Internal citrate ions reduce the membrane potential for contraction threshold in mammalian skeletal muscle fibers

An effect of internal citrate ions on excitation-contraction coupling in skeletal muscle is described. The threshold for contraction was measured in rat extensor digitorum longus, (EDL), and soleus muscle fibers using a two microelectrode voltage clamp technique with either KCl-filled or K3 citrate-...

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Bibliographic Details
Published in:Biophysical journal 1988-04, Vol.53 (4), p.609-616
Main Author: Dulhunty, A.F.
Format: Article
Language:English
Subjects:
Animals
Biological and medical sciences
Calcium - metabolism
Citrates - pharmacology
Fundamental and applied biological sciences. Psychology
In Vitro Techniques
Kinetics
Male
membrane potential
Membrane Potentials
Muscle Contraction - drug effects
Muscles - drug effects
Muscles - physiology
Potassium - pharmacology
Rats
Rats, Inbred Strains
skeletal muscle
Striated muscle. Tendons
Vertebrates: osteoarticular system, musculoskeletal system
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Summary:An effect of internal citrate ions on excitation-contraction coupling in skeletal muscle is described. The threshold for contraction was measured in rat extensor digitorum longus, (EDL), and soleus muscle fibers using a two microelectrode voltage clamp technique with either KCl-filled or K3 citrate-filled current electrodes. Contraction thresholds were stable for many minutes with KCl current electrodes. In contrast, thresholds fell progressively towards the resting membrane potential, by as much as -15 mV over a period of 10 to 20 min of voltage-clamp with citrate current electrodes. In addition, prepulse inhibition was suppressed, subthreshold activation enhanced and steady-state inactivation shifted to more negative potentials. Fibers recovered slowly from these effects when the citrate electrode was withdrawn and replaced with a KCl electrode. The changes in contraction threshold suggest that citrate ions act on the muscle activation system at an intracellular site, since the citrate permeability of the surface membrane is probably very low. An internal citrate concentration of 5 mM was calculated to result from citrate diffusion out of the microelectrode into the recording area for 20 min. 5 mM citrate added to an artificial cell lowered the free calcium concentration from 240 to 31 microM. It is suggested that citrate modifies excitation-contraction coupling either by acting upon an anion-dependent step in activation or by reducing the free calcium and/or free magnesium concentration in the myoplasm.
ISSN:0006-3495
1542-0086
DOI:10.1016/S0006-3495(88)83139-4