The expression of the mouse mammary tumor virus (MMTV) was studied in undifferentiated embryonal carcinoma cells (EC), in partially differentiated myoblasts derived from EC cells and in fully differentiated myotubes. Whereas no MMTV RNA could be detected in EC cells, relatively large amounts of tumor virus RNA were found in myoblasts. The MMTV RNA level was reduced in myotubes derived from myoblastic differenciation. DNaseI sensibility of MMTV DNA in chromatin allows us to show conformational changes in EC cell myoblasts and GR mammary cell chromatin which could be associated with MMTV RNA synthesis. The glucocorticoid hormones dexamethasone which stimulates the MMTV RNA synthesis in differentiated mammary cells did not affect this synthesis in myoblasts. By contrast, the reduced synthesis of MMTV RNA in myotubes was overcome with dexamethasone. The differential MMTV RNA synthesis is not related to different amount of receptor and its hormonal regulation is not associated with a conformational change of chromatin.

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PubMedDate not supplied

Immunoelectron microscopic visualization of the gap junction protein connexin 40 in the mammalian heart.

In mammalian myocardium, myocytes are electrically coupled by gap junctions, which are collections of membrane channels spanning the intercellular gap. Each channel is composed of twelve protein molecules--six in each membrane--enclosing a pore of 2 nm diameter. In addition to the main cardiac gap junction protein connexin 43 (cx43), two other constituents, connexin 40 (cx40) and connexin 45 (cx45), recently have been identified. In this study, the ultrastructural distribution of cx40 has been traced in several mammalian cardiac tissues, using a specific antibody and immunogold labeling. In rabbit, cx40 was found in gap junctions between myocytes of the right atrium as well as in the false tendons of the left ventricle. Labeling within the free ventricular wall could be obtained only with higher primary antibody concentrations. In rat atrial and ventricular working myocardium a similar distribution of label was observed. Double labeling of cx40 and cx43 in rat atrium, with 5 nm and 15 nm gold particles respectively, demonstrates limited patches of cx40 in gap junctions otherwise homogeneously labeled for cx43. Since cx40 has a much higher single channel conductance than cx43, topographical differences in their relative distribution might contribute to regional differences in conduction velocity of the cardiac impulse.

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