Biomedical subjects

G D Lamb

Publications and source records attributed to G D Lamb.

66 records · Page 4Linked to original sources

PubMed1989

Transient and persistent sodium currents in normal and denervated mammalian skeletal muscle.

1. Transient and persistent tetrodotoxin-sensitive sodium currents were recorded in response to depolarizing voltage pulses in voltage clamped segments of rat extensor digitorum longus muscle fibres at 20-25 degrees C in a triple Vaseline gap. 2. Appreciable persistent sodium current but little or no transient current was seen in response to depolarizations of up to 15 mV from a holding potential of -100 mV. 3. The maximum amplitude of both transient and persistent sodium currents occurred with depolarizations to -40 mV: the average peak amplitude of the transient current in fibres with a holding potential of -90 mV was -0.22 +/- 0.03 mA/microF (mean +/- 1 S.E.M., seven fibres) and the average amplitude of the persistent current was -0.94 +/- 0.10 microA/microF (mean +/- 1 S.E.M., twelve fibres). With a holding potential of -100 mV, the average amplitudes of the transient and persistent currents were -0.46 +/- 0.10 mA/microF (four fibres) and -1.4 +/- 0.22 microA/microF (five fibres), respectively. 4. The average maximum persistent sodium conductance in seven fibres held at -90 mV was 0.13 +/- 0.0078 microS and the potential for half-maximum conductance was -53 +/- 0.74 mV (mean +/- 1 S.E.M.). 5. When the transient sodium current was completely inactivated with 100 ms conditioning depolarizations to potentials more positive than -50 to -60 mV, there was little inactivation of the persistent current. 6. In six denervated fibres, the average amplitudes of the transient and persistent sodium currents generated by pulses to -40 mV from a holding potential of -90 mV were -0.11 +/- 0.01 mA/microF and -0.88 +/- 0.12 microA/microF, respectively (mean +/- 1 S.E.M.). It was concluded that there was a decrease in transient current but not persistent current amplitude following denervation and that the persistent current in denervated fibres with an increased input resistance could give rise to the spontaneous action potentials responsible for fibrillation.

Animals↗

PubMed1988

Paralysis of skeletal muscle by butanedione monoxime, a chemical phosphatase.

The chemical phosphatase butanedione monoxime (BDM) reversibly inhibited twitches and tetanic contractions in bundles of rat soleus fibres in a dose-dependent manner (2-20 mM) but had no effect on the amplitude or time course of action potentials. In addition, BDM reversibly reduced the amplitude of potassium contractures demonstrating a depressant effect on contraction not mediated by action potentials. BDM had no effect on asymmetric charge movement but depressed calcium currents across the surface membrane in voltage-clamped fibres. The most significant effect of BDM on excitation-contraction coupling was a reduction in the amplitude of the calcium transient associated with contraction in aequorin-injected fibres. While these experiments do not eliminate the possibility of a direct effect of BDM on contractile filaments, reduction of calcium release from the sarcoplasmic reticulum, at least at low concentrations of BDM (below 2 mM), would seem to be the main mechanism for the inhibition of contractions in rat skeletal muscle.

Aequorin↗

PubMed1988

Asymmetric charge movement and calcium currents in ventricular myocytes of neonatal rat.

