Relaxin, a new drug for heart failure therapy, exerts its cardiac actions through relaxin family peptide receptor 1 (RXFP1). Factors regulating RXFP1 expression remain unknown. We have investigated effects of activation of adrenoceptors (AR), an important modulator in the development and prognosis of heart failure, on expression of RXFP1 in rat cardiomyocytes and mouse left ventricles (LV). Methods: Expression of RXFP1 at mRNA (real-time PCR) and protein levels (immunoblotting) was measured in cardiomyocytes treated with a- and ?-AR agonists or antagonists. RXFP1 expression was also determined in the LV of transgenic mouse strains with cardiac-restricted overexpression of a1A-, a1B- or ?2-AR. Specific inhibitors were used to explore signal pathways involved in a1-AR mediated regulation of RXFP1 in cardiomyocytes. Results: In cultured cardiomyocytes, a1-AR stimulation resulted in 2-3 fold increase in RXFP1 mRNA (P?