Sequential activation of two mitogen activated protein (MAP) kinase isoforms in rat skeletal muscle following insulin injection.

Skeletal muscle is a major target of insulin action. The possible role of MAP kinase activation in insulin receptor signaling in muscle was examined. After a 48-hr fast, rats were injected intravenously with insulin or saline, muscles were excised after 3-20 min, homogenized, and MAP kinases were partially purified by ammonium sulfate precipitation and Mono Q chromatography. Activity was assayed as 32P-incorporation into myelin basic protein. Two activity peaks were identified; peak I eluted with approximately 0.1 M NaCl and peak II with approximately 0.2 M NaCl. Three min after insulin injection the activity of peak II increased > 2-fold, peak I was unchanged. After 10 min, the activity of peak II returned toward baseline, while peak I was activated approximately 3-fold. Immunoblots confirmed the presence of MAP kinases eluting with activity peaks I and II; the former as a approximately 41 kDa protein and the latter as a doublet of approximately 42 and approximately 44 kDa. The data suggest sequential activation of two MAP kinases in muscles; the isoform which activates/deactivates rapidly may represent ERK-1, while the more slowly responding isoform may be ERK-2.