Electrical responses of isolated Nitella protoplasm--excitations or artifacts?

Isolated protoplasmic droplets of the alga Nitella were investigated with microelectrodes under current clamp conditions. The following observations were made: 1. Long pulses of either polarity yielded almost symmetric current-voltage relations. Near the rather small resting potential (inside negative) the measuring points lay on a straight line corresponding to an apparent surface membrane resistance of 1.7 +/- 0.45 komega (mean +/- S.E.M., n=5). 2. Experiments with various pulse programs revealed no mechanism comparable to Na inactivation but stressed the electrical symmetry of the droplet with respect to the resting potential. 3. Changes of the [Ca2+] in the bathing medium between 0 to 10 mM as well as of the pH between 5 and 9 did not influence the responses. Replacing K+ by Na+ (or vice versa) or exchanging NO3 for acetylglycine or Cl- was also ineffective. These observations are not consistent with a normal excitable surface membrane. Similar responses are obtained with a RC network which is described and which may have its substrate in histological peculiarities of the protoplasmic droplet.