Effect of angiotensin converting enzyme inhibitors on the vasoconstrictor action of angiotensin I on isolated rat kidney

Metformin prevents the impairment of endothelium-dependent vascular relaxation induced by high glucose challenge in rabbit isolated perfused kidneys

High glucose concentrations are involved in the development of diabetic-associated vascular complications. We have previously reported that acute high glucose challenge, corresponding to post-prandial glycemia levels observed in patients with type 2 diabetes, blunts ACh-induced endothelium-dependent relaxation of the renal circulation of non-diabetic rabbits. Isolated perfused kidneys from non-diabetic rabbits were acutely exposed (3 h) to normal (5.5 mM--control group) or high (15 mM) D-glucose concentrations in the presence or absence of a continuous infusion of metformin (20 or 100 microM). Renal vascular reactivity was evaluated with endothelium-dependent (acetylcholine, ACh) and -independent (sodium nitroprusside, SNP) vasodilating agents. ACh-induced maximal renal vasodilation was reduced by high glucose infusion (15 mM) in comparison to the control group (25+/-3% and 41+/-3% respectively; P<0.01), being restored to 41+/-4% and 43+/-2% by a simultaneous 3-h infusion of 20 or 100 microM of metformin respectively (P>0.05). Perfusion of the kidneys with the angiotensin II-converting enzyme inhibitor captopril (10 microM) also significantly prevented the deleterious effects of high glucose challenge in the renal circulation. The use of a continuous infusion of N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM) did not affect the protective effect of metformin in the renal circulation (39+/-4%; P>0.05), while tetraethylammonium (TEA, 10 mM) partially blunted this effect (33+/-4, P<0.01). Renal vasodilation induced by SNP was not modified by simultaneous infusion of high glucose and/or metformin. It is concluded that the impairment of ACh-induced endothelium-dependent renal vasodilation observed after acute exposure to high glucose concentrations is abolished by metformin administration. These alterations of renal vascular reactivity can be accounted for, at least in part, by the activation of the renal renin-angiotensin system during hyperglycemia. The protective effects of metformin present some EDHF-dependent component and are not related to metabolic pathways dependent on nitric oxide.

Glucocorticoids regulate the expression of angiotensin AT1 receptors, in the human hepatoma cell line, PLC-PRF-5

The effects of different steroids on the expression of angiotensin AT1 receptors by the human hepatoma cell line, PLC-PRF-5 was studied. Dexamethasone and aldosterone decreased the specific binding of [3H]angiotensin II to intact PLC-PRF-5 cells by 57 +/- 4% and 54 +/- 2%, respectively, compared to control, untreated cells. EC50 values for dexamethasone, cortisol and aldosterone were 1.8 +/- 0.6, 40 +/- 6, and 310 +/- 20 nM, respectively, suggesting that these effects were mediated via a glucocorticoid receptor. Scatchard analysis revealed that dexamethasone decreased the number of angiotensin AT1 receptors expressed (50 +/- 4% relative to control) with no change in receptor affinity. Treating cells with dexamethasone in the presence of either an angiotensin converting enzyme inhibitor or an angiotensin II receptor antagonist did not prevent the reduction in angiotensin AT1 receptor expression, ruling out a mechanism involving a dexamethasone induced increase in endogenous angiotensin II production. A ribonuclease protection assay established that the steady state level of angiotensin AT1 receptor mRNA in dexamethasone treated cells was reduced to 34.7 +/- 8.4% of untreated cells. The decrease in the number of angiotensin AT1 receptors expressed on the cell surface after treatment with dexamethasone therefore seems likely to reflect the decreased steady state level of the mRNA coding for this receptor.

