Biomedical subjects
Publications and source records attributed to S Y Chiu.
Ionic currents in demyelinated segments of frog sciatic nerves have been detected by means of a patch clamp technique. Axons were examined 6-13 days after lysolecithin-induced demyelination. Both voltage-gated sodium channels and voltage-gated potassium channels could be demonstrated in the demyelinated axolemma.
Patch-clamp recording from the plasmalemma of rat cultured astrocytes reveals the presence of both voltage-dependent sodium and voltage-dependent potassium conductances. These conductances are similar but not identical to the corresponding conductances in the axolemma. Whereas the h infinity relation of the sodium channels has the same voltage dependence as in the nodal axolemma, the peak current-voltage relation is shifted by about 30 mV along the voltage axis in the depolarizing direction. It is speculated that the glial cells synthesize sodium and potassium channels for later insertion into the axolemma of neighbouring axons. The astrocytes also express a plasmalemmal voltage-dependent anion conductance that is turned on at about -40 mV (that is, near the resting potential of the cultured astrocytes). The channels involved are large enough to be just permeable to glutamate but not to ascorbate. It is suggested that the conductance of this channel for chloride plays a physiological role in the spatial buffering of potassium by glial cells.
Cultured Schwann cells from sciatic nerves of newborn rabbits and rats have been examined with patch-clamp techniques. In rabbit cells, single sodium and potassium channels have been detected with single channel conductances of 20 pS and 19 pS, respectively. Single sodium channels have a reversal potential within 15 mV of ENa, are blocked by tetrodotoxin, and have rapid and voltage-independent inactivation kinetics. Single potassium channels show current reversal close to EK and are blocked by 4-aminopyridine. From these results, and from comparisons of single-channel and whole-cell data, we show that these Schwann cells contain voltage-dependent sodium and potassium channels that are similar in most respects to the corresponding channels in mammalian axonal membranes. Cultured rat Schwann cells also have sodium channels, but at a density about 1/10th that of rabbit cells, a result in agreement with saxitoxin binding experiments on axon-free sectioned nerves. Saxitoxin binding to cultured cells suggests that there are up to 25,000 sodium channels in a single rabbit Schwann cell. We speculate that in vivo Schwann cells in myelinated axons might act as a local source for sodium channels at the nodal axolemma.
A theoretical analysis of the passive electrical properties of normal myelinated nerve suggests that the function of the voltage-dependent potassium channels in the internodal axolemma under the myelin sheath is to permit the generation of an internodal resting potential. Calculation shows that if this internodal potential were not present, the nodal potential would be reduced (by electrotonic short-circuiting) thus impairing the security of conduction. This impairment is particularly pronounced with smaller diameter fibres.
1. The central portion of single frog internodes under voltage-clamp conditions was acutely demyelinated with lysolecithin to resolve whether potassium channels are normally present only in the paranode (Chiu & Ritchie, 1980, 1981) or are in fact present throughout the entire internode of a myelinated nerve fibre. 2. The experiments were performed in the absence of nodes of Ranvier to ensure that all currents observed originated from the internode. 3. The demyelinating internodal segment showed a steady increase in the capacity and leakage for the first 40--50 min, after which a delayed outward current was revealed. This delayed current, which was blocked by TEA and caesium and exhibited a potassium-dependent reversal potential, seems to be a potassium current similar to that normally present in the frog node and in squid nerve. 4. The present method of acute demyelination revealed no detectable sodium currents in the internodal segments. 5. These results suggest that potassium channels, but virtually no sodium channels, are normally present throughout a frog internode covered by the myelin.
1. A study has been made of the ionic currents in voltage-clamped single rabbit nodes of Ranvier at 22-26 degrees C both under normal conditions, and after the nerve fibres had been acutely demyelinated by a variety of treatments designed to lossen the myelin from the axonal membrane. 2. The myelin-loosening treatments included application of various combinations of: lysolecithin (to dissolve the myelin); collagenase (to loosen the connective tissue in the nodal region); high-potassium Locke solution, hypertonic and hypotonic solutions (to induce axonal volume changes). 3. At a critical stage in such treatment (usually after 15-45 min) a large outward current suddenly appeared. 4. There was no substantial change in the size of the measured inward sodium current when measured at this critical stage. 5. The outward current was blocked by internal TEA and caesium ions, had a reversal potential that became more positive when the external potassium concentration was increased, was kinetically similar to the known potassium current in frog fibres, and was therefore assumed to be a potassium current. 6. The phase of large outward current, whenever it appeared, was always accompanied by the appearance of a slow transient capacitative component in the leakage current, which indicated a marked increase in the effective nodal capacity (of 10- to 60-fold). We suggest that the slow transient capacity current reflected charging of newly exposed axonal membrane, probably in the paranodal region, which was uncovered by the various acute demyelination treatments. This internodal membrane seems to contain mostly potassium channels and few, if any, sodium channels. 7. Newly dissected fibres occasionally showed large potassium currents before treatment, particularly if they were deliberately stretched during dissection; a marked slow capacity transient current was consistently present in these fibres. 8. The effects of acute paranodal demyelination on the sodium and potassium currents, and on the transient capacity currents, can be simulated by a model in which the node is coupled to a cable-like paranode which contains Hodgkin--Huxley type potassium channels and which has a much higher leakage resistance. 9. The functional significance of the presence of potassium channels in rhe internodal region (at least in the paranode) of mammalian fibres is discussed.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Horakova et al. were the first to observe that the phase of late outward current carried by potassium ions in frog and squid nerve is virtually absent in voltage-clamped rat nodes of Ranvier. This observation has been recently confirmed by Chiu et al. in rabbit nodes of Ranvier, suggesting that the nodal membrane in the mammal generally has few if any potassium channels. The present voltage-clamp experiments show that large potassium currents can, however, be produced in normal rabbit nodes of Ranvier by acute treatment designed to loosen the myelin from the axonal membrane. From this we conclude that whereas potassium channels are absent in the mammalian nodal membrane, they are normally present in the internodal axonal membrane at least in the paranodal region.
