Pepstatin inhibition of rat myocardial acid protease

The protease inhibitor, pepstatin, inhibited acid protease in rat heart homogenates with a Ki of 7 x 10(-11) M by a noncompetitive mechanism, but it had no effect on myocardial acid protease activity when given in high doses to intact animals. Exposure of heart slices to 10(-7) M pepstatin for two hours also failed to produce demonstrable acid protease inhibition, suggesting that myocardial acid protease activity is unaffected by this potent inhibitor unless the integrity of the cell is disturbed.