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R D Keynes

Publications and source records attributed to R D Keynes.

At least 19 recordsLinked to original sources
PubMed1999

The screw-helical voltage gating of ion channels.

In the voltage-gated ion channels of every animal, whether they are selective for K+, Na+ or Ca2+, the voltage sensors are the S4 transmembrane segments carrying four to eight positive charges always separated by two uncharged residues. It is proposed that they move across the membrane in a screw-helical fashion in a series of three or more steps that each transfer a single electronic charge. The unit steps are stabilized by ion pairing between the mobile positive charges and fixed negative charges, of which there are invariably two located near the inner ends of segments S2 and S3 and a third near the outer end of either S2 or S3. Opening of the channel involves three such steps in each domain.

Amino Acid Sequence↗
PubMed1998

On the slowly rising phase of the sodium gating current in the squid giant axon.

High-resolution records of the sodium gating current in the squid giant axon demonstrate the existence of a slowly rising phase that is first apparent at pulse potentials slightly below zero, and becomes increasingly pronounced at more positive potentials. At +80 mV the current reaches its peak with a delay of 30 microseconds at 10 degrees C. It is suggested that this current is generated by the first two steps labelled R-->P and P-->A in the S4 units of all four domains of the series-parallel gating system, activating the channel before its opening by the third steps A-->B in domains I, II and III in conjunction with hydration. The kinetics of the slowly rising phase can only be explained by the incorporation of an appropriate degree of voltage-dependent cooperativity between the S4 voltage-sensors for their two initial transitions.

Animals↗
PubMed1998

Modelling the activation, opening, inactivation and reopening of the voltage-gated sodium channel.

A model of the voltage-gated sodium channel is put forward suggesting that the four S4 voltage-sensors behave as screw-helices making a series of discrete transitions that carry one elementary charge for each notch of the screw helix. After the channel has been activated by the first two steps R in equilibrium with P in equilibrium with A in all four domains, followed by a voltage-independent rearrangement, it is opened by a third cooperative step A in equilibrium with B in domains I, II and III in conjunction with hydration. Inactivation is a voltage-dependent process controlled by the third step A in equilibrium with I in sensor IVS4, and the closing of the channel is brought about its dehydration. From the inactivated steady state the channel may be reopened by a fourth step, I in equilibrium with C in sensor IVS4 and rehydration. The computed kinetics of the model are shown to conform closely with those observed experimentally.

Animals↗
PubMed1997

Steps on the path to the origin of species.

The choice of Charles Darwin to serve as geologist on H.M.S. Beagle came about in a somewhat haphazard fashion, and by modern standards his technical qualifications for the post were not strong. However, during the voyage he was exposed to a wider range of phenomena, both in geology and in natural history, than any previous scientist, and his innate qualities of enquiring critically with an open mind into the why and wherefore of every one of his observations enabled him to make very effective use of his experience. By the end of the voyage he had found himself ready to abandon the doctrine of the fixity of species, and a few months later he opened the first of the series of notebooks on "Transmutation of Species" in which he recorded his private thinking. He quite quickly arrived at the Principle of Natural Selection as a mechanism for the creation of new species, but the process of building up adequate evidence in support of his theory was a slow one, and more than 20 years had passed before his great work was finally ready for publication.

Animals↗
PubMed1994

Bimodal gating of the Na+ channel.

Inactivation of voltage-gated ion channels, whether they are selective for Na+, K+ or Ca2+, probably never involves their total closure, and some flow of ion current persists if large enough test pulses are applied. Incomplete inactivation was first reported for the Na+ channels of the squid giant axon, but has since been observed in other types of peripheral nerve and, more recently, in muscle fibres and the neurons of mammalian brain. The phenomenon is therefore widespread and has important implications for the functioning of voltage-gated channels in a variety of situations. It is best described in terms of a gating mechanism that switches the channel from an initial mode in which it has a high probability of opening to one in which the probability is greatly lowered, but not reduced to zero.

Animals↗
PubMed1993

Properties of the voltage sensor for the opening and closing of the sodium channels in the squid giant axon.

