EXCLUSIVE

Ex-"American Gladiators" star Nitro was hospitalized for a HEART ATTACK on Wednesday, TMZ has learned ... and Nitro tells us he thinks STEROIDS played a factor.

Dan Clark says he experienced intense chest pains while working out at an L.A. gym that night ... and then his left arm went numb.

Nitro asked a buddy to drive him to a nearby urgent care center ... where they quickly realized he was having a heart attack and put him in an ambulance for a nearby hospital.

Once Nitro arrived to the hospital, he was rushed into emergency surgery to remove a blood clot in one of his arteries. During the procedure, doctors fitted Nitro with a brand new stent.

Nitro says the whole thing was a shock 'cause he's in peak physical condition ... and believes the heart attack may have been a result of the YEARS of steroid abuse from his days as a Gladiator.

According to Nitro, 'roids weren't viewed as taboo back in the AG heyday ... it was just part of the culture.

Clark says he's confident he'll make a full recovery and says the worst part about the whole ordeal ... he can't workout for a month.