Smooth-muscle myosin heavy-chain SM-B isoform expression in developing andadult rat lung

The smooth-muscle cells composing the vasculature and airways of the lung d isplay a variety of contractile protein phenotypes. To date, however, it ha s remained unclear how these phenotypes might contribute differentially to contractile activity. To address this issue, we made monospecific rabbit po lyclonal antibodies against the difference peptide for the SM-B smooth-musc le myosin heavy chain (SMMHC) and used these to investigate the distributio n of the SM-B isoform in lung. SM-B has a seven-amino acid insert in the he ad region that is known to result in a higher actin-activated adenosine tri phosphatase activity and tn vitro motility. During development, reactivity is first seen in the trachea and bronchi of saccular lung at the time of bi rth, when other SMMHC isoforms also are present. Immunoreactivity spreads d istally through the airways as development proceeds, reaching the level of alveolar septae in the adult. Although the smaller vessels of the pulmonary vasculature react strongly with the SM-B antibody, reactivity is infrequen tly observed in large pulmonary vessels. Adult tracheal smooth muscle is hi ghly and more uniformly reactive, commensurate with its relatively high max imal velocity of shortening. The differential expression of the SM-B isofor m in vascular and airway smooth muscles demonstrated in this study may prov ide the molecular basis for functional differences between these smooth-mus cle cell types and may provide one mechanism for adapting contractility in response to physiologic stresses in the lung.