Selective reinnervation of twitch and tonic muscle fibres of the frog

The electrical properties and innervation of piriformis muscle fibres and the conduction velocities and thresholds of the corresponding motor axons were studied. In normal muscles all fibres clearly fell into the category of twitch or tonic. Tonic muscle fibres were selectively reinnervated by small motor axons after crushing or cutting the piriformis nerve. Twitch fibres were reinnervated by large motor axons. Tonic fibres were also selectively reinnervated by small motor axons when the proximal stump of the piriformis nerve was cut and attached to the caudal end of the muscle. With this procedure the possibility of mechanical guidance by remaining neural sheaths was eliminated. Polyneuronal innervation in twitch fibres in piriformis muscles of normal frogs was 4.7% and in contralateral muscles of operated frogs with the piriformis nerve cut it was 22.6%. Unexpectedly three out of seventeen tonic fibres in contralateral muscles were simultaneously innervated by both small and large motor axons. It is concluded that reinnervation is highly specific for fibre type in frog skeletal muscles.

Electrical properties of cultured epithelioid cells (MDCK)

This is a study of the intracellular electrical potential, membrane resistance, and capacity of MDCK cells (epithelioid of renal origin) cultured in monolayers on a collagen couch. These monolayers have a transepithelial resistance of 256 +/- 12 (22) ohm cm2 (mean +/- standard error, and number of observations), and the cells have 61.6 +/- 6.3 (92) M omega across their plasma membrane. The electrical capacity of the cells is 45.1 +/- 2.9 (63) pF and is much higher than expected for a cell of its size (diameter 14 micrometers, height 5 micrometers) and cannot be attributed to intercellular coupling, as no evidence of this type of connection was found in 20 pairs of neighboring cells. On the contrary, the high capacity is in keeping with previous studies using electron microscopy showing microvilli and a high degree of lateral infolding. The relationship between resistance and capacity was 1981 +/- 177 (61) omega . microF. The cells have an intracellular potential of -40.5 +/- 15 (120) mV. Yet the shape of the distribution curve suggests that the actual value may be somewhat higher (some -50 mV). The current/voltage curve of the distribution curve suggests that the actual value may be somewhat higher (some -50 mV). The current/voltage curve shows a marked asymmetry, and in some cells the voltage becomes time-dependent for large, depolarizing current pulses.

Calcium-channel gating in frog skeletal muscle membrane: effect of temperature

Voltage-clamp experiments using the three micro-electrode method were performed to study the temperature dependence of the calcium current ICa in intact twitch skeletal muscle fibres of the frog. Contraction was blocked by recording in hypertonic sucrose solutions. For depolarizations smaller than 0 mV the decay of the transient, slow, inward current, recorded in the presence of external tetraethylammonium (TEA+) and by replacing Cl- for CH3SO3-, followed a complex time course. For larger depolarizations, after the initial inward current, there was a prominent, slow, outward current which showed two phases: after reaching a peak (time to peak 1.0 sec, peak amplitude 20-50 microA/cm2 at 20 mV) it slowly declined to a steady level in about 2-3 sec at 23 degrees C. The inward current was greatly reduced or abolished by the adding of 2 mM-Cd2+ or by replacing external Ca2+ with Mg2+. The amplitude and time course of slow, outward currents were not obviously modified by replacing Ca2+ with Mg2+, having the two described phases. However, in the presence of Cd2+ the first transient phase of the outward current was not detected and only outward currents slowly increasing to a steady level were observed. Reliable ICa records were obtained by further blocking K+ outward currents by incubating the muscles in a K+-free TEA+- and Cs+-containing solution prior to experiments. Tubular space clamp was improved by recording ICa from small fibres with 20-30 microns radius. The decay phase of ICa under a maintained depolarization in incubated muscles was fitted by a single exponential. The corresponding rate constant determined between 12 and 24 degrees C strongly depended on temperature, as expected for a gating process. The values for the activation energy and the corresponding Q10 (calculated for a 10-20 degrees C transition) were respectively: 17.5 +/- 1.0 kcal/mole and 2.9 +/- 0.2 at 0 mV, and 18.0 +/- 1.5 kcal/mole and 3.0 +/- 0.3 at -20 mV. The activation phase of ICa, analysed following the m alpha h Hodgkin-Huxley kinetic model, showed a similar temperature dependence with a Q10 of 3.0 +/- 0.3. The peak amplitude of ICa and the limiting Ca2+ permeability had a lower Q10 value of about 1.6. For a given temperature the rate constant of decay was independent of ICa peak amplitude in disagreement with a current-dependent process (intratubular Ca2+ depletion or intracellular Ca2+ accumulation) for the decay of ICa. In conclusion, our results favour a gating process (inactivation) as the principal mechanism underlying the decay phase of ICa under a maintained depolarization.