1. Calcium and sodium currents and non-linear capacitive currents were recorded from isolated ventricular cells from neonatal rats, using the whole-cell patch-clamp technique, usually with a holding potential of -100 mV. 2. When recording with internal and external solutions designed to suppress virtually all ionic currents except the calcium current, careful subtraction of all linear capacitive and ionic currents revealed that depolarizations elicited a small transient outward current which preceded the inward calcium current. This outward current was discernible just below the threshold potential for the calcium current and increased with larger depolarizations to a maximum for potentials of about +30 mV and above. 3. Elimination of the calcium current revealed that at each potential the transient outward current was accompanied by a roughly equal transient inward current upon repolarization. The properties of these currents indicate that they are non-linear capacitive currents. Best-fit Boltzmann curves of the 'on' charge (integral of the transient outward current) gave values for qmax, V and k of 3.9 nC/microF, -29.3 mV and 15.5 mV with internal Cs+. The maximum 'on' charge is similar to that found with calcium currents (4.3 nC/microF). Similar values were obtained with internal TEA+. 4. Boltzmann fits of conductance vs. voltage for the calcium channel gave mean values of -15.5 and 13.3 mV for V and k (with internal Cs+); the corresponding values for the sodium channel were -49.9 and 5.4 mV. 5. Pre-pulses (20 ms) to -60 mV inactivated 77% of the peak sodium current, but only inactivated about 10% of the peak calcium current and reduced the maximum 'on' charge (moved at potentials positive to -60 mV) by 19%. 6. With a holding potential of -100 mV, 10 microM-nifedipine blocked 89% of the calcium current, but had little effect on the amount of 'on' charge. The 'off' charge appeared to be slower in the presence of nifedipine. 7. These results and consideration of the number of calcium channels and high-affinity binding sites for dihydropyridines (DHP), suggest that a large part of the charge movement may be related to DHP binding sites and involved with gating calcium channels. Comparison with skeletal muscle suggests similarities in the mechanisms involved in excitation-contraction coupling.

Action Potentials↗

PubMed1987

Potassium contractures and asymmetric charge movement in extensor digitorum longus and soleus muscles from thyrotoxic rats.

Potassium contractures and asymmetric charge movement were recorded from extensor digitorum longus (EDL) and soleus muscle from normal rats and rats that had been made thyrotoxic by daily intraperitoneal injections of triiodothyronine (150 micrograms kg-1) for two to three weeks. Potassium contracture tension (relative to tetanic tension) in thyrotoxic rats was greater in EDL muscles and smaller in soleus muscles than in normal rats. As the relationship between membrane potential and potassium concentration was unaltered by thyroid treatment, it was concluded that the changes in potassium contracture tension were due to changes in excitation-contraction coupling. In thyrotoxic rats there was an average negative shift of -5 mV in the voltage sensitivity of tension in EDL fibres and a positive shift of 5 mV in soleus. As a result, the tension-membrane potential curves for EDL and soleus fibres essentially coincided. There was a corresponding average negative shift of -4 mV in the voltage sensitivity of asymmetric charge in EDL fibres, and a positive shift of 4 mV in soleus fibres from thyrotoxic rats. The dependence of asymmetric charge movement on membrane potential became essentially the same in EDL and soleus fibres from thyrotoxic rats. The maximum asymmetric charge in soleus fibres increased from an average value of 6.5 nC microF-1 in normal rats (n = 33) to 8.9 nC microF-1 in thyrotoxic rats (n = 32; p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1987

Asymmetric charge movement in polarized and depolarized muscle fibres of the rabbit.

1. The Vaseline-gap technique was used to record asymmetric charge movement in single white sternomastoid fibres of the rabbit, both when the fibres were normally polarized (holding potential, -90 mV) and when they were subjected to prolonged depolarization (holding potential, 0 mV). 2. In normally polarized fibres, 10 microM-nifedipine suppressed substantial charge movement (charge 1) without any prior depolarization, showing that the asymmetric charge does not have to be activated for the drug to exert its action. 3. Examination of the charge moved over a potential range from -140 to +10 mV confirmed that nifedipine affects only that charge generated at potentials more positive than -60 mV. 4. It was shown that there was charge movement in fibres subjected to prolonged depolarization (charge 2), with the greatest movement of charge occurring at about -71 mV, and that this charge was unaffected by nifedipine. 5. The total capacitive charge, that is, the sum of 'linear' capacitive and asymmetric charges, moved between -90 and -80 mV, was the same in polarized and depolarized fibres. Furthermore, the amount of asymmetric charge moved between potentials in the range -140 to -70 mV was the same in polarized and depolarized fibres. These observations indicate that the asymmetric charge moved between -140 and -70 mV in polarized fibres (charge 1) and depolarized fibres (charge 2) are in fact the same charge movement. 6. These results are inconsistent with the hypothesis that prolonged depolarization or nifedipine can cause a transition between charge 1 and charge 2. The results also suggest that the charge movement involved in calcium release may be quite different from that usually assumed.

Animals↗

PubMed1987

Calcium currents, charge movement and dihydropyridine binding in fast- and slow-twitch muscles of rat and rabbit.

