A primary factor that affects contractility is the concentration of free calcium ions within the cytoplasm. Calcium levels can be increased when a greater number of calcium channels responds to the voltage stimulus to open. The number of responding channels depends on the concentration of cytoplasmic cyclic AMP.
Cardiac nerves (solid CN line) of the sympathetic nervous system (SNS) release norepinephrine that binds to beta 1 receptors ( ß 1 triangle ). Epinephrine (E) from the adrenal cortex (not shown) also activates these sites. The result is disassociation of the G-stimulatory protein (Gs triangle) into subunits. The alpha subunit ( hexagon) detachs and diffuses through the membrane (shaded arrow) to active membrane-bound adenylcyclase (ACase).
The ensuing increase in cAMP causes a greater number of calcium channels to open when stimulated. The increased calcium ion concentration increases the number of cross-bridges between contractile proteins thus increasing the force of contraction. Increased force of contraction due to increased calcium ions is called 'contractility.'
Last Updated:7/16/2005