Patch-clamp studies on the anomalous mole fraction effect of the K+ channel in cytoplasmic droplets of Nitella: an attempt to distinguish between a multi-ion single-file pore and an enzyme kinetic model with lazy state.
Patch-clamp studies have been employed in order to check whether the assumption of a multi-ion single-file pore is necessary for the explanation of the anomalous mole fraction effect or whether this effect can also be explained by a single-barrier enzyme kinetic model. Experiments in the cell-attached configuration were done on the tonoplast membrane of cytoplasmic droplets of Nitella in solutions containing 150 mol m-3 of K+ plus Tl+ with seven different K+/Tl+ ratios. At first sight, the results seem to support the multi-ion single-file pore, because apparent open channel conductivity displays the anomalous mole fraction effect, whereas open-probability has not been found to be dependent on the K+/Tl+ ratio. Changes in open probability would be expected for a single-barrier enzyme kinetic model with a lazy state. On the other hand, the lazy-state model is more successful in explaining the measured I-V curves. The entire slope of the apparent open channel current-voltage curves rotates with changing K+/Tl+ ratios in the whole voltage range between -100 and +80 mV. Numerical calculations on the basis of multi-ion single-file pores could create the anomalous mole fraction effect only in a limited voltage range with intersecting I-V curves. The apparent absence of an effect on open probability which is postulated by the lazy-state model can be explained if switching into and out of the lazy state is faster than can be resolved by the temporal resolution of 1 msec.
Draber
S
S
Institut für Angewandte Physik, Kiel, Federal Republic of Germany.
Schultze
R
R
Hansen
U P
UP
D001692:Biological Transport / Q000502:physiology; D000460:Chlorophyta / Q000502:physiology*; D003593:Cytoplasm / Q000502:physiology; D004553:Electric Conductivity / Q000502:physiology; D004789:Enzyme Activation / Q000502:physiology*; D015640:Ion Channel Gating / Q000502:physiology; D007700:Kinetics; D008433:Mathematics; D008564:Membrane Potentials / Q000502:physiology; D008954:Models, Biological; D011188:Potassium / Q000493:pharmacokinetics*; D015221:Potassium Channels / Q000502:physiology*; D013793:Thallium / Q000493:pharmacokinetics*; D013997:Time Factors
D015221:Potassium Channels; D013793:Thallium; D011188:Potassium