Mechanical properties of porcine small coronary arteries in one-kidney, one-clip hypertension

The purpose of this study was to analyse the mechanical properties of small coronary arteries from pigs with one-kidney, one-clip hypertension and normotensive control animals and to determine whether these could contribute to vascular structural changes observed in hypertension. Small coronary artery wall tension-internal circumference relationships were determined for coronary arteries mounted in a myograph and used to calculate incremental elastic modulus-stress relationships for arteries from normotensive and hypertensive pigs. Wall tension-internal circumference relationships were shifted to the left in arteries from hypertensive pigs, but the stress-strain and incremental elastic modulus-stress relationships were not influenced by the increased blood pressure load. These data indicate that small coronary arteries from hypertensive pigs are not less distensible than those from normotensive controls. Therefore the previously reported increased media thickness:lumen diameter ratios in coronary arteries in this model of hypertension are not a consequence of abnormal elastic properties and can be ascribed to a remodelling process.

A comparison of the effect of angiotensin converting enzyme inhibition and angiotensin II receptor antagonism on the structural changes associated with hypertension in rat small arteries

OBJECTIVE

To investigate whether, when angiotensin converting enzyme inhibitors are administered to young, genetically hypertension-prone animals, the demonstrated attenuation of blood pressure development and prevention of the structural changes usually observed in small arteries is attributable to the prevention of angiotensin II production.

DESIGN

We have treated spontaneously hypertensive rats (SHR) aged 4-20 weeks with either lisinopril (1 or 10 mg/kg) or the angiotensin II receptor antagonist D 8731 (1, 20 or 50 mg/kg).

METHODS

Blood pressure was measured and structural parameters in small arteries from four vascular beds were examined using isometric myography.

RESULTS

At age 20 weeks lisinopril had attenuated blood pressure development and prevented cardiac hypertrophy (but not vascular hypertrophy) in a dose-dependent manner. The highest dose of lisinopril had reduced the blood pressure of the SHR to below that of the Wistar-Kyoto (WKY) rats and prevented most structural changes, but there was a slight reduction in body weight in those rats. Comparable blood pressure control with D 8731 was associated with similar structural parameters.

CONCLUSION

The prevention of hypertension-associated vascular structural alteration appears to be dependent upon the degree of blood pressure control.

Effect of one-kidney, one clip hypertension on the structure and function of porcine intramyocardial small arteries

OBJECTIVE

To determine the influence of experimental hypertension on the structure and function of porcine coronary small arteries.

METHODS

Miniature pigs underwent partial left renal artery constriction and contralateral nephrectomy. Blood pressures were recorded, using indwelling carotid artery catheters. After 4 weeks the pigs were killed, the heart was removed and subepicardial third-order branches of the left anterior descending artery were dissected and mounted in a myograph for morphological and functional assessment.

RESULTS

Final mean +/- SEM systolic and diastolic blood pressures were, respectively, 197 +/- 9 and 142 +/- 7 mmHg (n = 21) for the hypertensive pigs and 125 +/- 4 and 80 +/- 4 mmHg (n = 11) for the sham-operated control pigs. Hypertension was associated with significant left ventricular hypertrophy. The media thickness: lumen diameter ratio was increased significantly in hypertensive intramyocardial small arteries, caused mainly by remodelling (remodelling index 92%) rather than by medial growth. Maximal contractile responses to potassium and acetycholine were significantly depressed in the arteries from hypertensive pigs, whereas endothelium-dependent relaxation responses to bradykinin, substance P and serotonin were not significantly influenced by hypertension.

CONCLUSIONS

These results demonstrate that even short-term hypertension induces both structural and functional changes in left ventricular intramyocardial small arteries.

Influence of arterial diameter on vasomotor responses in the porcine coronary vasculature

OBJECTIVE

The aim was to determine whether regional differences in arterial responses to vasoconstrictor and vasorelaxant agonists exist within the minipig coronary vasculature.

METHODS

Hearts were obtained from miniature pigs (20-40 kg) immediately after death. First and third order arterial branches of the left anterior descending artery were dissected from within the subepicardium and mounted as ring preparations in a small vessel myograph for measurement of isometric tension under standardised conditions. Contractile responses to acetylcholine, noradrenaline, and U46619, and the relaxation responses to noradrenaline, bradykinin, and substance P were measured. Arterial tone was increased with KCl or acetylcholine prior to addition of vasodilator agonists.

RESULTS

First order branches were more sensitive to the constrictor influence of acetylcholine than third order branches [pD2 values 6.42(SEM 0.07), n = 13, and 6.26(0.07), n = 13, for first and third order respectively, p < 0.05]. U46619 did not induce contractile responses in arteries less than 210 microns in diameter. Noradrenaline only induced small contractile responses in the presence of propranolol following removal of the endothelium. In arteries preconstricted with 40 mM KCl, noradrenaline induced relaxation which was inhibited by propranolol and was uninfluenced by arterial calibre. In the presence of propranolol, noradrenaline-mediated relaxations of acetylcholine-preconstricted arteries were endothelium dependent and alpha 2 adrenoceptor mediated, and greater in first order than in third order branches [58(5)%, n = 9, and 26(8)%, n = 9, for first and third order branches respectively, p < 0.05]. Relaxations mediated by bradykinin and substance P were not influenced significantly by arterial calibre but were greater in arteries preconstricted with acetylcholine than with KCl.

