Changes of contractile responses due to simulated weightlessness in rat soleus muscle.
Some contractile and electrophysiological properties of muscle fibers isolated from the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles of rats were compared with those measured in SOL muscles from suspended rats.  In suspended SOL (21 days of tail-suspension) membrane potential (Em), intracellular sodium activity (aiNa) and the slope of the relationship between Em and log [K]o were typical of fast-twitch muscles.  The relation between the maximal amplitude of K-contractures vs Em was steeper for control SOL than for EDL and suspended SOL muscles.  After suspension, in SOL muscles the contractile threshold and the inactivation curves for K-contractures were shifted to more positive Em.  Repriming of K-contractures was unaffected by suspension.  The exposure of isolated fibers to perchlorate (ClO4-)-containing (6-40 mM) solutions resulted in a similar concentration-dependent shift to more negative Em of activation curves for EDL and suspended SOL muscles.  On exposure to a Na-free TEA solution, SOL from control and suspended rats, in contrast to EDL muscles, generated slow contractile responses.  Suspended SOL showed a reduced sensitivity to the contracture-producing effect of caffeine compared to control muscles.  These results suggested that the modifications observed due to suspension could be encounted by changes in the characteristics of muscle fibers from slow to fast-twitch type.
Elkhammari
A
A
Laboratory of General Physiology, National Veterinary School, Nantes, France.
Noireaud
J
J
Léoty
C
C
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D006614:Hindlimb; D007103:Immobilization; D008564:Membrane Potentials / Q000502:physiology; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D010472:Perchlorates / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology; D013757:Tetraethylammonium Compounds / Q000378:metabolism / Q000494:pharmacology; D018474:Weightlessness Simulation
D010472:Perchlorates; D013757:Tetraethylammonium Compounds; D002110:Caffeine; D012964:Sodium; D011188:Potassium; D002118:Calcium