface of protoplasmic drops

A protoplasmic drop isolated from an internodal cell of Nitella became electrically excitable in a solution containing 0.5 mM NaCl, 0.5 mM KNO₃, 1mM Ca(NO₃)₂ and 2mM Mg(NO₃)₂. A thermodynamic property of the excitable membrane was characterized in terms of tension at the surface of the protoplasmic drop. This was determined by the compression method and/or by the sessile-drop method. The surface tension of the membrane was obtained as a function of the composition of the salts in the external solution, and the time during the formative period of the excitable surface membrane. The results are summarized as follows:

1. 1. The surface of the protoplasmic drop increased with time starting from 0.003 dyne/cm and approached a steady value of about 0.1 dyne/cm within 1 h after the drop was placed in the test solution described above. The membrane became electrically excitable when the surface tension attained the steady value.

2. 2. Increase of concentration of either Na⁺ or K⁺ in the solution induced a sudden decrease of the surface tension, which followed a suppression of the excitability. The critical concentration of Na⁺ or K⁺ was about 10 mM.

3. 3. The surface tension remained constant at about 0.1 dyne/cm in a Ca²⁺ concentration ranging between about 0.1 and 10 mM. At this concentration the drop was excitable. Below and above this range of Ca²⁺ concentration, the surface tension changed sharply with concentration, and the excitability disappeared. At about 0.1 mM Ca²⁺ concentration a discrete variation of the surface tension was observed.

4. 4. The surface tension of the drop stayed constant at 0.1 dyne/cm in the range between 1 and 10 mM of Mg²⁺ concentration. Above and below this range of Mg²⁺ concentration, the surface tension increased sharply with the variation of Mg²⁺ concentration.

These results indicate that the protoplasmic drop retains its excitability in a limited range of salt composition in the external solution. This implies that the excitable membrane of the drop must be very labile in its structure against external perturbations such as electrical stimulus and/or slight variation of salt composition in the solution.