Induction of ventricular morphogenesis and atrial natriuretic factor synthesis by thyroid hormone

Thyroid hormones have been shown to increase atrial natriuretic peptide (ANP) synthesis in atria both in vitro and in vivo. In this study the effects of triiodothyronine (T3) administration on primary cultures of 6-day-old atrial and ventricular myocytes were studied. Levels of immunoreactive (ir-) ANP were determined by radioimmunoassay over 9 days of culture in cells cultured in the presence and absence of 10(-8) M T3. The proportion of immunostained nuclei was also determined and changes in cell characteristics noted over this time. Over 9 days T3 had no effect on the proportion of atrial cells immunostained for ANP, while cell and medium content of ir-ANP in treated wells doubled that of the untreated. In treated ventricular myocytes, cell and medium ir-ANP similarly increased; in addition, the proportion of immunostained cells increased by 1-2 orders of magnitude. The formation of dendritic processes was enhanced in these cultures, reflected in increased rates of spontaneous contractility. Thus it would appear that the 6-day-old myocardium exhibits profound differences in response to T3 administration, in that ventricular cells dedifferentiate and undergo morphological changes which are not seen in atriocytes.