Oscillations of an electrogenic pump in the plasma membrane of Neurospora

Self-referencing optrodes for measuring spatially resolved, real-time metabolic oxygen flux in plant systems

The ability to non-invasively measure metabolic oxygen flux is a very important tool for physiologists interested in a variety of questions ranging from basic metabolism, growth/development, and stress adaptation. Technologies for measuring oxygen concentration near the surface of cells/tissues include electrochemical and optical techniques. A wealth of knowledge was gained using these tools for quantifying real-time physiology. Fiber-optic microprobes (optrodes) have recently been developed for measuring oxygen in a variety of biomedical and environmental applications. We have adopted the use of these optical microsensors for plant physiology applications, and used the microsensors in an advanced sensing modality known as self-referencing. Self-referencing is a non-invasive microsensor technique used for measuring real-time flux of analytes. This paper demonstrates the use of optical microsensors for non-invasively measuring rhizosphere oxygen flux associated with respiration in plant roots, as well as boundary layer oxygen flux in phytoplankton mats. Highly sensitive/selective optrodes had little to no hysteresis/calibration drift during experimentation, and an extremely high signal-to-noise ratio. We have used this new tool to compare various aspects of rhizosphere oxygen flux for roots of Glycine max, Zea mays, and Phaseolus vulgaris, and also mapped developmentally relevant profiles and distinct temporal patterns. We also characterized real-time respiratory patterns during inhibition of cytochrome and alternative oxidase pathways via pharmacology. Boundary layer oxygen flux was also measured for a phytoplankton mat during dark:light cycling and exposure to pharamacological inhibitors. This highly sensitive technology enables non-invasive study of oxygen transport in plant systems under physiologically relevant conditions.

Oscillations in plant membrane transport: model predictions, experimental validation, and physiological implications

Although oscillations in membrane-transport activity are ubiquitous in plants, the ionic mechanisms of ultradian oscillations in plant cells remain largely unknown, despite much phenomenological data. The physiological role of such oscillations is also the subject of much speculation. Over the last decade, much experimental evidence showing oscillations in net ion fluxes across the plasma membrane of plant cells has been accumulated using the non-invasive MIFE technique. In this study, a recently proposed feedback-controlled oscillatory model was used. The model adequately describes the observed ion flux oscillations within the minute range of periods and predicts: (i) strong dependence of the period of oscillations on the rate constants for the H+ pump; (ii) a substantial phase shift between oscillations in net H+ and K+ fluxes; (iii) cessation of oscillations when H+ pump activity is suppressed; (iv) the existence of some 'window' of external temperatures and ionic concentrations, where non-damped oscillations are observed: outside this range, even small changes in external parameters lead to progressive damping and aperiodic behaviour; (v) frequency encoding of environmental information by oscillatory patterns; and (vi) strong dependence of oscillatory characteristics on cell size. All these predictions were successfully confirmed by direct experimental observations, when net ion fluxes were measured from root and leaf tissues of various plant species, or from single cells. Because oscillatory behaviour is inherent in feedback control systems having phase shifts, it is argued from this model that suitable conditions will allow oscillations in any cell or tissue. The possible physiological role of such oscillations is discussed in the context of plant adaptive responses to salinity, temperature, osmotic, hypoxia, and pH stresses.

Ultradian rhythms in Desmodium

The leaves of Desmodium gyrans (L.F.) DC show circadian movements in the terminal and ultradian movements of the lateral leaflets. The movements are due to swelling and shrinking of motor cells in special organs. The anatomy of these pulvini is described for the lateral leaflets. Data from electrophysiological recordings using microelectrodes inserted into the lateral pulvini, together with treatments that affect the proton pumps and ion channels, have been used to develop a physiological model of the ultradian leaflet movement. It explains the oscillations in the motor cells as being due to a change between a pump state and depolarization. During the pump state, ions are taken up, causing water influx and swelling of the motor cells. Depolarization causes loss of ions and water efflux (the motor cells shrink). The roles of calcium and the phosphatidyl inositol signal chain are discussed on the basis of experiments using chemical agents that affect these processes. Since calcium oscillations are known to occur in organisms in both time and space, an attempt has been made to simulate the situation in Desmodium pulvini by a model of specially coupled oscillators. Effects of different other treatments of the lateral pulvini are discussed. Oscillations in the minute range seem to be more common and some might be related to turgor regulation and ion uptake comparable to the situation in Desmodium. The ultradian control of the lateral pulvini and the circadian control of the terminal pulvini are apparently based on different mechanisms.

Small lipid-soluble cations are not membrane voltage probes for Neurospora or Saccharomyces

Small lipid-soluble cations, such as tetraphenylphosphonium (TPP+) and tetraphenylarsonium (TPA+) are frequently used as probes of membrane voltage (delta psi, or Vm) for small animal cells, organelles, and vesicles. Because much controversy has accompanied corresponding measurements on 'walled' eukaryotic cells (plants, fungi), we studied their transport and relation to Vm in the large-celled fungus Neurospora crassa-where Vm can readily be determined with microelectrodes-as well as in the most commonly used model eukaryotic cell, the yeast Saccharomyces cerevisiae. We found no reasonable conditions under which the distribution of TPP+ or TPA+, between the cytoplasm (i) and extracellular solution (o), can serve to estimate Vm, even roughly, in either of these organisms. When applied at probe concentrations (i.e., < or = 100 microM, which did not depolarize the cells nor deplete ATP), TPP+ stabilized at ratios (i/o) below 30 in both organisms. That would imply apparent Vm values positive to -90 mV, in the face of directly measured Vm values (in Neurospora) negative to -180 mV. When applied at moderate or high concentrations (1-30 mM), TPP+ and TPA+ induced several phases of depolarization and changes of membrane resistance (Rm), as well as depletion of cytoplasmic energy stores. Only the first phase depolarization, occurring within the perfusion-turnover time and accompanied by a nearly proportionate decline of Rm, could have resulted from TPP+ or TPA+ currents per se. And the implied currents were small. Repeated testing, furthermore, greatly reduced the depolarizing effects of these lipid-soluble ions, implicating an active cellular response to decrease membrane permeability.

Deficient cyclic adenosine 3',5'-monophosphate control in mutants of two genes of Neurospora crassa

Strains of Neurospora crassa mutant in either of two genes, Crisp-1 (cr1) and Frost (fr), showed no increase of cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels when subjected to several treatments which produce large increases of cyclic AMP in wild-type Neurospora. Evidently, the previously reported deficiencies of adenylate cyclase in these mutants were sufficient to block the normal increases. This fact suggests that both mutants could be used to help determine which control phenomena involve cyclic AMP and to interrupt the control of established cyclic AMP-regulated functions. Earlier studies had suggested an interdependence of the cyclic AMP level and the electric potential difference across the plasma membrane of Neurospora. Present experiments, therefore, employed several strains with the cr1 mutation to test for possible roles of cyclic AMP in recovery and oscillatory behavior of the Neurospora membrane potential. The results showed all such phenomena to be normal in the adenylate cyclase-defective strains, which demonstrates that variations of cyclic AMP are not obligatorily involved in the apparent control processes. Evidence is also presented that the induction of both glucose transport system II and the alternative oxidase do not require elevated cyclic AMP levels.

Effect of 2,4,5-trichlorophenol on hyphal membrane potentials in rhythmic mutants of Neurospora crassa

The uncoupler 2,4,5-trichlorophenol (TCP) was used to test for differences in maintaining the hyphal membrane potentials in the wild type and rhythmic mutants of Neurospora crassa. Trichlorophenol (0.1 mmol·1(-1)) resulted in a depolarization of 93 mV (wild type) and 144 mV in the mutant clock. A total recovery was achieved in both strains after washing out the uncoupler. The circadian conidiation mutant band was more sensitive than the two other strains and showed two different reaction patterns: one with delayed reaction, total breakdown and without recovery; the other one with nearly immediate reaction, slow but not entirely total decline and partial recovery. These differences are discussed in their relation to the circadian rhythm of conidiation.

Deficient cyclic adenosine 3',5'-monophosphate control in mutants of two genes of Neurospora crassa

Strains of Neurospora crassa mutant in either of two genes, Crisp-1 (cr1) and Frost (fr), showed no increase of cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels when subjected to several treatments which produce large increases of cyclic AMP in wild-type Neurospora. Evidently, the previously reported deficiencies of adenylate cyclase in these mutants were sufficient to block the normal increases. This fact suggests that both mutants could be used to help determine which control phenomena involve cyclic AMP and to interrupt the control of established cyclic AMP-regulated functions. Earlier studies had suggested an interdependence of the cyclic AMP level and the electric potential difference across the plasma membrane of Neurospora. Present experiments, therefore, employed several strains with the cr1 mutation to test for possible roles of cyclic AMP in recovery and oscillatory behavior of the Neurospora membrane potential. The results showed all such phenomena to be normal in the adenylate cyclase-defective strains, which demonstrates that variations of cyclic AMP are not obligatorily involved in the apparent control processes. Evidence is also presented that the induction of both glucose transport system II and the alternative oxidase do not require elevated cyclic AMP levels.