Kinetic properties of calcium channels of twitch muscle fibres of the frog

Calcium currents (ICa) were recorded in frog skeletal muscle fibres using the three-micro-electrode voltage-clamp technique. The sartorius muscle was bathed in TEA methanesulphonate saline with 350 mM-sucrose. 5 mM-3,4-diaminopyridine was added to the saline to minimize K+ currents. The I-V relationship for peak Ca2+ currents showed that ICa was detected at -40 mV and reached a maximum value at ca. -10 mV. No net inward current was recorded at potentials positive to ca. +40 mV. Remaining K+ currents (IK) were recorded by replacing 10 mM-Ca2+ with 5.5 mM-Co2+. They were not noticeably time-dependent up to +20 mV and would tend to diminish the amplitude of ICa without greatly affecting its time course. ICa tail currents could be separated from non-linear capacity currents. Tail currents were measured 5 msec after repolarization and extrapolated to the end of the pulse. ICa tail-current amplitudes at EK were measured with pulses of different durations. The envelope of tail-current amplitudes declined with a time course similar or identical to that of inward current during a maintained depolarization. Consequently, the decline of inward current cannot be explained by an increase of outward IK with time. ICa inactivated with 9 sec prepulses which did not elicit detectable ICa. The fitted h infinity curve had a mid point of -33.0 mV and a steepness of 6.3 mV. ICa between -30 mV and +20 mV could be described adequately using the Hodgkin-Huxley m3h relationship. The fitted m infinity curve had a mid point of -35.2 mV and a steepness of 9.9 mV. The limiting Ca2+ permeability PCa was 1.4 +/- 0.4 X 10(-4) cm/sec.

Calcium action potentials in rat fast-twitch and slow-twitch muscle fibres

Slow action potentials were evoked in twitch fibres of rat extensor digitorum longus (e.d.l.) and soleus muscles after drastically reducing the Cl and K conductances of the muscle fibres. Cl conductance was eliminated by exposing the muscles to a Cl-free saline in which methanesulphonate replaced Cl. K conductance was reduced by adding tetraethylammonium (TEA) and 3,4-diaminopyridine (3,4-DAP) to the Cl-free saline or by overnight incubation of the muscles in a saline containing Cs and TEA. The delayed rectifier was markedly blocked by TEA and 3,4-DAP. In contrast, the inward rectifier was blocked only by TEA. Depolarization with pulses of increasing amplitude triggered slow responses which had a threshold of -30 to -10 mV and a peak amplitude of 50-60 mV. In e.d.l. muscles the time course of the response was sustained for the duration of the pulses and was not affected by repeated stimulation. In soleus muscles the first evoked response was sustained in about 60% of the fibres and transient in the rest. Transient responses reached a peak amplitude and were followed by a hyperpolarization. Repeated stimulation irreversibly transformed the sustained responses of soleus fibres into transient ones. The responses were blocked when the Ca in saline was replaced by Mg (10 mM) or Co (5 mM) or by the addition of Cd (0.1-1.0 mM) or nifedipine (5-6 microM). Tetrodotoxin did not affect the responses. These results strongly suggest that Ca is the main carrier of current during the response. Nifedipine blocked both the Ca response and the subsequent hyperpolarization, suggesting that the latter is due to the activation of a Ca-dependent K conductance.

