Adrenergic receptors activate a G-protein-coupled cascade that includes the production of cAMP and activation of a protein kinase.
A number of proteins involved in excitation-contraction coupling are phosphorylated by the kinase, which alters their activity. These include:
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DHP receptors (L-type calcium channels) in the plasma
membrane;
-the ryanodine receptor and associated proteins in the sarcoplasmic reticulum membrane;
- thin filament proteins; in particular, troponin;
-thick filament proteins associated with the cross-bridges; and
-proteins involved in pumping calcium back into the
sarcoplasmic reticulum.
Due to these alterations, cytosolic calcium concentration rises more quickly and reaches a greater value during excitation, calcium returns to its pre-excitation value more quickly following excitation, and the rate of cross-bridge activation and cycling are accelerated.
The net result is the stronger, faster contraction observed during sympathetic activation of the heart.
this figure explains it clearly:
