Extraneuronal monoamine oxidase in rat heart: biochemical characterization and electron microscopic localization

Monoamine oxidase (MAO) increases in an age-weight relationship in the hearts of male rats. Accumulation of MAO is not related to the activities of such mitochondrial enzymes as succinic dehydrogenase or cytochrome oxidase which do not change with age. Our previous experiments, utilizing serotonin as a substrate, have determined that cardiac MAO in the young rat does not change after chemical sympathetectomy with 6-hydroxydopamine. In this study, rats of different ages were treated with 6-hydroxy-dopamine to investigate the neuronal vs. non-neuronal distribution of MAO in the heart. After sympathetectomy, various parts of the hearts and fractions of the hearts isolated by differential centrifugation were tested for changes in MAO activity with two different substrates (kynuramine and 14C-tryptamine). It was not possible to detect any changes in MAO activity in any parts or subcellular fractions of the heart as a result of denervation. Studies with clorgyline, the MAO inhibitor, in control and sympathetecomized animals revealed that rat cardiac MAO is mostly of the type A enzyme, which was originally thought to be neuronal. A histochemical technique for the electron microscopic demonstration of MAO with osmiophilic thiocarbamyl nitro blue tetrazolium was used in the rat heart in order to determine the ultrastructural location of the enzyme. Histochemical localization of MAO with the electron microscope using tryptamine as the substrate indicates that a substantial portion of rat cardiac MAO is located near the outer membranes of mitochondria within myocardial cells. This histochemical technique provides no evidence to support differential centrifugation data which suggests the presence of a sarcoplasmic reticular (microsomal) MAO in rat heart.