Morphometric changes in rabbit ventricular myocardium produced by long-term beta-adrenoceptor blockade.

In rabbits long-term beta-adrenoceptor blockade did not cause any change in the morphology of individual organelles in the ventricular myocardium. There were, however, striking and significant changes in their relative distribution: (1) the relative volume of the mitochondria was reduced and was replaced by an equivalent relative increase in sarcoplasm; and (2) the relative volume of vascular elements was increased and the interfibrillar volume was decreased, so that the path from capillary to cell wall was reduced. The relative volumes of myofibrils and nuclei were unchanged. The doses used were well within the range used clinically, so that the results could be relevant to the efficacy of beta-blockers in angina pectoris, because the relative increase in capillaries and shorter diffusion pathway for oxygen could raise oxygen availability. The reduced rate of growth of the heart in young rabbits during beta-blockade may also be relevant to the treatment of infants with hypertrophic obstructive cardiomyopathy by beta-blockers.