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J M Ritchie

Publications and source records attributed to J M Ritchie.

At least 91 records · Page 5Linked to original sources
PubMed1979

A pharmacological approach to the structure of sodium channels in myelinated axons.

Figure 6 summarizes the present state of our knowledge on the sodium channel in myelinated nerve fibers. Two sites have been discussed in detail: a metal cation binding site accessible by tetrodotoxin and saxitoxin from the outside surface only; and a second site accessible from the inside surface with which local anesthetics combine. Hydrogen ions gain access to this region of the sodium channel (and hence determine the relative local concentration of protonated drug) more readily from the extracellular fluid than from the axoplasm (Schwarz et al 1977). In addition, a variety of other sites have been mentioned, binding of drugs to which alters selectively the kinetics of opening and closing of the h and m gates. In myelinated nerve fibers these channels are packed tightly on the nodal membrane. The highest estimate for the sodium channel density in the mammalian node is 10,000 micron2. A re-evaluation of the effective nodal area, however, might reduce this value to 3000-5000/micron 2. This would still leave the nodal membrane rather crowded with sodium channels. Furthermore, the channel density would still be greater than the density of particles, sometimes believed to be sodium channels seen in freeze fracture studies (Rosenbluth 1976). One possibility for resolving this problem is that the units detected by X-ray inactivation (Levinson & Ellory 1973), and those seen in freeze-fracture studies (Rosenbluth 1976) represent not single sodium channels but groups of three. Catterall & Morrow (1978) in a comparison of the binding of saxitoxin and Leiurus sculpturatus scorpion toxin venom have concluded that there are three saxitoxin binding sites for each scorpion toxin binding site. On this basis, three saxitoxin molecules might act to block independently each of the three openings of the channels; while the the conformational change produced by the scorpion venom molecule would affect the inactivation process of all three channels.

Aconitine↗
PubMed1978

Increase in efflux of inorganic phosphate during electrical activity in small non-myelinated nerve fibres.

1. The movements of labelled phosphate were measured in garfish olfactory and in rabbit vagus nerves at rest and during activity. 2. In garfish olfactory nerve kept in solutions with 120 mM-sodium and 0.2 mM-phosphate the fractional loss of 32P was 9.82 X 10(-4) min-1. Lowering the sodium concentration of the washing fluid decreased the efflux; lowering the phosphate produced a transient increase with subsequent return towards the efflux in 0.2 mM-phosphate. 3. Stimulation at 0.50 sec produced an extra fractional loss of 12 X 10(-6) impulse-1. At 1/sec the effect was larger; at 5/sec it was about the same as at 0.5/sec. 4. After stimulation the effect of activity disappeared exponentially with a time constant of 4.4 min. 5. Lowering the sodium decreased the extra efflux with stimulation, whereas changing the phosphate concentration did not much affect the extra efflux. 6. In rabbit vagus nerve kept in 154 mM-sodium and 0.2 M-phosphate the fractional loss of 32P was 4.91 X 10(-4) min-1. Lowering the sodium or the phosphate decreased the resting efflux. 7. Stimulation of the vagus nerve at 15/sec produced an extra fractional loss of 0.87 X 10(-6) impulse-1. 8. The extra efflux with stimulation seems to result predominantly from an increase in intracellular inorganic phosphate resulting from increased break-down of ATP after activity.

Action Potentials↗
PubMed1977

Density of sodium channels in mammalian myelinated nerve fibers and nature of the axonal membrane under the myelin sheath.

The density of sodium channels in mammalian myelinated fibers has been estimated from measurements of the binding of [3H]saxitoxin to rabbit sciatic nerve. Binding both to intact and to homogenized nerve consists of a linear, nonspecific, component and a saturable component that represents binding to the sodium channel. The maximum saturable binding capacity in intact nerve is 19.9 +/- 1.9 fmol-mg wet-1; the equilibrium dissociation constant, Kt, is 3.4 +/- 2.0 nM. Homogenization makes little difference, the maximum binding capacity being 19.9 +/- 1.5 fmol-mg wet-1 with Kt = 1.3 +/- 0.7 nM. These values correspond to a density of about 700,000 sodium channels per node--i.e., about 12,000 per mum2 of nodal membrane. From the difference between the values of maximum saturable binding capacity in intact and homogenized preparation, given the statistical uncertainty of their estimate, it seems that the internodal membrane can have no more than about 25 channels per mum2. The significance of these findings for saltatory conduction and in demyelinating disease is discussed.

Animals↗
PubMed1977

The binding of labelled saxitoxin to the sodium channels in normal and denervated mammalian muscle, and in amphibian muscle.

1. The binding of [3H]saxitoxin to innervated and denervated rat diaphragm muscle, and to normal frog muscle, has been measured. 2. A saturable component of saxitoxin binding, which was inhibited by tetrodotoxin, was detected in all preparations, as well as a component of non-saturable binding. The values for the maximum saturable capacity, M, and the equilibrium binding constant, K, for normal rat diaphragm muscle were: M = 24-4 f-mole.mg wet-1, and K = 3 -8 NM. 3. Denervation of rat diaphragm muscle reduced the maximum binding capacity per unit weight to 16-5 f-mole.mg-1. The value of K remained virtually unchanged at 4-2 nM. 4. It is suggested that the decrease in density per unit weight does not reflect any change in the density of sodium channels per unit area of membrane. 5. Two varieties of the same species of frog, Rana pipiens, were examined. In one variety (Southern) the value of M was 25-6 f-mole.mg-1 and the value of K was 4-3 nM. In the Northern variety the maximum binding capacity was less, M being 14-6 f-mole.mg-1; the value of K was 3-8 nM.

