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Angiotensin-converting enzyme inhibitors (ACEi) reduce cardiovascular morbidity and mortality by improving coronary perfusion, reducing ventricular hypertrophy and remodeling, and preventing progression of coronary atherosclerosis. However, the cellular mechanisms underlying the beneficial effects of ACEi are not fully understood. We studied the in vivo effects of ACE inhibition with perindopril on cellular expression of ACE, AT(1) receptors and 2 nitric oxide synthase (NOS) isoforms, endothelial (eNOS) and inducible NOS (iNOS), in human blood vessels using quantitative in vitro autoradiography and immunocytochemistry. Seven patients with ischemic heart disease were treated with perindopril (4 mg/d) for up to 5 weeks before elective coronary bypass surgery, whereas controls did not receive the ACEi (n=7). Perindopril decreased plasma ACE by 70% and the plasma angiotensin II to angiotensin I ratio by 57% and reduced vascular ACE to approximately 65% of control levels in both endothelium and adventitia. By contrast, AT(1) receptor binding in vascular smooth muscle cells was increased by 80% in patients treated with perindopril as confirmed by immunocytochemistry. eNOS was expressed primarily in endothelial cells, whereas little iNOS expression occurred in vascular smooth muscle cells of untreated patients. Both eNOS and iNOS expression seemed to increase during perindopril treatment. These results suggest that suppression of angiotensin II formation in the vascular wall and increased expression of eNOS and iNOS during ACE inhibition may be beneficial in reversing endothelial dysfunction in patients with cardiovascular disease. Because vascular AT(1) receptor expression is increased during chronic ACE inhibition, more clinical studies are required to determine whether it is necessary to combine ACE inhibitors and AT(1) receptor antagonists in clinical management of heart failure, coronary heart disease, and hypertension

Adult Ren-2 gene transgenic rats, TGR(mRen-2)27, exhibit elevated circulating and kidney angiotensin II (Ang II) levels in the presence of severe hypertension. The aim of this study was to examine whether AT1 and AT2 receptors in the kidney and renal hemodynamic and tubular responses to blockade of these receptors were altered in the Ren-2 gene transgenic rats during the maintenance phase of hypertension. Renal AT1 and AT2 receptors were mapped by in vitro autoradiography (n=8), and the effects of blockade of these receptors on mean arterial pressure (MAP), heart rate (HR), and renal cortical (CBF) and medullary blood flows (MBF) were studied in anaesthetized, adult age-matched male homozygous TGR rats (n=12) and Sprague-Dawley (SD) rats (n=7). TGR rats showed higher basal MAP (P<0.001), heart and kidney weight (P<0.001), plasma renin activity (P<0.05) and plasma Ang II level (P<0.05), and CBF (P<0.05) and MBF (P<0.05) than SD rats. AT1 receptor binding was significantly increased in the glomeruli, proximal tubules, and the inner stripe of the outer medulla of TGR rats (P<0.01), while the AT2 receptor binding was low at all renal sites of TGR and SD rats. Immunohistochemistry revealed that this increased AT1 receptor labeling occurred mainly in vascular smooth muscle layer of intrarenal blood vessels including afferent and efferent arterioles, juxtaglomerular apparatus, glomerular mesangial cells, proximal tubular cells, and renomedullary interstitial cells (RMICs) in the transgenic rats. Blockade of AT1 receptors with losartan in TGR rats markedly reduced MAP to the normotensive level (P<0.001) without altering HR. Both CBF (P<0.005) and MBF (P<0.05) were significantly increased by losartan in the transgenic rats. By contrast, losartan only caused a smaller decrease in MAP and an increase in renal CBF in SD rats (P<0.05). PD 123319 was without any renal effect in both SD and TGR rats. These findings suggest that markedly increased AT1 receptors in renal vasculature, glomerular mesangial cells, and RMICs in the presence of fulminant hypertension and elevated circulating and tissue Ang II levels may play an important role in the maintenance of hypertension in the Ren-2 gene transgenic rats.

Angiotensin II receptor binding sites in type 1 interstitial cells in the inner stripe of the outer medulla are readily labeled in vitro by the radioligand but not in vivo after systemic radioligand administration. In anesthetized rats, we investigated if reduced vascular delivery due to angiotensin II-induced renal vasoconstriction or, alternatively, prior occupancy of these sites by endogenous angiotensins modulates angiotensin II subtype 1 receptor binding to renal medullary interstitial cells in vivo using electron microscopic autoradiography. Using 125I-angiotensin II, administered systemically, as a radioligand, binding in control rats occurred predominantly in the glomeruli and proximal tubules, while only low binding was observed in the inner stripe of the outer medulla. Pretreatment of rats with unlabeled [Sar1,Ile8]angiotensin II or with the angiotensin II subtype 1 receptor antagonist losartan before receiving the radioligand completely abolished binding to all sites. Renal vasodilatation induced by sodium nitroprusside or use of the radiolabeled antagonist analogue 125I-[Sar1,Ile8]angiotensin II did not alter binding to the inner stripe. In contrast, chronic salt loading or inhibition of angiotensin-converting enzyme by perindopril significantly increased binding not only to the cortical sites but also to the sites in the inner stripe of the outer medulla. Electron microscopic autoradiographs of the inner stripe detected binding in the interstitial cells only in rats treated with chronic salt loading or perindopril. These results suggest that endogenous angiotensins may modulate binding of circulating angiotensin II to the interstitial cells in vivo, and these angiotensin II receptor-bearing cells are more likely to be more responsive to interstitial angiotensin II than to the circulating hormone.

