Double sucrose-gap method applied to single muscle fiber of Xenopus laevis

Exploring the Coordination of Cardiac Ion Channels With Action Potential Clamp Technique

The patch clamp technique underwent continual advancement and developed numerous variants in cardiac electrophysiology since its introduction in the late 1970s. In the beginning, the capability of the technique was limited to recording one single current from one cell stimulated with a rectangular command pulse. Since that time, the technique has been extended to record multiple currents under various command pulses including action potential. The current review summarizes the development of the patch clamp technique in cardiac electrophysiology with special focus on the potential applications in integrative physiology.

Modular double sucrose gap apparatus for improved recording of compound action potentials from rat and mouse spinal cord white matter preparations

Compound action potential (CAP) recording is a powerful tool for studying the conduction properties and pharmacology of axons in multi-axonal preparations. The sucrose gap technique improves CAP recording by replacing the extracellular solution between the recording electrodes with a non-conductive sucrose solution to minimize extracellular shunting. The double sucrose gap (DSG), conferring similar advantages at the stimulation site, has been extensively used on guinea pig spinal cord white matter (WM) in vitro. Establishing the DSG methodology for WM preparations from smaller animals such as rats and mice is appealing due to their extensive use in basic and translationally oriented research. Here we describe a versatile modular DSG apparatus with rubber membrane separation of the compartments, suitable for WM strips from rat and mouse spinal cord. The small volumes of compartments (<0.1 ml) and the air-tight design allow perfusion rates of 0.5-1 ml/min with faster refreshment rates compared to commonly used 2-3 ml/min and larger compartments, providing economical usage of expensive pharmacological drugs. Our improved DSG design is particularly efficient for uncovering slower conducting, higher threshold CAP components, as demonstrated by recordings of C-wave (non-myelinated axons) in rat dorsal WM. In myelin-deficient Shiverer mice with genetically dysmyelinated axons, our DSG apparatus recordings revealed a multi-peak C-wave without preceding faster components. The improved stimulation and recording with our DSG apparatus, lowering the range of required stimulus intensities and reducing the artifact interference with recorded CAPs provide for critical technical advantages that allow for more detailed analysis of CAPs in relatively short preparations.

A gap isolation method to investigate electrical and mechanical properties of fully contracting skeletal muscle fibers

We describe here a single-gap isolation method that allows the simultaneous measurement of electrical activity and tension output from fully contracting segments of frog skeletal muscle fibers. By using single pulses and pulse trains of varying frequency (5-100 Hz), records obtained for both electrical and mechanical fiber response demonstrate that the physiological properties of the fiber segments have been preserved. Action potentials could be recorded free of movement artifacts, even while segments were in fused tetani and developing maximum tensions of more than 600 kN/m2. Single current pulses evoked action potentials that averaged 144 +/- 16 mV (mean +/- SD, n = 8) in amplitude and twitches that averaged 285 +/- 66 kN/m2 and 55 +/- 5 ms (mean +/- SD, n = 20) in magnitude and time to peak, respectively. Trains of action potentials elicited patterns of tension development that exhibited summation, unfused tetani, and fused tetani in a frequency-dependent manner. The AC and DC electrical properties of the single grease gap were modeled with a simple Thévenin equivalent circuit, which satisfactorily predicted the experimental results. Our methodology is easily implemented and potentially applicable to any muscle preparation in which fiber segments with an intact end attached to a piece of tendon can be dissected.

