MECHANISMS OF IMPAIRMENT OF ENDOTHELIUM-DEPENDENT RELAXATION TO ACETYLCHOLINE IN WATANABE HERITABLE HYPERLIPIDAEMIC RABBIT AORTAS

1. The mechanism of impairment of the endothelium-dependent relaxation in response to acetylcholine (ACh) in aortas from Watanabe heritable hyperlipidaemic (WHHL) rabbits was investigated using a modified sandwich (layered) technique. Intact aortas from WHHL rabbits or Japanese white (JW) rabbits as the control were used as donor strips of endothelium-derived relaxing factor (EDFU?) and endothelium-denuded aortas from JW rabbits were used as detector strips. The EDRF released from a donor strip could be directly detected as the relaxation response in a detector strip. 2. The endothelium-dependent relaxations in all rabbit arteries were almost abolished by treatment with N^G-nitro-l-arginine methyl ester (an inhibitor of nitric oxide synthase). 3. The ACh-induced endothelium-dependent relaxations in the donor strips were impaired in WHHL rabbits in comparison with relaxations in JW and heterozygous WHHL rabbits. Similarly, the relaxation in the detector strips induced by EDRF released from donor strips was reduced in WHHL rabbits. There was a good negative correlation between the aortic total cholesterol content in the donor strips and the degree of relaxation in the detector strips from WHHL rabbits. 4. The reduced relaxation in the detector strips when using donor strips with high cholesterol accumulation or atheromatous plaque was not affected by superoxide dismutase plus catalase (scavengers of superoxide anions), indomethacin (an inhibitor of cyclo-oxygenase), ONO-3708 (an antagonist of endoperoxide/ thromboxane receptor) and 97–139 (an antagonist of endothelin ET_A receptor). 5. These results suggest that the mechanism of impaired endothelium-dependent relaxations in atherosclerotic WHHL rabbit aortas may be due to the reduced amount of EDRF, probably nitric oxide, from the endothelium and not due to its inactivation by oxygen-derived free radicals or masking by increased production of endothelium-derived contracting factors.     

CLEARANCE OF CHYLOMICRON-LIKE LIPID EMULSIONS IS INCREASED IN NORMAL RABBITS BUT NOT IN HETEROZYGOUS WATANABE HERITABLE HYPERLIPIDAEMIC RABBITS FOLLOWING TREATMENT WITH CHOLESTYRAMINE OR PRAVASTATIN

1. Cholestyramine and pravastatin are two potent hypocholesterolaemic drugs which lower plasma cholesterol by increasing the clearance of low density lipoproteins by high affinity uptake mechanisms. 2. We gave heterozygous Watanabe heritable hyperlipidaemic rabbits (hz-WHHL) cholestyramine and/or pravastatin for a two week period to try and ameliorate slow clearance of chylomicron remnants, which occurs because of reduced expression of the apolipoprotein B₁₀₀/E receptor. 3. In hz-WHHL rabbits the clearance of chylomicron-like lipid emulsions, traced by the decrease in plasma cholesteryl oleate radioactivity was not improved following treatment with either of the cholesterol lowering drugs. 4. In contrast, control rabbits had significantly less chylomicron-like emulsion cholesteryl-ester radioactivity remaining at each time of blood sampling. 5. Similarly, the clearance of chylomicron-like emulsion triolein was enhanced in normal rabbits receiving cholestyramine or pravastatin, whereas there was no detectable increase in clearance in hz-WHHL rabbits. 6. Combined treatment with cholestyramine and pravastatin increased the rate of receptor-mediated uptake in vivo in control rabbits but not in hz-WHHL rabbits. 7. The plasma lipid profiles of control and hz-WHHL rabbits parallelled the patterns of chylomicron-like emulsion clearance. Moderate hypertriglyceridaemia was identified in hz-WHHL rabbits compared to controls and there was no change in plasma triglyceride or cholesterol following drug therapy. In contrast, control rabbits had decreased plasma lipids following cholestyramine or pravastatin treatment. 8. It appears that therapy with lipid lowering drugs increased chylomicron remnant clearance in control rabbits by up-regulation of the apolipoprotein BIOO/E receptor. In contrast, in hz-WHHL rabbits receptor activity did not appear to be increased sufficiently to ameliorate defective remnant clearance. We have suggested that a critical receptor density is required for efficient remnant clearance.     

