Usually it is percentage reversal of agonist induced tone. However, you have a problem because NE starts higher than in controls - and I'm guessing the total contraction is higher. A similar problem can arise for NOS  inhibitors in some vessels. You can try several things: 1. Reversal of just NE tone, 2. Reversal of Ba2++NE tone, and 3. On top of Ba2+ use a concentration of NE that gives comparable total contraction to control NE - a little trial and error will tell you the suitable concentration of NE to use here. We did this when arsenic diminished contraction (Edwards et al 2013 Toxicol). I think if you analyze all three approaches a clearer picture of what is happening will emerge. Each approach individually has its caveat, unfortunately. 

Thank you very much David C. Ellinsworth for your reply

I have also shared your question with a number of colleagues

Hi My it should be the TOTAL amount of vasoconstictor tone from both the agents as that is the tone you have to relax against. Total constrictor tone should matched to similar levels to your control curve (by adjusting the concentration of NA) if you add the same amount of NA as in the control curve onto an already constricted artery you are likely to have much higher total tone and run the very real risk of physiological antagonism (it is a real problem in aortic tissue). This means you may think your treatment has had an effect but if tone had been that high in the control curve your would have seen the same % relaxation.  Similarly the concentration of NA used in the control curve should produce a sub-maximal constriction (no more than 75% max) to prevent this phenomenon.

Also an observation what concentration of BaCl2 is that, when we add enough to block Kir (high micro M) there is no contraction, I presume you are adding a higher amount to block more K+ channels?)

Ali, I had the same problem with pig pulmonary arteries. Even in 5th generation arteries there was a huge constriction to L-NAME. I was trying to look at EDH mechanisms, and as it turned out even the 

Yep in that situation I would have included the L-NAME tone (in cerebral did not need any other vasoconstrictor!) Often I would add 0 calcium or papaverine at the end of the experiment and call that relaxation baseline and use that as the baseline for all constriction

i am adding concentration of Quercetin to aorta, pre contracted with PE, When i add quercetin then after some times rise in tension is observed , that too lasts for 10 minutes then it starts relaxing again. My question is why is this flavinoid showing rise in tension. buffer is krebs with calcium.

Quercetin blocks PLA2, which would decrease baseline arachidonic acid dilator products, e.g. prostacyclin, maybe EETs, raising tension.

then what should i do.. it is being reported as vasodilator. should i ignore that rise in tension. but i have noticed that at PE (5micro M) concentration Quercetin relaxes muscle.

Hmm quercertin isn't a very good dilator it does relax at high and not very physiologically relevant concentrations (above 10 microM) and tends to add extra constriction before doing this. In my hands at the concentrations it relaxes it seems to kill the vessels or at least prevent any further constriction following its application - even if well washed out.

what concentration of quercetin causes vaso dilation it is reported that it 10- 500uM of concentration has relaxed aorta.