Metabolic modulation of stoichiometry in a proton pump

The current-voltage characteristics of the ATP-dependent proton pump in the plasma membrane of Neurospora have been explored under varied metabolic conditions imposed by mutation and by differential respiratory inhibition. The reversal potential, or presumed equilibrium potential, for the pump was observed at about -400 mV under energy-replete conditions, and at about -200 mV during a stable metabolic downshift of 55 percent. Steady-state levels of adenine nucleotides and inorganic phosphate, however, were not affected by this partial energy restriction, so that under both normal and restricted conditions the apparent free energy of ATP hydrolysis remained near -500 mV. The results suggest that a normal pump stoichiometry of 1 H+ extruded/1 ATP split is modified to 2 H+/1 ATP, by chronic energy restriction.

Cable properties and compartmentation in Acetabularia

The electrical cable properties of three different compartmentation types of Acetabularia cells have been investigated. These three types were: normal cells, 'stumps' (filled with cytoplasm, no central vacuole) and 'tubes' (cytoplasm depleted vacuoles). The latter two types have been obtained by centrifugation of normal cells. Qualitatively, the characteristic biphasic voltage response upon rectangular current pulses is the same in these three types. Quantitatively, however, the two conductances which can be obtained from the biphasic voltage response as well as the apparent capacity of several F . m-2 which derives from the large time constant of the second phase, are drastically increased in stumps and decreased in tubes compared to normal cells. The resting potential is a few mV more negative in stumps, and more positive in tubes, than in normal cells. Based on the existence of the high resting potential and the apparent large capacity in the non-vacuolated stumps, it is concluded that the electrogenic Cl- pump of Acetabularia is located in the plasmalemma membrane and that the apparent large capacity is not a result of the complicated membraneous organisation of the vacuolar system. Several possibilities are discussed, in relation to the quantitative correlation between intracellular compartmentation and electrical membrane parameters.

Current-voltage relationships for the plasma membrane and its principal electrogenic pump in Neurospora crassa: I. Steady-state conditions

The nonlinear membrane current-voltage relationship (I-V curve) for intact hyphae of Neurospora crassa has been determined by means of a 3-electrode voltage-clamp technique, plus "quasi-linear" cable theory. Under normal conditions of growth and respiration, the membrane I-V curve is best described as a parabolic segment convex in the direction of depolarizing current. At the average resting potential of - 174 mV, the membrane conductance is approximately 190 micronhos/cm2; conductance increase to approximately 240 micronhos/cm2 at -300 mV, and decreases to approximately 130 micronhos/cm2 at 0 mV. Irreversible membrane breakdown occurs at potentials beyond this range. Inhibition of the primary electrogenic pump in Neurospora by ATP withdrawal (with 1 mM KCN) depolarizes the membrane to the range of -40 to -70 mV and reduces the slope of the I-V curve by a fixed scaling factor of approximately 0.8. For wild-type Neurospora, compared under control conditions and during steady-state inhibition by cyanide, the I-V difference curve--presumed to define the current-voltage curve for the electrogenic pump--is a saturation function with maximal current of approximately 20 muA/cm2, a half saturation potential near -300 mV, and a projected reversal potential of ca. -400 mV. This value is close to the maximal free energy available to the pump from ATP hydrolysis, so that pump stoichiometry must be close to 1 H+ extruded:1 ATP split. The time-courses of change in membrane potential and resistance with cyanide are compatible with the steady-state I-V curves, under the assumption the cyanide has no major effects other than ATP withdrawal. Other inhibitors, uncouplers, and lowered temperature all have more complicated effects. The detailed temporal analysis of voltage-clamp data showed three time-constants in the clamping currents: one of 10 msec, for charging the membrane capacitance (0.9 muF/cm/2); a second of 50-75 msec; and a third of 20-30 sec, perhaps representing changes of intracellular composition.

