Current-voltage curve of electrogenic Cl− pump predicts voltage-dependent Cl− efflux inAcetabularia

A simple recipe for setting up the flux equations of cyclic and linear reaction schemes of ion transport with a high number of states: The arrow scheme

The calculation of flux equations or current-voltage relationships in reaction kinetic models with a high number of states can be very cumbersome. Here, a recipe based on an arrow scheme is presented, which yields a straightforward access to the minimum form of the flux equations and the occupation probability of the involved states in cyclic and linear reaction schemes. This is extremely simple for cyclic schemes without branches. If branches are involved, the effort of setting up the equations is a little bit higher. However, also here a straightforward recipe making use of so-called reserve factors is provided for implementing the branches into the cyclic scheme, thus enabling also a simple treatment of such cases.

Impedance of the electrogenic Cl(-) pump inAcetabularia: Electrical frequency entrainements, voltage-sensitivity, and reaction kinetic interpretation

Reaction kinetic analysis of the electrical properties of the electrogenic Cl(-) pump inAcetabularia has been extended from steady-state to nonsteady-state conditions: electrical frequency responses of theAcetabularia membrane have been measured over the range from 1 Hz to 10 kHz at transmembrane potential differences across the plasmalemma (V m ) between -70 and -240 mV using voltage-clamp techniques. The results are well described by an electrical equivalent circuit with three parallel limbs: a conventional membrane capacitancec m , a steadystate conductanceg o (predominantly of the pump pathway plus a minor passive ion conductance) and a conductanceg s in series with a capacitancec p which are peculiar to the temporal behavior of the pump. The absolute values and voltage sensitivities of these four elements have been determined:c m of about 8 mF m(-2) turned out to be voltage insensitive; it is considered to be normal.g o is voltage sensitive and displays a peak of about 80 S m(-2) around -180 mV. Voltage sensitivity ofg s could not be documented due to large scatter ofg s (around 80 S m(-2)).c p behaved voltage sensitive with a notch of about 20 mF m(-2) around -180 mV, a peak of about 40 mF m(-2) at -120 mV and vanishing at -70 mV. When these data are compared with the predictions of nonsteady-state electrical properties of charge transport systems (U.-P. Hansen, J. Tittor, D. Gradmann, 1983,J. Membrane Biol. in press), model "A" (redistribution of states within the reaction cycle) consistently provides magnitude and voltage sensitivity of the elementsg o ,g s andc p of the equivalent circuit, when known kinetic parameters of the pump are used for the calculations. This analysis results in a density of pump elements in theAcetabularia plasmalemma of about 50 nmol m(-2). The dominating rate constants for the redistribution of the individual states of the pump in the electric field turn out to be in the range of 500 sec(-1), under normal conditions.

Action potentials in Acetabularia: measurement and simulation of voltage-gated fluxes

Amounts and temporal changes of the release of the tracer ions K+ (86Rb+), 22Na+, and 36Cl- as well as of H+ in the course of action potentials in Acetabularia have been recorded. New results and model calculations confirm in quantitative terms the involvement of three major ion transport systems X in the plasmalemma: Cl- pumps, K+ channels, and Cl- channels (which are marked in the following by the prefixes, P, K and C) with their equilibrium voltages XVe and voltage/time-dependent conductances, which can be described by the following, first approximation. Let the maximum (ohmic) conductance of each of the three populations of transporter species be about the same (pL, KL, CL = 1) but voltage gating be different: the pump (pVe about -200 mV) being inactivated (open, o----closed, c) at positive going transmembrane voltages, Vm; the K+ channels (KVe about -100 mV) are inactivated at negative going Vm; and the Cl- channels (CVe: around 0 mV), which are normally closed (c) at a resting Vm (near pVe) go through an intermediate open (o) state at more positive Vm before they enter a third "shut" state (s) in series. Model calculations, in which voltage sensitivities are expressed by the factor f = exp(VmF/(2RT], simulate the action potential fairly well with the following parameters (pkco: 10/fks-1, pkoc: 1000.f ks-1, Kkco: 200.f ks-1, Kkoc: 2/f ks-1, ckco: 500.f ks-1, fkoc: 5/f ks-1, Cks0: 0.1/f ks-1,Ckos: 20.f ks-1). It is also shown that the charge balance for the huge transient Cl- efflux, which frequently occurs during an action potential, can be accounted for by the observation of a corresponding release of Na+.

