MgADP Promotes Myosin Head Movement toward Actin at Low [Ca<sup>2+</sup>] to Increase Force Production and Ca<sup>2+</sup>-Sensitivity of Contraction in Permeabilized Porcine Myocardial Strips
Myosin cross-bridges dissociate from actin following Mg<sup>2+</sup>-adenosine triphosphate (MgATP) binding. Myosin hydrolyses MgATP into inorganic phosphate (P<sub>i</sub>) and Mg<sup>2+</sup>-adenosine diphosphate (ADP), and release of these hydrolysis products...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2022-12-01
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| Series: | International Journal of Molecular Sciences |
| Subjects: | |
| Online Access: | https://www.mdpi.com/1422-0067/23/23/15084 |
| Summary: | Myosin cross-bridges dissociate from actin following Mg<sup>2+</sup>-adenosine triphosphate (MgATP) binding. Myosin hydrolyses MgATP into inorganic phosphate (P<sub>i</sub>) and Mg<sup>2+</sup>-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<sup>2+</sup>] conditions (pCa 8.0, where pCa = −log<sub>10</sub>[Ca<sup>2+</sup>]), tension increased as [MgADP] increased from 0-5 mM. Complementary small-angle X-ray diffraction measurements show that the equatorial intensity ratio, I<sub>1,1</sub>/I<sub>1,0</sub>, 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<sup>2+</sup>-activated force-pCa measurements show that Ca<sup>2+</sup>-sensitivity of contraction increased with 5 mM MgADP, compared to 0 mM MgADP. These data show that MgADP augments tension at low [Ca<sup>2+</sup>] and Ca<sup>2+</sup>-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. |
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| ISSN: | 1661-6596 1422-0067 |