Microtubule reorganization is related to rate of heart myocyte hypertrophy in rat.

Since stimulation of heart hypertrophy by pressure overload was previously shown to be accompanied by a densification of microtubule network in rat heart myocytes, we verified that similar process occurred during postnatal growth in euthyroid rats and in hypothyroid rats whose growth was stimulated by 4 micrograms/day L-thyroxine (T4). For this purpose, tubulin, the constituent protein of microtubules, was immunolabeled in myocytes isolated at various times after birth. Myocyte hypertrophy was evaluated by myocyte size, the number of nuclei per cell and isomyosin expression. In hypothyroid rats, the microtubule network, which was underdeveloped, was most dense around the nucleus. During the phase of fast myocyte hypertrophy observed in euthyroid rats during late postnatal development and in hypothyroid rats after T4 administration, transient microtubule densification occurred in a myocyte subpopulation in which size was mainly determined by the rate of myocyte hypertrophy. The densification process and its kinetics resembled those observed when heart hypertrophy was induced by pressure overload. It is concluded that in rat heart myocytes undergoing hypertrophy, microtubule densification might be related to fast sarcomerogenesis, whether the stimulus is mechanical (e.g., pressure overload) or hormonal (e.g., T4).