The mechanisms underlying cooperative activation and inactivation of myocardial force extend from local, near-neighbor interactions involving troponin-tropomyosin regulatory units (RU) and crossbridges (XB) to more global interactions across the sarcomere. To better understand these mechanisms in the hearts of small and large mammals, we undertook a simplified mathematical approach to assess the contribution of three types of near-neighbor cooperative interactions, i.e., RU-induced, RU-activation (RU-RU), crossbridge-induced, crossbridge-binding (XB-XB), and XB-induced, RU-activation (XB-RU). We measured the Ca2+ and activation dependence of the rate constant of force redevelopment in murine- and porcine-permeabilized ventricular myocardium. Mathematical modeling of these three near-neighbor interactions yielded nonlinear expressions for the RU-RU and XB-RU rate coefficients (kon and koff) and XB-XB rate coefficients describing the attachment of force-generating crossbridges (f and f'). The derivation of single cooperative coefficient parameters (u = RU-RU, w = XB-RU, and v = XB-XB) permitted an initial assessment of the strength of each near-neighbor interaction. The parameter sets describing the effects of discrete XB-XB or XB-RU interactions failed to adequately fit the in vitro contractility data in either murine or porcine myocardium. However, the Ca2+ dependence of ktr in murine and porcine ventricular myocardium was well fit by parameter sets incorporating the RU-RU cooperative interaction. Our results indicate that a significantly stronger RU-RU interaction is present in porcine ventricular myocardium compared with murine ventricular myocardium and that the relative strength of the near-neighbor RU-RU interaction contributes to species-specific myocardial contractile dynamics in small and large mammals.
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