ION GRADIENTS AND CONTRACTILITY IN SKELETAL-MUSCLE - THE ROLE OF ACTIVE NA+, K+ TRANSPORT

Intensive contractile activity is associated with a significant net lo ss of K+ and a comparable gain of Na+ in the working muscle fibres. Th is leads to an increase in the interstitial and T-tubular K+ concentra tion and to a decrease in the T-tubular Na+ concentration. It is well established that the exposure of muscles to high extracellular K+ or l ow extracellular Na+ inhibits contractile performance. More importantl y. the combination of high extracellular K+ and low extracellular Nahas a much more pronounced inhibitory effect on force than the sum of the individual effects of the two ions. The inhibitory effects of high extracellular K+ or low extracellular Na+ can be alleviated within 5- 10 min by acute hormonal stimulation of the Na+, K+ pump. In contrast. reductions in the capacity for active Na+, K+ transport by pre-incuba tion of isolated muscles with ouabain or by prior K+ depletion of the animals significantly decreases contractile endurance during high-freq uency electrical stimulation. Thus. muscles from K+-depleted rats exhi biting a 54% reduction in Na+, K+ pump concentration showed a 110% inc rease in force decline during 30 s of 60 Hz stimulation. Reducing the Na+, K+ pump capacity to a similar extent by pre-incubation with ouaba in led to a comparable decrease in endurance. Moreover. reductions in the Na+, K+ pump capacity were associated with an increased intracellu lar accumulation of Na+ during electrical stimulation. These observati ons support the notion that excitation-induced decreases in Na+. K+ gr adients contribute to fatigue during intensive exercise and suggest th at the capacity for active Na+, K+ transport is a determining factor f or contractile endurance.