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K G Beam

Publications and source records attributed to K G Beam.

79 records · Page 5Linked to original sources
PubMed1976

A voltage-clamp study of the effect of two lidocaine derivatives on the time course of end-plate currents.

1. Voltage-clamped end-plate currents (e.p.c.s) have been studied in the glycerol-treated Rana pipiens sartorius nerve-muscle preparations in normal Ringer solution and in the presence of QX-222 and QX-314. 2. Both QX-222 and QX-314, the trimethyl and triethyl analogues, respectively, of lidocaine, greatly modify end-plate current kinetics. The altered e.p.c.s still show a true reversal potential, which is essentially the same as the reversal potential before drug treatment. The time course of the altered end-plate currents varies with both membrane potential and drug concentration. 3. In the presence of 0-1-1-0 mM QX-222, end-plate currents decay as the sum of three exponentials: I(t) =I1(0)e-k1t+I2(0)e-k2t+I3(0)e-k3t, where the subscipts 1, 2 and 3 refer to the rapidly, intermediately, and slowly decaying components, respectively. Both the amplitudes, Ij(0), and the decay rates, kj, depend upon membrane potential. 4. Hyperpolarization increases the relative size of the first and third components, i.e. I1(0) and I3(0) increase relative to I2(0). Depolarization increases the relative size of the second component. 5. Hyperpolarization causes a decrease in the decay rates k2 and k3 and causes a slight increase in the decay rate k1. Dependence of the three decay rates on membrane potential is well described by: kj=bjeajv. 6. The Q10 of each of the kj is about 3. 7. Raising QX-222 concentration, at any given membrane potential, augments I1(0) and I3(0) at the expense of I2(0). Raising concentration increases k1 and decreases k3; their voltage-dependence is little affected. 8. At all QX-222 concentrations tested the decay rate k2 is nearly the same as the decay rate of a normal e.p.c. recorded at an equivalent holding potential from the same fibre before drug exposure. 9. End-plate currents in the presence of 0-1 mM-QX-314 show a "major" or rapidly decaying phase and a very small, slowly decaying phase or "tail", but no intermediate component. Only the major component is discernible for end-plate currents in 0-5 mM-QX-314. 10. Voltage- and concentration-dependence of the decay rate of the major component in QX-314 is similar to k1 and QX-222. Voltage-dependence of the tails decay rate appears to be similar to k3. It is hypothesized that the second component in QX-222 represents currents of unaltered or normal conductance kinetics, and that the first and third components in QX-222, as well as the major component and tail in QX-314, represent current of "QX-altered conductance kinetics".

Animals↗
PubMed1976

A quantitative description of end-plate currents in the presence of two lidocaine derivatives.

1. Possible ways in which local anaesthetics may act to produce their characteristic alterations of end-plate current (e.p.c.s) are considered. 2. Following Magleby & Stevens (1972b), it is proposed that when acetylcholine (ACh) binds to its end-plate receptor it induces a conformational change which causes an increased ionic permeability and, thus, the measured e.p.c.; the reverse conformational change which returns the permeability to its resting state is postulated to be the rate-limiting event for e.p.c. decay whether or not local anaesthetics are present. Moreover, it is proposed that the ACh receptor had multiple binding sites and the local anaesthetics can bind to one or more of these sites thereby altering the conformational changes induced when ACh binds to remaining sites. 3. Equations based on this proposal are developed and are shown to provide an accurate description of the entire e.p.c. time couse as modified by eith QX-222 or QX-314. 4. Other models are also discussed; models in which local anaesthetic molecules bind only after receptor activation appear to be ruled out.

Animals↗
PubMedDate not supplied

Na channels in skeletal muscle concentrated near the neuromuscular junction.

Neuronal function depends crucially on the spatial segregation of specific membrane proteins, particularly the segregation associated with sites of synaptic contact. Understanding the factors governing this localization of proteins is a major goal of cellular neurobiology. A conspicuous example of synaptic specialization is the almost exclusive localization of vertebrate skeletal muscle acetylcholine (ACh) receptors to the subsynaptic membrane of the neuromuscular junction (for example, refs 1,2). The localization of other membrane proteins in skeletal muscle has been much less studied, but a knowledge of their distribution is crucial for understanding the factors governing regional specialization. We have explored the distribution in muscle of the voltage-gated Na channel responsible for the action potential using the loose patch-clamp technique, and have measured Na currents in 5-10 micron-diameter membrane patches as a function of distance from the end plate region of snake and rat muscle fibres. Here we report that the Na current density immediately adjacent to the endplate is 5-10-fold higher than at regions away from the endplate. The increased Na current density falls off rapidly with distance, reaching the background level 100-200 micron from the endplate. Although one might expect ACh receptors to be concentrated near the region of ACh release, such a concentration for Na channels, which propagate the impulse throughout the length of the cell, is surprising and suggests that factors similar to those responsible for concentrating ACh receptors at the endplate also operate to concentrate Na channels.

Action Potentials↗
PubMedDate not supplied

A lethal mutation in mice eliminates the slow calcium current in skeletal muscle cells.

Contraction of a vertebrate skeletal muscle fibre is triggered by electrical depolarization of sarcolemmal infoldings termed transverse-tubules (t-tubules), which in turn causes the release of calcium from an internal store, the sarcoplasmic reticulum (SR). The mechanism that links t-tubular depolarization to SR calcium release remains poorly understood. In principle, this link might be provided by the prominent slow calcium current that has been described in skeletal muscle cells of adult frogs and rats. However, blocking this current does not abolish the depolarization-induced contractile responses of frog muscle, and the function of this slow calcium current is unknown. Here we describe measurements of calcium currents in developing skeletal muscle cells of normal rats and mice, and of mice with muscular dysgenesis, a mutation that causes excitation-contraction (E-C) coupling to fail. We find that a slow calcium current is present in skeletal muscle cells of normal animals but absent from skeletal muscle cells of mutant animals. The effect of the mutation is specific to the slow calcium current of skeletal muscle; a fast calcium current is present in developing skeletal muscle cells of both normal and mutant animals, and slow calcium currents are present in cardiac and sensory neurones of mutant animals. We believe this to be the first report of a mutation affecting calcium currents in a multicellular organism. The effects of the mutation raise important questions about the relationship between the slow calcium current and skeletal muscle E-C coupling.

Animals↗