The majority of nitric oxide synthase in pig heart is vascular and not neural

As a physiological mediator of smooth muscle relaxation and possibly a non-adrenergic non-cholinergic (NANC) neurotransmitter, nitric oxide plays a role in regulating coronary artery blood flow and modulating myocardial contractility. Although recent attention has been directed toward neural tissue, we investigated the possibility that vessels, as well as nerves, may be a source of nitric oxide in the heart.

The NADPH-diaphorase method was used to localise the synthesis enzyme, nitric oxide synthase (NOS), in the pig heart.

All regions showed a similar staining pattern. Muscle fibres had virtually no NOS activity. The endocardium showed reaction product in a lattice configuration without much cytoplasmic staining, suggesting that endothelial cell membranes are the primary site of NOS activity. Every vessel contained reaction product in intima but none in the media or adventitia. Muscular arteries had more NOS activity than veins. Lighter staining capillaries coursed along each muscle fibre. Only a few scattered nerve processes and rare neuronal cell bodies with NOS activity were present in the heart; there was no particular spatial relationship between neural tissue and vessels.

(1) the majority of NOS activity in the pig heart is in vascular endothelium, consistent with the concept of nitric oxide as a regulator of coronary blood flow; less is present in the endocardium; (2) paucity of nerves with NOS activity probably indicates that this particular type of NANC neural tissue does not affect coronary blood flow directly; and (3) although muscle fibres have no discernible NOS activity, the rich vascular supply in close proximity may subserve some myocardial function of nitric oxide.