Efficacy of intrarenal ACE-inhibition estimated from the renal response to angiotensin I and II in humans

Recent studies on the nature of the renin-angiotensin system (RAS) in animals have led to the concept that systemic and intrarenal RAS can be influenced to different degrees by angiotensin converting enzyme (ACE) inhibitors. Assessment of efficacy of intrarenal ACE inhibition by ACE inhibitors in humans is necessarily indirect and has not been reported. We therefore monitored the renal response to acute angiotensin (Ang) I infusion in volunteers taking 20 mg enalapril twice daily, and related the responses to the obtained increments in plasma Ang II levels. Ang I infusion rates of 4, 8, 16, and 32 pmol/kg/min caused gradual increments in plasma Ang I (maximal change from 26 +/- 18 to 578 +/- 120 pmol/liter, P < 0.05) and, despite treatment with enalapril, also of Ang II (from 3 +/- 1 to 29 +/- 5 pmol/liter, P < 0.05). This was associated with large reductions in renal plasma flow (paraaminohippurate clearance), filtration fraction, maximal urine flow, sodium excretion, lithium and uric acid clearance, and increments in mean arterial pressure and plasma aldosterone (P < 0.05 for each variable). Strong correlations existed between the changes in either variable and the increment in plasma Ang II. Infusions of Ang II at 1 and 4 pmol/kg/min in the same subjects caused comparable increments in plasma Ang II and had similar physiological effects as found during the Ang I infusion. Analysis of covariance of the changes in plasma Ang II and each of the measured variables revealed no differences between Ang I and Ang II infusions.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of angiotensin-converting enzyme inhibitors in the rat in perfused hindlimbs and in vivo

To investigate the role of local renin angiotensin systems in the functional responses to angiotensin I and II, the effects of angiotensin I and angiotensin II on resistance were measured in perfused hindlimbs of rats under normal conditions and during infusion of enalaprilate, lisinopril, zabiciprilate and captopril at two infusion rates. The angiotensin-converting enzyme (ACE) inhibitors significantly increased ED50 and decreased maximal resistance changes of angiotensin I dose dependently, without effects on angiotensin II responses. Captopril increased the ED50 of angiotensin I significantly more than did the other ACE inhibitors at a low infusion rate. The ACE inhibitors, except for lisinopril, increased the ED50 of angiotensin I pressor responses in vivo to the same extent, and were similarly potent to inhibit plasma ACE activity at 15 min after injection. The ratio of the doses of the ACE inhibitors used in vivo was the same as in perfused hindlimbs. These results suggest that, in the hindlimbs, angiotensin I causes ACE-dependent vasoconstriction. Captopril may be more effective to inhibit local ACE than the other ACE inhibitors investigated.

Effect of angiotensin II antagonism on canine renal sympathetic nerve function

The objective of this study was to examine effects of nonpeptide angiotensin II (Ang II) receptor antagonists on renal vasoconstrictor responses to renal nerve stimulation (RNS) and intrarenal injection of norepinephrine in pentobarbital-anesthetized dogs. The subtype 1-selective Ang II receptor antagonists, DuP 753 (2-n-butyl-4-chloro-5-(hydroxymethyl)-1-[(2'-(1H- tetrazol-5-yl)biphenyl-4-yl)methyl]imidazole, potassium salt) and EXP3174 (2-n-butyl-4-chloro-1-[(2'-(1H-tetrazol-5-yl) biphenyl-4-yl)methyl]imidazole-5-carboxylic acid) given intra-arterially to the kidney caused dose-dependent reductions of renal vasoconstrictor responses to RNS but not to norepinephrine. In contrast, the subtype 2-selective Ang II receptor specific ligand, PD 123177 (1-[(4-amino-3-methylphenyl)methyl]-5-(diphenylacetyl)-4,5,6,7- tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylic acid), did not alter the renal vasoconstrictor responses to RNS, norepinephrine, and Ang II in doses of 10-100 micrograms/kg/min i.a. Captopril also reduced the renal vasoconstrictor responses to RNS but not to norepinephrine. However, saralasin did not alter the renal vasoconstrictor responses to RNS and norepinephrine, although it was as effective as DuP 753 and EXP3174 in blocking the renal vasoconstrictor response to Ang II. These results suggest that endogenous Ang II enhances renal adrenergic function at the prejunctional site in anesthetized dogs. Analogous to the Ang II receptor in vascular smooth muscle, the prejunctional Ang II receptor appears to be of subtype 1. Mechanisms accounting for the absence of the inhibition of Ang II-mediated renal adrenergic response by saralasin remain to be determined.

