Characterization and regulation of apamin-binding K⁺ channels in skeletal muscle

The pattern of development and regulation of the apamin receptor (afterhyperpolarization channel) was studied in cultures of skeletal muscle prepared from 1-2-day-old rat pups. Expression was measured by the specific binding of ¹²⁵I-apamin. Apamin binding was virtually undetectable until the time of fusion (3-4 days in culture) of single myoblasts into myotubes. Mature myotubes (5-7 days in vitro) displayed a B(max) of 7.4 fmol/mg protein and a K(d) of 376 pmol/L. When studied in mature muscle cells apamin binding was found to increase twofold in response to tetrodotoxin (TTX) and elevated K(o), which resulted in decreased Na(i). In contrast, treatments causing an increase in Na(i), such as monensin and veratridine, caused a decrease in apamin binding. The increase in apamin binding following TTX treatment was due mainly to synthesis of new channels, as the effect was blocked by cycloheximide. Alterations in cytosolic Ca²⁺ by calcium ionophore or Ca- channel blockers were without effect on apamin-sensitive channel expression. We conclude that afterhyperpolarization channel expression is regulated by the level of intracellular Na⁺ ions.