To establish if physiological manipulations of the renin–angiotensin system modulates tissue angiotensin converting enzyme (ACE), rats were fed low, normal, or high salt diets, or high salt and DOCA, for 3 weeks, and then plasma and tissue ACE levels were determined by radioinhibitor binding studies. Urinary sodium excretion (mmol/24 h) was (0.03 ± 0.01 (low salt). 0.58 ± 0.09 (normal salt). and 10.4 ± 1.3 (high salt), and plasma angiotensin II (pg/ml) was 73.5 ± 8.6 (low salt). 49.3 ± 8.5 (normal salt). 32.2 ± 8.5 (high salt). and 6.9 ± 0.6 (high salt plus DOCA) in the third week of dietary treatment. ACE was studied by Scatchard analysis of radioinhihitor binding in plasma and tissue homogenates. [125I]MK351A hound to ACE was measured under standardized conditions in the presence of MK35IA (10-13 to 10-5 M) and MK35IA binding sites and equilibrium dissociation constant calculated. There were no significant changes in binding site concentration in plasma, lung, aorta, epididymus, brain, or kidney preparations across the range of salt states studied. The equilibrium dissociation constant appeared uniform within organs. but varied between organs. Further studies were undertaken with captopril, enalapril, and cilazoprilic acid in kidney and lung preparations. Concentration of inhibitor required for equal displacement of hound [125I]MK351A was consistently greater for kidney than for lung. Binding studies of rat ACE using [125I]MK35IA showed that manipulation of salt status did not influence rat tissue ACE. Binding data suggest ACE from different tissues may have variations at the active site.
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