Regulation of human cardiac myosin heavy chain genes by cyclical mechanical stretch in cultured cardiocytes

We examined the effect of cyclical mechanical stretch on the regulation of cardiac myosin heavy chain genes using an isolated neonatal rat cardiocyte culture system. Cultured cardiocytes grown on a flexible membrane were deformed by vacuum to 20% of maximum elongation, at 60 cycles/min in a serum-free medium. Cyclical stretch did not cause myocyte damage as assessed by supernatant LDH measurement and trypan blue exclusion test. The levels of myosin heavy chain (MyHC) mRNA increased as early as 1 h after stretch, reaching 12-fold over the control in 24 h, as shown by Northern blot analysis. However, the proximal 5'-flanking regions of the α- and β-MyHC gene which were linked to chloramphenicol acetyltransferase (CAT) reporter gene did not exhibit enhanced CAT activity following cyclical stretch. Deletion of the chimeric constructs to shorten the 5'-flanking regions of the MyHC genes generated by polymerase chain reaction amplification did not enhance the CAT activity under cyclical stretch. This finding suggests that the stretch-reponse element of the α- and β-MyHC gene promoter is probably not present in the proximal region of either the α- or β-MyHC genes.