Metabolic response to anisoosmolarity of rat skeletal muscle in vitro

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