Membrane potential and contractures in segments cut from rat fast and slow twitch muscles

Contractile responses associated with depolarization caused by an increase in [K]0 or by voltage-clamp steps were compared for fast and slow mammalian twitch muscles. The contractions and contractures of isolated mammalian muscle fibres cut into 0.5-1.0 cm lengths are similar to those obtained from intact cells. The slope of the relationship between the membrane potential and the log [K]0 is similar in slow (46 mV +/- 0.8) and in fast fibres (48 mV +/- 1.1). This relation is not significantly modified in sodium-free or Cl-free solution. The K-contractures of cut sections of slow and fast fibres are transient and a full repriming of the response is only observed when the [K] x [Cl] product is kept constant. The contractile threshold in fast fibres is at 20-30 mM [K]0 (-52 to -43 mV) and in slow muscle at 10-15 mM [K]0 (-62 to -55 mV). Under voltage-clamp conditions, the contractile responses of both types of muscle show two components. In Na-free solution or in presence of TTX (5 x 10(-7) g/ml) the first component is abolished and the second remaining component is similar to that developing during K-contractures. In iliacus fibres, the contracture threshold is between -50.5 mV and -40.5 mV and in soleus fibres between -66 mV and -56 mV; these values are close to those obtained with K-rich depolarizing fluids. The S-shaped curve of the contracture vs membrane potential relation is similar to that found in frog muscles except that the contractile responses are graded over a larger range of membrane potentials (-50 to + 30 mV in fast and -55 to + 10 mV in slow muscle).

Activation and inactivation characteristics of the sodium permeability in muscle fibres from Rana temporaria

1. The steady-state and kinetic characteristics of the processes of activation and inactivation of the Na(+) permeability, P(Na), were measured in cut skeletal muscle fibres from Rana temporaria under voltage-clamp conditions.2. The specific resistance, r(ss), in series with the surface sarcolemma, was estimated as 6 Omega cm(2) by measuring the initial value of the membrane potential transient in response to current pulses under current-clamp conditions. To reduce the error in the potential across the sarcolemma introduced by r(ss), Na(+) currents were recorded using positive feed-back compensation, in the presence of tetrodotoxin (2.4-5 nm).3. P(Na)(t) was fitted with m(3)h kinetics assuming a voltage-dependent delay, deltat, to the start of the activation process.4. The P(Na)-V(p) curve exhibited saturation at potentials more positive than 30 mV. m(infinity), calculated as (P(Na), (infinity)/ P(Na))((1/3)) as a function of V(p), was a sigmoid curve with a mid point at -35 mV. The slope, dm(infinity)/dV(p), at this point was 0.032 mV(-1).5. Using a double-pulse protocol a non-exponential time course for the development of fast inactivation at small depolarizations was observed.6. The time constant for activation, tau(m), as a function of V(p), and tau(h) as a function of V(p), could be fitted with an approximately bell-shaped function, maximum of 430 mus at -43 mV and 925 mus at -78 mV respectively, at 15 degrees C.7. The mid-point potential of the h(infinity)-V(l) curve occurred at -58 mV, and h(infinity) approached 1 for V(1) values more negative than -103 mV.8. Using a double-pulse procedure the development of a slow inactivation of the Na(+) current was demonstrated. Its time course could be described in terms of a single exponential function, time constant equal to 0.58 s. The recovery from slow inactivation could be described by a similar exponential for recovery times smaller than 1 s.

Slow inward barium current and contraction on frog single muscle fibres

1. Excitation-contraction coupling process in isolated frog muscle fibres, under conditions which allow the development of a Ba permeability, has been investigated by the simultaneous recording of electrical and mechanical activity. 2. The sustained contraction elicited by a long lasting Ba action potential depends on two mechanisms. The first is potential dependent, the second which is inhibited by MnCl2 (10 mM), depends on the inward flux of Ba ions. 3. The relationship observed between the inward IBa and the peak tension resembles that which has been observed between ICa and the contraction on other muscular structures. 4. The relative tension progressively declines as the intracellular Ba concentration increases and the contractility ends after a series of depolarizing pulses (or Ba action potentials). This indicates that the Ba ions which enter the cell release Ca ions and replace them in the intracellular storage sites. 5. Following a pretreatment with caffeine, the inward IBa fails to induce a contraction. Moreover a muscle which has been loaded with barium until the contraction ceases, does not develop a contracture in presence of caffeine. These results show that the Ba induced Ca release is located at the level of the sarcoplasmic reticulum. 6. Calculations show that the amount of Ba ions necessary to abolish the contractility corresponds to the maximum ability of the sarcoplasmic reticulum for Ca binding. 7. Almost all the inward flux of Ba ions and the contraction are abolished by glycerol-treatment which suggests that the coupling occurs at the T-system level. The results are discussed in regard to the technical limitations of the voltage-clamp method.

