Physiological expression of neural marker proteins in the heart of young rats

The alpha-subunit of a GTP-binding protein Go (Go alpha), is a new marker protein of neurons and neuroendocrine cells. In our previous report, this protein was shown to be also distributed in the cardiac muscle of young rats, although the cardiac muscle is traditionally thought to be a non-neuronal tissue. To evaluate this unusual finding, expression of other neural marker proteins in the heart was examined. Immunohistochemical studies using antibodies against gamma-enolase, neurofilament and Go alpha protein revealed localization of these neural markers only in neurons and neuroendocrine cells in adult rats. In young rats, however, these neural markers were localized not only in these cells but also in the cardiac muscle. An enzyme immunoassay study of enolase isozymes showed that the heart of young rats was rich in neuron-specific gamma-enolase rather than in muscle-specific beta-enolase, while the heart of adult rats was deprived of the former and rich in the latter. These results suggest that the heart of young rats should be understood as a neuroendocrine tissue as well as a muscular tissue.