1. The Vaseline-gap technique was used to record slow calcium currents and asymmetric charge movement in single fibres of fast-twitch muscles (extensor digitorum longus (e.d.l.) and sternomastoid) and slow-twitch muscles (soleus) from rat and rabbit, at a holding potential of -90 mV. 2. The slow calcium current in soleus fibres was about one-third of the size of the current in e.d.l. fibres, but was very similar otherwise. In both e.d.l. and soleus fibres, the dihydropyridine (DHP), nifedipine, suppressed the calcium current entirely. 3. In these normally polarized fibres, nifedipine suppressed only part (qns) of the asymmetric charge movement. The proportion of qns suppressed by various concentrations of nifedipine was linearly related to the associated reduction of the calcium current. Half-maximal suppression of both parameters was obtained with about 0.5 microM-nifedipine. The calcium current and the qns component of the charge movement also were suppressed over the same time course by nifedipine. Another DHP calcium antagonist, (+)PN200/110, was indistinguishable from nifedipine in its effects of suppressing calcium currents and qns. 4. In all muscle types, the total amount of qns in each fibre was linearly related to the size of the calcium current (in the absence of DHP). On average, qns was 3.3 times larger in e.d.l. fibres than in soleus fibres. 5. In contrast to the other dihydropyridines, (-)bay K8644, a calcium channel agonist, did not suppress any asymmetric charge movement. 6. The potential dependence of the slow calcium current implied a minimum gating charge of about five or six electronic charges. The movement of qns occurred over a more negative potential range than the change in calcium conductance. 7. Experiments on the binding of (+)PN200/110 indicated that e.d.l. muscles had between about 2 and 3 times more specific DHP binding sites than did soleus muscle. 8. These results point to a close relationship between slow calcium channels, the qns component of the charge movement and DHP binding sites, in both fast- and slow-twitch mammalian muscle. qns appears to be part of the gating current of the T-system calcium channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1986

Differential effects of thyroid hormone on T-tubules and terminal cisternae in rat muscles: an electrophysiological and morphometric analysis.

Isometric twitches, passive electrical properties and the amounts of transverse (T) tubule system and terminal cisternae in extensor digitorum longus (EDL) and soleus muscle fibres were measured in normal rats and rats given daily injections of triiodothyronine (T3, 150 micrograms kg-1) for 15-25 days. Isometric twitches in both muscles were more rapid after the T3-treatment, particularly in soleus. Cable properties were measured using a three-microelectrode, end-of-fibre, voltage clamp technique. In order to increase the space constant of the T-tubule system, extracellular solutions were used that reduced ionic, particularly chloride, conductance. Fibre diameter was less than normal in the hyperthyroid rats. Membrane capacity, per cm2 of fibre surface, increased in both EDL and soleus muscles and there was a decrease in membrane resistance. The volume and surface area of the T-system and terminal cisternae were measured using standard morphometric techniques. Following T3-treatment the amount of T-tubule system per 100 micron3 of fibre volume, in both EDL and soleus fibres, was twofold higher than in normal fibres. The larger area of T-tubule membrane per unit volume was sufficient to account for the increase in membrane capacity. In contrast, the amount of terminal cisternae per 100 micron3 of fibre was unchanged in EDL following T3-treatment and there was only a small increase in soleus. As a consequence, the normal relationship between the T-tubules and terminal cisternae was changed in both muscles. There was an increase in the numbers of 'bare' T-tubules and an increased occurrence of diadic, pentadic and heptadic junctions between the membranes of the T-tubules and terminal cisternae. The results suggest that thyroid hormone has a differential effect on the synthesis of T-tubule and terminal cisternae membrane, resulting in a disproportionately large amount of T-tubule membrane.

Animals↗

PubMed1986

Asymmetric charge movement in contracting muscle fibres in the rabbit.

