42.5 Saturday, Jan. 5 Cardiac, Slow and Fast Troponin-T Isoform Expression Patterns in Dog and Rat Extraocular Muscles REISER, PJ*; BICER, S; Ohio State University; Ohio State University firstname.lastname@example.org
Mammalian extraocular muscles (EOMs) consist of two distinct layers. Global layer fibers insert directly onto the eyeball and orbital layer fibers insert onto an outer connective tissue complex. Orbital fibers appear to modulate the force vector associated with EOM contractions. EOMs express a large number of myosin heavy and light chain isoforms and this diversity is a major contributor to the broad range of eye rotation velocities. We previously reported (Reiser and Bicer, 2011 Annual Meeting of the Biophysical Society) an unusual isoform expression pattern of the thin filament protein, tropomyosin (Tm), in the orbital layer. Specifically, fast orbital fibers express all three isoforms of Tm (α, β and γ), whereas fast and slow global and limb muscle fibers consistently express two Tm isoforms, α and β or γ and β, respectively. Given the critical dependence of interactions between Tm and troponin-T (TnT) during muscle activation, the objectives of this study were to identify TnT isoforms in fast and slow fibers in the global and orbital layers of dog and rat extraocular muscles and to quantitate their relative amounts in homogenates of both layers. SDS-PAGE and immunoblotting results indicate that fast global and orbital fibers express only fast isoforms of TnT, but the relative amounts of the individual isoforms are different from those in limb skeletal muscles. Slow fibers in both layers express slow TnT isoforms and the relative amounts also differ from those in limb slow fibers. Unexpectedly, cardiac TnT isoforms were detected in slow orbital fibers. These results further distinguish extraocular muscle fibers from limb muscle fibers and suggest that unique calcium-activation properties, especially among orbital fibers, subserve EOM contractions that drive oculomotor functions.