Monoamine oxidase activities of dissociated cell fractions from rat ventricular muscle

Cell fractions enriched in cardiac muscle cells (myocytes), on the one hand, and in non-myocytes, on the other, were prepared by dissociation of rat ventricular tissue with collagenase. Amine oxidase activities in homogenates of these cell fractions and also in homogenates of the corresponding undissociated ventricular tissue were compared. In addition, the activity of alkaline phosphatase (AP), an enzyme found predominantly associated in the heart with non-myocytes, particularly capillary endothelial cells, was also measured. No significant difference in the activity of MAO-A (assayed with 1 mM 5-hydroxytryptamine) was found between myocyte and non-myocyte fractions. In contrast, the activities of alkaline phosphatase (AP) and also the semicarbazide-sensitive amine oxidase (SSAO), assayed with 1 microM benzylamine (BZ), were both significantly higher in non-myocytes, by several-fold, than in myocyte fractions. Studies of the inhibition by clorgyline of 1 mM BZ metabolism confirmed that both MAO-A and MAO-B can also contribute to BZ oxidation in the rat heart. These experiments indicated different ratios of MAO-A: MAO-B in the various cell fractions. The ratios of the percentage contributions of MAO-A and MAO-B, respectively, to the total metabolism of 1 mM BZ were 78:20 (myocytes), 43:52 (non-myocytes) and 57:32 (undissociated tissue). These results suggest that MAO-B, in addition to AP and SSAO, may be associated preferentially with non-myocyte constituents of the rat heart. Although cardiac myocytes appear to contain predominantly MAO-A, this enzyme form is also localized, with high activity, to the non-myocyte fraction. However, since the non-myocyte fraction is heterogeneous in its cell content, containing vascular components of the coronary microcirculation, as well as other cells of connective tissue origin, the exact cellular localization of the enzyme activities within this fraction has not yet been defined.