Hypotensive action of DuP 753, an angiotensin II antagonist, in spontaneously hypertensive rats. Nonpeptide angiotensin II receptor antagonists: X

In conscious 18-21-week-old spontaneously hypertensive rats, DuP 753, a nonpeptide angiotensin II receptor antagonist, given orally at 3 and 10 mg/kg or intravenously at 3, 10, and 30 mg/kg, reduced blood pressure dose dependently. It did not alter heart rate at these doses. At 10 mg/kg i.v., DuP 753 decreased blood pressure significantly for at least 24 hours, suggesting a long duration of the antihypertensive effect. Unlike saralasin, DuP 753 did not cause a transient increase in blood pressure. The acute antihypertensive efficacy of DuP 753 was greater than that of captopril. Our data indicate that, for captopril to reduce blood pressure to a similar extent as that of DuP 753, it would need to be supplemented by a diuretic. DuP 753 did not have an acute diuretic effect. Bilateral nephrectomy, but not inhibition of prostaglandin synthesis, abolished the antihypertensive effect of DuP 753, suggesting that the antihypertensive effect of DuP 753 is dependent on an active renin-angiotensin system. Furthermore, DuP 753 inhibited the pressor response to angiotensin II but not the responses to norepinephrine, vasopressin, and Bay K 8644 (a calcium agonist). As neither DuP 753 nor captopril decreased blood pressure acutely in Wistar-Kyoto normotensive rats, our results suggest that the renin-angiotensin system plays a significant role in the control of blood pressure in spontaneously hypertensive rats.

Hydrolysis of angiotensin I by peptidases in homogenates of rat lung and aorta

The hydrolytic cleavage of angiotensin I has been studied in homogenate preparations of rat lung and aorta using gradient elution HPLC to monitor the formation of peptide products. Fresh crude homogenate preparations produced a rapid breakdown of angiotensin I to largely unidentifiable fragment peptides. Neither His-Leu nor angiotensin II was observed in these preparations even in the presence of captopril (20 microM) and the amino-peptidase inhibitors, puromycin, amastatin and bestatin. However, in freeze-thawed homogenates, angiotensin II and His-Leu were detectable together with the tetrapeptide, angiotensin (1-4). The addition of captopril (20 microM) reduced the amount of angiotensin II produced but did not completely block its formation. Higher concentrations of captopril or the addition of enalaprilat or EDTA did not further reduce the amount of angiotensin II produced. In the presence of captopril a peptide corresponding to des-Leu(10)angiotensin I was formed in relatively large amounts (equivalent to 40% of angiotensin I catabolized). Homogenates purified by concanavalin A affinity chromatography gave a clean hydrolysis of angiotensin I to angiotensin II and His-Leu which was completely blocked by captopril. These results suggest an ACE-like activity in rat lung and aorta that is not sensitive to converting enzyme inhibitors.

Differential effects of oral trandolapril and enalapril on rat tissue angiotensin-converting enzyme

Trandolapril (3-100 micrograms/kg) and enalapril (10-300 micrograms/kg) were administered orally to conscious rats. Angiotensin-converting enzyme (CE) activity was inhibited in serum, heart ventricle, renal inner cortex, lung, aorta, adrenal cortex and adrenal medulla, but not in the striatum. Inhibition was maximal at 2 h and with trandolapril was maintained for 24 h. Blood pressure and heart rate were not affected by either compound. Trandolapril was 6-10-fold more potent than enalapril. Differences between trandolapril and enalapril in CE inhibition observed in heart ventricle, adrenal cortex and medulla could be due to the presence of more than one type of CE or CE-like activity.