Moderate elevation of extracellular potassium transiently inhibits regeneration of sensory axons in cultured adult sciatic nerves

The adult frog dorsal root ganglia (DRG) together with the sciatic nerve (ScN) has previously been shown to survive in organ culture for several days. If a local test crush is made at the beginning of culturing, there is an initial delay of about 3 days before the sensory axons start to grow into the distal nerve stump at a rate of about 0.6-0.9 mm/day. The present results showed that axonal growth was unaffected in preparations maintained for 8 days in medium containing 10 mM K⁺ (5 mM is the physiological level). In contrast, the outgrowth was markedly reduced by 15 mM K⁺ and still more by 20 and 25 mM K⁺. The growth inhibition was partially counteracted by nifedipine, a Ca²⁺-channel antagonist. Other experiments clearly showed that high K⁺ exerted its effects during the early phase of the regeneration and lacked effects at later stages. The possibility that Ca²⁺-binding proteins, e.g., calbindin, which showed immunohistochemical reactivity in different structures, contribute to the growth adaptation to high K⁺ will be considered. The generality of the findings was supported by inhibition of axonal outgrowth of adult mouse sciatic sensory axons by high K⁺.