External calcium and contractile activation during potassium contractures in twitch muscle fibres of the frog

Effects of external Ca2+ concentration reduction on the amplitude and time course of K+ contractures were studied in single muscle fibres. The resting potential, effective resistance, threshold for the Na current, action potential and K+-induced depolarizations did not change when 1.8 mM Ca2+ was replaced by 3 mM Mg2+ (3--6 microM Ca2+). Identical results were obtained after the addition of 5 mM EGTA (less than or equal to 10(-9) M Ca2+; Ca-free saline). The rate of tension development during the initial phase of K+ contractures was independent of external Ca2+ while the amplitude, the duration, and the time constant of spontaneous relaxation decreased progressively as Ca2+ concentration was diminished. The activation curve shifted by 3--5 mV towards more positive potentials while the inactivation curve shifted by 16--18 mV in the opposite direction and both curves became steeper in Ca-free saline. External Ca2+ may play a role in excitation--contraction coupling during K contractures either via the inward Ca current or via specific interactions between external Ca2+ ions and the coupling mechanism or both.

Occluding junctions of the Necturus gallbladder

The paracellular conducting pathway of the Necturus gallbladder was studied with electrophysiological and electromicroscopic methods. The first one consists of the passage of short (5 msec) and small (32 microA cm-2) current pulses associated with a voltage scanning of the plane of the epithelium at the apical surface with a microelectrode to detect the regions where current flows. The procedure shows that (a) the conductance is evenly distributed along the intercellular regions along the intercellular spaces of the cells where occluding junctions are located; (b) the field above the occluding junctions has the shape of a bell, so that the junction can be sensed at 1-2 micron from the region where the intercellular space is visualized by light microscopy; (c) the intersections between three cells, in spite of having 3 half-junctions contributing (instead of two), do not have a higher conductance than the rest of the occluding junction. Scanning electron microscopy shows that (a) cells are densely covered by microvilli which interdigitate above the region of the occluding junctions, and (b) are covered by a surface coat. With transmission electron microscopy, (a) the opening of the occluding junctions at the apical border appears irregular, and most of them oblique; (b) in the last microns the actual mouth of the junction may deviate from the course of the interspace. Freeze-fracture replicas indicate that (a) the occluding junction has a uniform width and little variations in the number of strands around the cell, except (b) at intersections between 3 cells where both, its width and the number of strands, increase toward the basal region.

Occluding junctions in MDCK cells: modulation of transepithelial permeability by the cytoskeleton

In MDCK cell monolayers the opening and resealing of occluding junctions can be induced by removal and restoration of calcium to the external medium. The overall changes in permeability of the occluding junctions in the monolayer can be monitored by the drop and recovery of the total transepithelial electrical resistance. We have investigated the effects of cytochalasin B (CB) on this process. When CB is added to sealed monolayers there is a gradual drop in the electrical resistance across the monolayer. This drop is accompanied by a slow disorganization of the microfilament pattern of these cells, including a disturbance of a ring of cortical microfilaments that is normally associated with the junctions. Cells in open monolayers treated with CB will not reseal and have an altered filament distribution. These cells do not have a continuous cortical ring. We have used a voltage scanning technique that uses a microelectrode to measure the resistance at selected points along the junction which surrounds a single cell. In untreated, closed monolayers, the junction is heterogeneous with alternating points of high and low conductance. In closed monolayers treated with CB, although there are low conductance points, we have observed an increased frequency of high conductance points that correlates with the change in the overall conductance. The frequency of high conductance points along the junction and the overall conductance both increase with time of exposure to CB. In an effort to understand the molecular basis for the permeability changes induced by EGTA and CB, we have looked for differences in the protein components of the cell membranes of open, closed, and CB-treated MDCK monolayers. This was done by radioiodinating the surface membrane proteins under control and experimental conditions that bring about permeability changes. No significant differences in the labeled protein patterns were found under these conditions. These results suggest that the permeability changes involve only a structural rearrangement of membrane components. In additions we have observed that about 36% of the surface label remains bound to the insoluble cytoskeletons obtained from cells in control and experimental conditions that alter the permeability of the tight junctions. The iodinated proteins attached to the CS include polypeptides with Mr of greater than or equal to 120K daltons as well as peptides with Mr = 56K, 50K, 36K, and 18K daltons.

