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W e have identified a previously undescribed intrinsic cardiac adrenergic (ICA) cell type in rodent and human heart. Northern and Western blot analyses demonstrated that ICA cell isolates contain mRNA and protein of enzymes involved in catecholamine biosynthesis. Radioenzymatic catecholamine assays also revealed that the catecholamine profile of adult rat ICA cell isolates differed from that of sympathetic neurons. Unlike sympathetic neuronal cells, isolated ICA cells have abundant clear vesicles on electron microscopy. Endogenous norepinephrine and epinephrine constitutively released by ICA cells in vitro affect the spontaneous beating rate of neonatal rat cardiac myocytes in culture. Finally, ICA cells could be identified in human fetal hearts at a developmental stage before sympathetic innervation of the heart has been documented to occur. These findings support the concept that these cells constitute an ICA signaling system capable of participating in cardiac regulation that appears to be independent of sympathetic innervation.