In conscious spontaneously hypertensive rats, intraxerebroventricular injection of captopril (2 mg/kg) resulted in a rapid hypotensive response that lasted several hours. The same dose given by intracerebroventricular injection had no significant effect on blood pressure (BP) of normotensive Wistar-Kyoto (WK) rats over 7 hours. There was no significant change in BP of conscious spontaneously hpertensive rats (SHR) in response to intracerebroventricular injection of vehicle and only a transitory fall in BP in response to intravenous injection of captopril (2 mg/kg). There was no significant differences between plasma renin activity (PRA) of conscious normotensive WKY rats and the PRA of SHR. These results suggest biochemical differences between the brains of SHR and normotensive WKY control rats. These differences could involve the brain renin-angiotensin system or other neuropeptides.

Angiotensin-converting enzyme (ACE) in rat brain closely resembled that in lung in its kinetics with the substrate Hip-His Leu, the inhibitors SQ 20,881 and SQ 14,225, and iun its Cl- activation profile. Modification of dietary NaCl intake was associated with marked changes in brain ACE activity. Sodium-loaded rats had lower activity of ACE in hypothalamus, striatum, and midbrain, and higher activity in spinal cord compared to controls. In sodium-restricted rats, ACE was elevated in pituitary and depressed in spinal cord. Chronic intravenous infusion of angiotensin (AII) was associated with a pattern of changes partly resembling sodium loading: ACE was depressed in hypothalamus and striatum but elevated in midbrain. After chronic intracerebroventricular infusion of AII, ACE was elevated in striatum and hippocampus, and depressed in spinal cord; a pattern of changes quite different from those associated with intravenous AII. These results show that ACE in several brain regions is sensitive to dietary sodium intake and support the hypothesis that angiotensin-containing neurons in these areas might be responsive to NaCl status of the animal. The observed changes in brain ACE do not seem to be explained in any simple manner by changes in circulating or central angiotensin II.

Intravenous injection of the converting enzyme inhibitor SQ14,225 (captopril, 2 mg/kg) reduced the blood pressure of anesthetized, spontaneously hypertensive rats (SHR) progressively over a 3-hour period. An indistinguishable fall in blood pressure occurred in SHR that were bilaterally nephrectomized 1 hour prior to injection of the converting enzyme inhibitor. In the nephrectomized animals, plasma renin activity (PRA) had fallen to less than 30% of its initial values at the time of injection. Injection of the vehicle alone had no effect on blood pressure in either anephric or intact SHR. The converting enzyme inhibitor produced no significant change in the blood pressure of either intact or anephric normotensive Wistar-Kyoto (NT-WK) rats. Infusions of Sar1-Ala8-angiotensin II (saralasin, 10 micrograms/kg-1/min-1) similarly reduced blood pressure of both intact and anephric SHR. These results indicate that captopril and saralasin lower blood pressure in the SHR by some mechanism(s) independent of the kidneys, circulating renin, or bradykinin potentiation. It is suggested that angiotensin II, locally produced at some critical tissue site(s), is involved in the maintenance of raised blood pressure in SHR.

Inhibition of angiotensin converting enzyme (ACE) in serum and tissues of rats was studied after administration of lisinopril, an ACE inhibitor. Tissue ACE was assessed by quantitative in vitro autoradiography using the ACE inhibitor [125I]351A, as a ligand, and serum ACE was measured by a fluorimetric method. Following oral administration of lisinopril (10 mg/kg), serum ACE activity was acutely reduced but recovered gradually over 24 hours. Four hours after lisinopril administration, ACE activity was markedly inhibited in kidney (11% of control level), adrenal (8%), duodenum (8%), and lung (33%; p less than 0.05). In contrast, ACE in testis was little altered by lisinopril (96%). In brain, ACE activity was markedly reduced 4 hours after lisinopril administration in the circumventricular organs, including the subfornical organ (16-22%) and organum vasculosum of the lamina terminalis (7%; p less than 0.05). In other areas of the brain, including the choroid plexus and caudate putamen, ACE activity was unchanged. Twenty-four hours after administration, ACE activity in peripheral tissues and the circumventricular organs of the brain had only partially recovered toward control levels, as it was still below 50% of control activity levels. These results establish that lisinopril has differential effects on inhibiting ACE in different tissues and suggest that the prolonged tissue ACE inhibition after a single oral dose of lisinopril may reflect targets involved in the hypotensive action of ACE inhibitors.