The Vaseline-gap technique was used to record asymmetric charge movement in small segments of muscle fibres from the white sternomastoid or the soleus muscle of the rabbit. At 22 degrees C, non-linear ionic currents (Na+, K+, Cl-, Ca2+) were virtually eliminated for potential steps to 0 mV or below by specific blocking agents or ion substitution. A Boltzmann fit of charge movement (Q) vs. potential (V) produced the mean values Qmax = 15.2 nC/microF, V = -26.8 mV and k = 15.3 mV for twenty-three sternomastoid fibres, and 4.8 nC/microF, -32 mV and 13.7 mV for seven soleus fibres. Qmax for the sternomastoid fibres was similar to that for other fast-twitch fibres when normalized by surface area rather than capacitance. Using a 55 ms step, the mean threshold potential (Vth) for contraction in twenty-eight fibres was -25.9 (+/- 2.9) mV (+/- S.E. of mean), and the mean amount of charge moved (qth) at the threshold potential was 8.5 (+/- 0.4) nC/microF. In some contracting fibres, a component of charge movement was observed which was analogous to q gamma in amphibian muscle in its time course and potential dependence. Addition of 80 mM-sucrose to the external solution increased the speed of both the asymmetric charge movement and the charging of the linear capacitance of each fibre. The effect was reversible. A clear relation between the time course of these two parameters was established, and this strongly indicated that the majority of the asymmetric charge was located in the transverse tubular system or beyond. Moreover, it was shown that at 22 degrees C nearly all asymmetric charge moved in less than 0.5 ms after depolarization of the T-system. Sucrose in the external solution affected the Q vs. V relation, steepening the curve and shifting it to more negative potentials, as well as slightly increasing Qmax. The actions of sucrose strongly suggest that it effectively dilates and/or shortens the transverse tubular system, probably by osmotic effects.

Action Potentials↗

PubMed1986

Components of charge movement in rabbit skeletal muscle: the effect of tetracaine and nifedipine.

The effects of tetracaine and nifedipine on asymmetric charge movement in rabbit muscle fibres were examined to investigate whether mammalian charge movement could be subdivided into several components. Tetracaine (0.05-0.2 mM) stopped contraction in every sternomastoid fibre examined (n = 9) and reduced the asymmetric charge (moved by depolarizing steps to 0 mV) by 15% (S.E. of mean 3%). Tetracaine had little effect on the charge moved at potentials more negative than the threshold potential (established in the absence of the drug). Application of the Ca2+ channel blocker nifedipine (2 or 10 microM), reduced the mean maximum asymmetric charge to 50% (+/- 4) of the control value in twenty-three sternomastoid fibres and to 32% (+/- 5) in four soleus fibres. Increasing the concentration of nifedipine to 120 microM had little further effect. The charge moved at potentials more negative than -60 mV was unaffected by nifedipine. A similar result was found with 30 microM-D600 (two fibres). 10 microM-nifedipine completely blocked Ca2+ currents (external [Ca2+] = 8 mM), but 0.15 microM-nifedipine only had a small effect on either the Ca2+ current or charge movement in the four fibres examined. Contractions could no longer be elicited in eleven of eighteen fibres within 6 min of the application of 2 or 10 microM-nifedipine. However, in the remaining seven fibres contractions could be elicited with unchanged thresholds over 30 min, even in the presence of 50 microM-nifedipine. Nifedipine did not noticeably effect q gamma. It is suggested that nifedipine might prevent contraction only when, for other reasons, the normal release of Ca2+ from the sarcoplasmic reticulum has been disrupted and contraction is dependent on the inflow of external Ca2+. The amount of asymmetric charge moved by depolarizing steps was about 50% greater with a holding potential of -110 mV than with one of -90 mV. This 'extra' charge was not suppressed by nifedipine.

Action Potentials↗

PubMed1983

Tactile discrimination of textured surfaces: psychophysical performance measurements in humans.

