TAC1 and hydrops fetalis: In a study designed to determine the mechanism for increased Na+ concentration in HF, Despa and coworkers found that increased [Na+]i in failing rabbit cardiomyocytes (that more closely resembles human cardiomyocytes than rodents) primarily is due to higher TTX-sensitive Na+ influx and not due to reduced NKA activity (Despa et al., 2002).