Diastolic dysfunction contributes significantly to age-associated congestive heart failure, which is a significant problem among the elderly. This project proposes fundamental research to answer key questions about passive stiffness in aged myocardium in which aspects of diastolic dysfunction are just beginning to emerge. The mechanisms most detrimental to diastolic function are those that stiffen tissue elastic properties, thus decreasing left ventricular (LV) chamber compliance and elevating late-diastolic LV pressure. Myocardial alterations contributing to such elevated chamber stiffness can arise both within the sarcomere lattice (titin) and the extracellular matrix (collagen). The research proposed here is organized around two experimental hallmarks: (1) Hypotheses will be tested using a nonhuman primate model (2) Diastolic dysfunction will be addressed using a broad, fully integrative set of measures, ranging from genetic expression to observations from an awake, behaving animal. By utilizing a primate model (Macaca fascicularis) - rather than a rodent model - the conclusions from this project will provide greater insight into human age-related diastolic dysfunction. In addition, the project will allow a primary focus on aging, since atherosclerosis and hypertension are absent with age in this primate model. The age being studied was selected because it is a transitional state - diastolic changes are evident but heart failure has not yet emerged. The project will test hypotheses about specific age-dependent changes at 4 levels of diastolic function: (1) In intact animals, at high end-diastolic pressure, LV chamber diastolic reserve diminishes with age;while for normal end-diastolic pressures, chamber passive elasticity is preserved. (2) At the level of myocardial tissue, sarcomere stiffness declines with age, and this compensates for increases in extracellular stiffness at moderate stretch - although at large stretch net tissue stiffness does increase. (3) In myocytes, cellular passive stiffness in the fiber direction declines significantly with age, whereas transverse to the fiber stiffness increases. (4) At the molecular level, more compliant isoforms of titin become dominant with age, while collagen content, fiber diameters, and crosslinking all increase. Cellular level elastic function will be quantified by (a) stretching isolated resting myocytes parallel to the sarcomere axis using carbon fibers and (b) nanoindentation of passive myocytes transverse to the sarcomere axis using atomic force microscopy.
