Conditional differentiation of heart- and smooth muscle-derived cells transformed by a temperature-sensitive mutant of SV40 T antigen.

To create muscle cell lines that conditionally differentiate in vitro we introduced a temperature-sensitive SV40 T antigen by retroviral infection into rat aortic smooth muscle cells (SMCs) and neonatal heart-derived cells. After G418 selection cell lines isolated were characterized at permissive (33 degrees C) and non-permissive (39 degrees C) temperatures. [3H]Thymidine uptake showed tht progression through the cell cycle is greatly reduced at 39 degrees C. Cytoskeletal proteins, such as actins and vimentin did not change significantly after temperature shift, while the number of desmin-positive SMCs significantly increased when cells were switched to 39 degrees C. Heart-derived muscle cells showed sarcomeric myosin heavy chain reactivity only when grown at 39 degrees C. After thrombin stimulation intracellular calcium in both cell types increased severalfold in 39 degrees C-cells but not in 33 degrees C-cells. Whole cell patch-clamp recordings of SMCs and heart-derived cells revealed a strong increase in nicardipine-sensitive Ca2+ current when cells were switched to 39 degrees C. Nicardipine-insensitive Ca2+ current also increased in both cell types at the non-permissive temperature. Na+ current in SMCs was large at 33 degrees C and small or not detectable at 39 degrees C and absent in heart-derived cells. Using a cDNA probe specific for the alpha 1 subunit of the dihydropyridine-sensitive Ca2+ channel we demonstrate a temperature-sensitive expression of the dihydropyridine receptor mRNA in smooth muscle-derived cells but not in heart-derived H10 cells. Our results suggest that upon downregulation of SV40 T antigen these cells become quiescent and exhibit a more differentiated phenotype. These cell lines may provide a useful tool to investigate ion channel- and receptor signal transduction, as well as cell cycle control in smooth and possibly cardiac muscle cell differentiation.