From what I know, there are hormones and amino acids in blood plasma, among other things. I don't think that it will affect your nitrogen balance that much, so that's not too much of a worry. And I doubt that it takes away that much of your hormones.
Actually, the best way to find out would be to look at your progress since you've been donating plasma. I doubt that it would make much of a difference.
I just googled your question and this article came up:
If you don't read the whole thing, at least read the summary on the last page or two.
Having a gainer shake (carb/protein) right before or right after might negate your loss.
That's my 10 cents. I might be completely off, but that's how I reasoned it. I'm curious to hear a more educated response also.