IDENTIFICATION OF ALPHA-CARDIAC MYOSIN HEAVY-CHAIN MESSENGER-RNA AND PROTEIN IN EXTRAOCULAR-MUSCLE OF THE ADULT-RABBIT

Extraocular muscles contain both fast-twitch and multiply-innervated, tonic-contracting fibres. In rat, these fibres collectively express nu merous myosin heavy chain isoforms including fast-type embryonic and n eonatal, adult slow twitch type I and fast twitch type II, and a fast isoform unique to extraocular muscle. Immunocytochemical and Western b lotting results are presented which suggest that, in rabbit, an additi onal species, the alpha-cardiac myosin heavy chain, is present. The im munoreactive species is found in all rabbit extraocular muscles and in the rotatory extraocular muscles is expressed in almost all fibres wh ich do not contain a fast myosin heavy chain. Positive identification of this isoform as the alpha-cardiac myosin heavy chain was obtained b y sequencing a cloned PCR product derived from extraocular muscle mRNA unique to the 3'-end of rabbit alpha-cardiac myosin heavy chain mRNA. This is the first unequivocal demonstration of alpha-cardiac myosin h eavy chain expression in extraocular muscle.