Action Potentials↗
PubMed1976

A new method for labelling saxitoxin and its binding to non-myelinated fibres of the rabbit vagus, lobster walking leg, and garfish olfactory nerves.

1. A new method of labelling saxitoxin (STX) is described, based on transfer of tritium from tritiated water to the toxin. 2. The radiochemical purity of the labelled toxin has been directly determined, rather than being based on indirect biochemical means, as in previous experiments with Wilzbach-labelled STX and TTX. 3. The specific activity of the labelled toxin, 66 d..m.f-mole-1, corresponds with one tritium atom per molecule STX, an improvement of about 300-fold over other means of labelling TTX and STX. 4. The binding of this toxin to rabbit, lobster and garfish olfactory nerve fibres has been re-examined. 5. The density of sodium channels calculated on the basis of the binding of the toxin is about four to six-times the values previously reported.

Animals↗
PubMed1975

Binding of tetrodotoxin and saxitoxin to sodium channels.

A useful first step in any chemical characterization of the sodium channels in nerve membrane would clearly be the identification of some measurable property of the channel that does not depend on the intactness of the tissue. To this end, tetrodotoxin and saxitoxin, which bind specifically to sodium channels, have been triated and their binding to rabbit, lobster and garfish non-lyelinated nerve fibres examined. In each case, a component of the binding curve was found that saturated at concentrations of a few nanomolar. In addition, non-specific binding, indicated by a linear dependence of the amount bound on concentration, occurred. A solubilized membrane preparation from garfish nerve shows the same specific binding component as that of the intact nerve. The saturable component of binding seems to reflect the sodium channel density in nerve, and this is extremely small, being about 27/mum-2 in the rabbit nerve and as small as 6/mum-2 in the garfish nerve.

Animals↗
PubMed1975

The movement of potassium ions during electrical activity, and the kinetics of the recovery process, in the non-myelinated fibres of the garfish olfactory nerve.

1. An electrophysiological study was made with the sucrose gap of the compound action potential of the non-myelinated fibres of the garfish olfactory nerve at temperatures between 8 and 22 degrees C. The efflux of 42K with stimulation, and the post-tetanic hyperpolarization following a period of repetitive stimulation, were also studied. 2. At 22 degrees C the compound action potential (at zero conduction distance) reached its peak in about 5 msec and lasted about 12 msec; the conduction velocity was 19.4 cm/sec. The corresponding extrapolated values at 0 degrees C were: time to peak, 16 msec; duration, 37 msec; conduction velocity, 4.2 cm/sec. 3. The rate constant for resting potassium efflux (kr) was 0.0108 min-1 at 22 degrees C and 0.00489 min-1 at 0 degrees C. 4. The extra fractional loss of potassium with stimulation (ks) was 2.02 x 10(-4) impulse-1 at 22 degrees C and 4.41 x 10(-4) impulse-1 at 0 degrees C. 5. Raising the external potassium concentration, or addition of ouabain (1 mM), raised the resting rate of efflux of potassium. 6. The post-tetanic hyperpolarization, which reflects the electrogenic extrusion of sodium ions, indicated a rate constant for active sodium efflux at 22 degrees C of 0.47 min=1. 7. The rate constant was decreased by a 10 degrees C fall in temperature by a factor of 1.73. 8. Increasing the external potassium concentration increased the rate constant, apparently by potassium combining with some site with an equilibrium dissociation constant of 1.2 mMM. 9. The rate constant was decreased slightly in acidic solutions and increased slightly in alkaline solutions. 10. 14C-labelled ouabain was found to bind to the garfish olfactory nerve with an equilibrium dissociation constant of about 0.5 muM. The maximum saturable binding capacity, 22.3 p-mole/mg dry, suggests that there are about 350 sodium pumping sites per square micron membrane.

Action Potentials↗
PubMed1975

The heat production associated with the passage of a single impulse in pike olfactory nerve fibres.

1. A study has been made of the temperature changes associated with the passage of a single impulse in the non-myelinated fibres of the pike olfactory nerve. 2. The initial heat occurs in two phases: a burst of positive heat, followed by an evolution of negative heat. The positive and negative heats, and the net initial heat, are temperature-dependent. 3. At 0 degrees C the measured positive initial heat is 44.2 mucal/g.impulse; and the corresponding negative initial heat is 48.9 mucal/g.impulse. There is thus a net initial heat that is negative, of about 4.7 mucal/g.impulse. 4. The positive heat has a positive temperature coefficient, being increased by a factor of 1.86 when the temperature is rasied from 0 degrees C to 10 degrees C. 5. The negative initial heat also increases when the temperature is raised, but less than the positive initial heat. As a result, the net initial heat tends to become positive at higher temperatures. 6. Because of temporal dispersion of the action potential over the face of the thermopile, the observed temperature changes are smaller than those that occur at a single point in the nerve close to the stimulating cathode. The value of the positive heat at 0 degrees C corrected for temporal dispersion is estimated to be about 62 mucal/g.impulse: the corresponding value for the negative heat is about 67 mucal/g.impulse. 7. All records were analysed in terms of only two phases of initial heat (one positive, one negative). No analysis required four phases; but it is unclear whether this finding reflects a true absence of four phases, or merely the inability of the recording equipment to resolve them. 8. The positive heat seems to be derived from two sources. First, there is a dissipation of the free energy stored in the membrane capacity. Secondly, there is an evolution of heat corresponding with a decrease in entropy of the membrane dielectric with depolarization.

Action Potentials↗