Sarcolemmal and sarcoplasmic reticular ATPase activities in the failing canine heart

The specific activity of both the sarcoleminal Na + -K + -activated and the sarcoplasmic reticular ATPase systems was studied in dogs with right ventricular hypertrophy and congestive heart failure induced by progressive pulmonary artery stenosis. In these enzyme preparations, mitochondrial or cross contamination of either of the cellular fractions studied did not appear to be a factor. In the sarcoplasmic reticular fraction, ouabain was without effect and succinic dehydrogenase assay showed less that 10% of the activity observed in pure mitochondrial preparations. In the sarcolemmal fraction, sodium azide did not significantly inhibit the ATPase activity, but ouabain inhibited 91% of it. The activity of the sarcolemmal ATPase system of the faihng heart did not differ from that of the control; however, the activity of failing heart sarcolemmal preparations was found to be only 61% inhibited by 10 -4 M ouabain, and the activity of control preparations was 91% inhibited. The sarcoplasinic reticular ATPase activity from right ventricles of dogs with congestive heart failure was significantly less ( P <0.001) than the respective control activity (average 15.9 compared to 28.1 µmole hr -1 mg -1 ), whereas the activity from left ventricles of failing hearts did not differ significantly from the respective control value. These findings suggest that a possible explanation for the diminished contractility of the chronically failing heart lies within the subcellular enzymatic ion-regulating systems.

Association of depressed myofibrillar adenosine triphosphatase and reduced contractility in experimental heart failure

Although previous studies have suggested a depression of myofibrillar ATPase in the presence of heart failure, their interpretation has been limited by failure to exclude mitochondrial contamination and lack of information regarding the contractility of the tissue studied. To explore this fundamental question, myofibrils were prepared from the right and left ventricles of 15 normal cats, 10 cats with right ventricular hypertrophy and 19 with right ventricular hypertrophy and right ventricular failure, the latter groups following graded pulmonary artery stenosis. The contractility of these hearts was assessed using their right ventricular papillary muscles isolated in a myograph. Myofibrillar ATPase in all groups was maximally activated by 5 mM ATP, 5 mM MgCl 2 at pH 7.0 and 37°C, while mitochondrial contamination was eliminated by sodium azide. In right ventricular failure, right ventricular myofibrillar ATPase was depressed by 39% from an average of 0.18±0.01 in normal cats to 0.11±0.01 µmole inorganic phosphate/mg protein per min (P<.001). In right ventricular hypertrophy, right ventricular myofibrillar ATPase was not significantly depressed (0.16±0.01 µmole/mg per min). Of note, left ventricular myofibrillar ATPase was also significantly (P<.05) depressed in right ventricular failure from 0.16±0.01 to 0.13±0.01 µmole/mg per min. Contractility of the associated right ventricular papillary muscles, expressed as maximum rate of force development at the apex of the length-active tension curve, was correlated with myofibrillar ATPase activity.