MECHANICAL STRETCH INDUCES ENHANCED EXPRESSION OF ANGIOTENSIN-II RECEPTOR SUBTYPES IN NEONATAL RAT CARDIAC MYOCYTES

Mechanical stress plays a pivotal role in the development of cardiac h ypertrophy during hemodynamic overload, and angiotensin (Ang) II secre ted from stretched myocytes plays an important role in mechanical stre tch-induced hypertrophy. In the present study, we examined stretch-ind uced expression of Ang II receptors in an in vitro stretch model using 1-day-old rat myocytes. Both Ang II type 1 receptor (ATI-R) and type 2 receptor (AT2-R) mRNA levels were upregulated by myocyte stretching with similar time courses: significant increases were evident 6 hours after stretching, maximal levels (2.8- and 3.3-fold, respectively) wer e observed at 12 hours, and these were sustained for up to 18 hours. A ng II receptor expression in fibroblast-rich cultures was not affected by stretching. Conditioned medium in which myocytes were stretched fo r 12 hours significantly downregulated AT1-R and AT2-R mRNA levels in recipient myocytes, and this effect was almost completely blocked by A T1-R antagonists but not AT2-R antagonists. Stretch-induced expression of AT1-R and AT2-R mRNAs was further increased by 27% and 31%, respec tively, after pretreatment with AT1-R antagonists, suggesting that Ang II secreted from stretched myocytes downregulates both AT1-R and AT2- R. Western blot and binding assays showed that the number of AT1-Rs an d AT2-Rs increased by 2.4- and 2.6-fold, respectively, without affecti ng receptor affinities. Inositol phosphate response to 0.5 mu mol/L An g II was enhanced 2.1-fold in stretched myocytes. Nuclear runoff assay s and treatment with actinomycin D revealed that stretch-induced upreg ulation of AT1-R was mainly due to increased transcription, whereas th at of AT2-R resulted from a stabilizing effect on AT2-R mRNA metabolis m. Stretch-induced changes in levels of Ang II receptors were inhibite d by genistein but not by H-7, staurosporin, and protein kinase C depl etion or by BAPTA-AM. Exposure to cycloheximide did not affect stretch -induced changes. These findings indicate that nonsecretory pathways a ctivated by myocyte stretching upregulate the expression of Ang II rec eptor subtypes transcriptionally and posttranscriptionally through mec hanisms involving stretch-activated tyrosine kinases independently of de novo protein synthesis and that the AT1-R-mediated action of Ang II is functionally enhanced in stretched cardiac myocytes.