MORPHOLOGICAL, HISTOCHEMICAL, AND MYOSIN ISOFORM ANALYSIS OF THE DIAPHRAGM OF ADULT HORSES, EQUUS-CABALLUS

The horse provides an interesting model for study of the structure and function of the mammalian diaphragm. Multiple regions of diaphragm fr om seven adult horses were prepared for histochemistry, immunocytochem istry, myosin heavy chain electrophoresis, and native myosin electroph oresis. Two additional adults were dissected to demonstrate myofiber a nd central tendon morphology and stained for acetylcholinesterase to d emonstrate motor endplates. All regions of the adult diaphragm were hi stochemically characterized by a preponderance of type I fibers with s ome type IIa fibers. Type IIb fibers were absent in all adult specimen s. Myosin heavy chain electrophoresis supported the histochemical stud y: two isoform bands were present on SDS gels that comigrated at the s ame rate as rat type I and IIa myosin heavy chain isoforms. No isoform was determined to comigrate with rat type IIb heavy chain isoforms. N ative myosin isoform analysis revealed two isoforms that comigrated wi th rat FM-4 and FM-3 (FM = fast myosin) and two isoforms that comigrat ed with rat SM-1 and SM-2 (SM = slow myosin) isoforms. In some samples , a third slow native myosin isoform was observed that comigrated at t he same rate as the SM-3 of the equine biceps brachii muscle. This dou blet (or ''triplet'') of slow isoforms is unique to some horse muscles compared with other adult animals studied. It is not known if these m ultiple slow native myosin isoforms confer some functional advantage t o the equine muscles. The adult equine diaphragm also differs in its m orphology by having a large central tendon compared to that in other m ammals, and is predominantly slow in fiber type and myosin isoform com position. (C) 1994 Wiley-Liss, Inc.