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J R Clay

Publications and source records attributed to J R Clay.

At least 37 records · Page 2Linked to original sources
PubMed1995

Asymmetric modulation and blockade of the delayed rectifier in squid giant axons by divalent cations.

The effects of intracellular magnesium ions and extracellular calcium and magnesium ions on the delayed rectifier potassium ion channel, IK, were investigated from intracellularly perfused squid giant axons. Cao+2 and Mgo+2 both blocked IK in a voltage-independent manner with a KD of approximately 100 and 500 mM, respectively. This effect was obscured at potentials in the vicinity of the resting potential (approximately -60 mV) by a rightward shift of the steady-state IK inactivation curve along the voltage axis. The addition of either calcium or magnesium ions to the extracellular solution also produced the well known shift of the IK activation curve along the voltage axis. Cao+2 was approximately twice as effective in this regard as Mgo+2. The IK activation kinetics were slowed by Cao+2, but deactivation kinetics were not altered, as shown previously. Similar results were obtained with Mgo+2. The addition of magnesium ions to the intracellular perfusate shifted the activation curve along the voltage axis in the negative direction (without producing block) by approximately the same among as the Mgo+2 shift of this curve in the positive direction. Moreover, Mgi+2 substantially slowed the deactivation kinetics, whereas the effects of Mgi+2 on activation kinetics at strongly depolarized potentials were relatively minor. At modest depolarizations, Mgi+2 significantly reduced the delay before IK activation. These results are essentially the mirror image of the effects on gating of extracellular divalent cations.

Animals↗
PubMed1995

A quantitative description of the E-4031-sensitive repolarization current in rabbit ventricular myocytes.

We have measured the E-4031-sensitive repolarization current (IKr) in single ventricular myocytes isolated from rabbit hearts. The primary goal of this analysis was a description of the IKr kinetic and ion transfer properties. Surprisingly, the maximum time constant of this component was 0.8 s at 33-34 degrees C, which is significantly greater than the value of 0.18 s previously reported under similar conditions in the original measurements of IKr from guinea pig ventricular myocytes. The primary, novel feature of our analysis concerns the relationship of the bell-shaped curve that describes the voltage dependence of the kinetics and the sigmoidal curve that describes the activation of IKr. The midpoint of the latter occurred at approximately +10 mV on the voltage axis, as compared to -30 mV for the point on the voltage axis at which the maximum time constant occurred. Moreover, the voltage dependence of the kinetics was much broader than the steepness of the activation curve would predict. Taken together, these results comprise a gating current paradox that is not resolved by the incorporation of a fast inactivated state in the analysis. The fully activated current-voltage relation for IKr exhibited strong inward-going rectification, so much so that the current was essentially nil at +30 mV, even though the channel opens rapidly in this voltage range. This result is consistent with the lack of effect of E-4031 on the early part of the plateau phase of the action potential. Surprisingly, the reversal potential Of /Kr was ~15 mV positive to the potassium ion equilibrium potential,which indicates that this channel carries inward current during the latter part of the repolarization phase of the action potential.

Animals↗
PubMed1994

A combined endodontic retrofill and periodontal guided tissue regeneration technique for the repair of molar endodontic furcation perforations: report of a case.

This is a report of a case in which the techniques of endodontic retrofill and guided tissue regeneration were used together to treat a perforation of the mesiofacial root of a maxillary left first molar (a strip perforation). After full-thickness flap reflection, the root received a retrograde filling and a large fenestration defect was surgically created to allow access to the furcal (distal) surface of the mesiofacial root. The roots were treated with tetracycline, the defect was filled with freeze-dried, demineralized, irradiated human cadaver bone, and the access fenestration was covered with a piece of barrier membrane. Healing was uneventful and the defect showed 100% bone fill at the time of reentry to retrieve the membrane 7 months after the initial surgery. Fifteen months after surgery the area appears clinically and radiographically healthy and the tooth has been employed as an abutment for a fixed partial denture.

Adult↗
PubMed1992

Pacemaker current in single cells and in aggregates of cells dissociated from the embryonic chick heart.

1. We have measured the time-dependent pacemaker current, I(f), in single cells, or small clusters of two or three cells dissociated from embryonic chick hearts with the whole-cell patch clamp technique, and in multicellular reaggregates of dissociated cells with the two-microelectrode voltage clamp technique. 2. We observed time-dependent current (I(f)) in the -90 to -60 mV range from aggregates of ventricular cells, as in our earlier results from this preparation, which we previously attributed to the potassium ion current mechanism, IK2. We also observed I(f) in single atrial cells and aggregates of atrial cells. 3. The range of activation of I(f) was -120 to -90 mV in atrial preparations (either single cells or aggregates). The activation range of I(f) in ventricular cells was also -120 to -90 mV which is approximately 30 mV negative to the I(f) activation range in ventricular cell aggregates. The reasons for this shift of I(f) in ventricular preparations are unknown. 4. The I(f) component clearly underlies the spontaneous pacemaker depolarization which is observed in ventricular heart cell aggregates during the first week of embryonic development. However, I(f) is not a significant factor underlying spontaneous activity in atrial preparations. The pacemaker current in these cells is a net inward background component, which is significantly reduced in amplitude with development, as is the I(f) component in the ventricle.

