Distribution of enkephalin-like immunoreactivity in the feline heart; its interrelation with substance P- and vasoactive intestinal polypeptide-like immunoreactive nerve fibers.

Distribution of enkephalin (Enk)-like immunoreactivity in the feline heart was investigated by indirect immunofluorescence method. Enk-like immunoreactive (Enk-IR) nerve fibers and cells predominated in the atria but were rare in the ventricles. Enk-IR cells were mainly distributed near sinoatrial (SA) node and between aorta and pulmonary artery. These cells were usually grouped in clusters near blood vessels or cardiac ganglia. Some Enk-IR cells appeared to extend their processes to other Enk-IR cells, cardiac ganglia, blood vessels and rarely cardiac muscles in the atria. Furthermore, substance P (SP)- and vasoactive intestinal polypeptide (VIP)-IR nerve fibers were closely associated with some Enk-IR cells. Moreover, the distribution and morphological profiles of Enk-IR cells were very similar to those of catecholamine (CA)-containing cells.