1. Asymmetrical displacement currents were recorded in the rabbit node of Ranvier by averaging the currents associated with depolarizing and hyperpolarizing pulses in the temperature range 15-25 degrees C with the ends of the fibre cut in 160 mM-CsCl. 2. The identification of the asymmetrical currents as intramembranous current is supported by the findings that (a) the "on' and "off' current transients were equal for short and small depolarizing pulses and (b) the total charge displaced reached saturation at (32-111 x 10(-15) C/node) when sufficiently large depolarizations were applied. 3. After a large depolarization to around 50 mV, the "off' response consisted of a fast phase followed by a slow phase. Lengthening the depolarization reduced the size of the fast response but enhanced the slow response. 4. The steady-state rearrangement of the charges can be described by a Boltzmann distribution of charges with an effective valence of 1.86 and a midpoint potential of -33 mV. The time course of rearrangement of these charges following a change in membrane potential could be fitted reasonably well with a single exponential though a double exponential might have been better at large depolarizations. 5. The steady-state activation curve for Na conductance was measured with various procedures to eliminate the effects of series resistance, which include decreasing peak Na current by TTX, and electronic compensation of the series resistance. The measured steepness of the Na-activation curve corresponds to moving a minimum charge of about 5e to open each Na channel at 22.7 degrees C. 6. The time constant (Tm) for activation of Na current and time constant (Ton) for declining phase of the asymmetry current were of the same order of magnitude but not identical over the potential range -50 to +25 mV. 7. The time course of charge displacement determined from the asymmetry current occurred earlier than did Na activation. Raising the charge displacement curve to second or third power did not yield a curve that matched the time course of Na activation. 8. If all the observed asymmetrical currents are related to the Na-gating system, an upper limit for the number of Na channels per rabbit node is 82,000 and a lower limit for the single channel conductance is 9.8 pS at 18.8 degrees C.
Explore the source record for details and available documents.
1. Voltage-clamp studies were carried out on single rabbit myelinated nerve fibres at 14 degrees C with the method of Dodge & Frankenhaeuser (1958). 2. A method was developed to allow the ionic currents through the modal membrane to be calibrated exactly under voltage-clamp conditions by measuring the resistance of the internode through which the current was injected. 3. The ionic currents in a rabbit node of Ranvier can be resolved into two components, a sodium current and a leak current. Potassium current is almost entirely absent. 4. The sodium currents in rabbit nodes were fitted by the Hodgkin-Huxley model using m2h kinetics. The kinetics of sodium currents in a rabbit node differ from that in a frog node under similar experimental conditions in two respects: (a) inactivation is faster, tau h for rabbit being 2-3 times smaller around -50 mV; (b) the P(Na) (E) curve for mammal is shifted 10-15 mV in the hyperpolarizing direction. 5. From the kinetics of sodium current, the non-propagating rabbit action potential was reconstructed at 14 degrees C. The transient inward sodium current is responsible for the fast initial depolarization phase of the action potential, while the repolarizing phase is accounted for by leak alone. The computed shape of the action potential was in good agreement with the experimentally obtained action potential. 6. At 14 degrees C, frog and rabbit nodes with similar diameters have similar measured gNa values.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
1. Kinetics of inactivation of sodium channels in myelinated nerve from Rana pipiens were studied at 4.5 degrees C using the voltage clamp technique of Dodge & Frankenhaeuser (1958).2. Potassium currents were blocked by cutting the internodes in 20 mM-TEA-Cl + 100 mM-KCl and by adding 12 mM-TEA-Cl to the external Ringer. Leakage and capacitative currents were subtracted electronically.3. Kinetics of recovery from inactivation of the sodium channels were studied by inactivating the channels with a large depolarizing prepulse and allowing the channels to recover at different potentials; the extent of recovery was measured by applying a test pulse at various times after the prepulse.4. Kinetics of development of inactivation were studied by two different methods. The first was to measure the decay of sodium current under a maintained depolarization. The second method was to measure the decay of the peak sodium current in a test pulse as a function of time after the onset of a maintained depolarization. These two methods yielded similar results for the kinetics of inactivation development.5. Contrary to expectations of the Hodgkin-Huxley formalism, the time course of recovery from and development of inactivation is not strictly exponential. Rather, recovery from complete inactivation shows an initial delay which depends on recovery potentials. Development of inactivation at a fixed potential exhibits at least two exponentials.6. The steady-state inactivation curve h(infinity)(E) is asymmetrical and is fitted better by 1/[1+exp (A(1)E+B(1)) +exp (A(2)E+B(2))] than by 1/[1+exp (AE+B)].7. Most of the above kinetic observation on inactivation can be fitted by the following modification of the h system of the Hodgkin-Huxley formalism: [Formula: see text]8. In the analysis it was not necessary to modify the concept of two separate processes, activation and inactivation, governing the opening and closing of the sodium channels.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.