Potassium activation and K spikes in muscle fibers of the mealworm larva (Tenebrio molitor)

Abstract

Muscle fibers of larval mealworms (Tenebrio molitor) can be set to a wide range of resting potentials (ca. –40 to –90 mv), while remaining responsive to electrical stimuli. The initial resting potential is maintained long after K⁺₀ is increased to levels well above the normal value (ca. 40 mEq/liter). However, spikes or graded responses are markedly affected by the level of K⁺₀. For levels between 40 and 150 mEq/liter the terminal portion of the responses may become prolonged depolarizations, and for K⁺₀ above about 200 mEq/liter positive overshoots occur. These changes follow the Nernst relation for K⁺₀ > 20 mEq/liter. Thus, the membrane of the muscle fiber at rest is not a K electrode, but changes to the latter state during a response, indicating occurrence of K activation. The "K spikes" which develop in high K⁺₀ lack an early depolarizing component, which is comparable to that subsumed under the Na activation and inactivation processes of the Hodgkin-Huxley theory. The K spikes may last for many seconds and are associated with increased membrane conductance throughout this time. The K spikes are probably terminated by a process of depolarizing K inactivation.