Passive load increased protein synthesis by 30% independently of Angiotensin II, whereas load-induced c-fos expression was Angiotensin II-dependent in adult cardiocytes.
Does passive load require Angiotensin II to induce protein synthesis and gene expression in isolated cardiac myocytes?
In isolated cardiocytes, load-induced protein synthesis and Na+-Ca2+ exchanger expression are independent of Angiotensin II, whereas load-induced c-fos expression is Ang II dependent.
This study introduced an improved model of loaded adult cardiocytes to address a proposed requirement for angiotensin II (Ang II) in the transduction pathway between load on the cardiac myocyte and its early anabolic responses of gene expression and acceleration of protein synthesis. The isolated cardiocytes were subjected to passive load by step increments of stretch and responded with proportional acceleration of protein synthesis in both adult and neonatal cardiocytes; this response was unaltered by 1 mumol/L Sar1, Ile8Ang II, an antagonist peptide to Ang II. Ang II from 1 nmol/L to 10 mumol/L did not increase protein synthesis after 4 hours in adult cardiocytes nor at 100 nmol/L in neonatal cardiocytes. However, 100 nmol/L Ang II did increase 3Hphenylalanine incorporation into neonatal cardiocyte protein over a 24-hour period by 10%, whereas passive load increased 3Hphenylalanine incorporation into protein by 30%, which was not blocked by Sar1, Ile8Ang II. Thus, the anabolic effect of load does not require ANG II to increase either 4-hour protein synthesis in both adult and neonatal cardiocytes or 24-hour 3Hphenylalanine incorporation into protein in neonatal cardiocytes. The genetic response of the cardiocyte to load was examined by assessing c-fos and Na+-Ca2+ exchanger mRNA levels, because there are rapidly expressed at the onset of cardiac pressure overload. The c-fos mRNA was increased fourfold within 1 hour after 100 nmol/L Ang II treatment of either adult or neonatal cardiocytes. This c-fos induction was blocked by Sar1, Ile8Ang II. One hour after loading of adult cardiocytes, induction of c-fos expression was increased threefold; this was also blocked by Sar1, Ile8Ang II. Thus, load-induced c-fos expression was Ang II dependent in adult cardiocytes. In contrast, exchanger mRNA levels were increased threefold 1 hour after loading of adult cardiocytes, but this increased expression was not blocked by Sar1, Ile8Ang II. For additional comparison, c-fos expression was induced by Ang II and phorbol myristate acetate, which did not induce exchanger expression; conversely, exchanger expression was induced by veratridine, which did not increase c-fos expression. Thus, separate c-fos and exchanger expression pathways can be differentiated in adult cardiocytes. This study demonstrated that Ang II is not required for load to initiate the anabolic processes of accelerated protein synthesis or enhanced Na+-Ca2+ exchanger expression pathways can be differentiated in adult cardiocytes. This study demonstrated that Ang II is not required for load to initiate the anabolic processes of accelerated protein synthesis or enhanced Na+-Ca2+ exchanger gene expression in cardiocytes; however, load induced c-fos expression is Ang II dependent.
Kent et al. (1996) studied Cardiac myocytes (in vitro). Passive load and Angiotensin II vs. Unloaded/control was evaluated on Protein synthesis and gene expression (c-fos and Na+-Ca2+ exchanger mRNA levels). Passive load increased protein synthesis by 30% independently of Angiotensin II, whereas load-induced c-fos expression was Angiotensin II-dependent in adult cardiocytes.