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J Bernhardt

Publications and source records attributed to J Bernhardt.

At least 73 records · Page 4Linked to original sources
PubMed1982

Single channel gating events in tracer flux experiments. II. Flux amplitude analysis.

Measurement of tracer ion flux from or into a collection of closed membrane structures (CMS) constitutes a broadly applicable technique for studying ion channel gating by specialized gating molecules in biological membranes. The amplitudes for the flux process reflect the overall change in tracer content due to flux during a period in which channels on at least some of the CMS were open. In practice, the attainment of a time-invariant, finite overall tracer content, indicating a cessation of flux, need not imply that flux has reached completion, i.e., that the CMS internal and external tracer concentrations have fully reached equilibrium. Less than maximum flux amplitudes arise when binding of control ligands leads to an inhibition or inactivation of the channel gating molecules prior to a complete equilibration of tracer. Analysis of the dependence of the flux amplitudes on control ligand concentration permits determination of characteristic parameters of the CMS that may vary with the methods of preparation (e.g., the distributions of CMS size and CMS content of gating units). Knowledge of these parameters in turn permits evaluation of the mean single channel flux amplitude contribution, which is functionally dependent on the rate constant ratio (k'eff/ki), where k'eff and ki are, respectively, the effective rate constants for tracer flux and for gating unit inactivation.

Acetylcholine↗
PubMed1982

Single channel gating events in tracer flux experiments. III. acetylcholine receptor-controlled Li+ efflux from sealed Torpedo marmorata membrane fragments.

Filter assay measurements of Li+ efflux from acetylcholine receptor-containing vesicular Torpedo marmorata membrane fragments (microsacs) are presented. Techniques are introduced for: (a) inducing a complete emptying of the Li+ content of all microsacs containing one or more functionally intact receptors, and (b) for determining the distribution of internal volumes of the microsacs using filtration with membrane filters of different pore sizes. The flux amplitudes resulting for acetylcholine receptor-controlled Li+ efflux, when receptors are inhibited by alpha-bungarotoxin or inactivated by a neuroactivator-induced desensitization process, were measured. Amplitude analysis was used to determine characteristic parameters of the microsacs that may vary with the technique of preparation (e.g., the distribution in size and receptor content), as well as the mean single channel flux amplitude contribution (e-kt)infinity, which represents the mean reduction of the Li+ content of a microsac due to efflux from a single receptor-controlled channel closing due to inhibition or inactivation of the receptor. The ratio keff/ki was found to lie in the range 0.1 less than keff/ki less than 0.5, where keff and ki are, respectively, the rate constant for Li+-Na+ exchange flux and for the slow inactivation reaction mode of the acetylcholine receptor induced by carbamoylcholine at high concentrations.

Animals↗
PubMed1981

Single channel gating events in tracer flux experiments I. Theory.

Tracer ion flux measurements are a commonly used method for studying ion transport through membranes of cellular systems, where the rate of ion flow is determined by gating processes which control the opening and closing of transmembrane channels. Due to recent advances in the theoretical analysis of tracer flux from or into closed membrane structures (CMS), the mechanism of gating reactions can, in principle, be derived from flux data. A physically well founded analysis is presented for the dependence of the total tracer ion content of a collection of CMS on the gating processes. For functionally uncoupled gating units a mean single channel flux contribution [equation, see text] can be defined, where k is the intrinsic single channel flux coefficient, t the time over which flux is measured, and p(tau,t) is the probability that a given channel was open for a total period tau during t. This quantity reflects the mean time course of the tracer content due to flux through a single channel. Expressions for are derived that explicitly take into account a distribution in the lifetime of open channels. On the basis of the results, kinetic and thermodynamic parameters of multiphasic gating reactions can be determined from the time course of the overall tracer content in a colleciion of CMS.

Journal Article↗
PubMed1981

Ion flow gating by the acetylcholine system: kinetics of isolated receptor and esterase and of receptor-mediated ion flux.

1. Basic features of the elementary bioelectric signals such as miniature-endplate-potentials are molecularly interpreted on the basis of relaxation kinetic data of isolated acetylcholine receptor and acetylcholinesterase. Electrophysiological and molecular data suggest an essentially sequential processing of acetylcholine by receptor and esterase. 2. Flux measurements with sealed biomembrane fragments containing acetylcholine receptor show that the ion-transporting conformation of the receptor-channel is a short-lived metastable state. In the presence of neuroactivators the receptors inactivate. The description of the flux-inactivation requires a cyclic reaction scheme similar to the desensitization scheme of KATZ and THESLEFF (1957). 3. The recently introduced concept of integrated flux rate coefficients permits us to derive gating mechanisms from flux data under well-defined experimental conditions: sealed biomembrane vesicles, activator concentration, type of transported ion. 4. With respect to activation and inactivation and the metastability of the ion-conducting conformation, there are fundamental similarities between the axonal Na+ ion channel and the acetylcholine receptor-channel.

