Prenatal expression of inwardly rectifying potassium channel mRNA (Kir4.1) in rat brain.

The levels and cellular localization of the mRNA encoding the inwardly rectifying potassium ion channel Kir4.1 were investigated in the embryonic rat brain by Northern blots and in situ hybridization. This transcript was absent at embryonic day 13 (E13), whereas it was clearly present in E14-15 preparations, principally in the neuroepithelium of the cerebral cortex, thalamus, and hypothalamus. At later embryonic stages (E17-20), Kir4.1 mRNA levels increased and expanded to the mantle zone, such as the cortical plate, hippocampus, thalamus, and hypothalamus. The early appearance of Kir4.1 mRNA in various brain regions suggests an involvement of the channel in cell proliferation, migration and differentiation in the rat CNS.

Action potentials occur spontaneously in squid giant axons with moderately alkaline intracellular pH.

This report demonstrates a novel finding from the classic giant axon preparation of the squid. Namely, the axon can be made to fire autonomously (spontaneously occurring action potentials) when the intracellular pH (pH(i)) was increased to about 7.7, or higher. (Physiological pH(i) is 7.3.) The frequency of firing was 33 Hz (T = 5 degrees ). No changes in frequency or in the voltage waveform itself were observed when pH(i) was increased from 7.7 up to 8.5. In other words, the effect has a threshold at a pH(i) of about 7.7. A mathematical model that is sufficient to mimic these results is provided using a modified version of the Clay (1998) description of the axonal ionic currents.

Nifedipine-associated gingival overgrowth: a survey of the literature and report of four cases.

A survey of the literature indicates that calcium channel-blocking medications are used for an ever-increasing number of medical problems. Their use may result in gingival overgrowth that can be of concern to the patient and dentist. Four cases are presented that illustrate several relevant points: (1) histologic examination may reveal factors overlooked in the medical history; (2) the condition exists in a variety of clinical manifestations; (3) the level of plaque control maintained by the patient is important to the management of the condition; and (4) responses vary to different approaches to treatment, including changes in medication, as well as nonsurgical and surgical therapies.

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.

Tobacco cessation: a practical dental service.

Tobacco use is a complex addiction that must be addressed in all aspects of health care. Despite the deleterious and costly outcomes of tobacco use, Americans still are smoking and using smokeless tobacco. Dentists are trained to detect oral lesions and periodontal problems that are related to tobacco use. Dentists also are in a position to help prevent the initiation of tobacco use by children and adolescents through the use of positive anti-tobacco messages. Over the past decade, tobacco cessation strategies have been modified for practical use in the dental setting.

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.

Determining K+ channel activation curves from K+ channel currents.

Potassium ion channels are generally believed to have current-voltage (IV) relations which are linearly related to driving force ( V - E(K)), where V is membrane potential and E(K) is the potassium ion equilibrium potential. Consequently, activation curves for K+ channels have often been measured by normalizing voltage-clamp families of macroscopic K+ currents with (V - E(K)), where V is the potential of each successive step in the voltage clamp sequence. However, the IV relation for many types of K+ channels actually has a non-linear dependence upon driving force which is well described by the Goldman-Hodgkin-Katz relation. When the GHK dependence on (V - E(K)) is used in the normalization procedure, a very different voltage dependence of the activation curve is obtained which may more accurately reflect this feature of channel gating. Novel insights into the voltage dependence of the rapidly inactivating I(A) channels Kv1.4 and Kv4.2 have been obtained when this procedure was applied to recently published results.

Potassium current in the squid giant axon.

The squid giant axon was the first preparation to be investigated with the voltage clamp technique over 30 years ago by Cole (1949) and Hodgkin et al. (1952). During the intervening years it has continued to serve as a useful preparation for the development of other new techniques such as internal perfusion (Baker et al., 1962), gating current measurements (Armstrong and Bezanilla, 1974), and patch clamp measurements (Conti and Neher, 1980). It also has served as a useful comparative preparation for investigations of sodium and potassium currents in other excitable membrane preparations. This article has focused on the activation kinetics and the instantaneous current-voltage relation of the potassium component. The squid axon is well suited for studies of IK, because it appears to have only a single type of potassium channel, and the leakage current is relatively small under ideal conditions. The IK component is activated in a sigmoidal manner following membrane depolarization. It deactivates with a single exponential time constant following return of the membrane potential to the holding level, although the deactivation time constant varies with changes in the external potassium concentration. There has not, as yet, appeared a self-consistent model which describes all of these results. The current-voltage relation is a nonlinear function of driving force, which is approximately described by the Goldman-Hodgkin-Katz equation, although a model of the IV based on single file diffusion of ions through a channel is more in tune with the modern view of the ion permeation process (Hodgkin and Keynes, 1955; Hille and Schwarz, 1978; Clay and Shlesinger, 1977, 1983, 1984). Further progress in this area will probably be achieved both by the traditional techniques and by the patch clamp technique. The traditional method is well suited for studying tail current kinetics and the slow inactivation process. The patch clamp technique is well suited for studying the distribution of channels in the membrane and the kinetics of channel gating in steady state conditions.

