Heart myosin adenosine triphosphatase and light subunits in experimental chronic aortic insufficiency in the rabbit

Abstract

Cardiac myofibrillar and myosin ATPases were studied in experimentally induced aortic insufficiency in the rabbit, in order to elucidate the pathogenesis of the defect in myofibrillar ATPase shown in chronically hypertrophied and/or failing hearts. The rabbits were killed 345 ± 32 days after the operation; 24% of them had clinical or anatomical signs of failure. They all had hypertrophic hearts (mean degree of hypertrophy, 64%) with dilatation. Heart myofibrillar Ca 2+ ATPase was found to be lowered in aortic insufficiency. Three different preparations of myosin were also studied. The purity of these preparations was assessed by urea or SDS polyacrylamide gel electrophoresis or by measuring Mg 2+-ATPase in a medium of low ionic strength. The first preparation (homogenized muscle) was highly contaminated by thin-filament proteins; the second (minced muscle) was rather pure but still contaminated by a nucleoprotein; the latter was separated by chromatography in the third preparation. Myosin Ca 2+-ATPase measured in a medium of low or high ionic strength at pH 7.0 was decreased in aortic insufficiency when all three preparations were tested. On electrophoresis myosin light subunits were normal in aortic insufficiency. The search for an inhibitor was unsuccessful: the two myosin ATPase activities were additive and ribonuclease treatment did not normalize hypertrophied heart myosin. The “nucleoprotein” peak separated from myosin during chromatography had an inhibitory effect on myosin ATPase but was present in controls as well as in aortic insufficiency. This work suggests that myosin itself is abnormal in chronic aortic insufficiency, but an abnormality residing in a very closely associated factor could not definitely be excluded. Because myosin subunits were only studied by electrophoresis, their structures and relative amounts would need more detailed study before final conclusions could be reached.