An ultrastructural investigation into the size dependency of contractility of isolated cardiac muscle.

The maximal stress (force per cross-sectional area), maximal rate of stress development, and cross-sectional area of 30 rabbit right ventricular papillary muscles were measured. Both mechanical parameters declined with increasing muscle size (the stress-area relation). Three large and three small muscles were fixed for electronmicroscopy. Stereological analyses of these six muscles were undertaken to test the hypothesis that the stress-area relation is caused by differences in the relative proportions of mitochondria and contractile matrix between muscles of large and small cross-sectional area. No difference in the proportion of these two cytoplasmic components was found. Other possible explanations underlying the stress-area relation are discussed.