Comparison of the effects of internal TEA+ and Cs+ on potassium current in squid giant axons.

Internal tetraethylammonium (TEA) and cesium ions block outward potassium current in nerve membrane in a voltage-dependent manner. Blockade with Cs+ occurs virtually instantaneously after membrane depolarization, whereas blockade with TEA+ occurs after a delay. The latter result suggested to Armstrong (1966, J. Gen. Physiol., 50:279-293; 1969, J. Gen. Physiol., 54:553-575) that potassium channels must open before TEA+ blockade can occur, which is in contrast to Cs+ blockade, which appears to be independent of channel gating. The results in this study concerning the effect of TEA+ on inward (tail) current argue against the Armstrong model. Specifically, TEA+ (partially) blocks inward current without altering the tail current time constant. This result indicates that TEA+ can occupy its binding site within the channel whether or not the channel gates are open. This alternative hypothesis can describe both the steady-state and time-dependent components of TEA+ blockade.

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.

Comparison of ion current noise predicted from different models of the sodium channel gating mechanism in nerve membrane.

The theoretical power density spectrum S(f) of ion current noise is calculated from several models of the sodium channel gating mechanism in nerve membrane. Sodium ion noise experimental data from the frog node of Ranvier [Conti, F., et al. (1976), J. Physiol. (London) 262:699] is used as a test of the theoretical results. The motivation for recent modeling has been evidence for a coupling between sodium activation and inactivation from voltage clamp data. The two processes are independent of one another in the Hodgkin and Huxley (HH) model [Hodgkin A.L., Huxley, A.F. (1952), J. Physiol. (London) 117:500]. The noise data is consistent with HH, as noted by Conti et al. (1976). The theoretical results given here appear to indicate that only one case of coupling models is also consistent with the noise data.

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.

Potassium channel kinetics in squid axons with elevated levels of external potassium concentration.

Potassium ion current in squid axons is usually modified by the effects of ion accumulation in the periaxonal space during voltage-clamp depolarization. The time course of potassium channel activation and ion accumulation usually overlap. A widely accepted procedure for circumventing the effects of accumulation in measurements of activation kinetics consists of measuring the difference in the current at the end of a depolarizing pulse and immediately following return of the membrane potential to the holding level. This instantaneous jump procedure is based upon the assumptions that the potassium channel current-voltage relation (IV) is a linear function of the driving force, and that the IV and the potassium channel-gating kinetics are both independent of ion accumulation. The latter assumption appears to be appropriate for activation kinetics. However, both assumptions concerning the IV are incorrect, in general. Consequently, the jump procedure provides a misleading view of gating kinetics for membrane depolarizations that produce net current flow. Jump conductance measurements for depolarizations that produce little or no net current indicate that the Hodgkin-Huxley n4 model of potassium channel kinetics is appropriate for the physiological range of membrane potentials.

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.

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).

Excitability of the squid giant axon revisited.

The electrical properties of the giant axon from the common squid Loligo pealei have been reexamined. The primary motivation for this work was the observation that the refractoriness of the axon was significantly greater than the predictions of the standard model of nerve excitability. In particular, the axon fired only once in response to a sustained, suprathreshold stimulus. Similarly, only a single action potential was observed in response to the first pulse of a train of 1-ms duration current pulses, when the pulses were separated in time by approximately 10 ms. The axon was refractory to all subsequent pulses in the train. The underlying mechanisms for these results concern both the sodium and potassium ion currents INa and IK. Specifically, Na+ channel activation has long been known to be coupled to inactivation during a depolarizing voltage-clamp step. This feature appears to be required to simulate the pulse train results in a revised model of nerve excitability. Moreover, the activation curve for IK has a significantly steeper voltage dependence, especially near its threshold (approximately -60 mV), than in the standard model, which contributes to reduced excitability, and the fully activated current-voltage relation for IK has a nonlinear, rather than a linear, dependence on driving force. An additional aspect of the revised model is accumulation/depeletion of K+ in the space between the axon and the glial cells surrounding the axon, which is significant even during a single action potential and which can account for the 15-20 mV difference between the potassium equilibrium potential EK and the maximum afterhyperpolarization of the action potential. The modifications in IK can also account for the shape of voltage changes near the foot of the action potential.