Potassium and caffeine contractures in fast and slow muscles of the chicken

1 K+ contractures, caffeine contractures and electrical properties were studied in slow (posterior latissimus dorsi; p.l.d.) and fast (anterior latissimus dorsi; a.l.d.) chicken muscles. 2. P.l.d. K+ contractures show a transient increase of tension that relaxes spontaneously. Contractures in a.l.d. show an initial component followed by a maintained tension. 3. A.l.d. K+ contractures of similar amplitude and time course were reproduced at 4 min intervals. In p.l.d., the interval needed for full recovery is about 30 min. In Cl-free saline p.l.d. and a.l.d. K+ contractures can be reproduced at 4 min intervals. 4. The time course of repolarization after a short exposure to 160 mM-KCl was much slower in p.l.d. than in a.l.d. In Cl-free saline the time course of repolarization becomes faster in p.l.d. 5. The membrane resistance was not modified in a.l.d. and was increased in p.l.d. by Cl-free saline. The calculated Cl- conductance in p.l.d. was about 70% of the total membrane conductance. 6. In a.l.d., Mn2+, D600 and external Ca2+ reduction greatly diminishes the maintained phase of the K+ contracture leaving the initial phase almost unmodified. Under similar conditions p.l.d. K+ contractures were slightly reduced. 7. P.l.d. caffeine contractures (10-40 mM) were not maintained and they were not modified by Ca-free saline, Cd2+, Co2+, Mn2+ and D600. 8. A.l.d. caffeine contractures (2-15 mM) were maintained and were highly dependent on external Ca2+. In addition they were greatly reduced by Cd2+, Co2+, Mn2 and D600. 9. It is suggested that caffeine contractures of a.l.d. are elicited by a Ca2+ entry into the muscle from the external fluid.

Effects of external calcium reduction on the kinetics of potassium contractures in frog twitch muscle fibres

1. The amplitude and time course of K contractures (Cl- constant) of single twitch muscle fibres of the frog have been analysed in three external Ca2+ concentrations. 2. The resting potential, effective resistance, threshold for the Na current and K-induced depolarizations were not modified by replacing 1.8 mM-Ca2+ by 3 mM-Mg2+ in absence (low-Ca saline: 3-6 micro M-Ca2+) or in the presence of 5 mM-EGTA (Ca-free saline: less than or equal to 10(-9) M-Ca2+). 3. The tension development during the initial phase of K contractures was independent of external Ca2+ while the amplitude, the duration and the time constant of spontaneous relaxation decreased progressively as Ca2+ concentration was diminished. 4. When the concentration of Mg2+ was increased to 5 mM in Ca-free saline K contractures were slower and smaller than those in 3 mM-Mg2+. 5. In Ca-free saline the activation curve (peak tension vs. logarithm of external K+ concentration) shifted by 3-5 mV towards more positive potentials while the inactivation curve (peak tension of the test contracture vs. logarithm of external K+ concentration during the conditioning period) shifted by 16-18 mV towards more negative potentials. Both curves became steeper in Ca-free saline. 6. The effects of external Ca2+ reduction were not modified by replacing all chloride for methanesulphonate. 7. Direct effects of external Ca2+ on excitation-contraction coupling during K contractures could involve the inward Ca current and/or specific interactions between external Ca2+ ions and the coupling mechanism.

Effect of glycerol treatment on the calcium current of frog skeletal muscle

1. Current and voltage clamp experiments were carried out on frog skeletal muscle fibres. For voltage clamp, the three micro-electrode technique near the fibre end was used. 2. Calcium spikes and currents were recorded in TEA sulphate saline. The addition of 400 mM-glycerol did not appreciably modify them. 3. Muscle fibers were detubulated with the glycerol method. They showed sodium propagating action potentials, with sodium and potassium currents of expected amplitudes. 4. Calcium spikes and currents were reduced or abolished in detubulated muscle fibres. 5. An analysis of fibre capacitance showed a linear correlation between the remaining ICa and the degree of electric discontinuity between the transverse tubular system and the surface membrane. 6. These results indicate that ICa is mainly located in the transverse tubular system. This localization is compatible with some role during mechanical activation.