A combination of data from standard I-V curves, and from steps applied either at the initial current peak or in the inactivated steady state, yielded values of the total probability of the two open states of the sodium channel, multiplied by a constant scaling factor, as a function of membrane potential. The probability function PFpeak was found to reach a maximum for pulses to 40-50 mV, but for larger test potentials it underwent a slight decline. The curve for its rise was shifted in a positive direction by several millivolts when the temperature was raised. Measurements of the probability function PFss in the final steady condition, when almost the whole population of channels was inactivated, but a small flow of Na+ current persisted, showed that the voltage sensor responsible for the actual opening of the channels carried 0.8 electronic charges, and that its equilibrium potential had been shifted nearly 100 mV by inactivation to lie close to 50 mV. The charge carried by the C<-->O voltage sensor was the same for all the dialysis and bathing solutions that were tested, but when dialysing with 350 mM NaF and bathing with full Na seawater plus 16 nM TTX, the equilibrium potential in the inactivated state was increased by about 25 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Activation, inactivation and recovery in the sodium channels of the squid giant axon dialysed with different solutions.

Comparisons were made between families of ion currents recorded in voltage-clamped squid axons dialysed with 20 mM NaF and 330 mM CsF or TMAF, and bathed in a solution in which four fifths of the Na was replaced by Tris. The permeability coefficient PNa,fast for the fast-inactivating current in the initial open state was calculated as a function of test potential from the size of the initial peak of INa. The permeability coefficient PNa,non for the non-inactivating open state was calculated from the steady-state INa that persisted until the end of the test pulse. Dialysis with TMA had no direct effect on the QV curve for gating charge. The reversal potential for INa,non was always lower than that for INa,fast, the mean difference being about -9 mV when dialysing with Cs, but only about -1 mV with TMA. Except close to threshold, PNa,fast was roughly halved by dialysis with TMA as compared with Cs, but PNa,non was substantially increased. The time constant tau h inactivation of the sodium system was slightly increased during dialysis with TMA in place of Cs, and there were small shifts in the steady-state inactivation curve, but the rate of recovery from inactivation was not measurably altered. The flattening off of the tau h curve at increasingly positive test potentials corresponded to a steady reduction of the apparent inactivation charge until a value of about 0.2e was reached for pulses to 100 mV. The instantaneous I-V relationship in the steady state was also investigated. The results have a useful bearing on the effects of dialysis with TMA, on the differences between the initial and steady open states of the sodium channel, and on the relative voltage-dependences of the transitions in each direction between the resting and inactivated states.

Animals↗
PubMed1992

The dual effect of internal tetramethylammonium ions on the open states of the sodium channel in the squid giant axon.

Voltage-clamp recordings of INa in squid axons dialysed with Cs or TMA, and bathed in low Na choline seawater, showed that, except close to threshold, the initial peak of fast-inactivating current was invariably decreased by TMA, whereas the non-inactivating current in the steady state was simultaneously increased. The results suggest that although TMA does not act directly on the movements of the voltage sensors that activate the sodium system, it blocks single-channel conductance in a voltage-dependent fashion in both the open states of the Na channel, while it has an entirely different type of action by increasing the probability of late openings in the steady state. Another difference between the two open states was that the sodium permeability coefficient had a Q10 of 1.8 in the initial open state, whereas in the steady state the effect of temperature was much smaller or even negative.

Animals↗
PubMed1992

A new look at the mechanism of activation and inactivation of voltage-gated ion channels.

Studies on the kinetics of activation and inactivation of the sodium channels of the squid giant axon, on the sodium gating current, and on the properties of the non-inactivating steady-state current, are briefly reviewed. Taken in conjunction with recent evidence on the structure of voltage-gated ion channels, they have led to the development of a series-parallel model of the sodium channel that can be regarded as a modernized version of the Hodgkin-Huxley model, with some novel features. It is suggested that activation results from conformational changes brought about by the four S4 voltage sensors operating in parallel, each of which makes two discrete steps to reach the fully activated state of the channel. There follows a voltage-independent hydration step, and the channel is ready to open. Inactivation is a potential-dependent process involving a third transition of voltage sensor S4d alone, which, rather than bringing a ball and chain blocking group into position to close the channels, serves to switch the system so that it passes from an initial activated mode, in which there is a high probability of arriving at an open state with a brief latency, to a second steady-state mode, in which the probability of opening is very much lower.

Animals↗
PubMed1991

The effect of tetrodotoxin on the sodium gating current in the squid giant axon.

