Cytoplasmic Ca2+, K+, Cl-, and NO3- activities in the liverwort Conocephalum conicum L. at rest and during action potentials

Intracellular Ca2+, K+, Cl-, and NO3(-) activities were measured with ion-selective microeletrodes in the liverwort Conocephalum conicum L. at rest, during dark/light changes, and in the course of action potentials triggered by light or electrical stimuli. The average free cytosolic Ca2+ concentration was 231 -/+ 65 nM. We did not observe any light-dependent changes of the free cytosolic Ca2+ concentration as long as no action potential was triggered. During action potentials, on average a 2-fold increase of the free cytoplasmic Ca2+ concentration was recorded. Intracellular K+ activity was 76 -/+ 10 millimole. It did not depend on K+ concentration changes in the bath solution between 0.1 and 10 millimole. The average equilibrium potential for K+ in the standard medium containing 1 millimole K+ was -110 mV, which differed significantly from the resting potential of -151 -/+ 2 mV. During action potentials, either a slight decrease or no changes in intracellular K+ actvity were recorded. The average Cl-activity was 7.4 -/+ 0.2 millimole in the cytoplasm and 43.5 -/+ 7 millimole in the vacuole. The activities of NO3(-) were 0.63 -/+ 0.05 millimole in the cytoplasm and 3.0 -/+ 0.3 millimole 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 Cl- and the NO3(-) activity showed either no change or a slight increase. Illumination caused a gradual return to previous values or no change. During action potentials a slight decrease of intracellular Cl- activity 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