Effect of calcium withdrawal on mechanical threshold in skeletal muscle fibres of the frog

1. Voltage-clamp experiments were performed on frog skeletal muscle fibres using two intracellular micro-electrodes. The threshold for the Na current and the strength-duration curve for mechanical threshold were determined. 2. The change in threshold for the Na current was studied as a function of the external Ca and Mg concentrations which ranged from 0.1 to 50 mM. 3. The resting potential, effective resistance and threshold for the Na current were unchanged when 1.8 mM-Ca was replaced by 3 mM-Mg, indicating that the surface potential and the electrical properties of the fibres were not modified. The additon of 5 mM-EGTA did not affect these parameters. 4. In Ca-free saline (3 mM-Mg and 5 mM-EGTA) the mechanical threshold was significantly increased for short pulses (less than or equal to 20 msec.). In isolated single muscle fibres this effect was observed shortly after applying the Ca-free saline, and was rapidly reversed upon the return to control saline. 5. In isotonic EGTA (85 mM-EGTA) the muscle fibres were depolarized and were unable to contract even if they were hyperpolarized to --90 mV for 12 min prior to stimulation. If 3 mM-Mg was added, most fibres contracted locally. 6. In single muscle fibres caffeine contractures were unmodified after a 30 min exposure to Ca-free saline. 7. It can be concluded that external Ca withdrawal impairs Ca release from the sarcoplasmic reticulum and that external Ca is not essential for triggering contraction.

Occluding junctions in a cultured transporting epithelium: structural and functional heterogeneity

MDCK cells (epithelioid of renal origin) form monolayers which are structurally and functionally similar to transporting epithelia. One of these similarities is the ability to form occluding junctions and act as permeability barriers. This article studies the junctions of MDCK monolayers formed on a permeable and transparent support (a disk of nylon cloth coated with collagen) by combining two different approaches: (i) Scanning of the electric field: the disk is mounted as a flat sheet between two Lucite chambers and pulses of 20--50 microA cm-2 are passed across. The apical surface of the monolayer is then scanned with a microelectrode to detect those points where the current is flowing. This shows that the occluding junctions of this preparation are not homogeneous, but contain long segments of high resistance, intercalated with sites of high conductance. (ii) Freeze fracture electron microscopy: the junctions are composed of regions of eight to ten strands intercalated with others where the strands are reduced to one or two ridges. The sites of high conductance may correspond to those segments where the number of junctional strands is reduced to 1 or 2. It is concluded that the occluding junctions of MDCK monolayers are functionally and morphologically heterogeneous, with "tight" regions intermixed with "leaky" ones.

Electrophysiological identification of two types of fibres in rat extraocular muscles

1. The synaptic potentials and electrical properties of rat inferior rectus muscles were examined in vitro. 2. In most fibres the spontaneous synaptic activity consisted of typical miniature end-plate potentials which had a normal distribution of amplitudes and rather uniform time courses. Suprathreshold and maximal nerve stimulation evoked unitary end-plate potentials (e.p.p.s). The synaptic activity of these fibres could be recorded only in the innervation zone of the muscle. These fibres were identified as being focally innervated. 3. Focally innervated fibres gave action potentials upon direct and indirect stimulation. They had an effective resistance (Reff) of 1.62 +/- 0.22 M omega (mean +/- S.E., twenty-two fibres) and a time constant (tau m) of 3.8 +/- 0.4 msec (twenty-one fibres). Voltage-current curves in control saline were linear between membrane potentials of -50 to -140 mV. 4. In a small number of fibres the spontaneous synaptic activity consisted of miniature small-nerve junction potentials which had a skewed distribution of amplitudes with predominance of smaller voltages and time courses with a wide range of variation. Nerve stimulation evoked composite small-nerve junction potentials (s.j.p.s) which could be resolved into unitary components by varying the strength of stimulation. S.j.p.s had a higher threshold than e.p.p.s. Synaptic potentials could be recorded outside the innervation zone, at various sites along the muscle length. These fibres were recognized as being multiply innervated with polyneuronal innervation. 5. Multiply innervated fibres lacked action potentials had a large Reff of 6.0 +/- 1.1 M omega (six fibres) and a prolonged tau m of 29.8 +/- 4.8 msec. Reff show a moderate decrease to hyperpolarization and a rather large decrease to depolarization which denote, respectively, the presence of anomalous and delayed reactification. 6. It is concluded that rat extraocular muscles contain at least two populations of muscle fibres that in terms of synaptic activity and electrical properties are comparable to twitch fibres of other mammalian muscles and to slow or tonic fibres of amphibians.

