NH₂-terminal-inserted myosin II heavy chain is expressed in smooth muscle of small muscular arteries

We demonstrate, using reverse transcriptase-polymerase chain reaction, that, whereas abdominal aorta from rabbit consists almost entirely of myosin heavy chain (MHC) mRNA with no insert at the 5'-terminal coding region, the distributing arteries (femoral and saphenous) begin to show MHC mRNA with the 21-nucleotide insert that encodes seven amino acids in the ATP-binding region located in the myosin head. The femoral/iliac artery contains >50% inserted mRNA, whereas the more distal saphenous artery contains >80% inserted mRNA. This insert is also present in the smooth muscle from rat tail artery but is absent in the smooth muscle from rat aorta. The actin-activated ATPase activity of myosin from the rabbit femoral/saphenous artery is 1.7-fold higher than that of the myosin from the aorta. A concomitant increase (about twofold) in the maximum shortening velocity of the saphenous artery, compared with that of the aorta, indicates that the preponderance of the inserted myosin is associated with both an increase in the actin -activated ATPase activity and a larger maximum velocity of shortening. Furthermore, analysis of the 17-kDa essential light chain from the aorta reveals near equal quantities of the 17-kDa light chain isoforms a and b, whereas the myosin from the femoral/saphenous artery contains predominantly the 17-kDa light chain a isoform. Together, these data indicate that the smooth muscle cells from the small distributing arteries are similar to those of visceral smooth muscle with respect to the expression of myosin isoforms, actin-activated myosin ATPase activity, and contractility.