The distribution of nerve fibers showing substance P-like immunoreactivity in the conduction system of the bovine heart: dense innervation in the atrioventricular bundle.

There is limited information on the distribution of nerve fibers containing substance P (SP) in the heart conduction system. Therefore, in the present study, the various parts of the conduction system of the bovine heart were examined by the use of an SP-antiserum and immunohistochemistry. Nerve fibers showing SP-like immunoreactivity (SP-LI) occurred in the proximities of conduction cells in all parts of the conduction system, but were present in greatly larger numbers in the AV bundle than in the other parts. The nerve fibers showed a predilection for certain regions of the bundles of conduction cells (Purkinje fiber bundles) in the AV bundle and the bundle branches and their ramifications. Nerve fibers showing SP-LI also occurred in the walls of the arteries and in association with some the ganglionic cells located in the regions of the conduction system. None of the ganglionic cells exhibited SP-LI. The observations are discussed in relation to what is known of the function of SP in the heart and of the distribution of sympathetic and parasympathetic nerve fibers in the conduction system. As SP is regarded as a marker of afferent fibers the observations support the view that afferent nerve fibers are present throughout the conduction system. It is likely that the existence of a significant SP-innervation in the conduction system is of importance for the function of this part of the heart.