Biomedical subjects

I A Newman

Publications and source records attributed to I A Newman.

9 recordsLinked to original sources

PubMed2001

Ion transport in roots: measurement of fluxes using ion-selective microelectrodes to characterize transporter function.

The transport of mineral ions into and out of tissues and cells is central to the life of plants. Ion transport and the plasma membrane transporters themselves have been studied using a variety of techniques. In the last 15 years, measurement of specific ion fluxes has contributed to the characterization of transport systems. Progress in molecular genetics is allowing gene identification and controlled expression of transporter molecules. However the molecular expression of transporter gene products must be characterized at the functional level. The ion-selective microelectrode technique to measure specific ion fluxes non-invasively is ideally suited to this purpose. This technique, its theory, its links with others and its application and prospects in plant science, are discussed. Ions studied include hydrogen, potassium, sodium, ammonium, calcium, chloride and nitrate. Applications discussed include: solute ion uptake by roots; gravitropism and other processes in the root cap, meristematic and elongation zones; Nod factor effect on root hairs; osmotic and salt stresses; oscillations; the effects of light and temperature. Studies have included intact roots, leaf mesophyll and other tissues, protoplasts and bacterial biofilms. A multi-ion capability of the technique will greatly assist functional genomics, particularly when coupled with imaging techniques, patch clamping and the use of suitable mutants.

Cell Membrane↗

PubMed1999

Natriuretic peptides and cGMP modulate K+, Na+, and H+ fluxes in Zea mays roots.

Recent evidence suggests that in plants, as in vertebrates, natriuretic peptides (NPs) regulate homeostasis. In this study noninvasive ion-selective vibrating microelectrodes were used to measure net fluxes of K+, Na+, and H+ in Zea mays root conductive tissue. Immunoreactant plant natriuretic peptides (irPNP) cause immediate net H+ influx and delayed net K+ and Na+ uptake. Delayed net K+ influx was also observed in response to 8-Br-cGMP, however, not accompanied by significant changes in net H+ fluxes. Furthermore, 8-Br-cGMP does not stimulate the plasma membrane H+-ATPase implying that cGMP directly affects cation channels. The data are consistent with NP and cGMP-dependent stimulation of nonselective cation channels with P(K) > P(Na) and point to a complex role for NPs in plant homeostasis.

Affinity Labels↗

PubMed1998

Osmotic sensitivity of Ca2+ and H+ transporters in corn roots: effect on fluxes and their oscillations in the elongation region.

Seedling roots of corn were treated with different concentrations of mannitol-containing solution for 1 to 1.5 hr, and net fluxes of Ca2+ and H+ were measured in the elongation region. H+ fluxes were much more sensitive to osmotic pressure than were Ca2+ fluxes. Oscillations of 7-min period in H+ flux, normally observed in the control, were almost fully suppressed at high osmotic concentrations. Net H+ flux was shifted from average efflux of 25 +/- 3 nmol m-2 sec-1 to average influx of 10 +/- 5 nmol m-2 sec-1 after the incubation in 100 mm mannitol. The larger the osmotic concentration, the larger was the H+ influx. This flux caused the unbuffered solution of pH 4.85 to change to pH 5.3 after mannitol application. It appears that the osmoticum suppresses oscillatory H+ extrusion at the plasma membrane. Discrete Fourier Transforms of the H+ flux data showed that, apart from suppression of the 7-min oscillations in H+ flux, mannitol also promoted the appearance of faster 2-min oscillations. Ca2+ influx slightly increased after mannitol treatment. In addition the 7-min oscillatory component of Ca2+ flux remained apparent thereby showing independence of H+ flux.

Biological Transport↗

PubMed1997

Proton and calcium flux oscillations in the elongation region correlate with root nutation.

The involvement of Ca2+ and H+ flux oscillations in root nutation was studied for decapped roots of corn (Zea mays L. cv. Aussie Gold) placed horizontally. Net ion fluxes were measured around the elongation and meristematic regions using a microelectrode ion flux measuring system. High correlation between H+ flux oscillations and root nutations was found in the elongation region. Two oscillatory components of H+ flux, with periods of about 90 min and 7 min, correlated with root circumnutations and micronutations, respectively. The periods of H+ flux oscillations and rhythmical root movements in this region could be modified similarly by external factors including pH. In the meristematic region no association between ion flux behaviour and nutation was apparent. Ion flux oscillations and nutations both decreased in amplitude as the growth rate at the measured location decreased. Possible involvement of ion flux oscillations in root circumnutation is discussed. It is concluded that a model involving an internal oscillator must be developed to explain the H+ flux involvement in root nutations.

Calcium↗

PubMed1987

Fluxes of h and k in corn roots : characterization and stoichiometries using ion-selective microelectrodes.

We report here on an experimental system that utilizes ion-selective microelectrodes to measure the electrochemical potential gradients for H(+) and K(+) ions within the unstirred layer near the root surface of both intact 4-day-old corn seedlings and corn root segments. Analysis of the steady state H(+) and K(+) electrochemical potential gradients provided a simultaneous measure of the fluxes crossing a localized region of the root surface. Net K(+) influx values obtained by this method were compared with unidirectional K(+) ((86)Rb(+)) influx kinetic data; at any particular K(+) concentration, similar values were obtained by either technique. The ionspecific microelectrode system was then used to investigate the association between net H(+) efflux and net K(+) influx. Although the computed H(+):K(+) stoichiometry is dependent upon the choice of diffusion coefficients, the values obtained were extremely variable, and net K(+) influx rarely appeared to be charge-balanced by H(+) efflux. In contrast to earlier studies, we found the cortical membrane potential to be highly K(+) sensitive within the micromolar K(+) concentration range. Simultaneous measurements of membrane potential and K(+) influx, as a function of K(+) concentration, revealed similar K(m) values for the depolarization of the potential (K(m) 6-9 micromolar K(+)) and net K(+) influx (K(m) 4-7 micromolar K(+)). These data suggest that K(+) may enter corn roots via a K(+)-H(+) cotransport system rather than a K(+)/H(+) antiporter.

