Maybe, either that or the plant would stop producing as much and let its current supply spread out and be its defense. Or it would just leave it as it is and resume normal alkaloid production on the new growth. Or maybe each time it is stressed it would produce even more, thus adding even more alkaloid protection.

This is all speculation of course, it depends on what the cactus actually does with its current supply of alkaloids. It may be able to break it down and reuse it as nutrients to allow growth to resume/get better.

I wish there was someway of testing the amount of alkaloids present in a growing cactus so that it could be harvested at its peak, something like this http://www.enasco.com/product/C08709N , but these only test for nutrients not alkaloids, on top of that I doubt they would be able to tell you concentrations.

I think it would just need to be done through harvesting and experimentation, like what we plan on doing, to figure out what works best.