Measurement of angiotensin converting enzyme induction and inhibition using quantitative in vitro autoradiography: TISSUE selective induction after chronic lisinopril treatment

Angiotensin converting enzyme (ACE) inhibitors lead to induction of ACE in animals and humans. This complicates the use of ACE enzymatic activity as an index of inhibition in plasma or tissues after chronic administration of ACE inhibitors. We have, therefore, developed a method for ACE measurement by in vitro autoradiography using an 125l-labelled inhibitor to quantitate total ACE and the concentration of free (not inhibited) ACE in tissues after prolonged administration of ACE inhibitors to rats. Measurements made on unprocessed tissue sections reflect residual free ACE activity in the presence of the unlabelled inhibitor. In a parallel series of adjacent sections, the ACE inhibitor is dissociated from the enzyme by reversibly denaturing the enzyme by zinc chelation. This is followed by reconstitution of the active enzyme by zinc ion replacement and measuring total enzyme concentration. This technique permits measurement of the extent of ACE inhibition and induction. This method was evaluated in tissues of rats following chronic oral administration of lisinopril (10mg/kg per day) for 2 weeks. The pattern of ACE inhibition was similar to that seen in our previous acute studies. However, induction of ACE was found to be organ specific; plasma total ACE increased 1.75-fold and total ACE in the lung increased by 30% compared with untreated animals, but there was no demonstrable change in total ACE concentration in the kidney, adrenal or aorta. Despite this, during chronic treatment with lisinopril, ACE activity in all of these organs was inhibited with low levels of free ACE. There was neither induction nor inhibition of ACE in the testis. Thus, the pattern and degree of tissue ACE induction and inhibition by chronic lisinopril treatment varies amongst different tissues, suggesting that ACE may be under tissue specific controls. The lungs appear to be the main source of increased serum ACE during chronic ACE inhibition.