Circulating levels of Metrnβ are regulated in humans. (a) Plasma levels of Metrnβ in humans <60 yr old (mean age, 45 ± 1.35; n = 33) and >60 yr old (mean age, 80 ± 1.48; n = 23; P < 0.0001). (b) Plasma levels of Metrnβ in control (mean age, 60 ± 2.63; n = 56) and HF (mean age, 67 ± 1.7; n = 49) patients (P = 0.0093). Results are expressed as mean ± SEM. Data were analyzed by nonparametric Mann-Whitney test; **, P <0.001; ***, P < 0.0001 versus control patients. (c) Schematic representation of the transcriptional regulation and the endocrine and autocrine/paracrine actions of Metrnβ in the myocardium.