Adrenomedullin-RAMP2 and -RAMP3 Systems Regulate Cardiac Homeostasis during Cardiovascular Stress

Adrenomedullin (AM) is a peptide hormone with multiple physiological functions, which are regulated by its receptor activity–modifying proteins, RAMP2 and RAMP3. We previously reported that AM or RAMP2 knockout (KO) (AM–/–, RAMP2–/–) is embryonically lethal in mice, whereas RAMP3–/– mice are apparently normal. AM, RAMP2, and RAMP3 are all highly expressed in the heart; however, their functions there are not fully understood. Here, we analyzed the pathophysiological functions of the AM-RAMP2 and AM-RAMP3 systems in hearts subjected to cardiovascular stress. Cardiomyocyte-specific RAMP2–/– (C-RAMP2–/–) and RAMP3–/– showed no apparent heart failure at base line. After 1 week of transverse aortic constriction (TAC), however, C-RAMP2–/– exhibited significant cardiac hypertrophy, decreased ejection fraction, and increased fibrosis compared with wild-type mice. Both dP/dtmax and dP/dtmin were significantly reduced in C-RAMP2–/–, indicating reduced ventricular contractility and relaxation. Exposing C-RAMP2–/– cardiomyocytes to isoproterenol enhanced their hypertrophy and oxidative stress compared with wild-type cells. C-RAMP2–/– cardiomyocytes also contained fewer viable mitochondria and showed reduced mitochondrial membrane potential and respiratory capacity. RAMP3–/– also showed reduced systolic function and enhanced fibrosis after TAC, but those only became apparent after 4 weeks. A reduction in cardiac lymphatic vessels was the characteristic feature in RAMP3–/–. These observations indicate the AM-RAMP2 system is necessary for early adaptation to cardiovascular stress through regulation of cardiac mitochondria. AM-RAMP3 is necessary for later adaptation through regulation of lymphatic vessels. The AM-RAMP2 and AM-RAMP3 systems thus play separate critical roles in the maintenance of cardiovascular homeostasis against cardiovascular stress.