The significance of active Na+,K+ transport in the maintenance of contractility in rat skeletal muscle.
The effects of reduced Na+,K+ pump capacity on contractile endurance and excitation-induced changes in intracellular Na+ content were investigated in isolated rat soleus and extensor digitorum longus muscles. Pre-incubation with 10(-5) M ouabain increased the rate of force decline measured over the first 5-20 s of tetanic contraction from 0.32 to 0.94% s-1 and 1.4 to 4.6% s-1 in soleus and extensor digitorum longus muscles, respectively. Soleus muscles from K(+)- deficient rats exhibited 54% reduction in the concentration of Na+,K+ pumps and the force decline during 30 s of 60 Hz stimulation was increased from 0.53 to 1.15% s-1. A similar change was induced in control muscles when a comparable reduction in the concentration of functional Na+,K+ pumps was elicited by pre-incubation with ouabain (10(-6)-2 x 10(-6) M). In soleus, the force decline during 60 s of 60 Hz stimulation showed linear correlation to the increase in intracellular Na+ content. In extensor digitorum longus, force decline and increase in Na+ content during 60 Hz stimulation were both four times faster than in soleus as measured over 15 s of excitation. These results indicate that during maximal contractions the Na+,K+ pump capacity is one of the determinants for the contractile endurance in skeletal muscle. Furthermore, the maintenance of contractile force seems to be a function of the rate of Na(+)-influx and this relationship may account for the difference in endurance between slow-twitch and fast-twitch muscles.
Nielsen
O B
OB
Department of Physiology, University of Aarbus, Denmark.
Clausen
T
T
D000818:Animals; D004558:Electric Stimulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000378:metabolism / Q000502:physiology*; D009929:Organ Size / Q000502:physiology; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D011191:Potassium Deficiency / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000502:physiology*
D004791:Enzyme Inhibitors; D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium