Distribution and neuropeptide content of nitric oxide synthase-containing nerve fibers in arteries and conduction system of the rat heart.

The rat heart receives its blood supply not only from the coronary arteries but also from the accessory coronary arteries that supply mainly the atria. The distribution and chemical nature of nitric oxide synthase-containing nerve fibers in the conduction system as well as the coronary and accessory coronary arteries were investigated using immunohistochemistry and NADPH diaphorase (NADPH-d) histochemistry. A few NADPH-d-positive nerve fibers were observed mainly around the main trunk of the coronary arteries, while NADPH-d-positive fibers were found along the entire course of the accessory coronary arteries from their main branches to the arteriolar level. A double-staining method demonstrated that NADPH-d-positive fibers innervating both the coronary and accessory coronary arteries contained vasoactive intestinal polypeptide or neuropeptide Y. NADPH-d-positive fibers were relatively abundant in the sinus node and penetrating bundle but were very sparse in the atrioventricular node and right bundle branch. No NADPH-d-positive fibers were detected in the left bundle branch. Some of the NADPH-d-positive fibers innervating the penetrating bundle exhibited distinct immunoreactivity to calcitonin gene-related peptide. These results suggest that nitric oxide may play a role as a neurotransmitter and/or neuromodulator in the neural control of the cardiac blood flow and impulse conduction.