]> concentration of vasoconstrictor at which the force of blood vessel contraction is half the maximum value (EC50) JSmith 2014-06-09T18:33:14Z A calculated value for the concentration of a vasoconstrictor at which the energy produced by the active reduction in the diameter of a blood vessel in response to the application of the stimulus is halfway between the baseline value and the highest achievable value of such stimulus-induced contractile energy. A vasoconstrictor is any agent or condition that narrows blood vessels by constricting the smooth muscle in the vessel walls. vasoconstrictor half maximal effective concentration (EC50) calculated blood vessel contractile force measurement