| Myocardial hypertrophy is the heart's growth and remodeling response to a hemodynamic overload. Though this growth process is initially compensatory and reversible, a chronic overload, such as that caused by hypertension or a regurgitant valve, often leads to further remodeling and heart failure. Given the prevalence of hypertension in the U.S. (1 in 3 adults) and abroad, there is a clear need for a complete understanding of the mechanical signals and molecular transduction mechanisms responsible for adult cardiac myocyte growth and remodeling. A long-accepted mechanical stimulus hypothesis proposes that elevated systolic and diastolic stresses trigger the hypertrophic responses to pressure overload and volume overload, respectively. However, this has not been tested directly in a controlled environment with a physiologic specimen. Our unique muscle culture system provides us with complete control of 1D mechanics of an intact muscle prep in defined culture media for up to 36h. This duration is sufficient to induce measurable overload-induced changes to myocyte dimensions and the underlying biology.;Preliminary muscle culture studies and in vivo volume overload studies led us to propose an alternative to the stress hypothesis: (1) that the amount of cyclic shortening in cardiac myocytes controls myocyte shape; and (2) the amount of myocyte stretch controls myocyte size. We cultured RV papillary muscles from adult male LBN-F1 rats for 36h to test our hypothesis that a reduced amount of cyclic myocyte shortening stimulates a reduction in the myocyte length/diameter ratio (L/D). Muscles were cultured under low (< 5%) or physiologic (15%) shortening from a pre-stretched diastolic length (15% above slack). To stimulate protein synthesis, the alpha1-adrenergic agonist, phenylephrine, was added to half of the muscles from each shortening group. Based on our hypothesis, we predicted that the additional protein content from phenylephrine treatment would be assembled preferentially in parallel under low shortening (reduced L/D), but balanced between series and parallel under physiologic shortening (constant L/D).;In low shortening muscles, we detected a significantly reduced L/D with phenylephrine treatment, while in physiologic shortening muscles, L/D did not change with phenylephrine treatment. Surprisingly, this shape effect in phenylephrine-treated muscles was not accompanied by an increase in myocyte volume, though phenylephrine did stimulate expression of the fetal gene, atrial natriuretic peptide. Despite no change in volume, we detected two additional significant effects by two-way ANOVA: (1) phenylephrine had an effect of reducing myocyte length (independent of shortening amount) while (2) there was in interaction between phenylephrine and shortening, such that cross-sectional area trended toward an increase in phenylephrine-treated low shortening muscles and a decrease in phenylephrine-treated physiologic shortening muscles (compared to untreated muscles under low and physiologic shortening, respectively). This result agreed with our prediction that the effects of phenylephrine on myocyte cross-sectional area and L/D are shortening dependent.;In addition to identifying mechanical stimuli, we are interested in elucidating the cellular and molecular transduction mechanisms of myocyte growth and remodeling. As such, we cultured muscles for 12h under physiologic and reduced amounts of shortening at two different mean lengths, to identify genes whose expression is regulated by shortening at high, low, or both high and low mean length. A reduction in cyclic shortening induced a prominent fibrosis gene expression response comprised of ECM and other extracellular components, a hallmark of the response to pressure overload. This result suggested that fibroblasts and the ECM might play a central role in transverse myocyte remodeling under reduced shortening. In this study, we also identified the ERM protein, radixin, and the 90kDa heat shock protein, HSP90, as candidate "shape control" genes that were up-regulated with reduced shortening at both low and high mean length.;Finally, I assessed the feasibility of modeling a thermodynamic mechanism of myocyte remodeling based on the work of T.L. Hill, whose theory predicts addition of subunits into a tensile protein aggregate and removal from an aggregate under compression or below some threshold tension. I developed a model representation of the cardiac sarcomere as an adjustable length spring, and found that under certain parameter combinations, Hill's theory predicts axial addition of sarcomeres that agrees with the in vivo myocyte shape literature. |
