Three types of membrane excitations in the marine diatom Coscinodiscus wailesii

Three types of electrical excitation have been investigated in the marine d iatom Coscinodiscus wailesii. I: Depolarization-triggered, transient Cl- co nductance, G(Cl)(t), followed by a transient, voltage-gated K+ conductance, G(K), with an active state a and two inactive states i(1) and i(2) in seri es (a-i(1)-i(2)). II: Similar C-Cl(t) as in Type-I but triggered by hyperpo larization; a subsequent increase of G(K) in this type is indicated but not analyzed in detail. III: Hyperpolarization-induced transient of a voltage- gated activity of an electrogenic pump (i(2)-a-i(3)), followed by G(Cl)(t) as in Type-IT excitations. Type-III with pump Sating is novel as such, G(Cl )(t) in all types seems to reflect the mechanism of InsP(3)(-); and Ca2+-me diated G(Cl)(t) in the action potential in Chara (Biskup et al., 1999). The nonlinear current-voltage-time relationships of Type-I and Type-III excita tions have been recorded under voltage-clamp using single saw-tooth command voltages (voltage range: -200 to +50 mV, typical slope: +/-1 Vs(-1)). Fits of the: corresponding models to the experimental data provided numerical v alues of the model parameters. The statistical significance of these soluti ons is investigated. We suggest that the original function of electrical ex citability of biological membranes is related to osmoregulation which has p ersisted through evolution in plants, whereas the familiar and osmotically neutral action potentials in animals have evolved later towards the novel f unction of rapid transmission of information over long distances.