Toshiyuki Maki

Effect of neutral endopeptidase inhibitor on endogenous atrial natriuretic peptide as a paracrine factor in cultured cardiac fibroblasts

Cardiac remodelling is a fundamental response to hypertension, myocardial infarction and chronic heart failure, and involves cardiac fibroblast proliferation and production of extracellular matrix components such as collagen. The present study was performed to examine the role of endogenous atrial natriuretic peptide (ANP) as a possible paracrine factor for cardiac fibroblasts, and to examine the effects of three neutral endopeptidase (NEP) inhibitors, thiorphan, phosphoramidon and ONO‐BB‐039‐02 (ONO‐BB) on endogenous ANP‐induced changes in collagen synthesis by cultured neonatal rat cardiac fibroblasts. Each NEP inhibitor singly had no significant effect on collagen synthesis by cardiac fibroblasts, except for maximum concentration (10−3 M) of thiorphan. Exogenous ANP inhibited collagen synthesis in a concentration‐dependent manner (10−8–10−6 M). Thiorphan (10−4 and 10−3 M) and phosphoramidon (10−5 and 10−4 M) enhanced the ANP (10−7 M)‐induced decrease in collagen synthesis. ONO‐BB (10−5 and 10−4 M) slightly enhanced the ANP‐induced decrease in collagen synthesis. Myocyte‐conditioned medium (MC‐CM), as well as exogenous ANP, inhibited collagen synthesis dose‐dependently. The decrease in collagen synthesis at 100% MC‐CM was augmented by thiorphan (10−3 M), phosphoramidon (10−4 M) and ONO‐BB (10−4 M). HS‐142‐1, a natriuretic peptide receptor antagonist, significantly reduced the MC‐CM plus thiorphan‐ and MC‐CM plus ONO‐BB‐induced decrease in collagen synthesis, by 92 and 62%, respectively and showed a tendency to attenuate the MC‐CM plus phosphoramidon‐induced decrease in collagen synthesis by 40%. Our observations suggested that endogenous ANP released from cardiomyocytes inhibited collagen synthesis as a paracrine factor and that NEP inhibitors enhanced the activity of this peptide in cardiac fibroblasts.

Production and autocrine/paracrine effects of endogenous insulin-like growth factor-1 in rat cardiac fibroblasts

Insulin-like growth factor (IGF)-1 appears to play an important role in cardiac hypertrophy or remodeling. However, the role of endogenous IGF-1 in the growth of cardiac myocytes and fibroblasts remains unclear. This study investigated the major site of the production of cardiac IGF-1 and the local effects of endogenous IGF-1 secreted from cardiac cells. A significant expression of IGF-1 mRNA was found in cultured neonatal and adult rat cardiac fibroblasts, but not in myocytes. In addition, an in vivo examination by in situ hybridization histochemical analyses demonstrated the IGF-1 transcripts in the interstitial fibrotic tissue of the ventricle. Time-dependent secretion of IGF-1 protein was also observed in cultured cardiac fibroblasts. An antibody against IGF-1 decreased collagen synthesis in cardiac fibroblasts under basal conditions. Fibroblast-conditioned medium, as well as exogenous IGF-1, increased protein synthesis in cardiac myocytes, and this increase was inhibited by antibodies against IGF-1 and IGF-1 receptor, IGF binding protein-3, and IGF-1 receptor antagonist. These observations suggest that IGF-1 is produced and released mainly from cardiac fibroblasts and that endogenous IGF-1 promotes collagen synthesis by cardiac fibroblasts and hypertrophy of myocytes as an autocrine and a paracrine factor. Cardiac IGF-1 may function as an endogenous modulator of cardiac hypertrophy or remodeling.