Dual innervation of end-plate sites and its consequences for neuromuscular transmission in muscles of adult Xenopus laevis

1. Electrophysiological study of dually innervated end-plate sites was carried out in Xenopus laevis pectoral muscle fibres. End-plate potentials (e.p.p.s) and miniature end-plate potentials (m.e.p.p.s) have been recorded in Mg-blocked preparations. 2. The mean quantal content (m) of each e.p.p. at dually innervated end-plates was significantly smaller than the corresponding values obtained at singly innervated ones. At a given doubly innervated end-plate site the values of m tended to be inversely related, so that the compound value of m (obtained by adding them) was in the same range as that found in singly innervated junctions. These findings were taken to suggest the existence of an upper limit in the average amount of transmitter released at a synaptic site. 3. It was found that neither intermittent presynaptic conduction block, nor particular muscle fibre properties could account for the low values of m in dual end plates. The small size of the nerve terminals appears to be the most likely explanation. 4. The sensitivity to acetylcholine and muscle fibre electrical properties were investigated; no differences were found between fibres with sub- or suprathreshold e.p.p.s. 5. The nature of the factors responsible for this presumed small size of the nerve endings (competition between nerve endings for a limited synaptic space on the muscle membrane or reciprocal interaction between closely located terminals) as well as the possible origins of polyinnervation are discussed.

Contractile activation by voltage clamp depolarization of cut skeletal muscle fibres

1. Single frog skeletal muscle fibres bathed in a relaxing solution were cut close to the tendon and mounted across a single Vaseline gap so that a short segment of intact terminated fibre extended beyond one side of the gap. 2. A compensating circuit, set with a micro-electrode in the terminated fibre segment, was used both to correct total current for external current crossing the gap and to correct pool voltage for the voltage drop across the fibre segment in the gap. 3. The micro-electrode was then removed and the fibre voltage-clamped using the compensating circuit. This allowed movement without damage under controlled voltage. 4. Strength-duration curves for contraction thresholds of cut fibres exposed externally to TTX Ringer solution and internally to a predominantly K glutamate solution were similar to strength-duration curves reported for intact fibres. 5. The change from TTX Ringer to a predominantly (TEA)2SO4 external solution shifted the strength-duration curve for cut fibre contraction thresholds in the negative direction as reported for intact fibres. 6. When studied at 3-4 degrees C, fibres from warm-adapted frogs appeared to have higher contraction thresholds than fibres from cold-adapted frogs. 7. Delayed rectifier currents recorded from cut fibres were similar to those reported for intact fibres.

An improved vaseline gap voltage clamp for skeletal muscle fibers

A Vaseline gap potentiometric recording and voltage clamp method is developed for frog skeletal muscle fibers. The method is based on the Frankenhaeuser-Dodge voltage clamp for myelinated nerve with modifications to improve the frequency response, to compensate for external series resistance, and to compensate for the complex impedance of the current-passing pathway. Fragments of single muscle fibers are plucked from the semitendinosus muscle and mounted while depolarized by a solution like CsF. After Vaseline seals are formed between fluid pools, the fiber ends are cut once again, the central region is rinsed with Ringer solution, and the feedback amplifiers are turned on. Errors in the potential and current records are assessed by direct measurements with microelectrodes. The passive properties of the preparation are simulated by the "disk" equivalent circuit for the transverse tubular system and the derived parameters are similar to previous measurements with microelectrodes. Action potentials at 5 degrees C are long because of the absence of delayed rectification. Their shape is approximately simulated by solving the disk model with sodium permeability in the surface and tubular membranes. Voltage clamp currents consist primarily of capacity currents and sodium currents. The peak inward sodium current density at 5 degrees C is 3.7 mA/cm2. At 5 degrees C the sodium currents are smoothly graded with increasing depolarization and free of notches suggesting good control of the surface membrane. At higher temperatures a small, late extra inward current appears for small depolarizations that has the properties expected for excitation in the transverse tubular system. Comparison of recorded currents with simulations shows that while the transverse tubular system has regenerative sodium currents, they are too small to make important errors in the total current recorded at the surface under voltage clamp at low temperature. The tubules are definitely not under voltage clamp control.

