COCAINE ALTERS MYOSIN ISOFORM EXPRESSION IN THE RAT SOLEUS

The purpose of this study was to determine whether chronic cocaine adm inistration alters the expression of myosin isoforms in the rat soleus . Forty-five adult Sprague-Dawley rats were divided into three groups: chronic cocaine (n = 15), 12.5 mg/kg cocaine-HC1 injected intraperito neally twice daily for 14 days and one injection of cocaine (12.5 mg/k g ip) on day 15; acute cocaine (n = 15), saline injections twice daily for 14 days and one injection of cocaine (12.5 mg/kg ip) on day 15; a nd chronic saline (n = 15), saline injections twice daily for 14 days and one saline injection on day 15. Myosin isoform content of the sole us (native and heavy chains) was identified by electrophoresis. The so lei samples from the chronic saline and acute cocaine animals containe d slow myosin only. However, solei samples from the chronic cocaine gr oup contained slow myosin and two to three other myosin isoforms and t he associated heavy chains Pi a and Pi x. Therefore, chronic cocaine a dministration causes in the rat soleus a shift in myosin expression fr om slow isoforms to fast isoforms.