A quantitative study of nerve distribution in the conduction system of the guinea pig heart

Quantitative measurements of relative nerve density were achieved using computer-assisted image analysis of immunohistochemically and histochemically defined nerves in the conduction system of the guinea pig heart. All regions of the conduction system possessed a similar density of nerve fibres and fascicles displaying immunoreactivity for the general neuronal marker protein gene product 9.5 (PGP 9.5), and this was 3 to 4-fold higher than in the adjacent myocardium. Acetylcholinesterase (AChE) positive and tyrosine hydroxylase (TH)-immunoreactive nerves were the main subtypes identified in the sinus and atrioventricular nodes, representing 40-45% of the stained area occupied by PGP 9.5-immunoreactive nerves. AChE-positive nerves were the dominant subtype identified in the left and right bundle branches, but were equal in proportion to TH-immunoreactive nerves in the penetrating bundle. Neuropeptide Y-immunoreactive nerves represented the main peptide-containing subpopulation in the nodal tissues, displaying a similar pattern of distribution and relative density to those nerves demonstrating TH immunoreactivity. Substance P and calcitonin gene-related polypeptide immunoreactive nerves were present throughout the conduction system and represented the main peptide-containing subpopulation in the ventricular conduction tissues. Nerve fibres showing immunoreactivity for either somatostatin or vasoactive intestinal polypeptide exhibited distinct patterns of distribution and comprised a relatively minor component of the innervation. The innervation of the guinea pig conduction tissues thus exhibits a uniform distribution and it comprises putative parasympathetic nerves and intrinsic neurons (AChE positive), sympathetic efferent nerves (NPY and TH-immunoreactive nerves) as well as other peptide-containing nerves, some of which (substance P and calcitonin gene-related polypeptide) are considered to represent afferent nerves. The distribution and density of nerve subpopulations in the guinea pig conduction system differ from those observed in the human conduction system, which suggests that the guinea pig may be an inappropriate model for comparative functional studies.