CONCLUSIONS

In isolated minipig coronary arteries the vasoconstrictor responses to acetylcholine and U46619, and the endothelium dependent, noradrenaline mediated relaxations, differ according to the branching order studied. These data provide further evidence for a regional heterogeneity of vascular responses in the porcine coronary vasculature.

New nonpeptide angiotensin II receptor antagonists. 3. Synthesis, biological properties, and structure-activity relationships of 2-alkyl-4-(biphenylylmethoxy)pyridine derivatives

A novel series of nonpeptide angiotensin II (AII) receptor antagonists is reported, derived from linkage of the biphenylyltetrazole moiety found in previously described antagonists via a methyleneoxy chain to the 4-position of a 3-substituted 2,6-dialkylpyridine. When evaluated in an in vitro binding assay using a guinea pig adrenal membrane preparation, compounds in this series generally gave IC50 values in the range 0.005-0.5 microM. A variety of substituents was found to be effective at the 3-position of the pyridine ring. On intravenous administration in a normotensive rat model, the more potent compounds inhibited the AII-induced pressor response with ED50 values in the range 0.1-1.0 mg/kg. One of the compounds, 2-ethyl-5,6,7,8-tetrahydro-4-([2'-(1H-tetrazol-5-yl)biphenyl-4y l] methoxy)quinoline (26), demonstrated good oral activity in two rat models. At doses in the range 1-10 mg/kg po in AII-infused, conscious, normotensive rats, the compound exhibited a dose-related inhibition of the pressor response with a good duration of action at the higher doses. In a renal hypertensive rat model compound 26 showed a rapid and sustained lowering of blood pressure at a dose of 5 mg/kg po. Based on its profile, this compound, designated ICI D6888, has been selected for evaluation in volunteers.

New nonpeptide angiotensin II receptor antagonists. 2. Synthesis, biological properties, and structure-activity relationships of 2-alkyl-4-(biphenylylmethoxy)quinoline derivatives

A novel series of nonpeptidic angiotensin II (AII) receptor antagonists is reported, derived from linkage of the biphenylcarboxylic acid or biphenylyltetrazole moiety found in previously described antagonists via a methyleneoxy chain to the 4-position of a 2-alkyl quinoline. When evaluated in an in vitro binding assay using a guinea pig adrenal membrane preparation, compounds in this series generally gave IC50 values in the range 0.01-1 microM. Structure-activity studies showed the quinoline nitrogen atom and a short alkyl chain at the quinoline 2-position to be essential for receptor binding. On intravenous administration in a normotensive rat model, the more potent compounds inhibited the AII-induced pressor response with ED50 values in the range 0.1-2.0 mg/kg. One of the compounds, 2-ethyl-4-[[2'-(1H-tetrazol-5-yl)biphenyl-4-yl]methoxy]quinoline (5g), demonstrated good oral activity in two rat models. At doses in the range 1-10 mg/kg in AII-infused, normotensive rats, the compound exhibited a dose-related inhibition of the pressor response with a good duration of action at the higher doses. In a renal hypertensive rat model, compound 5g showed a rapid and sustained lowering of blood pressure at a dose of 5 mg/kg. On the basis of its profile, this compound, designated ICI D8731, has been selected for clinical evaluation.

New nonpeptide angiotensin II receptor antagonists. 1. Synthesis, biological properties, and structure-activity relationships of 2-alkyl benzimidazole derivatives

On the basis of an extension of the literature lead 1, a series of benzimidazoles have been synthesized and shown to be angiotensin II (AII) receptor antagonists. The structure-activity relationships of these new antagonists have been explored and the key binding interactions defined. Molecular mechanics calculations were carried out on analogues of imidazole AII antagonists and conformationally restricted analogues were synthesized. The benzimidazole antagonists displaced AII in binding studies in vitro with IC50 values in the range 10(-5)-10(-7) M and antagonized the hypertensive effects of AII in vivo (rats) following intravenous administration with ED50 values in the range of 5-20 mg/kg.

1,2,4-Triazolo[4,3-a]pyrazine derivatives with human renin inhibitory activity. 1. Synthesis and biological properties of alkyl alcohol and statine derivatives

A series of 1,2,4-triazolo[4,3-a]pyrazine derivatives with human renin inhibitory activity, which incorporate (1S,2S)-2-amino-1,3-dicyclohexyl-1-hydroxypropane, statine (Sta), and (3S,4S)-4-amino-5-cyclohexyl-3-hydroxy-pentanoic acid (ACHPA) transition-state mimetics, have been prepared. Structure-activity relationships for renin inhibitory activity in the series are consistent with the 2-[8-isobutyl-6-phenyl-1,2,4-triazolo[4,3-a]pyrazin-3-yl]-3-(3 pyridyl)propionic acid moiety 10b acting as a non-peptidic replacement for the P4-P2 (Pro-Phe-His) residues of the natural substrate angiotensinogen. Compounds 12m, 12o and 12q were potent inhibitors of partially purified human renin (IC50 values 1.7, 6.8, and 3.7 nM, respectively), and also effectively lowered blood pressure in anesthetized, sodium depleted marmosets following intravenous administration. On oral administration however, no blood pressure lowering activity could be detected, and absorption studies in bile duct cannulated rats indicate that this may be due primarily to poor oral absorption, rather than rapid biliary excretion. The reason for the observed poor oral activity is not clear, but it seems unlikely that poor aqueous solubility or metabolic instability to gut enzymes are rate-determining, and other factors such as high molecular weight may also be very important.

