Nobuharu Akatsuka

Stretch may enhance the release of endothelium-derived relaxing factor in rabbit aorta

The effect of vascular stretch on the release of EDRF was studied by measuring tissue cGMP levels of rabbit. Aortic rings of rabbit were quick-frozen in liquid nitrogen during varying resting tensions, and cGMP contents were determined by radio-immunoassay. The tissue cGMP levels significantly elevated with the increase in resting tension in endothelium-intact rings, but not in endothelium-denuded rings. Deprivation of extracellular calcium abolished the stretch-induced elevation of tissue cGMP levels in endothelium-intact segments. These stretch-induced endothelium dependent tissue cGMP elevations were unaffected by Ca2+ channel blockers, nicardipine and diltiazem. Data suggest that vascular stretch may release EDRF via mechanism dependent on extracellular calcium, but probably not through voltage-dependent calcium channel.

Vascular smooth muscle relaxation induced by epidermal growth factor is endothelium-dependent

Epidermal growth factor (EGF) is released from platelets during aggregation. Because we thought that EGF played a role in vascular tone, we investigated its vascular reactivity using isolated rat aortic strips with and without the endothelium. In the presence of endothelium, EGF relaxed vascular smooth muscle precontracted with 40 mM K+, 10(-5) M prostaglandin F2 alpha or 10(-6) M norepinephrine. The relaxation induced by EGF was more prominent on the prostaglandin F2 alpha- and norepinephrine-induced contractions than on the K(+)-induced contraction. Atropine (10(-5) M) and aspirin (10(-5) M) had no effect on the EGF-induced relaxation, but methylene blue (10(-5) M) partly abolished the relaxation evoked by EGF. These results suggest that EGF relaxes vascular smooth muscle in the presence of the endothelium. They also suggest that EGF has an effect on the endothelium to produce relaxing factor independent of cyclooxygenase; the releasing factor activates soluble guanylate cyclase, resulting in relaxation of vascular smooth muscle through the production of cyclic GMP.

Vascular smooth muscle relaxation by endothelium-dependent β1-adrenergic action

Abstract 1. Norepinephrine (NE) (10 −5 M) in rabbit aorta relaxed ring segments with endothelium precontracted with 10 −6 M NE, but not segments without endothelium. 2. The relaxation was inhibited with metoprolol and methylene blue, but not inhibited with yohimbine and indomethacin. 3. NE (10 −5 M) significantly elevated tissue c-GMP levels in segments with endothelium. 4. These studies suggest that the vascular relaxation by high doses of NE is mediated by the release of endothelium-derived relaxing factor (EDRF) induced by the stimulation of β 1 -adrenoceptor.

Acetylcholine-induced endothelium-dependent vascular smooth muscle relaxation in nitroglycerin-tolerant isolated rat aorta.

SummaryNitroglycerin (NTG) tolerance is recognized clinically, and its pharmacological mechanism has been thought to be due to a decrease in the accumulation of cyclic GMP (cGMP) which is a second messenger of NTG. Endothelium-derived relaxing factor (EDRF) also relaxes vascular smooth muscle through the activation of soluble guanylate cyclase and the production of cGMP. The purpose of this study was to investigate acetylcholine (ACh)-induced endothelium-dependent relaxation and cGMP response in NTG-tolerant isolated rat aorta. Ring strips prepared from the thoracic aorta of male Wistar rats were mounted in tissue baths and contracted with 10−6 M norepinephrine. NTG and ACh relaxation responses were compared before and after 1 h treatment with 5×10−4 M NTG. The chronological changes in tissue cGMP levels by 10−6 M NTG and ACh were compared between a control group (untreated) and NTG-tolerant group (treated with 5×10−4 M NTG for 1 h). The NTG dose-response curve shifted markedly to the right, but the ACh dose-response curve shifted to the left after the induction of NTG tolerance. In the control group, both NTG and ACh elevated the tissue cGMP levels, but in the NTG-tolerant group only ACh elevated cGMP significantly. However, in the NTG-tolerant group, the cGMP increase induced by ACh was smaller than that in the control group.These results suggest that NTG tolerance does not decrease, but rather augments ACh-induced endothelium-dependent vascular smooth muscle relaxation in isolated rat aorta.

Effect of captopril on acetylcholine-induced relaxation in the presence of nitroglycerin tolerance in isolated rabbit aorta.

We investigated the effects of angiotensin converting enzyme inhibitors on acetylcholine-induced endothelium-dependent vasodilation in the presence of nitroglycerin tolerance in rings of rabbit thoracic aorta mounted in tissue baths and precontracted with 10(-6) M norepinephrine. The vasorelaxant effects of acetylcholine were measured before and after 1 h treatment with 5 x 10(-4) M nitroglycerin. The acetylcholine dose-response curve shifted to the right after the induction of nitroglycerin tolerance. Pretreatment with captopril (a sulfhydryl angiotensin converting enzyme inhibitor), but not with M-1 (a metabolite of delapril and a nonsulfhydryl angiotensin converting enzyme inhibitor) restored acetylcholine-induced relaxation. Pretreatment with N-acetylcysteine also restored reduced acetylcholine-induced relaxation. These results suggest that the sulfhydryl group plays a major role in restoration of reduced acetylcholine-induced vasodilation in the presence of nitroglycerin tolerance.

Comparison of functional responses of canine coronary artery and saphenous vein.

SummaryFunctional responses of canine circumflex coronary arteries and saphenous veins before and after grafting surgery were assessed following physiologic and pharmacologic intervensions. Developed tension of ringed segments of vessels was recorded isometrically. Dose-dependent responses to norepinephrine revealed significantly greater responses of saphenous veins than coronary arteries (maximal responses were 2.79±0.45 and 0.44±0.34 g, respectively; p<0.001). Sensitivity to norephinephrine [as evaluated by 50% effective dose (ED50) value] was 3.3 times greater in the saphenous veins (p<0.001). STA2, a synthetic thromboxane A2 analog (see Methods), produced similar contractions of both vessels [maximal responses were 2.13±0.37 g in saphenous vein and 1.64±0.85 g in coronary artery; p < not significant (NS)], while sensitivity to STA2 of saphenous veins was 3.1 times greater than that of coronary arteries (p<0.001). In contrast to the foregoing responses, coronary arteris demonstrated significantly greater responses to potassium than saphenus veins (maximal responses were 2.16±0.71 g and 1.40±0.56 g, respectively; p<0.001). Moreover, coronary arteries revealed 1.6 times greater sensitivity than saphenous veins (p<0.001). When saphenous veins were transplanted into the femoral artery, the segments of the grafts (2 weeks or 3 months after surgery) revealed 4.0 and 1.7 times greater sensitivity (denervation supersensitivity) to norepinephrine and potassium than those of the control veins (p<0.01, respectively). Contractile tension response, however, decreased by at least 50% 3 months after surgery, most probably due to a thickened vessel wall and a reduced compliance. Thus, the functional characteristics of the saphenous vein, which were totally different from those of the coronary artery, seemed to be preserved even after grafting except for denervation supersensitivity, which may modify the differences between the two vessels. Those differences may be relevant to the control of the coronary circulation after coronary bypass surgery.