The presence and molecular forms of cardiodilatin immunoreactivity in the human and rat right atrium

A sensitive and specific radioimmunoassay has been developed for cardiodilatin, the N-terminal peptide sequence of the atrial natriuretic peptide (ANP) prohormone. Cardiodilatin-immunoreactivity (-IR) concentrations in the human right atrial appendage were found to correlate with ANP-IR concentrations, determined by an established radioimmunoassay, (cardiodilatin-IR = 13.2 ±1.2 nmol/g, ANP-IR = 19.8 ± 2.0 nmol/g, r = 0.80, p < 0.001). Characterisation of the cardiodilatin-IR in the human and rat right atrium by gel permeation and fast protein liquid chromatography revealed only two cardiodilatin-IR molecular forms. The larger more hydrophobic form, the majority of the cardiodilatin-IR, contained in addition ANP-IR and therefore represents the prohormone. The smaller, less hydrophobic form, lacked ANP-IR and thus represents the cleaved N-terminal peptide sequence of the prohormone. These findings indicate that the prohormone is the major molecular form in the human and rat atrium. Furthermore, they demonstrate that a single large N-terminal peptide, cardiodilatin, derived from the prohormone, may exist as a distinct molecular form in the atrium of these species.