DIFFERENT PHENOTYPES AMONG SLOW BETA MYOSIN HEAVY CHAIN-CONTAINING FIBERS OF RABBIT MASSETER MUSCLE - A NOVEL TYPE OF DIVERSITY IN ADULT MUSCLE/

Differences in the phenotype of different mammalian muscle fibres are usually attributed to differences in the expression of the product of different myosin heavy chain (MyHC) genes, which are known as isoforms . We studied differences in phenotype among fibres containing a single MyHC isoform (slow/beta) of the masseter muscle of adult rabbits. Fou r different monoclonal antibodies to slow/beta MyHC were used to stain serial sections from muscles in males and females. All antibodies rec ognize a single band on immunoblots and stain the same set of fibres i n rabbit postcranial muscles. However, differential staining was obser ved in the masseter muscles. Antibody BA-D5 reacts with the most fibre s, antibody A4.951 reacts with a subset of these fibres, and antibody A4.840 reacts with a subset of the A4.951-positive fibres. Antibody S5 8 reacts only with an even smaller subset of fibres. Even though diffe rential staining using four antibodies might allow for the expression of as many as 15 different staining patterns, or phenotypes, only four were observed in >99% of over 30 000 fibres studied. In females, near ly 40% of the fibres stain exclusively with antibody BA-D5, while in m ales, fewer than 8% of the fibres express this phenotype. The proporti ons of fibres of the other phenotypes do not differ so strikingly with gender. We conclude that an epitope diversity exists among muscle fib res in the adult rabbit masseter and that it is not necessarily a cons equence of differences in gene expression. We feel that it is a regula ted process and that, at least for some phenotypes, this regulation ma y be hormonally influenced. (C) Chapman & Hall Ltd.