ENHANCEMENT OF ENDOTHELIUM-DEPENDENT RELAXATION IN THE AORTA FROM STROKE-PRONE SPONTANEOUSLY HYPERTENSIVE RATS AT DEVELOPMENTAL STAGES OF HYPERTENSION

1. Endothelium-dependent relaxation (EDR) in aortic rings from young (8 weeks) and adult (16 weeks and 20 weeks) stroke-prone spontaneously hypertensive rats (SHRSP) was investigated in comparison with age-matched Wistar-Kyoto rats (WKY). 2. At 8 weeks, acetylcholine (3×10^?-⁹-^?10^?-⁵ mol/L) and ionomycin (4×10^?8-10^?6 mol/L)-induced EDR in SHRSP aortae was significantly enhanced compared to that in WKY aortae. Mechanical denudation of the endothelium completely abolished, and pretreatment of aortae with N^G-monomethyl L-arginine (1 mmol/L), an inhibitor of nitric oxide formation, greatly reduced the relaxation in both strains. Indomethacin (10^?5 mol/L), a cyclo-oxygenase inhibitor that blocks the production of endothelium-derived contracting factors, did not significantly alter the relaxation by acetylcholine at this age. There was no difference in endothelium-independent relaxation of denuded aortae by sodium nitroprusside (10^?9-10^?6 mol/L) and 8-bromoguanosine 3‘, 5‘-cyclic monophos-phate (10^?6-10^?3 mol/L). 3. In adult SHRSP with established hypertension, however, the acetylcholine (10^?8-^?10^?5 mol/L)-induced relaxation markedly diminished at any of the concentrations tested compared to that observed in 8 week old SHRSP and WKY at 8–20 weeks of age. This finding differed from other observations where the relaxation in SHRSP was impaired only at higher concentrations of acetylcholine. Indomethacin pretreatment of aortae from 20 week old SHRSP restored acetylcholine-induced EDR to a level comparable with that in age-matched WKY. 4. These results suggest that the aorta from young SHRSP releases more endothelium-derived relaxing factors in response to acetylcholine and ionomycin, but the relaxation greatly diminishes in adult SHRSP with established hypertension. This may be due to counteraction of endothelium-derived contracting factors released from the endothelium with functional changes resulting from the long duration of the hypertension.     

NORADRENALINE SENSITIVITY AND CALCIUM FLUXES IN ARTERIES FROM RABBITS WITH PERINEPHRITIS HYPERTENSION

1. Contractions of isolated vascular and cardiac preparations taken from rabbits with perinephritis (one kidney, one wrapped) hypertension were compared with those of preparations from control operated animals. 2. Significantly increased sensitivity to noradrenaline, which acts on alpha 1-adrenoceptors, was found in mesenteric arterial rings but not in aortic rings. The degree of hypersensitivity was the same in the presence and absence of cocaine, suggesting that there is no increase in uptake of noradrenaline into adrenergic nerves in this model of hypertension. In contrast to these agonist-induced contractions, no increased sensitivity was found to potassium chloride, suggesting that hypersensitivity is specific for receptor mediated rather than membrane potential mediated effects. 3. No hypersensitivity to noradrenaline was found in the isolated left or right atria, which suggests that the hypertension is associated with changes in excitation-contraction coupling in blood vessels but not in cardiac muscle. 4. Hypertension increased basal 45Ca uptake in the mesenteric artery but not in the aorta. However, there was no significant difference between preparations from normotensive and hypertensive rabbits in 45Ca uptake or efflux stimulated by noradrenaline or KCl. 5. Increased basal 45Ca uptake could contribute to the increased sensitivity to noradrenaline found in the mesenteric artery in rabbit perinephritis hypertension.