Different effects of raised [K+]o on membrane potential and contraction in mouse fast- and slow-twitch muscle.
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.
Cairns
S P
SP
Department of Physiology, School of Medicine, University of Auckland, New Zealand.
Hing
W A
WA
Slack
J R
JR
Mills
R G
RG
Loiselle
D S
DS
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism*; D005260:Female; D008564:Membrane Potentials; D051379:Mice; D009119:Muscle Contraction / Q000187:drug effects; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D009994:Osmolar Concentration; D011188:Potassium / Q000378:metabolism*; D013997:Time Factors
D002110:Caffeine; D011188:Potassium