Marked sympathetic innervation in the regions of the bundle branches shown by catecholamine histofluorescence

The distribution of sympathetic nerve fibers in the regions of the bundle branches of bovine and rat hearts was examined by the glyoxylic acid-induced method for histofluorescence demonstration of catecholamines and by acetylcholinesterase (AChE) histochemistry. The studies on the bovine heart were concentrated on the nerve fascicles, the arteries and the ganglionic cells, and showed that (1) the AChE-positive nerve fascicles that occur just outside the bundle branches show a positive catecholamine-fluorescence reaction to varying degrees, the paraarterial nerve fascicles showing a uniform reaction; (2) the AChE-positive nerve fascicles within the bundle branches contain a few, mainly varicose, sympathetic nerve fibers; (3) extensive plexuses of sympathetic nerve fibers supply the arterial branches; and (4) sympathetic nerve fibers occur close to some of the ganglionic cells. The pattern of distribution of sympathetic nerve fibers in the region of the SA node was found to be essentially the same. The studies on the rat heart showed that (1) the septal arteries that occur in the proximity of the bundle branches are accompanied by the sympathetic component of innervation; and (2) there is a substantial number of varicose sympathetic nerve fibers within the bundle branches. These observations show that there is a pronounced sympathetic innervation in bundle branch regions and suggest that the paraarterial route is the most important for sympathetic nerve fibers to reach these regions.