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Cyclic AMP stimulates Ca(2+)-ATPase-mediated Ca2+ extrusion from human platelets.
J S Johansson1, L E Nied, D H Haynes
1Department of Molecular and Cellular Pharmacology, University of Miami School of Medicine, FL.
Biochimica Et Biophysica Acta
|March 23, 1992
Summary
Elevating cyclic AMP (cAMP) in human platelets enhances calcium (Ca2+) extrusion via the Ca(2+)-Mg(2+)-ATPase pump. This process lowers resting Ca2+ levels and increases extrusion rates, potentially inhibiting platelet aggregation.
Area of Science:
- Cell Physiology
- Biochemistry
- Pharmacology
Background:
- Platelet aggregation is regulated by intracellular calcium levels.
- Cyclic AMP (cAMP) is a key signaling molecule involved in platelet function.
- Understanding the mechanisms of calcium regulation in platelets is crucial for developing anti-thrombotic therapies.
Purpose of the Study:
- To investigate the effect of elevated cAMP on active calcium (Ca2+) extrusion across the plasma membrane of human platelets.
- To determine how cAMP influences the kinetics of Ca2+ extrusion mechanisms.
- To elucidate the role of cAMP in regulating resting intracellular Ca2+ concentrations ([Ca2+]cyt).
Main Methods:
- Utilized quin2, a fluorimetric indicator, to measure free Ca2+ in the cytoplasmic compartment of human platelets.
- Induced cAMP elevations using dibutyryl-cAMP or forskolin.
- Measured Ca2+ extrusion progress curves and analyzed rate vs. [Ca2+]cyt characteristics.
- Quantified the effects on the Ca(2+)-Mg(2+)-ATPase and Na+/Ca2+ exchanger components of Ca2+ transport.
Main Results:
- Forskolin (10 microM) increased the Vm of the plasma membrane Ca(2+)-Mg(2+)-ATPase by 1.6-fold without altering its Km or Hill coefficient.
- Dibutyryl-cAMP (1 mM) stimulated the Ca(2+)-Mg(2+)-ATPase component by 2.0-fold.
- Forskolin reduced resting [Ca2+]cyt from 112 nM to 96 nM, attributed to increased pump activity and passive Ca2+ leakage.
- No effect of forskolin was observed on the linear, non-saturable component (Na+/Ca2+ exchanger).
Conclusions:
- Elevated cAMP, induced by forskolin or dibutyryl-cAMP, stimulates the Ca(2+)-Mg(2+)-ATPase, enhancing active Ca2+ extrusion from platelets.
- This cAMP-mediated enhancement of Ca2+ extrusion contributes to lowering resting intracellular Ca2+ levels.
- The findings suggest two mechanisms by which cAMP inhibits platelet aggregation: reduced resting [Ca2+]cyt and increased Ca2+ removal capacity.