[The myogenic basis of smooth muscle motility (author's transl)]

Many mammalian smooth muscle tissues are able to produce spontaneous, myogenic activity. Five types of phasic-rhythmic activity can be distinguished: 1) Spikes: brief depolarizations of the membrane which trigger calcium release and contraction; 2) oscillations of the membrane potential of the second-rhythm type (SR) generating the spikes; 3) various organ-specific rhythms such as gastric and ureteral peristalsis which can be grouped together as basic organ-specific rhythms (BOR); 4) slower fluctuations of the minute-rhythm type (MR); 5) an hour-rhythm (HR) as the slowest type. In addition, some tissues generate tonic activity by special processes which can operate without spike discharges of the cell membrane. A selective blockade of phasic and tonic components is possible with some members of the group of so-called calcium antagonists. This indicates that two different calcium activation systems exist in the membrane of smooth muscle cells (P- and T-systems). Selective P- and T-blockade offers new possibilities for pharmacological influences on the smooth muscle system.

Excitation-contraction coupling in multiunit tracheal smooth muscle during metabolic depletion: induction of rhythmicity

Multiunit canine tracheal smooth muscle responded to carbachol with graded depolarization and tonic contraction. The same concentration of carbachol, after metabolic depletion by substrate removal, produced rhythmic contractions and action potentials. Similar mechanical effects were also observed with acetylcholine or histamine. These effects were reversed by reintroducing glucose or beta-hydroxybutyrate, but not by 3-O-methylglucose, which is not metabolized; hence, the structural requirements for glucose, per se, or any osmotic effect were ruled out. Sensitivity to extracellular Ca2+ was increased. A Ca2+-influx blocker, D-600, in low concentration (2 X 10(-8) M) abolished the rhythmic contractions without affecting the tonic contraction. Progressive metabolic depletion in presence of carbachol led to fluctuations in membrane potential with a crest of depolarization and appearance of action potentials, each of which resulted in a small contraction. Many of the small contractions partially fused to form the major rhythmic contractions which appeared at a frequency of one per minute. Rhythmicity could not be produced by increasing extracellular K+ concentration (20-120 mM) in presence of atropine (13(-7) M), but instead a tonic contraction occurred. These results suggest changes in excitation-contraction coupling mechanism with agonists like acetylcholine, carbachol, or histamine during substrate deprivation.

"Metabolic" action potentials in Acetabularia

The transient depolarizations in Acetabularia which fulfill the essential criteria of an action potential (all-or-none characteristics, triggering by depolarization, propagation, etc.) are investigated. These action potentials are analyzed by conductance measurements and voltage clamp experiments on the basis of the analog circuit of the membrane (Gradmann, D. 1975, J. Membrane Biol. 25:183). It is concluded that these action potentials do not arise by permeability changes of the passive diffusion channels, but by the active pathway of the electrogenic pump, which consists of a voltage source EP of about --20 mV in series with two nonlinear conducting elements P1 and P2, the latter and EP being shunted by a large quasi capacity CP of some mF cm-2. The nonlinear current-voltage relationship of the carrier system (P1) is not changed during the action potential but has an effect on its time course. However, the elements P2 and CP, which probably reflect metabolic entities, are suggested to control the action potentials.

"Action potentials" in Neurospora crassa, a mycelial fungus

Occasional spontaneous "action potentials" are found in mature hyphae of the fungus Neurospora crassa. They can arise either from low-level sinusoidal oscillations of the membrane potential or from a linear slow depolarization which accelerates into a rapid upstroke at a voltage 5-20 mV depolarized from the normal resting potential (near-180 mV). The "action potentials" are long-lasting, 1-2 min and at the peak reach a membrane potential near-40 mV. A 2-to 8-fold increase of membrane conductance accompanies the main depolarization, but a slight decrease of membrane conductance occurs during the slow depolarization. Two plausible mechanisms for the phenomenon are (a) periodic increases of membrane permeability to inorganic ions, particularly H+ or Cl- and (b) periodic decreases in activity of the major electrogenic pump (H+) or the Neurospora membrane, coupled with a nonlinear (inverse signoid) current-boltage relationship. Identification of action potential-like disturbances in fungi means that such behavior has now been found in all major biologic taxa which have been probed with suitable electrodes. As yet there is no obvious function for the events in fungi.