As summarized in Figure 6C, purified IgGs (0.1 μmol/L) from β3-AR Abs positive HF sera manifested agonist-like effects upon cardiomyocyte contraction similar to β3-AR agonist BRL37344 (0.1 μmol/L), as evidenced by a decrease in PTA (peak twitch amplitude, % cell length). This evidence concerns the gene ADRB3 and hydrops fetalis.