AJP - Cell Physiology, Vol 273, Issue 2 C598-C611, Copyright © 1997 by American Physiological Society

ARTICLES

Different effects of raised [K+]o on membrane potential and contraction in mouse fast- and slow-twitch muscle

S. P. Cairns, W. A. Hing, J. R. Slack, R. G. Mills and D. S. Loiselle
Department of Physiology, School of Medicine, University of Auckland, New Zealand.

Increasing extracellular K+ concentration ([K+]o) from 4 to 7-14 mM reduced both tetanic force and resting membrane potential (Em) in isolated slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles of the mouse. The tetanic force-[K+]o relationships showed a greater force loss over 8-11 mM [K+]o in soleus than EDL, mainly because the Em was 2-3 mV less negative at each [K+]o in soleus. The tetanic force-resting Em relationships show that force was reduced in two phases: phase 1 (Em < -60 mV), a 20% force decline in which the relationships superimposed in soleus and EDL, and phase 2 (Em -60 to -55 mV), a marked force decline that was steeper in EDL than soleus. Additionally in phase 2, longer stimulation pulses restored tetanic force; the twitch force-stimulation strength relationship was shifted toward higher voltages; caffeine, a myoplasmic Ca2+ concentration elevator, increased maximum force; and twitch force fell abruptly. We suggest that 1) the K(+)-depressed force is due to reduced Ca2+ release resulting from an altered action potential profile (phase 1) and inexcitable fibers due to an increased action potential threshold (phase 2), and 2) K+ contributes to fatigue in both fast- and slow-twitch muscle when it causes depolarization to about -60 mV.

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