1,2,4-Triazolo[4,3-a]pyrazine derivatives with human renin inhibitory activity. 2. Synthesis, biological properties and molecular modeling of hydroxyethylene isostere derivatives

A series of inhibitors of human renin have been synthesized, derived from combination of a 2-(8-propyl-6-pyridin-3-yl-1,2,4-triazolo[4,3-a]pyrazin-3-yl)- 3-pyridin- 3-ylpropionic acid moiety 6c with the hydroxyethylene isostere of the scissile amide bond (2S,4S,5S)-5-amino-6-cyclohexyl-4-hydroxy-2-isopropylhexanoic acid (ChaOH--Val). The more potent members of this series showed good inhibitory activity against partially purified human renin, 7d, for example, having an IC50 of 0.2 nM. Structure-activity relationships for these compounds were consistent with their binding to the S4-S2' sites of human renin. Analogues 7e and 7h-k with a variety of substituents at the C-terminus all had in vitro IC50S less than 1 nM. In contrast with the majority of previously reported inhibitors of similar potency, these compounds contain no natural amino acid fragments. When administered intravenously to anesthetized, sodium-depleted marmosets at doses of 0.3-3.0 mg/kg, compound 7d caused a marked reduction in mean arterial pressure. Following oral administration at 30 mg/kg in the same animal model, 7d again elicited a significant fall in mean arterial pressure, accompanied by suppression of plasma renin activity lasting up to 3 h after dosing.

In vivo comparison of the renin inhibitor H77 with the angiotensin-converting enzyme inhibitor captopril

Changes in blood pressure and heart rate in response to the renin inhibitor H77 and the angiotensin-converting enzyme inhibitor captopril were compared in conscious dogs during progressive sodium depletion. There were significant positive correlations between plasma renin activities immediately prior to administration of the inhibitors and the subsequent reductions in blood pressure produced by the inhibitors. The slopes and intercepts were similar for the two inhibitors, suggesting that both H77 and captopril were operating predominantly via inhibition of the renin-angiotensin system. Although the effects of H77 and captopril on heart rate and blood pressure were quantitatively similar, a small additional action of captopril was observed in dogs that had been sodium restricted for 12 days. Captopril injected during H77 infusion also had a small additional hypotensive action and caused a significant further increase in heart rate.

Non-specific inhibition of pressor agents in vivo by the renin inhibitor pepstatin A

The specificity of pepstatin A as an inhibitor of the cardiovascular actions of renin injected into anaesthetized rats has been investigated. Pepstatin A 70 micrograms/kg/min partially inhibited the pressor response to injected renin without affecting the pressor responses to injected angiotensin II, phenylephrine or vasopressin. Pepstatin A 150 micrograms/kg/min also produced partial inhibition of injected renin, but in addition caused significant inhibition of the other pressor agents. This was in contrast to the effects of the angiotensin converting enzyme inhibitor captopril, 100 micrograms/kg i.v., which caused greater inhibition of the renin pressor response than pepstatin A without affecting the pressor response to injected angiotensin II, phenylephrine or vasopressin. Finally the direct acting vasodilator hydralazine was found to have a similar non-specific inhibitory effect to pepstatin A on the pressor responses to injected pressor agents. It is concluded that pepstatin A reduces the pressor responsiveness to injected pressor agents and that this non-specific cardiovascular activity limits the usefulness of pepstatin A as a pharmacological tool to inhibit renal renin in vivo.

Effects of the renin inhibitor H77 on the pressor responses to injected pig, rat and dog renins in vivo

The potency of the substrate analogue inhibitor H77 has been investigated in vivo in anaesthetized rats and conscious dogs. ID50 values have been determined against rat, dog and pig renins injected into rats and against dog renin injected into dogs. In the rat H77 was most potent against injected dog renin (ID50 0.53 mg/kg/h), of intermediate potency against injected pig renin (ID50 1.1 mg/kg/h) and least potent against injected rat renin (ID50 40 mg/kg/h). H77 was more potent against dog renin injected into dogs than into rats (ID50 against dog renin in dogs 0.056 mg/kg/h). At infusion rates up to 0.5 mg/kg/h in dogs and 50.0 mg/kg/h in rats H77 was without effect on the pressor response to angiotensin I. Hence H77 appears to be a specific inhibitor of dog, rat and pig renins in vivo, the dose of H77 depending upon the type of renin injected and the species of the recipient animal.