Psychophysical experiments were designed to assess the tactile discriminative abilities of human subjects when touching textured surfaces. Plastic strips were produced which had raised dots in a square arrangement (standard surface) or in one of a number of rectangular arrangements (modified surfaces) in which the spacing of the dots differed from the standard surface by some constant amount in one direction. Subjects were presented with pairs of surfaces and asked to discriminate whether each pair consisted of (a), two identical standard surfaces, or (b), a standard surface and a modified surface. Performance measurements were analysed using decision theory. When subjects moved their fingers over the surfaces (active touch) their responses were virtually unbiased, and there was a linear relationship between discriminative performance and the difference between the spacing of the dots on the two surfaces. At the 75% correct level, subjects could distinguish surfaces in which the period of the dots differed by only 2%. Performance was virtually independent of the method of movement used, despite large differences in the velocity profiles of the various movements. Experiments in which the surfaces were moved under the subject's stationary finger (passive touch) displayed the same linear relationship between performance and period difference as in the active-touch experiments. Furthermore, the discriminative performance levels were very similar in the two types of experiments. In the passive-touch experiments, subjects could distinguish smaller differences in period in the surface dimension parallel to (along) the direction of movement than they could distinguish in the dimension perpendicular to (across) the direction of movement. The hypothesis is advanced that normal active discrimination of surfaces is made possible by using similar movements in successive surface contacts and a relatively simple neural code.

Adolescent↗

PubMed1983

Tactile discrimination of textured surfaces: peripheral neural coding in the monkey.

Recordings were made from single mechanoreceptive afferents in the median or ulnar nerve of the anaesthetized monkey while the appropriate digital pad was stimulated by a textured surface moving at a constant velocity tangentially across the skin. The surface was swept across the afferent's receptive field many times, each time having been displaced sideways (laterally) by a small amount. The neural responses showed a temporal rhythm directly related to the period of the raised dots on the surface in the dimension parallel to the direction of movement. The responses also displayed direct dependence on the position of the dots and the period size in the lateral surface dimension. It was clear that only information from a large number of afferents could enable the discrimination of the textured surfaces examined. For large-dot (2.0 mm period) surfaces, increases of up to 8% in the period of the dots, in either surface dimension, produced a roughly linear increase in the mean response rate of every rapidly adapting (r.a.) afferent. There was virtually no change in the response rate of any slowly adapting (s.a.) afferent when the period of the dots was increased by similar small amounts, and the pacinian afferents (p.c.) displayed a wide range of behaviour to such period changes. In contrast, the mean response rate of p.c. afferents seemed most able of the three populations to transmit information about changes in the period in the case of small-dot (1.0 mm period) surfaces. The adequacy of a number of neural codes in accounting for all the psychophysical discrimination reported in the preceding paper (Lamb, 1983) was examined. A strong case could be made for a code involving the total or mean number of impulses evoked (a rate code), based on the r.a. afferent responses for the large-dot surfaces and, less certainly, on the p.c. afferent responses for the small-dot surfaces.

Action Potentials↗

PubMed1981

Neural mechanisms of spatial tactile discrimination: neural patterns evoked by braille-like dot patterns in the monkey.

1. The experiments reported here were designed to investigate the responses of cutaneous mechanoreceptive afferents to spatially configured dot patterns scanned across the skin. Braille-like patterns were selected because the discrimination of Braille characters must depend on spatial patterning rather than some other facet of the afferent discharge. 2. A multifactorial experimental design was used in which each afferent fibre was studied using every combination of six dot patterns, two dot sizes, two dot spacings, two contact forces and two scanning velocities. Two other factors, scanning direction relative to the skin ridges and intermittent versus continuous scanning, were studied. 3. Beside the general question concerning the response properties of the mechanoreceptive afferents, three major questions were addressed here. (i) What is the critical spatial dimension at which neural spatial patterning breaks down and below which tactual discrimination must depend on facets of the afferent discharge other than spatial neural patterning? (ii) Which mechanoreceptive population sets this critical dimension? (iii) Why is tactual discrimination enhanced by lateral scanning? 4. The results presented here suggest that the critical dimension, below which spatial neural patterning breaks down, is of the order of 1.0 mm and that the slowly adapting (SA) afferent fibres are responsible for this limit. 5. At dimensions above approximately 1.0 mm the spatial contrast between peaks and troughs in the SA discharge is markedly enhanced during scanning. When the skin is stationary the discharge rates in the SA population drop rapidly to low levels. A second possible reason for enhanced tactual discrimination during scanning is related to the increased spatiotemporal information in a coherent pattern of neural activity moving across a discrete population of afferent fibres. 6. The effects of variations in conduction velocity are analysed and it is shown that they place serious constraints on the transmission of spatiotemporal information.

Animals↗