When and why should I use KCl normalization in vascular reactivity studies (contraction curves)?
KCl is often used as an initial stimulus, for reasons I don't fully understand. Some people say it "conditions" the tissue prior to conducting your experiment.
Normalizing against an agonist induced response is a different matter. What you're calculating here is the ability of an agonist/pressure to contract the vessel relative to the ability of that vessel to contract to depolarization. Under certain disease states the ability of a vessel to contract to an agonist may be altered. This can be due to a number of reasons, e.g. impaired NO bioavailability, receptor dysfunction or downregulation, altered Ca2+ dynamics. Therefore it's helpful to compare the response to a "pure" stimulus, such as depolarization, so you can ascertain whether it is simply due to an altered capacity to contract, rather than alterations in signalling events leading to contraction.
It's a handy, low cost, easily acquired piece of information.
Dave
To add to Ellinsworth's answer, you should also use it again at the end of the experiment to ensure your tissue is still viable. There is the possibility that the experiment itself (incubations, washes, treatments, time, etc) could damage the tissue and alter results.
