AN INTRINSIC ADRENERGIC SYSTEM IN MAMMALIAN HEART

We have identified a previously undescribed intrinsic cardiac adrenerg ic (ICA) cell type in rodent and human heart. Northern and Western blo t analyses demonstrated that ICA cell isolates contain mRNA and protei n of enzymes involved in catecholamine biosynthesis. Radioenzymatic ca techolamine assays also revealed that the catecholamine profile of adu lt rat ICA cell isolates differed from that of sympathetic neurons. Un like sympathetic neuronal cells, isolated ICA cells have abundant clea r vesicles on electron microscopy. Endogenous norepinephrine and epine phrine constitutively released by ICA cells in vitro affect the sponta neous beating rate of neonatal rat cardiac myocytes in culture. Finall y, ICA cells could be identified in human fetal hearts at a developmen tal stage before sympathetic innervation of the heart has been documen ted to occur. These findings support the concept that these cells cons titute an ICA signaling system capable of participating in cardiac reg ulation that appears to be independent of sympathetic innervation.