Myosin light chain 1 isoforms in slow fibers from global and orbital layers of canine rectus muscles.

Purpose: Initial results of an examination of the low molecular mass (< or =45 kDa) protein composition of canine rectus muscle homogenates, based on gel electrophoresis, revealed a distinct difference between the global and orbital layers in the myosin light chain (MLC)-1 region. The objectives of the present study were, therefore, to identify isoforms of MLC1 in homogenates of the global and orbital layers of adult canine rectus muscles and to determine the MLC1 isoform expression pattern among single muscle fibers isolated from both layers.

Methods: Muscle homogenates and single fibers from the global and orbital layers of canine rectus muscles were analyzed, using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF-MS) was used to identify a protein band in the orbital layer that comigrated with MLC1 in the adult canine atrium.

Results: Adult canine extraocular rectus muscles expressed embryonic skeletal/atrial MLC1 (MLC1(E/A)), in addition to the fast-type MLC1 (MLC1F) and slow-type MLC1 (MLC1S) isoforms expressed in limb skeletal muscles. MLC1(E/A) was detected in slow fibers of the orbital but not the global layer, and MLC1S was detected in slow fibers in only the global but not the orbital layer. Densitometric analysis of gel bands from homogenates supported these results, with significantly greater amounts of MLC1S in the global layer and of MLC1(E/A) in the orbital layer.

Conclusions: MLC1(E/A) is expressed in rectus muscles of adult dogs. Furthermore, two types of slow fibers, distinguished on the basis of MLC1 isoform expression, exist in separate layers of canine rectus muscles.