The maintenance of resting potentials in glycerol‐treated muscle fibres

SUMMARY1. The electrical and the mechanical activity of isolated frog muscle fibres have been simultaneously recorded in a physiological solution which allows the development of a large tubular sodium current.2. Under such experimental conditions, fibres develop long-lasting action potentials and strong mechanical responses.3. In voltage-clamp experiments a slow inward current is revealed for depolarizations higher than + 20 mV from the resting potential. This current increases until + 40 to + 50 mV and then decreases to reverse near + 90 mV. The amplitude of the mechanical response increases with the potential to reach an optimum value between + 40 and + 50 mV and then decreases to stabilize when the depolarization is near + 90 mV.4. In the presence of picrotoxin the slow inward current is reversibly inhibited and the tension-depolarization curve has an S-shape as found in normal physiological conditions. 5. The dependence of a part of the contraction upon the slow inward current is reinforced by the fact that in a 50 % sodium solution the amplitude of the current and that of the contraction are reduced in the same proportion. 6. Detubulated fibres failed to generate such a sodium inward current. 7. When sodium ions are replaced by lithium ions a slow inward lithium current develops but it does not induce a mechanical response.8. Tetracaine reversibly inhibits the current-dependent component of the contraction without affecting the potential-dependent one.9. It is concluded that the contraction recorded in the present experimental conditions is the sum of two components: one is potential-dependent and the other depends on a sodium-induced calcium release mechanism.

Section: Nature and Localization Of The Currentmentioning

confidence: 99%

Existence of a sodium‐induced calcium release mechanism of frog skeletal muscle fibres.

Potreau¹,

Raymond²

1982

“…3) was estimated to be 4-8 + 0 9 (absolute error) p-moles/cm2. Localization of the site of development of the calcium permeability Experiments were performed on four fibres detubulated according to the method of Howell & Jenden (1967) as modified by Eisenberg, Howell & Vaughan (1971). The results have shown that no calcium permeability can be elicited when T-tubules are disrupted (Fig.…”

Section: Effects Of High Concentration Of Calcium On the T-v Curvementioning

confidence: 99%

Calcium‐dependent electrical activity and contraction of voltage‐clamped frog single muscle fibres.

Potreau¹,

Raymond²

1980

SUMMARY1. The electrical and mechanical activities of isolated frog muscle fibres have been simultaneously recorded under conditions which allow the development of a calcium permeability (chloride-free solution containing 72 mM-calcium levulinate instead of 115 mM-NaCl).2. Long-lasting calcium action potentials and large and sustained contraction occur without inhibition of the potassium permeability.3. The relationship observed between the slow inward calcium current and the amplitude of the contraction, under voltage-clamp conditions, resembles that found between IBa and tension in frog skeletal muscle and between Ica and the contraction in frog heart. A part of the mechanical response which is not abolished by manganese seems to be potential-dependent. close to 4-8 p-moles/cm2. This value is 10-20 times larger than the calcium influx measured during a normal action potential. 6. On detubulated fibres the calcium permeability fails to develop: this indicates that this current originates from the T-system. The close correspondence between IC. and the contraction, and between the time course of the contraction at the end of the depolarizing steps suggests that the potential of the tubular membrane is better clamped than in normal physiological conditions.

“…After soaking the muscle for 50 min at room temperature, an isotonic Ringer was introduced. Calcium and magnesium were elevated to 5 mm (Eisenberg, Howell & Vaughan, 1971 (Campbell, 1976 (Campbell, 1976) (Hodgkin & Horowicz, 1959;Armstrong & Lee, 1971;Venosa & Horowicz, 1973). It is known that for intact fibres at the resting potential, the chloride conductance is twice as large as the potassium conductance (Hodgkin & Horowicz, 1959).…”

Section: Methodsmentioning

confidence: 99%

The interaction of potassium with the activation of anomalous rectification in frog muscle membrane.

Hestrin¹

1981

SUMMARY1. Inward rectification of frog muscle membrane was analysed with the Vaseline gap method.2. Hyperpolarization, under voltage clamp, produced inward potassium currents, which had a component that activated with a time constant, TK* 3. The activation time constant TK of the inward potassium currents was voltage dependent. For a given external potassium concentration, the time constant was maximal for potentials near the potassium equilibrium potential, EK.4. The potassium chord conductance gK, had a sigmoidal voltage dependency, increasing initially e-fold per 11-6 mV of hyperpolarization.5. When the internal potassium concentration was fixed, raising external potassium induced a shift of the rK-V and the 9K-V relations in the positive direction along the voltage axis. That shift was comparable to the change in EK.6. No shift of the TK -V and the gK -V relations was observed when the internal potassium was reduced from 150 to 50 mm. 7. Changes of internal sodium concentration between 5 and 100 mm did not significantly effect the magnitude of inward rectification.