Myosin Heavy Chain Isoform Patterns Do Not Correlate with Force-Velocity Relationships in Pulmonary Arterial Compared with Systemic Arterial Smooth Muscle

Abstract

Velocity of shortening is dependent on the myosin heavy chain (MHC) isoform pattern in both skeletal and cardiac muscle (Pagani and Julian, 1984). Furthermore, it has been reported that a shift in MHC isoform ratio occurs with certain physiological or pathophysiological changes such as hypertrophy and/or hyperplasia of striated muscles (Litten et al., 1974). Such shifts in MHC isoform proportions accompany concomitant changes in shortening velocity and ATPase activity (Alpert et al., 1979; Alpert and Mulieri, 1980). At least two different MHC isoforms have been reported to exist in various different smooth muscles (Sparrow et al., 1987). The 200 kDa form and the 204 kDa form have been designated MHC₁ and MHC₂, respectively. The ratio of MHC₁:MHC₂ has been shown to vary dependent on smooth muscle type, animal species, stage of development, and under certain different physiological or pathophysiological conditions for the same muscle type (Sparrow et al., 1987; Mohammed and Sparrow, 1988). However, no functional correlation has yet been made between MHC isoform ratio and shortening velocity for smooth muscle. Therefore, the purpose of this study was to compare force-velocity (F-V) relationships and MHC isoform ratios from two different arterial muscles (pulmonary versus caudal) from the same species (rat).