Dexamethasone up‐regulates skeletal muscle maximal Na + ,K + pump activity by muscle group specific mechanisms in humans

Dexamethasone, a widely clinically used glucocorticoid, increases human skeletal muscle Na + ,K + pump content, but the effects on maximal Na + ,K + pump activity and subunit specific mRNA are unknown. Ten healthy male subjects ingested dexamethasone for 5 days and the effects on Na + ,K + pump content, maximal activity and subunit specific mRNA level (α1, α2, β1, β2, β3) in deltoid and vastus lateralis muscle were investigated. Before treatment, maximal Na + ,K + pump activity, as well as α1, α2, β1 and β2 mRNA levels were higher ( P < 0.05) in vastus lateralis than in deltoid. Dexamethasone treatment increased Na + ,K + pump maximal activity in vastus lateralis and deltoid by 14 ± 7% ( P < 0.05) and 18 ± 6% ( P < 0.05) as well as Na + ,K + pump content by 18 ± 9% ( P < 0.001) and 24 ± 8% ( P < 0.01), respectively. Treatment with dexamethasone resulted in a higher α1, α2, β1 and β2 mRNA expression in the deltoid ( P < 0.05), but no effects on Na + ,K + pump mRNA were detected in vastus lateralis. In conclusion, dexamethasone treatment increased maximal Na + ,K + pump activity in both vastus lateralis and deltoid muscles. The relative importance of transcription and translation in the glucocorticoid‐induced regulation of Na + ,K + pump expression seems to be muscle specific and possibly dependent on the actual training condition of the muscle, such that a high Na + ,K + pump maximal activity and mRNA level prior to treatment prevents the transcriptional response to dexamethasone, but not the increase in Na + ,K + pump content and maximal activity.