Voltage clamp with double sucrose gap technique. External series resistance compensation

In this paper we deal with the double sucrose-gap voltage clamp technique. To perform a reliable clamp or to analyze the intracellular potential distribution, any external series resistance in the artificial node must be taken into account for it induces an instability in the external potential as soon as a current develops. A circuit was designed to compensate for this error, it has been found effective on an analog model and on experimental uni- or multicellular preparations. The attenuation in series resistance frequently causes ringing in the step response. This behavior was studied theoretically and also simulated with analog models where a selective bridged-T network was found to represent the electrical characteristics of the preparation when associated with the chamber and control electronics. A residual series resistance was found and is considered to be a part of the preparation. Characteristics necessary to obtain best results are proposed, for a preparation to be studied in experiments utilizing the double sucrose gap technique with external series resistance compensation.

Spread of the junction potential in the T-system in hagfish slow muscle fibres

The membrane constants of the slow muscle fibres from the Atlantic hagfish were determined by analyzing the responses to sinusoidal currents at different frequencies. The apparent low frequency membrane resistance was 20-50 komega-cm2 and was not significantly altered by replacing external chloride with methyl sulphate. The low frequency capacitance was 2.1-2.4 muF/cm2 and decreased to about 1 muF/cm2 at 100 Hz. A model of the T-system consisting of a number unbranched T-tubules was fitted to the impedance locus. About 15 tubules per sarcomere with a length approximately 8 times the fibre radius gave good fit to the observed impedance locus. The electrical characteristics of the T-system allow an efficient electrotonic spread of the junction potential through the fibre cross-section.

The spread of the action potential through the T-system in hagfish twitch muscle fibres

The input impedance of twitch muscle fibres of the Atlantic hagfish has been measured with sinusoidal transmembrane currents. The apparent specific membrane resistance and capacitance decreased markedly with frequency, and were relatively independent of fibre diameter. A model of the T-system based on anatomical observations, was used to predict the input impedance in the normal solution (artificial sea water). The changes in input impedance produced by glycerol treatment, low chloride solution, reduced pH and isotonic solutions with low ionic strength were easily interpreted in terms of the same model. The model predicts severe attenuation of the action potential if conducted electrotonically by the transverse tubules towards the center of the fibre.

Electrical characteristics of frog atrial trabeculae in the double sucrose gap

The electrical behavior of small single frog atrial trabeculae in the double sucrose gap has been investigated. The currents injected during voltage clamp experiments did not behave as predicted from the assumption of spatial uniformity of the voltage across a Hodgkin-Huxley membrane. Much of the difference is due to the geometrical complexities of this tissue. Nonetheless, two transient inward currents have been identified, the faster of which is blocked by tetrodotoxin (TTX). The magnitude of the slower transient varies markedly between preparations but always increases in a given preparation with increase of external calcium. The fast transient current traces, at small to intermediate depolarizations, are often marred by the presence of notches and secondary peaks due most probably to the loss of space clamp conditions. In many preparations these could be removed by reducing the current magnitude through application of a partially-blocking dose of TTX. Conversely, in the preparations whose fast transient was fully blocked by TTX, notches and secondary peaks in the slow transient could by induced through increasing calcium concentration and thereby the slow current magnitude. Previously used techniques for the measurement of the reversal potential of the fast inward transient have been shown to be invalid. In so far as they can be measured, the reversal potentials of the fast and slow inward transient are in the same neighborhood, i.e. around 120 mV from rest. The true values may be quite a bit apart. The total charge flow in the capacitive transient was measured for different sized nodes and preparations. From these data and estimates of plasma membrane area per unit trabecular volume, specific membrane capacitances of around 3 muF/cm2 were calculated for small bundles. The apparent ion current densities on this basis are approximately 1/10 of those measured in axons. The capacitive current occurring in small bundles decayed as the sum of at least three exponential functions of time. On the basis of these data and the anomalously large stable node widths, we suggest a coaxial core model of the preparation with the inner elements in series with an additional large extracellular resistance.

