Before doing experiments requiring pre constriction it helps to know the the concentration of PE that will cause half the maximal possible constriction with this agent; it doesn't really matter if you express this as a EC50 or as as a pEC50 it is just the pEC50 is a visually easier way to express this valuer particularly as as concentration response curves are normally done on a semi log scale. This most quickly done by conducting a cumulative concentration response curve to PE start adding at a low concentration that does not cause constriction increase the final bath concentration by approximately 3 fold each time ("half log units" of final bath concentration is a good idea i.e. 1nM then 3nM then10nM,30nM etc) continue doing this until you cannot elicit any more contraction. Draw a concentration response curve on a semi log scale i.e log concentration of PE (x axis) vs response (y axis) the concentration that causes 50% of the maximal response is the EC50. Read as log EC50 from the graph, the pEC50 is just the postive number of this value (often referred to as giving a measure of agonist potency)

Why is this important? you can study the effect of treatments that affect PE induced contraction (changes in the pEC50 inducate either an increase or decrease in contractile ablity). The other important consideration is that if you want to study agents that relax PE preconstriction you need to avoid over constricting the vessel (physiological antagonism) by choosing a concentration of PE that is is somewhere between the EC50 and EC80 (i.e. causes between 50 and 80% of maximal constriction to PE). If you constrict more than this you may underestimate the ability of a dilator to cause relaxation. Think of a scale or see saw  the more weight (constriction) you put on one side the more effort you have to make to reverse this pre-constriction. Thus if you over constrict it can look like vasodilator agents only work at higher concentrations (greater levels of stimulus known as physiological antagonism as there is no effect at the level of the dilators receptor rather the receptor needs to generate a greater stimulus to overcome the constriction).

Almost any good pharmacology text book will help explain these concepts in the opening chapters.I hope that helps and was the answer you were looking for.

Thank you Alister J Mcneish

very useful scientific information