Inhibition of the Na+/K+ causes extra sodium to accumulate inside the cell. The buildup the Na+ concentration inside the cell will cause the gradient from inside the cell to the outside of the cell to decrease slightly. This action will make it more difficult for calcium to leave the cell via the Na+/Ca++ antiporter. Increase the amount of calcium in the sarcoplasm. More calcium available for Troponin to use will increase the force developed. Decreasing contractility is done primarily by decreasing the influx of calcium or maintaining lower calcium levels in the cytosol of cardiac myocytes during an action potential. This is done by a number of mechanisms: Parasympathetic activation. If the heart is experiencing anoxia, hypercapnia (increased CO2) or acidosis, the heart cells will enter a state of dysfunction and not work properly. Correct sarcomere crossbridges will not form the heart becomes less efficient (leading to myocardial failure). Loss of parts of the myocardium. Heart attack can cause a section of the ventricular wall dies off, that portion cannot contract and there is less force developed during systole. Inotropy A measurable relative increase in contractility is a property of the myocardium similar to the term "inotropy". Contractility may be iatrogenically altered by the administration of inotropic agents. Drugs that positively render the effects of catecholamines such as norepinephrine and epinephrine that enhance contractility are considered to have a positive inotropic effect. The ancient herbal remedy digitalis appears to have both inotropic and chronotropic properties that have been recorded encyclopedically for centuries and it remains advantageous today. Model as a contributing factor Under one existing model , the five factors of myocardial performance are considered to be Heart rate Conduction velocity Preload Afterload Contractility By this model, if myocardial performance changes while preload, afterload, heart rate, and conduction velocity are all held constant, then the change in performance must be due to a change in contractility. However, changes in contractility alone generally do not occur. Other examples: An increase in sympathetic stimulation to the heart increases contractility and heart rate. An increase in contractility tends to increase stroke volume and thus a secondary increase in preload. An increase in preload results in an increased force of contraction by Starling's law of the heart; this does not require a change in contractility. An increase in afterload will increase contractility (through the Anrep effect). An increase in heart rate will increase contractility