Heterogeneity of smooth muscle myosin heavy chain expression at the single cell level

Expression of the smooth muscle myosin heavy chain (SM-MHC) isoforms was examined in individual rabbit arterial smooth muscle cells with the use of reverse transcription-polymerase chain reactions (RT-PCR). The RT-PCR amplification protocol used oligonucleotide primers complementary to regions that flank the alternative exon that encodes the nine unique amino acids found in the carboxy terminal domain of SM2. RT-PCR products of SM1 and SM2 mRNA differ in length and electrophoretic mobility. Partial DNA sequencing of the PCR products confirmed SM1 and SM2 identity. Densitometric analyses of adjacent samples extracted from the same tissues and processed for SM2-to-SM1 protein and PCR-amplified SM2-to-SM1 RNA ratios exhibited a high correlation (R = 0.92). RT-PCR-amplified SM2-to-SM1 mRNA ratios of individual adult rabbit arterial cells ranged from 0.0 to 1.8 (n = 59), whereas multicellular vascular samples varied much less (0.4-0.6, n = 5). These results indicate that individual cells within a blood vessel differ significantly in SM-MHC expression. This difference may be important for the regulation of contraction in these vessels.