Type 2X-myosin heavy chain is coded by a muscle fiber type-specific and developmentally regulated gene.

Expression of cardiac troponin I (TnIcardiac) and slow skeletal troponin I (TnIslow) genes was analyzed at the mRNA and protein level in the developing rat heart. TnIslow mRNA was detectable by in situ hybridization in the embryonic cardiac tube as early as the 13-somite stage (Embryonic Day 10). In contrast, TnIcardiac transcripts were first detected in the primordial atrium and ventricle of 11-day-old embryos, but were absent in the outflow tract region. TnIslow mRNA levels decreased after birth in atria and later in ventricles but persisted even in adult life in myocytes of the conduction system. TnIslow protein was detected by specific antibodies in atrial myocytes beginning from Embryonic Day 11; in contrast, ventricular myocytes were unreactive until Embryonic Day 18. Western blot analysis of 16-day-old fetal hearts confirmed the expression of TnIcardiac in atrial but not in ventricular myocardium. Slot blot analysis showed that at this stage equivalent amounts of TnIslow and TnIcardiac mRNAs are expressed in atria and ventricles. Similar differences in the expression of TnIslow and TnIcardiac mRNAs and proteins were observed in cultures of embryonic atrial and ventricular myocytes. The results suggest serial rather than simultaneous activation of TnIslow and TnIcardiac genes and they show that different regions of the developing heart differ in their patterns of TnIcardiac expression due to the operation of distinct mechanisms that separately affect the accumulation of TnIcardiac mRNA and protein.