Ion fluxes in Acetabularia: vesicular shuttle

Ion flux relations in the unicellular marine alga Acetabularia have been investigated by uptake and washout kinetics of radioactive tracers (22Na+, 42K+, 36Cl- and 86Rb+) in normal cells and in cell segments with altered compartmentation (depleted of vacuole or of cytoplasm). Some flux experiments were supplemented by simultaneous electrophysiological recordings. The main results and conclusions about the steady-state relations are: the plasmalemma is the dominating barrier for translocation of K+ with influx and efflux of about 100 nmol.m-2.sec-1. K+ passes three- to sevenfold more easily than Rb+ does. Under normal conditions, Cl- (the substrate of the electrogenic pump, which dominates the electrical properties of the plasmalemma in the resting state) shows two efflux components of about 17 and 2 mumol.m-2.sec-1, and a cytoplasmic as well as vacuolar [Cl-] of about 420 mM ([Cl-]o = 529 mM). At 4 degrees C, when the pump is inhibited, both influx and efflux, as well as the cellular [Cl-], are significantly reduced. Na+ ([Na+]i: about 70 mM, [Na+]o: 461 mM), which is of minor electrophysiological relevance compared to K+, exhibits rapid and virtually temperature-insensitive (electroneutral) exchange (two components with about 2 and 0.2 mumol.m-2.sec-1 for influx and efflux). Some results with Na+ and Cl- are inconsistent with conventional (noncyclic) compartmentation models: (i) equilibration of the vacuole (with the external medium) can be faster than equilibration of the cytoplasm, (ii) absurd concentration values result when calculated by conventional compartmental analysis, and (iii) large amounts of ions can be released from the cell without changes in the electrical potential of the cytoplasm. These observations can be explained by the particular compartmentation of normal Acetabularia cells (as known by electron micrographs) with about 1 part cytoplasm, 5 parts central vacuole, and 5 parts vacuolar vesicles. These vesicles communicate directly with the central vacuole, with the cytoplasm and with the external medium.

A Cl(-)-translocating adenosinetriphosphatase in Acetabularia acetabulum. 1. Purification and characterization of a novel type of adenosinetriphosphatase that differs from chloroplast F1 adenosinetriphosphatase

ATPases were solubilized from membranes of Acetabularia acetabulum using nonanoyl-N-methylgluconamide and purified by ion-exchange and gel permeation chromatography. Three fractions of ATPase, Mono Q-I, -II, and -III, were separated. Activity in fraction Mono Q-I was very labile and could not be accurately determined. Fractions Mono Q-II and -III had specific activities of 0.6 and 6 units/mg of protein, respectively. By SDS-polyacrylamide gel electrophoresis, isoelectric focusing, and peptide mapping, it was shown that fractions Mono Q-II and -III consisted of the same polypeptides with molecular masses of 54K (a-subunit) and 50K (b-subunit). Fractions Mono Q-II and -III had the following catalytic properties: pH optimum at 6.0; substrate specificity, ATP = GTP = ITP much greater than UTP = CTP (Km for ATP 0.6 mM); divalent cation requirement, Mn2+ = Mg2+ greater than Co2+ greater than Zn2+ much greater than Ca2+, Ni2+. Both activities were inhibited by monovalent anions, while monovalent cations had neither inhibitory nor stimulatory effects. Orthovanadate inhibited both activities to 50% at 1 mM, and the most effective inhibitor of both was azide (95% inhibition at 100 microM). An enzyme-phosphate complex was formed after incubation of fraction Mono Q-III with [gamma-32P]ATP. The CF1-ATPase subcomplexes were isolated from the same organism and compared with the fraction Mono Q-III. Data supported the difference of fraction Mono Q-III from CF1-ATPase.

Electrogenic Cl- pump in Acetabularia

Measurements of this transmembrane potential difference (V) under various conditions have demonstrated the operation of an electrogenic Cl- pump in the outer plasma membrane (plasmalemma) of the unicellular marine alga Acetabularia. In preparations of partly purified membranes (containing plasmalemma), there is Cl- stimulated, N,N'-dicyclohexylcarbodiimide-insensitive, vanadate-sensitive ATPase activity with a pH optimum around pH 6.5. These properties are consistent with the assumption that the electrogenic Cl- pump is an ATPase. In order to investigate electrical details of the "Mitchellian" type of charge-translocating enzyme, steady-state current-voltage curves of the electrogenic pump (Ip(V)) were measured in vivo under dark and light conditions and analysed by two-state reaction kinetic model. This model with the resulting parameters predicts V-sensitive, undirectional Cl- effluxes through the pump. The predictions of this model agree with the experimental results. Green light causes a fast decrease of V, which is explained as a disturbance of the pump cycle. Relaxation studies on this effect and reaction kinetic analysis of Ip(V) under different external Cl- concentrations are used to develop a consistent three-state model of the pump that includes the order of and absolute rate constants of individual reactions, states of charge, stoichiometry, voltage-sensitivity and density of the pump molecules in the membrane.