MgADP promotes a catch-like state developed through force-calcium hysteresis in tonic smooth muscle.
Tonic rabbit femoral artery and phasic rabbit ileum smooth muscles permeabilized with Triton X-100 were activated either by increasing [Ca2+] from pCa > 8.0 to pCa 6.0 (calcium-ascending protocol) or contracted at pCa 6.0 before lowering [Ca2+] (calcium-descending protocol). The effects of, respectively, high [MgATP]/low [MgADP] [10 mM MgATP + creatine phosphate (CP) + creatine kinase (CK)] or low [MgATP]/[MgADP] (2 mM MgATP, 0 CP, 0 CK) on the "force-[Ca]" relationships were determined. In femoral artery at low, but not at high, [MgATP]/[MgADP] the force and the ratio of stiffness/force at pCa 7.2 were significantly higher under the calcium-descending than calcium-ascending protocols (54% vs. 3% of Po, the force at pCa 6.0) (force hysteresis); the levels of regulatory myosin light chain (MLC20) phosphorylation (9 +/- 2% vs. 10 +/- 2%) and the velocities of unloaded shortening V0 (0.02 +/- 0.004 l/s with both protocols) were not significantly different. No significant force hysteresis was detected in rabbit ileum under either of these experimental conditions. [MgADP], measured in extracts of permeabilized femoral artery strips by two methods, was 130-140 microM during maintained force under the calcium-descending protocol. Exogenous CP (10 mM) applied during the descending protocol reduced endogenous [MgADP] to 46 +/- 10 microM and abolished force hysteresis: residual force at low [Ca2+] was 17 +/- 5% of maximal force. We conclude that the proportion of force-generating nonphosphorylated (AMdp) relative to phosphorylated cross-bridges is higher on the Ca2+-descending than on the Ca2+-ascending force curve in tonic smooth muscle, that this population of positively strained dephosphorylated cross-bridges has a high affinity for MgADP, and that the dephosphorylated AMdp . MgADP state makes a significant contribution to force maintenance at low levels of MLC20 phosphorylation.
Khromov
A
A
Department of Molecular Physiology and Biological Physics, University of Virginia Health Sciences Center, Charlottesville, Virginia 22906-0011, USA.
Somlyo
A V
AV
Somlyo
A P
AP
D000244:Adenosine Diphosphate / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002463:Cell Membrane Permeability; D002614:Chelating Agents; D003979:Diazonium Compounds; D005263:Femoral Artery / Q000502:physiology; D007082:Ileum / Q000502:physiology; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000502:physiology*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000502:physiology*; D017830:Octoxynol; D010642:Phenoxyacetates; D010766:Phosphorylation; D010782:Photolysis; D011817:Rabbits
D002614:Chelating Agents; D003979:Diazonium Compounds; D010642:Phenoxyacetates; C063685:diazo-2; D000244:Adenosine Diphosphate; D017830:Octoxynol; D002118:Calcium