Manganese superoxide dismutase (Mn-SOD) is induced in ischemic hearts 24 h after ischemic preconditioning, when tolerance to ischemia is acquired. We examined the relationship between Mn-SOD induction and the protective effect of preconditioning using cultured rat cardiac myocytes. Exposure of cardiac myocytes to brief hypoxia (1 h) decreased creatine kinase release induced by sustained hypoxia (3 h) that follows when the sustained hypoxia was applied 24 h after hypoxic preconditioning (57% of that in cells without preconditioning). The activity and content of Mn-SOD in cardiac myocytes were increased 24 h after hypoxic preconditioning (activity, 170%; content, 139% compared with cells without preconditioning) coincidentally with the acquisition of tolerance to hypoxia. Mn-SOD mRNA was also increased 20-40 min after preconditioning. Antisense oligodeoxyribonucleotides corresponding to the initiation site of Mn-SOD translation inhibited the increases in the Mn-SOD content and activity and abolished the expected decrease in creatine kinase release induced by sustained hypoxia after 24 h of hypoxic preconditioning. Sense oligodeoxyribonucleotides did not abolish either Mn-SOD induction or tolerance to hypoxia. These results suggest that the induction of Mn-SOD in myocytes by preconditioning plays a pivotal role in the acquisition of tolerance to ischemia at a later phase (24 h) of ischemic preconditioning. (J. Clin. Invest.

Although reciprocal intercellular signaling may occur between endocardial or microvascular endothelium and cardiac myocytes, suitable in vitro models have not been well characterized. In this report, we describe the isolation and primary culture of cardiac microvascular endothelial cells (CMEC) from both adult rat and human ventricular tissue. Differential uptake of fluorescently labeled acetylated low-density lipoprotein (Ac-LDL) indicated that primary isolates of rat CMEC were quite homogeneous, unlike primary isolates of human ventricular tissue, which required cell sorting based on Ac-LDL uptake to create endothelial cell-enriched primary cultures. The endothelial phenotype of both primary isolates and postsort subcultured CMEC and their microvascular origin were determined by characteristic histochemical staining for a number of endothelial cell-specific markers, by the absence of cells with fibroblast or pericyte-specific cell surface antigens, and by rapid tube formation on purified basement membrane preparations. Importantly, [3H]-thymidine uptake was increased 2.3-fold in subconfluent rat microvascular endothelial cells 3 days after coculture with adult rat ventricular myocytes because of release of an endothelial cell mitogen(s) into the extracellular matrix, resulting in a 68% increase in cell number compared with CMEC in monoculture. Thus biologically relevant cell-to-cell interactions can be modeled with this in vitro system.

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