Evidence for a novel source of relaxin: atrial cardiocytes.

Antibody-directed, complement-induced erythrocyte lysis (reverse hemolytic plaque assay) around atrial cardiocytes was used to determine whether this cell type possesses the capacity to secrete the insulin-like hormone relaxin. After 2h of incubation, 33 +/- 4% (n = 3) of cardiocytes derived from the atria of neonatal rats secreted detectable amounts of immunoreactive relaxin (i.e. formed plaques) when cultured in monolayers. Increased culture time of cardiocytes failed to increase the fraction of cardiocytes that secreted relaxin. The cumulative amount of relaxin secreted after 3h of incubation (plaque area) was 31% greater (P < 0.05) than the amount of hormone present after 1h of incubation, evidence of sustained peptide secretion by cultured cardiocytes. These data suggest that the source of the endogenous ligand for the specific and high-affinity relaxin receptors located in rat atria is the atrial cardiocyte itself. Therefore, relaxin may act via autocrine and/or paracrine routes to regulate cardiovascular structure and/or function.