Neuropeptide Y-like immunoreactivity in relation to the distribution of sympathetic nerve fibers in the heart conduction system.

The distribution of nerve fibers showing neuropeptide Y-like immunoreactivity (NPY-LI) was compared with the distribution of the sympathetic nerve fibers in all parts of the conduction system and adjacent atrial and ventricular tissue of the bovine heart. Tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) served as markers for the sympathetic nerve fibers. NPY-LI was detected in most of the sympathetic nerve fibers that were present in nerve fascicles and that were associated with conduction cells and arterial walls in all regions examined. This phenomenon was more apparent when staining for NPY was compared with staining for DBH than with staining for TH. It was also found that some nerve varicosities exhibiting NPY-LI in association with arterial walls and local ganglia did not show DBH-LI. Furthermore, some of the ganglionic cells located in the regions of the conduction system showed NPY-LI but not DBH- or TH-LI. The observations are discussed in relation to what is known of the subcellular localization of NPY, TH and DBH. It is furthermore concluded that an NPY-like peptide is present in the sympathetic innervation of all parts of the conduction system and ordinary myocardial tissue, but that this peptide is also present in nerve fibers in the heart that do not represent sympathetic fibers. The observations raise important questions for further research aimed at determining the effects of NPY in relation to the function of the conduction system and in relation to the functions of not only sympathetic but also non-sympathetic nerve fibers in the heart.