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M R Blatt

Publications and source records attributed to M R Blatt.

40 records · Page 3Linked to original sources
PubMed1986

Interpretation of steady-state current-voltage curves: consequences and implications of current subtraction in transport studies.

A problem often confronted in analyses of charge-carrying transport processes in vivo lies in identifying porter-specific component currents and their dependence on membrane potential. Frequently, current-voltage (I-V)--or more precisely, difference-current-voltage (dI-V)--relations, both for primary and for secondary transport processes, have been extracted from the overall membrane current-voltage profiles by subtracting currents measured before and after experimental manipulations expected to alter the porter characteristics only. This paper examines the consequences of current subtraction within the context of a generalized kinetic carrier model for Class I transport mechanisms (U.-P. Hansen, D. Gradmann, D. Sanders and C.L. Slayman, 1981, J. Membrane Biol. 63:165-190). Attention is focused primarily on dI-V profiles associated with ion-driven secondary transport for which external solute concentrations usually serve as the experimental variable, but precisely analogous results and the same conclusions are indicated in relation to studies of primary electrogenesis. The model comprises a single transport loop linking n (3 or more) discrete states of a carrier 'molecule.' State transitions include one membrane charge-transport step and one solute-binding step. Fundamental properties of dI-V relations are derived analytically for all n-state formulations by analogy to common experimental designs. Additional features are revealed through analysis of a "reduced" 2-state empirical form, and numerical examples, computed using this and a "minimum" 4-state formulation, illustrate dI-V curves under principle limiting conditions. Class I models generate a wide range of dI-V profiles which can accommodate essentially all of the data now extant for primary and secondary transport systems, including difference current relations showing regions of negative slope conductance. The particular features exhibited by the curves depend on the relative magnitudes and orderings of reaction rate constants within the transport loop. Two distinct classes of dI-V curves result which reflect the relative rates of membrane charge transit and carrier recycling steps. Also evident in difference current relations are contributions from 'hidden' carrier states not directly associated with charge translocation in circumstances which can give rise to observations of counterflow or exchange diffusion. Conductance-voltage relations provide a semi-quantitative means to obtaining pairs of empirical rate parameters.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

A potassium-proton symport in Neurospora crassa.

Combined ion flux and electrophysiological measurements have been used to characterized active transport of potassium by cells of Neurospora crassa that have been moderately starved of K+ and then maintained in the presence of millimolar free calcium ions. These conditions elicit a high-affinity (K1/2 = 1-10 microM) potassium uptake system that is strongly depolarizing. Current-voltage measurements have demonstrated a K+-associated inward current exceeding (at saturation) half the total current normally driven outward through the plasma membrane proton pump. Potassium activity ratios and fluxes have been compared quantitatively with electrophysiological parameters, by using small (approximately 15 micron diam) spherical cells of Neurospora grown in ethylene glycol. All data are consistent with a transport mechanism that carries K ions inward by cotransport with H ions, which move down the electrochemical gradient created by the primary proton pump. The stoichiometry of entry is 1 K ion with 1 H ion; overall charge balance is maintained by pumped extrusion of two protons, to yield a net flux stoichiometry of 1 K+ exchanging for 1 H+. The mechanism is competent to sustain the largest stable K+ gradients that have been measured in Neurospora, with no direct contribution from phosphate hydrolysis or redox processes. Such a potassium-proton symport mechanism could account for many observations reported on K+ movement in other fungi, in algae, and in higher plants.

Biological Transport↗
PubMed1986

Simultaneous Measurements of Cytoplasmic K Concentration and the Plasma Membrane Electrical Parameters in Single Membrane Samples of Chara corallina.

The electrophysiological properties of cytoplasm-rich fragments (single membrane samples) prepared from internodal cells of Chara corallina were explored in conjunction with K(+)-sensitive microelectrode and current-voltage (I-V) measurements. This system eliminated the problem of the inaccessible cytoplasmic layer, while preserving many of the electrical characteristics of the intact cells. In 0.1 millimolar external K concentration (K(o) (+)), the resting conductance (membrane conductance G(m), 0.85 +/- 0.25 Siemens per square meter (+/-standard error)) of the single membrane samples, was dominated by the proton pump, as suggested by the response of the near-linear I-V characteristic to changes in external pH. Initial cytoplasmic K(+) activities (a(K+)), judged most reliable, gave values of 117 +/- 67 millimolar; stable a(K+) values were 77 +/- 31 millimolar. Equilibrium potentials for K(+) (Nernst equilibrium potential) (E(K)) calculated, using either of these data sets, were near the mean membrane potential (V(m)). On a cell-to-cell basis, however, E(K) was generally negative of the V(m), despite an electrogenic contribution from the Chara proton pump. When K(o) (+) was increased to 1.0 millimolar or above, G(m) rose (by 8- to 10-fold in 10 millimolar K(o) (+)), the steady state I-V characteristics showed a region of negative slope conductance, and V(m) followed E(K). These results confirm previous studies which implicated a K(o) (+)-induced and voltage-dependent permeability to K(+) at the Chara plasma membrane. They provide an explanation for transitions between apparent K(o) (+)-insensitive and K(o) (+)-sensitive (;K(+) electrode') behavior displayed by the membrane potential, as recorded in many algae and higher plant cells.

Journal Article↗