Using immunocytochemistry, electrophoresis and immunoblotting, we stud
ied the expression of fast and slow myosin heavy chain isoforms in adu
lt ferret muscles during quiescent and breeding periods. Adult cremast
er muscle expressed slow and fast myosin heavy chain in relatively sim
ilar amounts during the quiescent period. During the breeding period,
the expression of slow myosin heavy chain I, significantly decreased,
and fast myosin heavy chain II, was predominant. No alteration of the
MHC pattern in EDL and soleus muscles was detected between the quiesce
nt and breeding periods. The possible involvement of androgens and mec
hanical factors in the regulation of myosin heavy chain expression in
adult cremaster muscle is discussed.