A calculated measurement in which the difference between the contractile force of a blood vessel in response to a specified dosage of potassium chloride and the baseline contractile force exerted by that vessel in the absence of any external stimuli, is divided by the baseline contractile force measurement and the result expressed as a percentage. Potassium chloride is the salt of the potassium cation (K+) and the chlorine/chloride anion (Cl-). It is used as an eletroylyte replenisher and has mild vasoconstrictive properties when applied directly to blood vessels.