Isoform switching from SM-B to SM-A myosin results in decreased contractility and altered expression of thin filament regulatory proteins.

We previously generated an isoform-specific gene knockout mouse in which SM-B myosin is permanently replaced by SM-A myosin. In this study, we examined the effects of SM-B myosin loss on the contractile properties of vascular smooth muscle, specifically peripheral mesenteric vessels and aorta. The absence of SM-B myosin leads to decreased velocity of shortening and increased isometric force generation in mesenteric vessels. Surprisingly, the same changes occur in aorta, which contains little or no SM-B myosin in wild-type animals. Calponin and activated mitogen-activated protein kinase expression is increased and caldesmon expression is decreased in aorta, as well as in bladder. Light chain-17b isoform (LC(17b)) expression is increased in aorta. These results suggest that the presence or absence of SM-B myosin is a critical determinant of smooth muscle contraction and that its loss leads to additional changes in thin filament regulatory proteins.