Volume density and distribution of mitochondria in myocardial growth and hypertrophy

The volume density and distribution of mitochondria relative to capillaries have been measured using the technique of point counting in concentric rings, on normal and hypertrophied guinea pig myocardium. Right ventricular hypertrophy was produced by hypobaric hypoxia. In control guinea pigs right ventricular weight increased linearly with body weight. In control and hypoxic guinea pigs fiber cross sectional area (FCSA) and ventricular weight were related by a single straight line. Capillary density decreased hyperbolically with FCSA in both control and hypoxic animals. Capillary density was significantly higher (P less than 0.001) in the right ventricles of the hypoxic animals than in controls when the fibers were small but this difference disappeared as the fibers hypertrophied beyond FCSA values of 260 microns2. Volume density of mitochondria reached a peak (29-35%) close to the capillary but then decreased significantly in the spaces farthest from the capillary (mean 25.2%). There was no significant difference in peak mitochondrial volume density in controls versus hypoxic animals. There was thus an increase in the total volume of mitochondria in these myofibers that was in direct proportion to the increases in FCSA and heart volume. The highest volume density of mitochondria was found at a distance that is approximately 15% of the total distance over which O2 must diffuse in myocardial tissue and the peak volume density of mitochondria was thus a function of capillary density. The speculation is presented that the distribution of mitochondria may be related both to oxygen supply to the mitochondria and to the movement of high energy phosphate compounds out of mitochondria for use by myofibrils.