Mechanical activation in slow and twitch skeletal muscle fibres of the frog.

SUMMARY1. Short (0-81-6 mm) lumbricalis fibres of Rana pipiens were voltage clamped by a two-micro-electrode technique at 5 TC in sucrose hypertonic Ringer solution (SHR). Terminated linear cable analysis suggests that if the current electrode is placed near the centre of the fibre length and the voltage-sensing electrode is placed 0-19 times the fibre length from the current electrode, the fibre can be adequately voltage clamped and the conductance may be simply calculated as I/V for fibre length constants from 1-0 to 0-15 mm.2. In SHR solution lumbricalis fibres have action potentials with peak amplitudes of only + 2 to 7 mV and a slow, gradual repolarization, distinct from the action potentials observed in sartorius muscle. In 60 mM-Na+ SHR the inward Na current could be adequately controlled over the fibre length, providing an estimated Na conductance (GNa) of 8-9 mS/cm2. The magnitude of GNa and GK (delayed rectifier) in lumbricalis fibres was approximately 20 % of that reported for sartorius and semitendinosus, although the resting conductances were similar.3. Fibres demonstrated delayed rectifier currents with complex patterns of activation suggesting two components of conductance (fast, GKrf and slow, GKS) which were combined together in varied amounts: (a) GK,f activated rapidly to a maximum within 80 ms at 0 mV as previously described (Adrian, Chandler & Hodgkin, 1970a); (b) GKS activated gradually with depolarizations below -50 mV and achieving peak currents at about 400 ms at 0 mV.4. In about 10 % of lumbricalis fibres studied, GKS occurred in isolation with a peak magnitude of 1-4+004 mS/cm2 (± S.D.). GKS activation kinetics and tail currents are described by a squared two-state (12) Hodgkin-Huxley model and have a Q10 of 2-8. These currents inactivated with a time constant of 5-7 s at 0 mV. Isolated ,Ks with identical kinetics was also observed in certain sartorius fibres studied with the three-electrode voltage clamp.5. The fractional amount of GKs in the combined delayed rectifier (GKS + GKf) currents could be estimated from analysis of the late activation phase with depolarization. Combined delayed currents were described by summing GKJf currents using a n4 model with GK s currents defined by the 12 model.

Section: Discussionmentioning

confidence: 60%

“…Since frog slow fibres maintain a contraction with little decline in tension throughout a 200 s depolarization (Gilly & Hui, 1980a), there appears to be no large population of slow fibres in lumbricalis muscles.…”

Section: Resting Fibre Characteristicsmentioning

confidence: 99%

Ionic conductances in frog short skeletal muscle fibres with slow delayed rectifier currents.

Lynch¹

1985

“…These observations suggest that charge movement 185 may regulate Ca release from the sarcoplasmic reticulum. Since tension in a depolarized slow fibre can be maintained for a very long time (Gilly & Hui, 1980a), it was of interest to determine whether the charge movements in slow fibres inactivate during long depolarizations. We tried to depolarize slow fibres by changing the holding potential, VH, in the depolarizing direction.…”

Section: Charge Movement In Slow Fibresmentioning

confidence: 99%

Voltage‐dependent charge movement in frog slow muscle fibres.

Gilly¹,

Hui²

1980

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SUMMARY1. Voltage-clamp experiments on frog slow and twitch fibres were carried out using the three-micro-electrode technique. Potassium currents were blocked by tetraethylammonium. Contraction was blocked by 2 mM-tetracaine.2. After subtracting the linear capacitive and leakage currents, the A V(test -control) traces from slow fibres show 'on' and 'off' charge movements simriilar to those observed in twitch fibres.3. The time integrals of the 'on' and 'off' transients, Q0,n and Qff, in slow fibres are, as in twitch fibres, almost equal in magnitude but opposite in direction.4. The charge-voltage distribution is well fitted by a sigmoid curve given by Qmax I 1+exp[-(V-V)/k]' which has been successfully applied to twitch fibres. Data from three fibres gave V = -25 mV, k = 13 mV, and Qmax = 7 nC/1tF. Thus, intramembranous charge in slow fibres has the same steady-state voltage distribution as that in twitch fibres, but the quantity of maximum movable charge is only 1/4 to 1/3 as large.5. Charge movement in slow fibres does not inactivate completely when the fibres are held at -20 to 0 mV for durations as long as 30 min. 6. These results show that charge movement exists in slow fibres and may serve the same function in regulating contractile activation as that postulated for twitch fibres. The lack of complete inactivation may be consistent with the ability of slow fibres to maintain maximal tension during prolonged depolarizations.

“…*, normal Ringer (solution A); *, normal Ringer solution with 2 mM-tetracaine; 0 high Ca2+ Ringer (solution B) with 2mM-tetracaine. Fibre diameters were determined optically (Gilly & Hui, 1980a). The continuous line has been drawn for slow fibres according to the equation: Cm = 1-9 + 0.05 d (see text), and the dashed line for twitch fibres according to the equation: Cm = 1-9+0-07d (Peachey, 1965).…”

Section: Resultsmentioning

confidence: 99%

“…Experiments were carried out in the same manner as those described in the preceding paper (Gilly & Hui, 1980a), except that fibre type identification was based solely on microscopic appearance and electrical characteristics, since in most experiments contraction was abolished by a 2 times hypertonic 2H20 solution. Slow fibres were all from warm-adapted frogs; twitch fibres were from cold-adapted animals.…”

Section: Methodsmentioning

confidence: 99%

Membrane electrical properties of frog slow muscle fibres.

Gilly¹,

Hui²

1980

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SUMMARY1. Pyriformis slow (and sartorius twitch) fibres from Rana temporaria were studied with a three-micro-electrode voltage-clamp technique to obtain an approximate measurement of membrane current density at a fibre end. In most experiments, a modified Ringer solution containing 2H20 and 230 mM-sucrose was used to reduce movement.2. Linear membrane properties of slow fibres obtained with this method are consistent with results from previous studies. Measured Cm (/ZF/cm2) increases with fibre diameter in a manner consistent with a tubular location of part of the fibre capacitance.3. Voltage steps to -50 mV and more positive potentials result in outward membrane currents in both slow and twitch fibres. These currents develop along similar sigmoid time courses and are blocked by tetraethylammonium (TEA+) ions. The reversal potential for delayed current channels in slow fibres varies with external K+ concentration, suggesting that the delayed current in slow fibres, as in twitch, is carried by K+ ions.4. Maximum GK' GK1, in slow fibres is an order of magnitude smaller than in twitch fibres. The steady-state GK-V curve of slow fibres is very broad (e-fold for , 15 mV), saturating at very positive voltages, whereas the GK of twitch fibres varies more steeply with voltage.5. No evidence of inward currents was seen in slow fibres during pulses of duration up to 96 msec. 6. Slow outward currents, which do not inactivate appreciably, are seen in slow fibres during long (10 sec) pulses. Tail currents following such long pulses are very slow. The reversal potential shifts to more positive values with increasing pulse duration.