Munechika Tominaga

Angiotensin converting enzyme (ACE) in the kidney: contribution to blood pressure regulation and possible role of brush-border ACE.

With regard to the concept of the local action of the renin angiotensin system (RAS) involved in blood pressure regulation, the presence of angiotensin converting enzyme (ACE) in a variety of organs suggests that locally produced angiotensin II (ANG II) shares, at least to some extent, the actions of this peptide on respective target organs of ANG II. However, renal ACE is less well understood for its relationship between blood pressure and enzyme activity. In our present studies, with a single oral administration of enalapril to spontaneously hypertensive rats, the inhibition of renal cortical and aortic ACE, but not plasma ACE, coincided with a reduction in blood pressure. Development of high blood pressure in stroke-prone, spontaneously hypertensive rats (SHRSP) from 7 to 22 weeks of age was accompanied by an increase in ACE activity in the renal cortex. Aortic and pulmonary ACE also tended to increase with age, but was less prominent. Isolated brush-border membranes contained abundant ACE, both in Wistar-Kyoto rats and SHRSP, and the levels of ACE in renal cortical homogenates closely correlated to the levels of brush-border ACE. Thus, changes in renal cortical ACE activity in response to the ACE inhibition and in cases of SHRSP in relation to aging are apparently associated with changes in blood pressure. It is likely that renal cortical ACE activity reflects the enzyme activity in the brush borders. Thus, brush-border ACE should probably be taken into account when discussing possible roles of renal ACE.

Angiotensin-Converting Enzyme in the Rat Kidney

While it is known that angiotensin-converting enzyme (ACE) in the kidney is concentrated at the brush borders of the proximal tubule, the role of tubular ACE in renal physiology is not well understood. The active site of tubular ACE is exposed on the luminal surface of the brush borders and may hydrolyze peptides in the glomerular filtrate. However, a positive correlation between blood pressure and renal ACE activity was observed in spontaneously hypertensive rats, as well as in cases of ACE inhibition. Determination of ACE activity in different renal zones and immunohistochemistry demonstrated that ACE predominates in the inner cortex and that the proximal tubule in the outer cortex contains less ACE. Perhaps the inner cortex is the area responsible for alteration of renal ACE activity, since only ACE activity in the inner cortex increased following administration of the ACE inhibitor captopril. This would suggest that the induction of ACE occurs in the inner cortex. Renal ACE activity is also affected by oxidation. Thus, the activity increased when diamide, an oxidizing agent, was added to the crude extract of renal cortex and when oxygen was introduced into the extract. Therefore, tissue oxidation may be one factor affecting renal ACE activity.

Angiotensin converting enzyme predominates in the inner cortex and medulla of the rat kidney.

Regional distribution of angiotensin converting enzyme(ACE) in the rat kidney was studied. The ACE activities in the inner cortex and outer medulla were about 10 and 5 times those in the outer cortex, respectively. The activity in the inner medulla or papilla was much the same as that in the outer cortex. Immunofluorescence was greatest in the proximal tubules in the inner cortex, while the outer medulla and the inner medulla or papilla showed a weak fluorescence. The brush border membranes isolated from the inner cortex also possessed about 10 times the ACE activity seen in the outer cortex. The results indicate that the major source of renal ACE is not the proximal convoluted tubules in the outer cortex, but rather the brush border membranes of proximal tubules in the inner cortex. The contribution of ACE in the inner cortex would therefore be predominant.

Oxidation-induced increase in activity of angiotensin converting enzyme in the rat kidney

The activity of angiotensin converting enzyme(ACE) in crude extracts of the rat renal cortex was increased when the oxidizing agent diamide was added to the extract. The maximal activity was obtained at concentrations over 1 mM, and the value was twice or more the activity in the absence of the pretreatment. The activity of ACE was also increased by the diamide-pretreatment of the isolated membrane fraction of the renal cortex, thereby indicating that the increase in activity was not due to oxidation of endogenous glutathione (GSH) that may lower the ACE activity, but rather that ACE itself was oxidized. When O2 was included in the extract for 2 h, the ACE activity also increased to about twice the original activity. Lineweaver-Burk plots analysis demonstrated that, after oxidation with diamide and O2, the Vmax was increased but the Km remained unchanged. We conclude that the action of ACE in the kidney functions may differ in relation to oxidation of the tissue.