The effect of tetrodotoxin (TTX) on the sodium gating current in the squid giant axon was examined by recording the current that flowed at the pulse potential at which the ionic current fell to zero, first in the absence and then in the presence of TTX. The addition of 1 microM TTX to the bathing solution had no consistent effect on the size of the initial peak of the gating current, but resulted in small changes in the timecourse of its subsequent relaxation which were mainly caused by a reduction of about one quarter in the component that has a delayed onset and may possibly arise from changes in the state of ionization of groups in the channel wall when the lumen fills with water. Our findings suggest that the binding of TTX at the outer face of the sodium channel does not interfere with the mechanisms of activation and inactivation by the voltage sensors, but has an allosteric effect on the access of internal cations to the inside of the channel.

Animals↗
PubMed1990

Kinetic analysis of the sodium gating current in the squid giant axon.

A critical study has been made of the characteristics of the kinetic components of the sodium gating current in the squid giant axon, of which not less than five can be resolved. In addition to the principal fast component Ig2, there are two components of appreciable size that relax at an intermediate rate, Ig3 alpha and Ig 3 beta. Ig3 alpha has a fast rise, and is present over the whole range of negative test potentials. Ig3 beta is absent below -40 mV, exhibits a delayed onset and disappears on inactivation of the sodium system. There are also two smaller components, Ig1 and Ig4, with very fast and much slower relaxation time constants, respectively.

Animals↗
PubMed1990

A series-parallel model of the voltage-gated sodium channel.

A series-parallel model of the kinetics of the voltage-gated sodium channel is described. It goes some way towards reconciling the time-courses of the gating and macroscopic sodium currents in the squid giant axon with the molecular structure of the channel.

Animals↗
PubMed1988

Kinetics of activation of the potassium conductance in the squid giant axon.

A quantitative re-investigation of the time course of the initial rise of the potassium current in voltage-clamped squid giant axons is described. The n4 law of the Hodgkin-Huxley equations was found to be well obeyed only for the smallest test pulses, and for larger ones a good fit of the inflected rise required use of the expression (1-exp[-t/tau n1])X-1(1-exp[-t/tau n2]), where both of the time constants and the power X varied with the size of the test pulse. Application of a negative prepulse produced a delay in the rise resulting mainly from an increase of X from a value of about 3 at -70 mV to 8 at -250 mV, while tau n1 remained constant and tau n2 was nearly doubled. The process responsible for generating this delay was switched on with a time constant of 8 ms at 4 degrees C, which fell to about 1 ms at 15 degrees C. Analysis of the inward tail currents at the end of a voltage-clamp pulse showed that there was a substantial external accumulation of potassium owing to the restriction of its diffusion out of the Schwann cell space, which, when duly allowed for, roughly doubled the calculated value of the potassium conductance. Computations suggested that the principal effect of such a build-up of [K]o would be to reduce the fitted values of tau n1 and tau n2 to two-thirds or even half their true sizes, while the power X would generally be little changed; but it would not affect the necessity to introduce a second time constant, nor would it invalidate our findings on the effect of negative prepulses.

Animals↗
PubMed1986

The conductance and density of sodium channels in the cut-open squid giant axon.

Non-stationary Na current fluctuations in small voltage-clamped patches of cut-open squid giant axon were analysed by an ensemble-average technique to yield the single Na channel conductance gamma Na and the Na channel density in the patch. gamma Na appeared to be voltage independent over the range -30 to +40 mV and had a mean value of 4.4 +/- 1.1 pS in 514 mM-Na/20 mM-Na at 5 kHz band width and temperature between 3.5 and 5.0 degrees C. gamma Na did not change significantly at band widths to 20 kHz. gamma Na in reduced Na solutions, 103 mM-Na/4 mM-Na, at 3.5-5.0 degrees C had a mean value of 1.2 +/- 0.3 pS. Internal solutions containing 50 mM-tetraethylammonium (TEA) depressed both gamma Na and the mean Na currents by roughly the same factor, compared with solutions without TEA. The reduced gamma Na had a mean value of 2.2 +/- 0.7 pS. The mean Na channel density in the standard 514 mM-Na/20 mM-Na solution was estimated to be 180 +/- 100 microM-2. The densities in the other solutions mentioned above were not significantly different from this value.

Action Potentials↗