Inward calcium current in twitch muscle fibres of the frog

1. Voltage clamp experiments using the three micro-electrode voltage clamp technique were performed on sartorius muscles of the frog. 2. By blocking potassium currents with tetraethylammonium and replacing chloride ions with sulphate a slow inward current was detected. 3. The slow inward current is mainly carried by calcium, since it is abolished by cobalt and D-600, it depends on external calcium, and is not affected by removing external sodium or by tetrodotoxin (TTX). 4. The slow inward current has a mean threshold of -40 mV, reaches a mean maximum value at ca. 0 mV of 81 microamperemetercm-2 and has a mean reversal potential of +38 mV. 5. The calcium current is inactivated by the application of 2 sec conditioning prepulses according to a sigmoid curve with V(h) = -42 mV and k = 6.2 mV. 6. The slow time course of this calcium current makes it rather unlikely that it participates in contraction during a twitch, but it might be activated during long depolarizations as potassium contractures.

Action potentials in slow muscle fibres of the frog during regeneration of motor nerves

1. Pyriformis muscles of Rana temporaria were denervated by crushing the sciatic nerve inside the pelvis. At different times during regeneration of the nerve, slow muscle fibres were examined for the presence of all-or-none responses. 2. During the early period of re-innervation (35-60 days), slow muscle fibres were found to be non-selectively re-innervated by (foreign) fast-conducting motor axons. All slow fibres during this period were able to produce full sized action potentials, with overshoot. 3. Action potentials markedly decreased in amplitude and eventually disappeared completely between day 61-110 following denervation; during this period, slow muscle fibres were re-innervated by slowly-conducting motor axons. 4. Functional re-innervation by slowly-conducting motor axons, in the late stages of re-innervation, was not a necessary condition for the suppression of the action potential. Slow fibres innervated by fast motor axons, as well as denervated slow fibres, lost the action potential simultaneously with slow fibres which were re-innervated by slowly-conducting motor axons. 5. It is suggested that (small) slowly-conducting motor axons can exert a 'trophic' influence on the slow fibre membrane, independent of their synaptic function.

Calcium dependent electrical activity in twitch muscle fibres of the frog

When twitch muscle fibres of the frog were equilibrated in chloride free saline with 2.5–20 mM tetraethylammonium sulphate [(TEA) 2 SO 4 ], the action potential was followed by a long depolarizing response. This response was greatly reduced by adding chloride ions (10 mM). In 20 mM (TEA) 2 SO 4 the response consisted of an initial depolarization of – 23 mV lasting several seconds, followed by a slow delayed spike reaching + 23 mV. After the delayed spike the cell remained continuously depolarized creeping to a steady depolarization of + 25 mV (mean values). The response can be attributed to an increase in membrane conductance to calcium since the phenomenon was abolished by removing external calcium or by adding cobalt or D-600, and was not greatly affected by reducing or removing external sodium or by adding tetrodotoxin.

Re-innervation of twitch and slow muscle fibres of the frog after crushing the motor nerves

1. The conduction velocities of individual motor axons innervating twitch and slow muscle fibres of the frog were determined by intracellular recording of junctional potentials elicited by stimulating the motor nerves at two different points. 2. In normal pyriformis muscles twitch and slow fibres were found to be innervated by two distinct populations of motor axons. Twitch fibre axons conducted at 10-18-7 m/sec, while the conduction velocities of slow fibre axons ranged from 0-5 to 5 m/sec (at 7-9 degrees C). The thresholds for electrical stimulation were significantly lower in the fast than in the slow axons population. 3. Following denervation by crushing the sciatic nerve fast axons which re-innervated the muscle had lower conduction velocities than normal but could still be identified. These lower conduction velocities were measured proximal to the site of the crush and did not recover over a period of 446 days. 4. Fast motor axons regenerated more quickly than slow axons and re-innervated twitch as well as slow muscle fibres non-selectively. About 1 month later slow axons re-established synaptic contacts with slow (and some twitch) muscle fibres. Simultaneous re-innervation by fast and slow motor axons was occasionally observed in slow muscle fibres. Finally, the slow muscle fibres were innervated by slow axons only, while synapses of fast axons could no longer be found in this type of muscle fibre. 5. Action potentials were observed in denervated as well as in re-innervated slow muscle fibres; they disappeared as re-innervation progressed. 6. It is concluded that non-selective re-innervation of slow muscle fibres is present in the frog; it is, however, a transient phenomenon followed by restoration of the original innervation pattern.