Animals↗
PubMed1991

A paradox concerning ion permeation of the delayed rectifier potassium ion channel in squid giant axons.

1. The fully activated current-voltage relation (I-V) of the delayed rectifier potassium ion channel in squid giant axons has a non-linear dependence upon the driving force, V-EK, as I have previously demonstrated, where V is membrane potential and EK is the equilibrium potential for potassium ions. 2. The non-linearity of the I-V relation and its dependence upon external potassium ion concentration are both well described, phenomenologically, by the Goldman-Hodgkin-Katz (GHK) flux equation, as I have also previously demonstrated. As illustrated below, this result can be modelled using the Eyring rate theory of single-file diffusion of ions through a channel in the low-occupancy limit of the theory. 3. The GHK equation analysis and the low-occupancy limit of the Eyring rate theory are both consistent with the independence principle for movement of ions through the channel, which is at odds with tracer flux ratio results from the delayed rectifier, published elsewhere. Those results suggest that the channel is multiply occupied by two, or perhaps three, ions. 4. The resolution of this paradox is provided by a triple-binding site, multiple-occupancy model in which only one vacancy, at most, is allowed in the channel. This model predicts current-voltage relations which are consistent with the data (and with the phenomenological prediction of the GHK flux equation). The model is also consistent, approximately, with the tracer flux ratio results.

Animals↗
PubMed1990

Phase resetting of embryonic chick atrial heart cell aggregates. Experiment and theory.

The influence of brief duration current pulses on the spontaneous electrical activity of embryonic chick atrial heart cell aggregates was investigated experimentally and theoretically. A pulse could either delay or advance the time of the action potential subsequent to the pulse depending upon the time in the control cycle at which it was applied. The perturbed cycle length throughout the transition from delay to advance was a continuous function of the time of the pulse for small pulse amplitudes, but was discontinuous for larger pulse amplitudes. Similar results were obtained using a model of the ionic currents which underlie spontaneous activity in these preparations. The primary ion current components which contribute to phase resetting are the fast inward sodium ion current, INa, and the primary, potassium ion repolarization current, IX1. The origin of the discontinuity in phase resetting of the model can be elucidated by a detailed examination of the current-voltage trajectories in the region of the phase response curve where the discontinuity occurs.

Action Potentials↗
PubMed1990

Effects of tetrodotoxin on heart cell aggregates. Phase resetting and annihilation of activity.

The influence of relatively low concentrations of tetrodotoxin (TTX) on phase resetting of spontaneous activity of embryonic chick atrial heart cell aggregates by brief duration current pulses was investigated experimentally and theoretically. The maximal upstroke velocity, Vmax, of the spontaneous action potential was reduced by TTX in a concentration-dependent manner for [TTX] less than 10(-7) M. However, the beat rate was unaffected in this concentration range. Application of a depolarizing current pulse of brief duration during a critical region of the spontaneous cycle annihilated activity in some preparations exposed to [TTX] approximately 10(-7) M. These results were analyzed with the model of electrical activity described in the previous paper (Clay, J.R., R.M. Brochu, and A. Shrier. 1990. Biophys. J. 58:609-621) based on a tonic block of the INa channel by TTX with a dissociation constant, KD, of 50 nM.

Action Potentials↗
PubMed1990

IK inactivation in squid axons is shifted along the voltage axis by changes in the intracellular pH.

The inactivation curve of the delayed rectifier in internally perfused squid giant axons is shifted along the voltage axis by changes in the pH of the internal perfusate. The amplitude of the shift is 9.5 mV per pH unit (6 less than or equal to pHi less than or equal to 10). No saturation of the effect was observed at either end of the pH range. This result suggests that the inactivation gating mechanism has several titratable groups accessible to protons from the intracellular side of the membrane.

Animals↗
PubMed1989

Slow inactivation and reactivation of the K+ channel in squid axons. A tail current analysis.

