Vascular smooth muscle updated.

• This brief review of the rapidly developing research on vascular smooth muscle presents the state of the art as I see it from within my own frame of reference. For a more objective, detailed insight into the workings of vascular smooth muscle, several substantial reviews and compendiums may be read (1-7). The mechanical events responsible for the contraction of vascular smooth muscle are associated with its contractile proteins. These proteins not only develop the mechanical force responsible for the contraction but also act as the enzyme that catalyzes the release of energy by which this force is developed. They are both the spark plug and the piston of the contractile machine. The contractile proteins of vascular smooth muscle are arranged in well-organized thick and thin filaments (8-10). The thick filaments, presumably bundles of myosin molecules, average 15.5 nm in diameter and have lateral projections suggestive of cross-bridges extending toward adjacent thin filaments. The thin filaments, presumably fibrous actin, average 5-8 nm in diameter and appear to be attached to dense bodies that are usually connected to the cell membrane. Contraction of vascular smooth muscle most probably is effected by some version of the Huxley sliding filament mechanism. The most easily interpretable studies of the functions of the contractile proteins are those performed in isolation with the determinants of the enzymatic and physical responses tightly controlled. There is a qualitative similarity between the actomyosin of vascular smooth muscle and the actomyosin of skeletal muscle (11) evidenced by the observation that a hybrid actomyosin can be prepared by combining myosin from one of these types of muscle with actin from the other; this hybrid provides a functionally active enzyme. Although the adenosinetriphosphatase (ATPase) activity of skeletal muscle actomyosin is many times faster than the activity of vascular smooth muscle actomyosin, the speed of activity of the hybrid preparation is determined by the source of the myosin. This observation correlates with the extensive studies by Barany (12) showing that a direct parallel exists between the maximum velocity of shortening of a muscle and the ATPase activity of its actin-activated myosin. These observations bear the important implication that the shortening velocity of vascular smooth muscle has the actomyosin ATPase activity as its rate-limiting factor. This slow release of chemical energy is reflected in the slow physical changes in the actomyosin molecule observed in superprecipitation studies (13) or in studies of contraction velocity of …