The relative contributions of the folds and caveolae to the surface membrane of frog skeletal muscle fibres at different sarcomere lengths

The plasmalemmal area of striated muscle fibres is greater than the apparent surface area (A = circumference x length) because of variable folds and the invaginations of the caveolae and T-tubules. Freeze-fracture replicas of the surface membrane of sartorius and semitendinosus muscles from Rana pipiens have been used to determine the numbers and distribution of folds and caveolae at different sarcomere lengths. (1) The plasmalemma folds are variable in size and shape, but are always oriented perpendicular to the long axis of the fibre. The folds vary with stretch, being more prominent at short sarcomere lengths. The caveolae are elliptical invaginations of the plasmalemma which open to the outside by a narrow "neck" of approximately 20 nm. The caveolar lumen has an average long dimension of 81.6 +/- 11.7 nm and an average short dimension of 66.9 +/- 7.9 nm. The caveolar "necks" only can be seen in freeze-fracture replicas and these are distributed in two circumferential bands on either side of the Z-line, and in longitudinal bands separated by distances of 1-5 mum. In the sartorius muscle, at a sarcomere length of 2.8 mum, there is an average number of thirty-seven caveolae per square micrometer of fibre surface. (2) During passive stretch the opening of folds provides membrane for the necessary increase in surface area up to a sarcomere length of about 3.0 mum. This length is defined as the critical sarcomere length (Sc). The number of caveolae remains constant at all sarcomere lengths less than Sc and thus their "necks" have been used as membrane markers to determine the amount of folding at different sarcomere lengths. The membrane area contained in folds and caveolae is expressed as a fraction of the apparent surface area (A). For example, in the sartorius muscle, at a sarcomere length of 2.4 mum, the membrane area, excluding the T-tubules, is: A + 0.1A (folding) + 0.7A (caveolae) = 1.8A. (3) For stretch beyond Sc membrane is provided by the opening of caveolae. At a sarcomere length of about 8 mum all the caveolae are open and the fibres rupture with further stretch. (4) The relative contributions of folds and caveolae vary with sarcomere length in a way that is consistent with assumptions of constant volume and plasmalemma area. The maintenance of constant plasmalemma area, even after excessive stretch, suggests that the plasmalemma is relatively inelastic in this situation.

Action potential in the transverse tubules and its role in the activation of skeletal muscle

The double sucrose-gap method was applied to single muscle fibers of Xenopus. From the "artificial node" of the fiber, action potentials were recorded under current-clamping condition together with twitches of the node. The action potentials were stored on magnetic tape. The node was then made inexcitable by tetrodotoxin or by a sodium-free solution, and the wave form of the action potential stored on magnetic tape was imposed on the node under voltage-clamp condition (simulated AP). The twitch height caused by the simulated AP's was always smaller than the twitch height produced by the real action potentials, the ratio being about 0.3 at room temperature. The results strongly suggest that the transverse tubular system is excitable and is necessary for the full activation of twitch, and that the action potential of the tubules contributes to about 70 % of the total mechanical output of the normal isotonic twitch at 20 degrees C. Similar results were obtained in the case of tetanic contraction. At a temperature near 10 degrees C, twitches produced by the simulated AP were not very different (85 % of control amplitude) from the twitches caused by real action potentials. This indicates that the excitability of the tubules becomes less necessary for the full activation of twitch as the temperature becomes lower.