MgADP Promotes Myosin Head Movement toward Actin at Low [Ca 2+ ] to Increase Force Production and Ca 2+ -Sensitivity of Contraction in Permeabilized Porcine Myocardial Strips

• Published in: International journal of molecular sciences 2022-12, Vol.23 (23)
• Main Authors: Awinda, Peter O, Ma, Weikang, Turner, Kyrah L, Zhao, Jing, Gong, Henry, Thompson, Mindy S, Campbell, Kenneth S, Irving, Thomas C, Tanner, Bertrand C W
• Format: Article
• Language: English
• Subjects: Actins - metabolism; Adenosine Diphosphate - metabolism; Adenosine Diphosphate - pharmacology; Adenosine Triphosphate - metabolism; Animals; Calcium - chemistry; Head Movements; Kinetics; Muscle Contraction; Myocardial Contraction; Myosins - metabolism; Swine
• ISSN: 1422-0067
• DOI: 10.3390/ijms232315084

Myosin cross-bridges dissociate from actin following Mg -adenosine triphosphate (MgATP) binding. Myosin hydrolyses MgATP into inorganic phosphate (P ) and Mg -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 [Ca ] conditions (pCa 8.0, where pCa = -log [Ca ]), tension increased as [MgADP] increased from 0-5 mM. Complementary small-angle X-ray diffraction measurements show that the equatorial intensity ratio, I /I , also increased as [MgADP] increased from 0 to 5 mM, indicating myosin head movement away from the thick-filament backbone towards the thin-filament. Ca -activated force-pCa measurements show that Ca -sensitivity of contraction increased with 5 mM MgADP, compared to 0 mM MgADP. These data show that MgADP augments tension at low [Ca ] and Ca -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.