Single rabbit stomach smooth muscle cell myosin heavy chain SMB expression and shortening velocity

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Abstract
Isolated single smooth muscle cells (SMCs) from different regions of the rabbit stomach were used to determine a possible correlation between unloaded shortening velocity and smooth muscle (SM) myosin heavy chain (MHC) S1 head isoform composition (SMA, no head insert; SMB, with head insert). α-Toxin-permeabilized isolated single cells were maximally activated to measure unloaded shortening velocity and subsequently used in an RT-PCR reaction to determine the SMA/SMB content of the same cell. SM MHC SMA and SMB isoforms are uniquely distributed in the stomach with cells from the fundic region expressing little SMB (38.1 ± 7.3% SMB; n = 16); cells from the antrum express primarily SMB (94.9 ± 1.0% SMB; n = 16). Mean fundic cell unloaded shortening velocity was 0.014 ± 0.002 cell lengths/s compared with 0.036 ± 0.002 for the antrum cells. Unloaded shortening velocity in these cells was significantly correlated with their percent SMB expression ( r2= 0.58). Resting cell length does not correlate with the percent SMB expression ( n = 32 cells). Previously published assays of purified or expressed SMA and SMB heavy meromyosin show a twofold difference in actin filament sliding speed in in vitro motility assays. Extrapolation of our data to 0–100% SMB would give a 10-fold range of shortening velocity, which is closer to the ∼20-fold range reported from various SM tissues. This suggests that mechanisms in addition to the MHC S1 head isoforms regulate shortening velocity.

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