Muscle fiber shortening and force generation are transduced into the volume and pressure pump function required of the ventricle by the geometric organization of the myocytes and their mechanical coupling. This coupling is accomplished through a highly organized system of interstitial proteins. It is known that this extracellular matrix undergoes adaptive changes in response to alterations in the load on the ventricle, but the mechanisms of this growth are poorly understood. This proposal brings together a group of physiologists, biochemists and molecular biologists to address the role of extracellular proteins in left ventricular hypertrophy. The studies in this proposal are designed to explore the gene expression, maturation, and functional consequences of the adaptive growth of the extracellular matrix in models of acute and gradual onset pressure overload hypertrophy. Four specific questions will be addressed. 1) How do fibronectin and collagen gene expression change in response to acute pressure overload? Northern blot and S1 nuclease protection assays will be used to measure mRNA levels following acute aortic banding in a rat model. In situ hybridization will be utilized to determine the cellular source of these mRNAs. The level of transcription will be evaluated with nuclear runoff assays. Finally, net protein synthesis will be surveyed in isolated hearts perfused with radiolabeled amino acids. 2) In order to identify specific mechanical consequences of pressure overload that may function as stimuli to growth of the extracellular matrix, we will determine which alterations in myocardial finite deformation or estimated wall stress correlate with the changes in mRNA levels for fibronectin and collagen. For these experiments we will apply both methods described above and the techniques of three dimensional strain analysis developed in our laboratory in an a large animal (pig) model. 3) We will examine how changes in the amount, concentration, and extent of crosslinking of collagen alter the residual strain, the diastolic pressure-strain relationship, and the systolic fiber and cross-fiber strains of the myocardium. 4) Finally, we will examine how the gene expression and functional consequences of gradual onset pressure overload differ from those of acute load imposition, using aortic banding in weaning pigs. The results of these studies will increase our understanding of the factors altering cardiac performance in hypertrophy.
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