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D Colquhoun

Publications and source records attributed to D Colquhoun.

108 records · Page 6Linked to original sources
PubMed1974

The binding of tetrodotoxin and alpha-bungarotoxin to normal and denervated mammalian muscle.

1. The binding of [(3)H]tetrodotoxin and [(125)I]iodo-alpha-bungarotoxin to innervated and denervated rat diaphragm muscle has been measured.2. A saturable component of tetrodotoxin binding, which was inhibited by saxitoxin, was detected in addition to considerable non-saturable binding. The saturable component had an equilibrium constant of K = 6.1 nM (limits 4.7-7.8 nM) and binding capacity M = 2.5 f-mole/mg wet wt. (limits 2.1-2.8 f-mole/mg).3. If the saturable component consisted of one-to-one binding of tetrodotoxin to sodium channels, the density of sodium channels would be about 21/mum(2) of surface membrane, a figure similar to that found in other excitable membranes.4. After denervation the specific tetrodotoxin binding, as measured by the ratio M/K, fell by a factor of 2.8. This change appeared to be due to a fall in binding capacity rather than a decrease in affinity.5. After denervation the maximum rate of rise of the action potential fell by 27% and became partially resistant to tetrodotoxin. The maximum rate of rise was at first reduced by tetrodotoxin in similar concentrations to those affecting normal muscle, but even large concentrations which completely blocked normal muscle only reduced the maximum rate of rise by a factor of about 2.6. Detubulation with glycerol did not appreciably affect the tetrodotoxin sensitivity of normal or denervated muscle.7. Tetrodotoxin resistance was not observed after denervation of the frog sartorius muscle.8. [(125)I]iodo-alpha-bungarotoxin binding amounted to 3.8 +/- 0.7 f-mole/mg in innervated muscle and 44.5 +/- 2.1 f-mole/mg in denervated muscle. Most of the uptake was inhibitable by (+)-tubocurarine.9. The increase in the labelled bungarotoxin binding is much larger than the specific tetrodotoxin binding of innervated muscle, which renders implausible the possibility that the acetylcholine receptors which appear after denervation are related to the tetrodotoxin resistant-sodium channels.

Action Potentials↗
PubMed1972

The interaction at equilibrium between tetrodotoxin and mammalian non-myelinated nerve fibres.

1. As a preliminary to chemical studies an estimate has been made of the equilibrium dissociation constant (K) for the interaction of tetrodotoxin (TTX) with the non-myelinated fibres of the rabbit desheathed vagus nerve.2. TTX causes a parallel shift to the right of the curves obtained when either the height or the conduction velocity of the compound action potential are plotted against the logarithm of the external sodium concentration.3. A model has been formulated based on the independence principle and the Hodgkin-Huxley theory, and the experimental results shown to be consistent with it. On this basis, and on the assumptions that one TTX molecule blocks one sodium channel, and that binding is Langmuir, K was estimated to be about 3-5 nM at about 20 degrees C. Other simple non-Langmuir models gave essentially similar low values for K.4. An alternative method of computing K that makes rather different assumptions gives a similar low value.5. Despite the low value for K, a TTX concentration of at least 100 nM is needed to block conduction completely and this seems to be related to the fact that conduction is not completely blocked until the external sodium concentration falls below 7% of its value in normal Locke.6. The minimum sodium concentration needed to support conduction increased with temperature.

Action Potentials↗
PubMed1972

The binding of labelled tetrodotoxin to non-myelinated nerve fibres.

1. Tritiated tetrodotoxin has been prepared and purified, and its binding to rabbit, lobster, and garfish non-myelinated nerve fibres examined.2. In each case a component of the binding curve was found that saturated at concentrations of a few nanomolar.3. In addition, non-specific binding, indicated by a linear dependence of the amount bound on concentration, occurred.4. The kinetics of wash-in and wash-out of the radioactive toxin were consistent with a model in which all binding was rapid and reversible and in which diffusion into and out of the nerve bundle was rate-limiting. This was shown by numerical solution of the appropriate non-linear diffusion equation. An extension of the limited biophase model that allows for non-specific binding was shown to give good semiquantitative approximations to the proper diffusion equation; and (unlike the latter) the extension was shown to have a relatively simple solution.5. A number of pharmacological agents were tested for competition with, or perturbation of, tetrodotoxin binding: sodium, calcium and hydrogen ions, lidocaine, batrachotoxin and saxitoxin. Apart from a small calcium effect, only saxitoxin, whose effect on sodium current is similar to that of tetrodotoxin, was found to interfere with binding. Increasing saxitoxin concentrations resulted in reduced amounts of tetrodotoxin binding in a manner consistent with a competition between the two toxins for the same site.

Animals↗
PubMed1970

Rapid histamine assays: a method and some theoretical considerations.

1. An automatic method for doing assays on superfused isolated tissues has been developed using the Autoanalyser sample turntable.2. Responses in the range used for assays are independent of the preceding responses when elicited at 1 min intervals. A longer interval was found to be necessary when using the conventional immersion method.3. The index of precision was similar to that reported for other superfusion methods.4. The assay method behaved as though it were assaying histamine selectively in two samples of material released by the anaphylactic reaction of passively sensitized guinea-pig lung. Responses to anaphylactically released material were also independent of the preceding response at 1 min intervals.5. Some theoretical considerations are discussed concerning the arrangement of doses, and the interpolation of results, in 2 + 1 dose assays. The object was to maximize speed while still allowing a valid estimate of the potency and its error.

Anaphylaxis↗
PubMedDate not supplied

Conductances of single ion channels opened by nicotinic agonists are indistinguishable.

Hypotheses concerning the mechanism by which acetylcholine-like agonists cause ion channels to open often suppose that the receptor-ionophore complex can exist in either of two discrete conformations, open and shut. On the basis of noise analysis it has been reported that certain agonists open ion channels of lower conductance than usual, though many potent agonists give similar conductances, and hence that differences in the conductance of ion channels opened by different agonists may contribute to differences in efficacy. Here we have reinvestigated this question by recording single ion channel currents evoked by acetylcholine-like agonists on embryonic rat muscle in tissue culture and on adult frog muscle endplate. Ten different agonists (Fig. 1) were tested, including several that noise analysis has suggested have a low conductance. The single-channel conductance was found to be the same, within a few per cent, for all 10 agonists. It seems that noise analysis has given erroneously low conductances in some cases. Therefore efficacy differences do not depend on differences in single-channel conductance evoked by various agonists but presumably on the position of the open-shunt equilibrium of the agonist-channel complexes.

Acetylcholine↗
PubMedDate not supplied

Unstable angina--a definitive role for statins in secondary prevention.

Unstable angina is a life-threatening disease. The overall incidence is estimated to be between 0.1 and 0.2% and, of those admitted to hospital, up to 5% die within the first six weeks. With such a high prevalence and poor prognosis, there is still a clear need for improved secondary preventive treatment. Several landmark statin trials (CARE, LIPID and 4-S) have now shown the benefits of lowering cholesterol, starting months after an acute coronary event, and several smaller studies (such as L-CAD, PTT and RECIFE) have demonstrated the clinical benefits of very early statin treatment. Recent research suggests the underlying cause of the disease, atherosclerosis, is a dynamic and potentially reversible process. Statins may tip this balance in favour of resolution by cholesterol lowering and through pleiotropic effects independent of lipid lowering. Pravastatin, when given very early in the clinical course, has already been shown to improve both plaque stability and endothelial function, as well as clinical outcome, suggesting it as the drug of choice in early secondary prevention.

Angina, Unstable↗