I have been training for seven years, weight 235, BF about 10%. I have a hard time gaining weight so I have never done any serious cardio except pad and bag work for boxing. I plan on entering a mixed martial arts event in the near future and want to get my cardio fitness up. How many high intensity sesion per week is required to see an improvement? I grapple twice a week, box once or twice a week and lift twice a week on a 14 day split (it works for me). I dont want to lose any weight, if anything I would like to gain another 10lbs, and no I am not natural.
If your not natural and having dificulty gaining weight, it is more than likely poor diet. Without more information it is hard to tell you where to go without it. As for your cardio, I would throw in a 3-5 rounds of rope work twice a week. It also sound like you need more ring time. I prefer to get my cardio on the mat or in the ring, as it also improves my skills.
Agree with KO. Check you diet. And the best way to training for this kind of thing is to do this kind of thing (SAID principle). The Roman Army practiced with double weigth swords and shields. It is an old practice, but it works. Best of Luck.
Well I can say this about the cardio. As far as for “getting your fitness up” grappling and kick/boxing are gonna be the best for you. Instead of just going for 3 minute rounds go to failure with a sparring buddy. Do marathon rounds. I train in Kajukembo so we grapple all the time ( using Bjj techiniques tho) Our Dojo has a “spar night” once a week for a month before tournies and we all go all out hard as we can for long long periods of time to drain us completely. This doesn’t seem like it would do much for your cardio but once you get used to 10-15 min grappling rounds the 3 min rounds at the tournies are nothing but butter. The reason this really works so well IMHO is becuse you are not focused on cardio. When you are on the treadmill running the time ticks by sooooo slow. You concentrate on nothing else but the task at hand and the burning lungs and muscles. When you are grappling or kick/boxing you are worried about much larger things like not getting caught in that armbar or triangle choke. You go at it hard for short bursts but you go at it harder on the ground than you ever would on the tread. Hey, its just my opinion and since you grapple I think you know what I mean. If you really want a workout just ask your Sifu, s/he’ll get you in shape 
The problem I have with 15 minute rounds, is that it does not condition you to get the job done in 3. You have to pace yourself to go 15, no matter how many times you do it. It’s like trying to sprint a mile, no matter how hard you try, you will slow down.
I agree with the other responses in the sense that you want to train sport-specifically, however if you don’t have a substantial level or aerobic fitness you may potentially struggle with the marathon rounds, personally you need to work on your stroke volume and aerobic recovery rates then progress into the marathon rounds. A great method for improving aerobic fitness- specifically stroke volume is hill training. This is gruelling however the results are tremendous at increasing your stroke volume without excessive use of your aerobic energy system which would evidently accelerate beta oxidation and subsequently losing weight.