Cardiac function is determined by the dynamic interaction of various cell types and the extracellular matrix that composes the heart. This interaction varies with the stage of development and the degree and duration of mechanical, chemical, and electrical signals between the various cell types and the ECM. Understanding how these complex signals interact at the molecular, cellular, and organ levels is critical to understanding the function of the heart under a variety of physiological and pathophysiological conditions. Quantitative approaches, both in vivo and in vitro, are essential to understand the dynamic interaction of mechanical, chemical, and electrical stimuli that govern cardiac function. The fibroblast can thus be a friend in normal function or a foe in pathophysiological conditions.

The cardiac extracellular matrix, composed predominantly of collagenous fibers, forms a stress-tolerant network that facilitates the distribution of forces generated in the heart and provides for proper alignment of cardiac myocytes. Although considerable information exists regarding the morphological organization of the heart extracellular matrix, little is known about the regulation of the synthesis and accumulation of extracellular matrix components. A potentially significant factor in the cardiovascular system is mechanical stimulation including changes in physical tension and pressure. We recently have developed an in vitro model system to elucidate the effects of mechanical stretch on isolated populations of heart cells. In the present study, we have used biochemical and molecular biological techniques to analyze changes in collagen synthesis by cardiac fibroblasts in response to mechanical stretch. These studies show that the ratio of collagen type III to collagen type I increases in mechanically stretched cells. They also show that type III collagen mRNA levels are increased in response to cyclic mechanical stretch for durations as short as 12 hours. Type I collagen mRNA levels were not found to change under the stretch conditions used in this study. Our results emphasize the potential regulatory role of mechanical stimulation in the expression of specific genes in the heart and support previous studies indicating this to be an intriguing in vitro model of cardiac hypertrophy.

The interaction between components of the extracellular matrix and the cell surface of cardiac myocytes appears to be regulated in part by receptors belonging to the integrin superfamily. The expression of the integrins was investigated at different stages of development of the heart as well as during cardiac hypertrophy. The characterization of the membrane proteins showed that a beta 1-integrin and associated alpha-chains were responsible for the interaction with collagen, laminin, and fibronectin. Immunoprecipitation data indicated that the presence of specific alpha-chains varied with development. These data were correlated with the ability of the isolated myocytes to attach to specific components of the extracellular matrix. The expression of the alpha 1-chain was prominently associated with the recognition of interstitial collagens. The presence of the alpha 1-chain was also associated with stages when collagen synthesis was increased, especially during fetal and neonatal growth and cardiac hypertrophy. Immunohistochemical localization with the antiserum against beta 1-integrin demonstrated its specific localization near the Z lines of cardiac myocytes. The localization both in vitro and in vivo indicated that the beta 1-integrin may play a role in myofibrillogenesis during development. The present immunohistochemical, cell adhesion, and biochemical data clearly indicate that integrins play a major role in the regulation of the interaction between cardiac myocytes and the extracellular matrix during development and disease.

Abstract-Correlative data suggest that cardiac mast cells are a component of the inflammatory response that is important to hypertension-induced adverse myocardial remodeling. However, a causal relationship has not been established. We hypothesized that adverse myocardial remodeling would be inhibited by preventing the release of mast cell products that may interact with fibroblasts and other inflammatory cells. Eight-week-old male spontaneously hypertensive rats were treated for 12 weeks with the mast cell stabilizing compound nedocromil (30 mg/kg per day). Age-matched Wistar-Kyoto rats served as controls. Nedocromil prevented left ventricular fibrosis in the spontaneously hypertensive rat independent of hypertrophy and blood pressure, despite cardiac mast cell density being elevated. The mast cell protease tryptase was elevated in the spontaneously hypertensive rat myocardium and was normalized by nedocromil. Treatment of isolated adult spontaneously hypertensive rat cardiac fibroblasts with tryptase induced collagen synthesis and proliferation, suggesting this as a possible mechanism of mast cell-mediated fibrosis. In addition, nedocromil prevented macrophage infiltration into the ventricle. The inflammatory cytokines interferon-␥ and interleukin (IL)-4 were increased in the spontaneously hypertensive rat and normalized by nedocromil, whereas IL-6 and IL-10 were decreased in the spontaneously hypertensive rat, with nedocromil treatment normalizing IL-6 and increasing IL-10 above the control. These results demonstrate for the first time a causal relationship between mast cell activation and fibrosis in the hypertensive heart. Furthermore, these results identify several mechanisms, including tryptase, inflammatory cell recruitment, and cytokine regulation, by which mast cells may mediate hypertension-induced left ventricular fibrosis.

Matrix remodeling, critical to embryonic morphogenesis and wound healing, is dependent on the expression of matrix components, their receptors, and matrix proteases. The collagen gel assay has provided an effective model for the examination of the functional role(s) of each of these groups of molecules in matrix remodeling. Previous investigations have indicated that collagen gel contraction involves the beta 1 integrin family of matrix receptors and is stimulated by several growth factors, including TGF-beta, PDGF, and angiotensin II. In particular, collagen gel remodeling by human cells involves the alpha 2 beta 1 and, to a lesser extent, the alpha 1 beta 1 integrin complexes. The present studies were undertaken to determine the role of the alpha 1 integrin chain, a collagen/laminin receptor, in collagen gel contraction by rodent and avian fibroblasts. A high degree of correlation was found between the expression of the alpha 1 beta 1 integrin complex and the relative ability of cells to contract collagen gels. Further studies using antibodies and antisense oligonucleotides against the alpha 1 integrin indicated a significant role for this integrin chain in contraction of collagen gels by rat cardiac fibroblasts. In addition, antibodies to the alpha 1 integrin chain inhibited migration of these fibroblasts on a collagen substratum, suggesting that at least one role of this integrin is in migration of cells in collagen gels. These results indicate that the alpha 1 beta 1 integrin complex plays a significant role in cellular interactions with interstitial collagen that are involved in matrix remodeling such as is seen during morphogenesis and wound healing.