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 codin g region, the distributing arteries (femoral and saphenous) begin to s how MHC mRNA with the 21-nucleotide insert that encodes seven amino ac ids in the ATP-binding region located in the myosin head. The femoral/ iliac artery contains >50% inserted mRNA, whereas the more distal saph enous 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 f rom the rabbit femoral/saphenous artery is 1.7-fold higher than that o f the myosin from the aorta. A concomitant increase (about twofold) in the maximum shortening velocity of the saphenous artery, compared wit h that of the aorta, indicates that the preponderance of the inserted myosin is associated with both an increase in the actin-activated ATPa se activity and a larger maximum velocity of shortening. Furthermore, analysis of the 17-kDa essential light chain from the aorta reveals ne ar equal quantities of the 17-kDa light chain isoforms a and b, wherea s the myosin from the femoral/saphenous artery contains predominantly the 17-kDa light chain a isoform. Together, these data indicate that t he smooth muscle cells from the small distributing arteries are simila r to those of visceral smooth muscle with respect to the expression of myosin isoforms, actin-activated myosin ATPase activity, and contract ility.