Increasing MgADP to 5 mM augmented tension at low [Ca2+] and increased Ca2+-sensitivity of contraction in permeabilized porcine myocardial strips.
MgADP augments tension at low [Ca2+] and Ca2+-sensitivity of contraction in porcine myocardial strips, suggesting it destabilizes the myosin OFF state and enhances cross-bridge binding to actin.
Myosin cross-bridges dissociate from actin following Mg2+-adenosine triphosphate (MgATP) binding. Myosin hydrolyses MgATP into inorganic phosphate (Pi) and Mg2+-adenosine diphosphate (ADP), and release of these hydrolysis products drives chemo-mechanical energy transitions within the cross-bridge cycle to power muscle contraction. Some forms of heart disease are associated with metabolic or enzymatic dysregulation of the MgATP-MgADP nucleotide pool, resulting in elevated cytosolic MgADP and impaired muscle relaxation. We investigated the mechanical and structural effects of increasing MgADP in permeabilized myocardial strips from porcine left ventricle samples. Sarcomere length was set to 2.0 µm at 28 °C, and all solutions contained 3% dextran T-500 to compress myofilament lattice spacing to near-physiological values. Under relaxing low Ca2+ conditions (pCa 8.0, where pCa = −log10Ca2+), tension increased as MgADP increased from 0-5 mM. Complementary small-angle X-ray diffraction measurements show that the equatorial intensity ratio, I1,1/I1,0, also increased as MgADP increased from 0 to 5 mM, indicating myosin head movement away from the thick-filament backbone towards the thin-filament. Ca2+-activated force-pCa measurements show that Ca2+-sensitivity of contraction increased with 5 mM MgADP, compared to 0 mM MgADP. These data show that MgADP augments tension at low Ca2+ and Ca2+-sensitivity of contraction, suggesting that MgADP destabilizes the quasi-helically ordered myosin OFF state, thereby shifting the cross-bridge population towards the disordered myosin ON state. Together, these results indicate that MgADP enhances the probability of cross-bridge binding to actin due to enhancement of both thick and thin filament-based activation mechanisms.
Awinda et al. (2022) studied this question. Increasing [MgADP] vs. 0 mM MgADP was evaluated on Tension at low [Ca2+] and Ca2+-sensitivity of contraction. Increasing MgADP to 5 mM augmented tension at low [Ca2+] and increased Ca2+-sensitivity of contraction in permeabilized porcine myocardial strips.