Growth factors and hormones may play an autocrine/paracrine role in mechanical stress-induced cardiac hypertrophy. Using an in vitro model of mechanical stress, i.e. stretch of cardiomyocytes and cardiac fibroblasts, we tested the involvement of growth factors and hormones in this process. We found that conditioned medium (CM) derived from 4 h cyclicly (1 Hz) stretched cardiomyocytes increased the rate of protein synthesis in static cardiomyocytes by 8 +/- 3%. Moreover, CM derived from 2 h stretched fibroblasts increased the rate of protein synthesis in static fibroblasts as well as in static cardiomyocytes by 8 +/- 2 and 6 +/- 2%, respectively. Analysis of CM using size-exclusion HPLC showed that cardiomyocytes and fibroblasts released at least three factors with MW < or = 10 kD, their quantities being time-dependently increased by stretch. Subsequent analyses using immunoassays revealed that cardiomyocytes released atrial natriuretic peptide (ANP) and transforming growth factor-beta1 (TGFbeta1) being increased by 45 +/- 17 and 21 +/- 4% upon 4 h of stretch, respectively. Fibroblasts released TGFbeta1 and very low quantity of endothelin-1 (ET-1). The release of TGFbeta1 was significantly increased by 18 +/- 4% after 24 h of stretch in fibroblasts. Both cell types released no detectable amount of angiotensin II (Ang II). In conclusion, upon cyclic stretch cardiomyocytes and fibroblasts secrete growth factors and hormones which induce growth responses in cardiomyocytes and fibroblasts in an autocrine/paracrine way. TGFbeta secreted by cardiomyocytes and fibroblasts, and ANP secreted by cardiomyocytes are likely candidates. We found no evidence for the involvement of Ang II and ET-1 in autocrine/paracrine mechanisms between cardiac cell types.