Action Potentials'

lntracellular Caz+, K+, CI-, and NOsactivities were measured with ion-selective microeledrodes in the liverwort Conocephalum conicum 1. at rest, during dark/light changes, and in the couese of adion potentials triggered by light or electrical stimuli. l h e average free cytosolic Caz+ concentration was 231 f 65 nM. We did not observe any light-dependent changes of the free cytosolic Caz+ concentration as long as no adion potential was triggered. During adion potentials, on average a 2-fold increase of the free cytoplasmic Caz+ concentration was recorded. lntracellular K+ adivity was 76 f 10 mM. It did not depend on K+ concentration changes in the bath solution between 0.1 and 10 mM. The average equilibrium potential for K+ in the standard medium containing 1 mM K+ was -110 mV, which differed significantly from the resting potentia1 of -151 f 2 mV. During adion potentials, either a slight decrease or no changes in intracellular K+ activity were recorded. The average CIadivity was 7.4 f 0.2 mM in the cytoplasm and 43.5 f 7 mM in the vacuole. l h e adivities of NO,were 0.63 I: 0.05 mM in the cytoplasm and 3.0 f 0.3 mM in the vacuole. For both anions the vacuolar activity was 5 to 6 times higher than the cytoplasmic activity. After the light was switched off both the CIand the NOsadivity showed either no change or a slight increase. lllumination caused a gradual return to previous values or no change. During action potentials a slight decrease of intracellular CIactivity was recorded. It was concluded that in Conocephalum, as in characean cells, chloride channels are involved in the depolarization phase of the action potentials. We discuss a model for the ion fluxes during an action potential in Conocephalum.