A variety of agonists including phenylephrine (PE) induce hypertrophy in neonatal ventricular cardiomyocytes. Here we report that signals provided by extracellular matrix proteins (ECM) augment the PE-induced hypertrophic response of cardiomyocytes and provide evidence that ECM-dependent signaling is mediated in part by the protein tyrosine kinase, focal adhesion kinase (FAK). Addition of PE to cultured neonatal cardiomyocytes stimulated sarcomeric organization, increased cell size, and induced atrial natriuretic factor in cardiomyocytes plated on the ECM protein laminin or fibronectin. In contrast, cardiomyocytes plated on the non-adhesive substrate gelatin exhibited a reduced capacity to undergo these PE-stimulated hypertrophic changes. In cardiomyocytes cultured on ECM, PE stimulated a rapid increase in tyrosine phosphorylation of focal adhesion proteins including FAK, paxillin, and p130 Crk-associated substrate and subsequent formation of peripheral focal complexes. Inhibition of the PEinduced hypertrophic response by genistein and herbimycin-A indicated a requirement for protein tyrosine kinases in PE signaling. To determine whether activation of FAK is required for PE-induced hypertrophy, a dominant-interfering mutant form of FAK, termed FRNK (FAK-related non-kinase), was ectopically expressed in cardiomyocytes using a replication-defective adenovirus expression system. FRNK expression attenuated PE-stimulated hypertrophy as assessed by cell size, sarcomeric organization, and induction of atrial natriuretic factor. These data indicate that the signal transduction pathways leading to cardiomyocyte hypertrophy are strongly influenced by and/or dependent upon an integrin-mediated signaling process requiring FAK.Cardiomyocyte hypertrophy, characterized by increased volume and myofibrillar protein content, is important for the normal developmental growth of the heart. However, in the fully developed adult heart, pressure or volume overload, myocardial infarction, or hormonal imbalance can lead to chronic pathological hypertrophy characterized by induction of immediate early genes, re-activation of an embryonic gene program, and reorganization of myocyte cytoskeletal architecture (1).The ability of integrins, extracellular matrix receptors, to regulate cytoskeletal architecture has been well characterized, and integrin signaling has clearly been implicated in hormone and growth factor-induced alterations in gene transcription in variety of cell types (2). Integrins are a family of heterodimeric transmembrane receptors (composed of ␣ and  subunits) containing extracellular ligand binding domains that show binding specificity for extracellular matrix (ECM) 1 components and a short cytoplasmic domain that serves to couple integrins with the actin cytoskeleton (3). Several studies have shown that cardiac hypertrophy is accompanied by alterations in ECM components and cardiac integrin expression (4 -6). Specifically, enhanced expression of collagen III, fibronectin, osteopontin, and  1 , ␣ 3 , and ␣ 5 integrin subu...