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cGMP-dependent protein kinase decreases calcium sensitivity of skinned cardiac fibers
Abstract:
Chemically skinned (Lubrol WX) cardiac muscle fibers produce half-maximum isometric tension at pCa 6.18 (pH 6.7) in presence of MgATP (10 mM). After addition of cGMP (5 microM) and cGMP-dependent protein kinase (0.1 microM), the pCa required for half-maximum activation is 5.96, while maximum tension is not affected. Similar shifts in the tension/pCa-relationship have been observed after incubation of skinned cardiac muscle fibers with cAMP of catalytic subunit of the cAMP-dependent protein kinase. The shift in the Ca2+-sensitivity is associated with an increased incorporation of radioactivity into a Mr 28000 band (presumably troponin-I) and a Mr 145000 band.
Insights
cGMP and cAMP signaling pathways enhance cardiac muscle calcium sensitivity by altering protein phosphorylation. This modification increases the responsiveness of muscle fibers to calcium ions without affecting maximum contraction force.
Area of Science:
- Cardiovascular Physiology
- Molecular Cardiology
- Muscle Biology
Background:
- Cardiac muscle contraction is regulated by calcium ions (Ca2+).
- Phosphorylation of key regulatory proteins can modulate Ca2+-sensitivity in muscle fibers.
- Cyclic nucleotides like cGMP and cAMP are important intracellular signaling molecules.
Purpose of the Study:
- To investigate the effect of cGMP and cAMP signaling on the Ca2+-sensitivity of chemically skinned cardiac muscle fibers.
- To determine if these signaling pathways influence the maximal isometric tension produced by cardiac muscle.
- To identify potential protein targets involved in the observed changes in Ca2+-sensitivity.
Main Methods:
- Chemically skinned cardiac muscle fibers (Lubrol WX) were prepared.
- Isometric tension was measured at varying levels of free Ca2+ (pCa).
- The effects of adding cGMP and cGMP-dependent protein kinase, or cAMP and the catalytic subunit of cAMP-dependent protein kinase, were assessed.
- Protein phosphorylation was analyzed using radioactivity incorporation assays.
Main Results:
- cGMP and cGMP-dependent protein kinase shifted the tension/pCa relationship to lower pCa values (increased Ca2+-sensitivity), with a half-maximum activation shift from pCa 6.18 to 5.96.
- Maximum isometric tension remained unaffected by cGMP treatment.
- Similar shifts in Ca2+-sensitivity were observed with cAMP and the catalytic subunit of cAMP-dependent protein kinase.
- Increased radioactivity incorporation was noted in a Mr 28,000 protein (likely troponin-I) and a Mr 145,000 protein, suggesting phosphorylation.
Conclusions:
- cGMP and cAMP signaling pathways, via their respective kinases, enhance the Ca2+-sensitivity of cardiac muscle fibers.
- This enhanced Ca2+-sensitivity is associated with the phosphorylation of specific cardiac proteins, including troponin-I.
- These findings highlight a mechanism by which cyclic nucleotide signaling can modulate cardiac contractility at the molecular level.