Journal Article↗

PubMed1984

A Quantitative Simulation Model for H-Amino Acid Cotransport To Interpret the Effects of Amino Acids on Membrane Potential and Extracellular pH.

The H(+) cotransport of neutral and acidic amino acids induces transient depolarizations of oat coleoptile (Avena sativa L., var Victory) plasma membranes. The depolarizations, which are completed within 1 or 2 minutes, are followed by repolarizations that are nearly completed within another 2 or 3 minutes. Cysteine induced a two-phased alkalinization of the tissue free space during the electrical changes. The first phase was a rapid, linear increase in pH that coincided with the depolarization; the second phase was a slower, also linear, increase in pH that coincided with the repolarization. Reacidification did not occur until cysteine was withdrawn. Five other acidic, basic, and neutral amino acids also induced persistent alkalinization of the free space.The notable features of these measurements are that free-space pH was measured more directly than previously, that pH changes corresponded in time to the electrical potential changes, and that reacidification of the free space did not occur. The latter observation indicates that net H(+) efflux did not occur during repolarization and that the repolarizing current was carried by some other ion. We propose that repolarization could have depended upon depolarization-induced changes in passive K(+) fluxes combined with an enhanced H(+) extrusion that increased until it equaled, but did not exceed, the enhanced influx of H(+).In support of the feasibility of our hypothesis, we present a quantitative simulation model for cotransport. The simulation model also provides an interpretation of the unique electrical effects of histidine and the basic amino acids. In addition, the model focuses attention upon the difficulties of interpreting H(+)-anion cotransport.

Journal Article↗

PubMed1981

Rapid electric responses of oats to phytochrome show membrane processes unrelated to pelletability.

The electric potential difference changes observed on etiolated oat coleoptiles in response to phytochrome transformation have been further studied using contacts on the coleoptile surface. Results are given, at 0.4 second resolution, for the first 1.5 minutes after saturating flashes of light each lasting 1 second.Responses to initial red (662 nanometers), to far red (about 700 nanometers and above) 10 minutes later, and to second red 10 minutes later still, all have time courses that are approximately Gaussian sigmoid in shape. The response to far red is of opposite sign to the response to red. Approximate magnitudes of the three changes, 1 minute after the light flash, are +6 millivolts for red, -10 millivolts for far red, +3.5 millivolts for the second red. It is argued that the observations reflect a hyperpolarization of the plasmalemma of coleoptile cells following red light and depolarization following far red. The response to red is not produced by a change in membrane permeability to K(+). The mechanism could include a change to Na(+) or Cl(-) permeability or a modulation of an electrogenic pump: enhanced H(+) extrusion, Ca(2+) extrusion, or Cl(-) uptake. The response to far red could be produced by the reverse of one of those changes.The Gaussian curve is fitted to the data to determine the time at which the responses begin. Each response begins 4.5 seconds after the start of the flash of light. These delays are not related to the time course of phytochrome pelletability or redistribution in the cell. The delays may be due to some interaction of the transformed phytochrome with the plasma-lemma. Alternatively, the transformed phytochrome may interact quickly with some other structure which initiates a signal that takes 4.5 seconds to reach the plasmalemma.

Journal Article↗

PubMed1972

Phytochrome-mediated Electric Potential Changes in Oat Seedlings.

Brief exposures to red light induce far red-reversible changes of 5 to 10 millivolts magnitude in the upper 1 centimeter of etiolated Avena coleoptiles. The changes begin within 15 seconds of the start of illumination and they continue for at least 12 minutes. The changes were measured using a flowing solution of 10 mm KCl to contact the surface of the coleoptile. A dark-grown coleoptile shows no change in response to far red light unless it first receives red light treatment. The second of two red light exposures is ineffective without an intervening far red treatment. Some characterization of these electric responses to light is presented.

Journal Article↗

PubMed1970

Auxin transport in Avena: I. Indoleacetic Acid-C distributions and speeds.

Measurements have been made on the initial stages of the transport of carbon-14-labeled indoleacetic acid in the coleoptile of Avena sativa L. Concentrations of mobile and immobilized indoleacetic acid are related to both distance and time during the first 2 hours after application of the indoleacetic acid at several concentrations to the top of the decapitated coleoptile.At the lowest concentration of indoleacetic acid applied (0.3 mum), the graphs of concentration against distance are linear from near the top of the coleoptile to the front of the indoleacetic acid stream. The indoleacetic acid possesses a definite front moving at constant speed. In the region of the front the graphs are linear even with concentrations above 1 mum applied.A large proportion of the mobile indoleacetic acid in the coleoptile does not move with the speed of the front of the stream. The rate of immobilization of indoleacetic acid by a cell increases with time after the arrival of the indoleacetic acid. Each cell must pass on to the next nearly all the indoleacetic acid it initially receives.Effects of indoleacetic acid application for a short time are investigated, as is the effect of triiodobenzoic acid on indoleacetic acid transport. It is deduced that diffusion does not play a major part in the normal transport of auxin in the coleoptile.

Journal Article↗