ISA is Intrinsic Sympathomemetic Activity, which means they block Catacholomines(adrenaline), just like any Beta Blocker, but the ISA properties keeps
the heart at a higher rate than
the equivalent dose of NON-ISA Beta
Blockers.
The two ISA Beta Blockers I know of are Acebutolol and Pindolol.
Acebutolol is a selective Beta Blocker, Pindolol is non-selective.
I read that 200 mg of Acebutolol is equivalent to 50 mg of Atenolol. With 75 mg of Atenolol, my resting heart rate was 55.