Potassium current inactivation and reactivation in squid axons were measured from tail current amplitudes after voltage clamp prepulses to the potassium equilibrium potential, EK, in seawater containing elevated levels of potassium ion concentration, Ko. Little or no inactivation resulted with prepulses lasting less than 100 ms. Longer pulses caused the current to inactivate in two phases, one between 0.1 and 1 s, and a second phase between 5 and 100 s. Inactivation was incomplete. The time constant of the tail current after a prepulse to EK was independent of pulse duration (0.1-120 s). Inactivation was independent of Ko (10 less than or equal to Ko less than or equal to 300 mM), and it was independent of membrane potential, V, for -40 less than or equal to V less than or equal to 0 mV. Reactivation was measured with a three-pulse protocol. The reactivation time course was sigmoidal with a delay of approximately 100 ms before significant reactivation occurred. These results were described by a model consisting of three inactivated states arranged in a linear sequence. The rate constants of the model are of the form (A + B exp (CV), or 1/(A + B exp (CV], which are required to describe the non-inactivating conductance component.

Animals↗
PubMed1988

Repolarization current in embryonic chick atrial heart cells.

1. We have measured the delayed rectifier potassium current, IK, with the whole-cell patch-clamp technique from single cultured cells from the atria of 6- to 11-day-old chick embryonic hearts. 2. The IK component was activated with depolarizing voltage-clamp steps positive to -30 mV (holding potential in the -60 to -40 mV range). Maximum activation of the IK conductance occurred at +25 mV, based on deactivation, or tail current amplitudes upon return to the holding potential. Activation and tail current kinetics could both be described by single-exponential functions of time. 3. The IK kinetics were voltage dependent, with a maximum time constant, tau n, of approximately 2 s at V = -20 mV. 4. The IK reversal potential measurements suggest that this current is carried predominantly by potassium ions. 5. The IK results from single cells, or clusters of two or three cells, were comparable to our recent measurements of IK (IX2) in heart cell aggregates (Shrier & Clay, 1986). However, we did not obtain clear evidence in single cells for the IX1 repolarization current, in contrast to the aggregate results. 6. Computer simulations based on our IK measurements demonstrate that this component is sufficient to initiate repolarization of the action potential in single cells. However, it is not sufficient to reproduce the latter phase of repolarization for potentials negative to -30 mV. Addition of a relatively small IX1 component (2% in absolute terms compared to the aggregate work) is sufficient to account for this part of the action potential.

Action Potentials↗
PubMed1986

Potassium ion accumulation slows the closing rate of potassium channels in squid axons.

Potassium ion accumulation in the periaxonal space between squid axonal membrane and the Schwann cell surrounding the axon slows the rate of potassium channel closing to a degree that is consistent with the effect on channel closing of an equivalent change in the bulk external potassium concentration. The alteration of channel gating is independent of membrane potential, V, for V less than or equal to -60 mV, which suggests that the effect is mediated at a site on the outer surface of the membrane, rather than a site within the channel.

Animals↗
PubMed1986

Repolarization currents in embryonic chick atrial heart cell aggregates.

Outward membrane currents in aggregates of atrial cells prepared from 7-12-d-old chick embryonic hearts were measured with the two microelectrode voltage-clamp technique. Two outward current components, Ix1 and Ix2, were found in the plateau potential range of the action potential. The Ix1 component is activated between -50 and -20 mV; the Ix2 component is activated between -15 and +20 mV. The Ix1 component inwardly rectifies, whereas Ix2 has an approximately linear current-voltage relation. These preparations lack a time-dependent pacemaker current component, even though they beat spontaneously with an interbeat interval of approximately 1 s. A mathematical model of electrical activity is described based on our measurements of time-dependent outward current, and measurements in the literature of inward current components.

Animals↗
PubMed1986

Conditioning hyperpolarization delays in squid axon potassium channels.

Activation of potassium conductance in squid axons with membrane depolarization is delayed by conditioning hyperpolarization of the membrane potential. The delayed kinetics superpose with the control kinetics almost, but not quite, exactly following time translation, as demonstrated previously in perfused axons by Clay and Shlesinger (1982). Similar results were obtained in this study from nonperfused axons. The lack of complete superposition argues against the Hodgkin and Huxley (1952) model of potassium conductance. The addition of a single kinetic state to their model, accessible only by membrane hyperpolarization, is sufficient to describe this effect (Young and Moore, 1981).

Animals↗
PubMed1986

Potassium channel blockade: A mechanism for suppressing ventricular fibrillation.

The suppression of ventricular fibrillation by antidysrhythmic drugs is well correlated with their ability to block potassium channels in nerve and cardiac membranes. Blockade of potassium channels reduces electrical inhomogeneities in action potential and conduction parameters that lead to ventricular fibrillation. These actions tend to effectively decrease the electrical size of the heart, which suggests a mechanism for antifibrillatory drug action. The receptor sites for antifibrillatory drug action (IK blockade) appear to be on the outside of the cardiac membrane whereas receptors for antiarrhythmic drug action (INa blockade) appear to be on the inside of the cardiac membrane.

4-Aminopyridine↗