Distribution of calcitonin gene-related peptide-like immunoreactivity in the bovine conduction system: correlation with substance P

The role of calcitonin gene-related peptide (CGRP) in the heart conduction system is unclear. In the present study, the distribution of CGRP in relation to that of substance P (SP) was examined in the bovine conduction system using immunohistochemical methods. Varicose nerve fibres showing CGRP-like immunoreactivity (LI) were frequently observed in the nerve fascicles, some of these fibres often also showing SP-LI. A few fibres exhibiting CGRP-LI were also observed in the intrinsic ganglia. In blood vessel walls and particularly in the conduction tissue, i.e., in association with the nodal cells and the Purkinje fibres, there were only a few varicose fibres showing both CGRP- and SP-LI, whilst there was a large number of varicose fibres showing only SP-LI. The observations show that the main morphologic correlate for the occurrence of CGRP-effects in the bovine conduction system is varicose nerve fibres located in the nerve fascicles. The observations also suggest that CGRP has effects at the intrinsic ganglia and that SP predominates over CGRP in the innervation of blood vessel walls and the conduction tissue.