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

Purpose: To determine the composition of myosin heavy chain (MHC) isoforms in rat extraocular muscles (EOMs) during postnatal development.

Methods: 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.

Results: 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.

Conclusions: 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.