Phosphorylation of uterine smooth muscle myosin permits actin-activation.

Myosin was purified from ovine uterine smooth muscle. The 20,000 dalton myosin light chain was phosphorylated to varying degrees by an endogenous Ca2+ dependent kinase. The kinase and endogenous phosphatases were then removed via column chromatography. In the absence of actin neither the size of the initial phosphate burst nor the steady state Mg2+-dependent ATPase activity were affected by phosphorylation. However, phosphorylation was required for actin to increase the Mg2+-dependent ATPase activity and for the myosin to superprecipitate with actin. Ca2+ did not affect the Mg2+-dependent ATPase activity in the presence or absence of action or the rate or extent of superprecipitation with actin once phosphorylation was obtained. These data indicate that: 1) phosphorylation of the 20,000 dalton myosin light chain controls the uterine smooth muscle actomyosin interaction, 2) in the absence of actin, phosphorylation does not affect either the ATPase of myosin or the size of the initial burst of phosphate and, 3) Ca2+ is important in controlling the light chain kinase but not the actomyosin interaction.

Contractile properties of actomyosin from human blood platelets.

Actomyosin was purified from human blood platelets and used to form threads via extrusion. A sensitive tensiometer was employed to measure isometric tension and velocity of isotonic shortening of the threads in the presence of MgATP. Using fully phosphorylated myosin, we obtained values for maximum isometric tension (Po) and maximum velocity of contraction (V max) that were similar to those reported for threads composed of skeletal muscle actomyosin. Po was found to be directly proportional to the level of phosphorylation of the 20,000-dalton myosin light chain. We also studied the effect of phosphorylation on superprecipitation of platelet actomyosin. Fully phosphorylated myosin produced rapid clearing and superprecipitation, while myosin with a low level of bound phosphate underwent rapid clearing but did not superprecipitate. We have concluded from these results that: 1) the interaction between platelet actin and myosin produces tension and motion that is similar to that produced by skeletal muscle actin and myosin and 2) phosphorylation of the 20,000-dalton myosin light chain in important in controlling the production of force by platelet actin and myosin.

Congenital heart malformations associated with disproportionate ventricular septal thickening.

Asymmetric septal hypertrophy, or ASH, is a genetically determined myocardial disorder that is transmitted as an autosomal dominant trait. ASH is characterized by a disproportionately thickened ventricular septum that contains numerous hypertrophied, bizarrely-shaped and disorganized cardiac muscle cells. Disproportionate hypertrophy of the ventricular septum has also been observed in association with certain congenital cardiac malformations. To determine whether such congenital cardiac malformations are part of the disease spectrum of genetically determined ASH, cardiac pathologic observations were made in eight patients with disproportionate septal thickening (ventricular septal to posterobasal left ventricular free wall thickness ratios of 1.5 to 2.5) and the following three categories of associated lesions: 1) parachute deformity of the mitral valve (occurring either as an isolated lesion or with ventricular septal defect, coarctation of the aorta, supravalvular ring of the left atrium, or double outlet right ventricle); 2) complete interruption of the aortic arch; and 3) ventricular septal defect. The arrangement of cardiac muscle cells in the disproportionately thickened ventricular septum was normal in six of the eight patients; in the other two patients (one with parachute deformity of the mitral valve and one with ventricular septal defect) numerous bundles of hypertrophied cardiac muscle cells were interlaced in a disorganized fashion among more normally arranged bundles of cells. First degree relatives of six of the eight patients were studied by echocardiography and found to have normal ventricular wall thicknesses and septal-free wall ratios. It is concluded that disproportionate ventricular septal thickening may occur in patients with a variety of congenital heart malformations, but that such a finding is not necessarily a manifestation of the disease spectrum of genetically determined ASH.