Fig. 3. RNase protection assay of ET_A and ET_B mRNAs expressed in embryonic chick hearts at various stages during development. (A) Autoradiogram of ³²P-labelled probes protected by ET_A, VMHC1 and GAPDH mRNAs from whole heart (wh), atrium (at), right ventricle (rv) and left ventricle (lv). VMHC1 and GAPDH served as internal controls to normalize the data of ET receptor mRNAs. Developmental stages are indicated by days of incubation. (B) Quantitation of ET_A mRNA levels expressed in ventricular and atrial chambers during heart development. The data from three different experiments were normalized and averaged. (C) Autoradiograms of RNase protection assay for ET_A and ET_B mRNAs. (D) Quantitation of changes in expression levels of ET_A and ET_B mRNAs in atrial and ventricular chambers during Purkinje fiber differentiation. The data are presented as the ratio of ET_B mRNA to ET_A mRNA. (E) Left ventricle-dominant expression of ET_B in embryonic hearts. Bars indicate standard deviation.