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The Role of the Myofilaments in the Relaxation of Cardiac Myocardium

Monasky, Michelle

Abstract Details

2010, Doctor of Philosophy, Ohio State University, Biophysics.

It is well known that the rate of intracellular calcium ([Ca2+]i) decline is an important factor governing relaxation in unloaded myocardium. However, it remains unclear to what extent, under near physiological conditions, the intracellular calcium transient amplitude and kinetics contribute to the length dependent increase in force and increase in duration of relaxation. We hypothesize that myofilament properties, rather than calcium transient decline, primarily determines the duration of relaxation in adult mammalian myocardium.

To test this hypothesis, first, we simultaneously measured force of contraction and calibrated [Ca2+]i transients in isolated, thin rabbit trabeculae, at various lengths at 37 °C. Time from peak tension to 50% relaxation (RT50) increased significantly with length, while time from peak calcium to 50% decline (CD50) was not prolonged. At optimal length, increasing the extracellular calcium concentration increased both developed force and calcium transient amplitude, but RT50 remained unchanged, while intracellular calcium decline actually accelerated. Thus, an increase in muscle length will result in an increase in both force and duration of relaxation, while the latter is not governed primarily by the rate of [Ca2+]i decline.

Next, in order to elucidate additional determinants of relaxation other than the sarcoplasmic reticulum (SR) at various frequencies within the in vivo range, we altered the calcium handling in rat and rabbit by inhibiting the sarcoplasmic reticulum. We chose rat and rabbit for this study because of how their sources for activator calcium differ. Trabeculae, iontophoretically loaded with bis-fura-2, were subjected to ryanodine and cyclopiazonic acid to inhibit SR function. Simultaneous force and [Ca2+]i measurements were obtained at 1-4 Hz in rabbit and at 4-8 Hz in rat before and after SR inhibition. Inhibition of the SR resulted in increased diastolic and peak calcium levels as well as decreased developed force in both species. Calcium transient amplitude decreased in rat, but increased in rabbit after SR inhibition. Time to peak tension, time from peak tension to 50% relaxation, time to peak calcium, and time from peak calcium to 50% calcium decline were all prolonged. Results suggest that L-type calcium channel current is responsible for increases in calcium with increasing frequency, and that the SR amplifies this effect in response to increased L-type current. The response of the myofilaments to alterations in calcium handling plays a critical role in the final determination of force, and may differ between species. These results imply the balance between force relaxation and calcium decline is significantly different in larger mammals, necessitating a critical re-evaluation of how myocardial relaxation is governed, specifically regarding frequency-dependent activation.

Next, we set out to investigate myofilament calcium sensitivity and phosphorylation status of myofilament proteins after a step-wise change in cardiac muscle length. Twitch force-calcium relationships and steady state force-[Ca2+]i relationships were measured at various muscle lengths at 37°C using potassium induced contractures. The calcium concentration at 50% maximal force (EC50) significantly decreased with increase in muscle length and maximal active force development significantly increased, while no significant change in the myofilament cooperativity coefficient was found. Phosphoprotein analysis Pro-Q diamond staining as well as phosphorylation-specific antibodies revealed increased phosphorylation of tropomyosin, troponin I, and myosin light chain-2 at longer muscle lengths. Specifically, TnI phosphorylation at Ser22/23 was increased. (Abstract truncated)

Paul Janssen, Ph.D. (Advisor)
Jonathan Davis, Ph.D. (Committee Member)
Peter Reiser, Ph.D. (Committee Member)
Mark Ziolo, Ph.D. (Committee Member)

Recommended Citations

Citations

  • Monasky, M. (2010). The Role of the Myofilaments in the Relaxation of Cardiac Myocardium [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1273778670

    APA Style (7th edition)

  • Monasky, Michelle. The Role of the Myofilaments in the Relaxation of Cardiac Myocardium. 2010. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1273778670.

    MLA Style (8th edition)

  • Monasky, Michelle. "The Role of the Myofilaments in the Relaxation of Cardiac Myocardium." Doctoral dissertation, Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1273778670

    Chicago Manual of Style (17th edition)