Spontaneous action potential initiation and propagation in developing cell segments of Acetabularia mediterranea

Abstract

The spontaneous action potentials which occur during the redevelopment of anucleate isolated stalk segments (ISS’s) of the unicellular alga Acetabularia mediterranea have been studied using a novel non-invasive technique. This involved measurement of spatial samples of the time dependent potential in the external medium (sea water) and an inverse transformation to give the current distribution at the cell surface. A detailed description of the method of regularization needed to handle this ill-conditioned inversion problem is presented, along with the results of test experiments to demonstrate its validity. The initiation region and subsequent propagation (if any) of each action potential was then analysed from the computed transmembrane currents. The results showed that the occurrences of the spontaneous action potentials followed a rhythm which had a period of ~24-30 hours. These action potentials initiated more frequently at the ends of the ISS’s than in the middle region. Contrary to the findings of Novak and Bentrup (1972) in which the cell segments had been maintained in unnatural conditions (a partitioned cuvette with electrical isolation between partitions) which could very well have resulted in perturbation of the cell environment, our results show no firm correlation between the regions of action potential initiation and the site of the redeveloping apex. Generally both propagating and non-propagating action potentials were observed in the same ISS, the ratio of which seems to depend on the length of the cell. The analysed initiation sites and the propagation behaviour of the spontaneous action potentials generated by the ISS’s during the redevelopmental process do not provide any support to the symmetry-breaking role of the propagating action potentials as proposed by Novak and Bentrup.