DIFFERENT EFFECTS OF RAISED [K-POTENTIAL AND CONTRACTION IN MOUSE FAST-TWITCH AND SLOW-TWITCH MUSCLE(](0) ON MEMBRANE)

Increasing extracellular K+ concentration ([K+](o)) from 4 to 7-14 mM reduced both tetanic force and resting membrane potential (E-m) in iso lated slow-twitch soleus and fast-twitch extensor digitorum longus (ED L) muscles of the mouse. The tetanic force-[K+](o) relationships showe d a greater force loss over 8-11 mM [K+](o) in soleus than EDL, mainly because the E-m was 2-3 mV less negative at each [K+](o) in soleus. T he tetanic force-resting E-m relationships show that force was reduced in two phases: phase 1 (E-m < -60 mV), a 20% force decline in which t he relationships superimposed in soleus and EDL, and phase 2 (E-m -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 elev ator, increased maximum force; and twitch force fell abruptly. We sugg est that 1) the K+-depressed force is due to reduced Ca2(+) release re sulting from an altered action potential profile (phase 1) and inexcit able 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.