Postnatal development of myosin heavy chain isoforms in rat extraocular muscles.

To determine the composition of myosin heavy chain (MHC) isoforms in rat extraocular muscles (EOMs) during postnatal development. The MHC composition of rat EOMs at postnatal day 0 (P0), postnatal day 14 (P14), and adults was evaluated at mRNA levels by competitive polymerase chain reaction and MHC composition of each six EOM in adult rats. EOMs at P0 revealed predominant expression of neonatal MHC (75.5%) with a lesser percentage of embryonic MHC (12.8%) and 2A MHC (11.5%). 2X MHC was expressed at low levels and other MHC isoforms were not detected. At P14, EOMs expressed mostly 2X MHC (42.4%) and 2A MHC (27.4%). Expression levels of neonatal MHC (14.1%) and embryonic MHC (4.9%) decreased. 2B MHC (8.2%), EOM MHC (1.9%), and beta-cardiac MHC (1.1%) were detected at low levels. In the adult rats, EOMs contained over 80% of three fast MHC isoforms, such as 2X MHC (29.9%), 2A MHC (29.3%), and 2B MHC (24.5%). Each of six adult EOM showed slightly different expression levels of MHC composition. A strong correlation exists between the composition of fast MHC isoforms and muscle development. MHC isoform followed a neonatal MHC-2X MHC-2B MHC isoform switching pattern after birth. Postnatal development of EOMs had a slightly different expression pattern for MHC isoforms and may have different regulatory roles related to their functional requirement.