Acetylcholine↗
PubMed1979

The direct influence of electromagnetic fields on nerve- and muscle cells of man within the frequency range of 1 Hz to 30 MHz.

By using several biophysical approximations and considering man as free space model limiting order-of-magnitude values for external electric and magnetic field strengths which may be hazardous for human beings were calculated. Danger may occur by excitation processes below 30 kHz for field strengths exceeding these limiting values; for frequencies larger than 60 kHz, thermal effects are predominant before excitation occurs. The external electric field strength necessary for causing action potentials in the central nervous system exceeds by far the corona forming level. But excitation is possible by strong alternating magnetic fields. Furthermore, by comparing the electrically and magnetically induced currents with the naturally flowing currents in man caused by the brain's and heart's electrical activity, a "lower boundary-line" was estimated. Regarding electric or magnetic field strengths undercutting this boundary-line, direct effects on the central nervous system may be excluded. Other mechanisms should be responsible for demonstrated biological effects.

Action Potentials↗
PubMed1979

[Biological effects of electromagnetic fields (author's transl)].

This résumé deals with thermal and nonthermal effects of electromagnetic fields on man. In consideration of two aspects a limitation is necessary. Firstly, there will be discussed only direct and immediate influences on biological cells and tissues, secondly, the treatment is limited to such phenomena, for which biophysical aproximations, based on experimental data, could be developed. Hazards for the human being may occur only by thermal effects within the microwave range. Regarding frequencies below approximately 30 kHz, excitation processes cannot be excluded in exceptional cases. Thermal effects are predominant, between 30 kHz and 100 kHz, before excitations can appear. Furthermore, by comparing the electrically and magnetically induced currents with the naturally flowing currents in man caused by the brain's and heart's electrical activity, a "lower boundaryline" was estimated. Regarding electric or magnetic field strengths undercutting this boundary-line, direct effects on the central nervous system may be excluded; other mechanisms should be responsible for demonstrated biological effects. The paper closes referring to some reports--presently discussed--on experimental findings of biological effects, which are observed as a result of the influence of electromagnetic fields of small field strengths.

Brain↗
PubMed1978

Kinetic analysis of receptor-controlled tracer efflux from sealed membrane fragments.

A detailed kinetic analysis is presented for activator-receptor-mediated efflux of tracer substances from vesicular membrane systems in general and from sealed fragments of excitable membranes in particular. Rate constants and amplitudes, as the primary measurable quantities of the efflux kinetics, are expressed in terms of fundamental properties of vesicular membrane systems containing receptors of chemical gating systems. The experimental determination and theoretical analysis of single contributions to a complex receptor-controlled efflux has been treated for the acetylcholine receptor system; also the effect of "pharmacological densensitization" on efflux is explicitly formulated. The dependence of the measured efflux parameters on the concentration of activators can be used to derive the kinetic and thermodynamic constants for receptor activation and inactivation processes; a general kinetic scheme and two limiting cases are analyzed. The efflux of (22)Na from "excitable microsacs" of Torpedo marmorata is treated as an example, and the power of the rigorous analytical method is demonstrated. In particular, the analysis of efflux amplitudes from only a few data points offers an alternative to the longer lasting measurements for obtaining efflux curves when a safety factor is involved, as in the case of tracer ions like (22)Na.

Journal Article↗
PubMedDate not supplied

Cellular signalling by lipoproteins in cultured smooth muscle cells from spontaneously hypertensive rats.

We investigated pivotal signalling responses of cultured aortic smooth muscle cells (VSMC) from the spontaneously hypertensive rat (SHR) and the normotensive Wistar Kyoto rat (WKY) to lipoproteins. Low-density lipoprotein (LDL) and high-density lipoprotein (HDL3) stimulated a time- and dose-dependent accumulation of inositol phosphates in VSMC. SHR and WKY VSMC exhibited comparable half-maximal dose requirements (approximately 13 micrograms/ml for LDL and approximately 14 micrograms/ml for HDL3), although, at any given dose, the response of SHR VSMC was significantly greater than WKY VSMC. Simultaneous addition of LDL and HDL3 to VSMC resulted in additive stimulatory effects on phosphoinositide catabolism. Pertussis toxin pretreatment of VSMC completely negated the stimulatory effects of LDL and HDL3 on IP accumulation. [32P]-ADP ribosylation and immunoblotting studies revealed the guanine nucleotide-binding (G protein) substrate(s) for pertussis toxin to be a Gi protein(s). SHR and WKY VSMC did not differ with respect to levels of Gi alpha or G beta, and thus, the amplified responsiveness in SHR VSMC cannot be attributed to alterations in levels of pertussis toxin-sensitive G protein. The spectrum of signalling responses elicited by LDL and HDL3 are similar to those elicited by vasoactive hormones, and thus lipoproteins may, via stimulation of phosphoinositide catabolism, 45Ca2+ uptake and Na+/H(+)-exchange, directly regulate smooth muscle cell growth and contraction.

Animals↗