Die Energetik des Säugetierherzens

Alterations in cardiac oxygen consumption under chronic pressure overload. Significance of the isoenzyme pattern of myosin

Hypertension and resulting left ventricular hypertrophy was induced in young male Wistar rats (60 to 70 days old) by narrowing of one renal artery (Goldblatt II). 8 and 24 weeks after operation, myocardial oxygen consumption was measured on a modified in situ heart-lung preparation with nearly isovolumetric left ventricular contractions. Measured myocardial oxygen consumption was related to left ventricular wall stress. The myosin isoenzyme pattern of each heart was determined with pyrophosphate gel electrophoresis. Oxygen consumption related to wall stress averaged over the entire heart cycle amounted to 15 mumoles O2/g X min 8 weeks after operation, and 24.4 mumoles O2/g X min in age-matched controls (delta 38%, p less than 0.0005). When wall stress was averaged over systole, oxygen consumption of the hypertrophied hearts amounted to 0.112 mumoles O2/g x beat, and 0.149 mumoles O2/g x beat in the controls (delta 25%, p less than 0.05). The proportion of VM-3 (the cardiac myosin isoenzyme of lowest ATPase activity) increased from 26.3% in the controls to 30.1% in the Goldblatt hearts (delta 14%, n.s.). 24 weeks after operation, oxygen consumption related to wall stress averaged over the entire heart cycle amounted to 16.1 mumoles O2/g x min, in age-matched controls 20.5 mumoles O2/g x min (delta 21%, p less than 0.05). When wall stress was averaged over systole, oxygen consumption of the Goldblatt hearts amounted to 0.080 mumoles O2/g x beat, and in the controls 0.107 mumoles O2/g x beat (delta 25%, p less than 0.0005). The proportion of VM-3 increased from 33.5% in the controls to 43.2% in the hypertrophied hearts (delta 29%, p less than 0.05). The present findings indicate that the reduced oxygen consumption of the pressure-loaded heart should be attributed to a redistribution of myosin isoenzymes. The transformation of myocardium into a slower, but more efficiently working muscle due to an increase in VM-3 can be interpreted as an adaptational process.

Mechanochemistry of cardiac muscle. II. The isotonic contraction

The aim of this study was to evaluate utilization of chemical energy in relation to myocardial mechanics in variably afterloaded contractions of cardiac muscle by determining total energy utilization (∼P) in the absence of energy production. Right ventricular papillary muscles of cats were equilibrated at 26°C in a myograph in Krebs' solution while contracting isometrically (12/min). Following treatment with iodoacetate and N 2 to inhibit completely ATP production from glycolytic and aerobic metabolism, the muscles were stimulated to contract isotonically 20 to 75 times with varying after loads. They were then rapidly frozen in liquid N 2 -cooled isopentane, and concentrations of ATP, creatine phosphate, inorganic phosphate and creatine were measured.

The efficiency of energy utilization for the performance of internal work was 0.0067 µmoles ∼P/g-cm of work and for external work was 0.0031 µmoles ∼P/g-cm. In addition, resting energy utilization was 0.662 µmoles/g/min and activation energy was estimated to be 0.040 µmoles/g/contraction. These findings provide a demonstration of the Fenn effect in cardiac muscle and explain the well-known discrepancy in energy cost when cardiac work is increased by increasing pressure load as opposed to increasing volume load.

The Stimulus to Hypertrophy of the Myocardium

Opinion is still divided concerning the fundamental nature of the stimulus to myocardial hypertrophy in various clinical and experimental conditions. Some of the current hypotheses, such as nutritional deficiency of the myocardium, excessive hormonal secretion, increased external work, myocardial dilatation, and increased myocardial energy expenditure were discussed, and an attempt was made to identify a unifying concept. The hypothesis that chronic increase in myocardial metabolic rate per beat per unit mass of tissue constitutes the stimulus was favored most on the grounds that it was more inclusive than any other in explaining hypertrophy. This view was far from proved, however, and deserves serious attention and experimental verification by future investigators in this field.