3‐Iodothyronamine is a novel suppressor of fetal sheep cardiomyocyte proliferation in vitro

OBJECTIVES: Fetal cardiomyocyte (CMC) maturation results in cells losing ability to divide. In late‐gestation sheep CMCs thyroid hormone (T 3 ) decreased proliferation and changed signaling protein levels in favor of cell cycle arrest (Chattergoon et al , 2007). 3‐Iodothyronamine (T 1 AM) is a naturally occurring relative of T 3 that does not bind classical thyroid receptors (TRs). We hypothesized that T 1 AM may depress CMC proliferation similar to T 3 . METHODS: Isolated right and left ventricular fetal sheep CMCs were cultured with T 1 AM (1nM, 10nM) and Bromodeoxyuridine (BrdU; 10μM) for 48 hours. Two ages were studied—100 days gestational age (dGA) and 135dGA—to determine developmental variations. BrdU uptake (index of proliferation) in fixed cells was analyzed. RESULTS: Basal BrdU uptake in serum free medium (SFM) was 10–12% in 100dGA CMCs versus 1–2% in 135dGA CMCs (p<0.01). BrdU uptake in response to 10% serum medium (SM) increased to 20% in 100dGA cells versus 8–10% in 135dGA CMCs (p<0.01). Similar to T 3 , T 1 AM does not affect cells exposed to SFM but decreases BrdU uptake by CMCs exposed to SM by 50% (p<0.05). CONCLUSIONS: These data suggest T1AM is as potent an inhibitor of fetal CMC proliferation as T 3 . Because T 1 AM does not activate TRs, non‐classical signaling of T 1 AM and T 3 may be important in CMC growth and maturation. Grant support: NICHD (P01HD34430) and Minority Supplement (3PO1 HD034430 ‐09S1).