Metabolic response to anisoosmolarity of rat skeletal muscle in vitro

Isolated rat hepatocytes exhibit an insulin-like anabolic response to hypoo smotic incubation and a glucagon-like catabolic response to hyperosmotic in cubation. Recently, a distinct glycogenic response to hypoosmotic treatment was likewise reported for cultured rat myotubes. The present study examine s the effects of anisoosmolar exposure on glucose metabolism in freshly iso lated rat soleus muscle strips. Under the same experimental conditions as u sed for cultured myotubes, hypoosmolarity reduced net glycogen synthesis to 52%, while hyperosmolarity stimulated glycogen storage to 231% of isoosmol ar control (nmol glucose incorporated into glycogen g(-1) x h(-1): hypoosmo lar, 34 +/- 3; isoosmolar, 65 +/- 8; hyperosmolar, 150 +/- 11; p < 0.01 eac h vs. isoosmolar). The responses of native skeletal muscle to anisoosmolari ty are therefore in opposition to what has been described for hepatocytes a nd cultured myotubes. Further experiments on rat skeletal muscle revealed t hat the observed lack of a glycogenic response to hypoosmolarity persisted independent of medium composition, specifically with regard to prevailing g lucose and K+ concentrations. In conclusion, hypoosmotic exposure inhibits glycogen synthesis in isolated rat soleus muscle, which clearly argues agai nst the hypothesis that osmotic changes and cell swelling may be physiologi cally relevant stimulators of muscle glycogen synthesis.