Effect of Oxidation on the Activity of Angiotensin Converting Enzyme in the Rat Kidney, Heart, and Brain

The activities of angiotensin converting enzyme (ACE) in crude extracts of renal cortex, heart and brain of the rat were increased when the oxidizing agent diamide was added to the extracts and then the activity determined. By pretreatment of the extracts with 10 mM diamide, the activities of ACE in the extracts of heart, brain and renal cortex were about 500, 290 and 240% of the control value, determined without the diamide-pretreatment, respectively. In the lung and aorta, increments in the activity after oxidation were less than 20% of the control. No such increase was observed in the plasma. Similar results were obtained when the extracts were exposed to O2. The activity was also increased by oxidation with diamide and O2, when an extract of the human renal cortex was used. Thus, the activity of ACE in the kidney, heart and brain can be increased by oxidation.

Enhancement of Angiotensin-Converting Enzyme Activity in the Inner Cortex of Rat Kidney by Captopril

The rat kidney was separated into the outer cortex, inner cortex, outer medulla and inner medulla or papilla, and the distribution of angiotensin-converting enzyme (ACE) in response to consecutive administrations of captopril was studied. In normal animals, the ACE activity in the inner cortex and outer medulla was about 10 and 3 times higher than in the outer cortex, respectively, and the activity in the inner medulla was much the same as that in the outer cortex. Captopril was given in doses of 30 and 100 mg/kg/day for 7 days, and the renal ACE activity on the 8th day was examined. Since it was assumed that the ACE activity might be lower than the full activity when the tissue contained captopril, diamide, a sulfhydryl oxidant, was used to eliminate the effect of any captopril remaining in the tissue extracts. However, this compound at concentrations over 1 mM enhanced the activity of ACE itself to about twice the value when assayed without diamide. Thus, oxidation with sufficient concentrations of diamide resulted in an enhancement of a maximal activity of ACE. When all samples were pretreated with 10 mM diamide and the ACE activity determined, captopril caused a dose-related increase in the ACE activity, but only in the inner cortex. We conclude that ACE locates predominantly in the inner cortex of the rat kidney and it is this area which has an altered ACE activity in response to captopril.

Contribution of Renal Angiotensin Converting Enzyme(ACE) to Blood Pressure Regulation: Possible Role of Brush Border Ace

The role of renal angiotensin converting enzyme (ACE) in blood pressure regulation is not well understood. In our studies, both acute and chronic treatment of hypertensive rats SHR and SHRSP with ACE inhibitors Enalapril and SA446 had a blood pressure lowering effect that coincided with an inhibition of renal cortical and aortic ACE, but not plasma ACE. Further, ACE activities in the renal cortex and aorta were found to increase with aging of the SHRSP, therefore concomitantly with hypertension development. In the kidney, brush border membranes (BBM) contained abundant ACE. We found that the activities of ACE in the renal cortex closely correlated to the activities in isolated BBM, in Wistar Kyoto rats and in the SHRSP. Thus, renal cortical ACE activity and blood pressure correlated in cases of ACE inhibition and hypertension development. Since the ACE activity in the renal cortex appeared to reflect the enzyme activity in BBM, the brush border ACE may have to be taken into account, in view of the relationship between renal ACE and blood pressure.

Regional Distribution of Angiotensin Converting Enzyme in the Rat Kidney

Angiotensin converting enzyme (EC 3.4.15.1:ACE) plays a crucial role in the renin-angiotensin cascade by its catalytic action of the conversion of angiotensin(ANG) I to ANG II, although the enzyme also degrades bradykinin and some other peptides. ACE in the lung and plasma can influence the plasma ANG II level, however, the role of ACE in other organs such as kidney, brain, vascular wall, heart etc. have been considered in view of the tissue(or local) renin-angiotensin system that appreciates the significance of production of ANG II at the site of its actions1,2.