Changes in actin synthesis and alpha-actin-mRNA content in rat muscle during immobilization.

The fractional rates of actin synthesis in adult rat gastrocnemius muscle from control and 6-h hindlimb-immobilized animals were determined by the constant-infusion technique. The rate of actin synthesis in gastrocnemius muscle was significantly decreased from control values during the 6th h of hindlimb immobilization. The content of alpha-actin-specific mRNA was then estimated in adult rat gastrocnemius muscle from control, 6-h, 72-h, and 7-day immobilized animals by "dot blot" hybridization. RNA extracted from control and immobilized animals was secured on nitrocellulose filters and hybridized to 32P-labeled plasmid p749 (containing a cDNA sequence produced from rat alpha-actin mRNA). The relative content of alpha-actin-specific mRNA in gastrocnemius muscle was significantly decreased at 7 days of immobilization but not at 6 or 72 h of immobilization. It is concluded that a change in the content of alpha-actin mRNA does not contribute significantly to the rapid onset of the decrease in actin synthesis rate observed after 6 h of immobilization. An alteration in the translation of alpha-actin-specific mRNA must occur to account for the early decline in actin synthesis during immobilization.