Tetsuhiko Nagao

Potentiation of endothelium-dependent relaxations to bradykinin by angiotensin I converting enzyme inhibitors in canine coronary artery involves both endothelium-derived relaxing and hyperpolarizing factors.

Studies were designed to investigate the mechanisms underlying the augmentation by angiotensin I converting enzyme (ACE) inhibitors of the endothelium-dependent relaxations evoked by bradykinin. Isometric tension, tissue levels of cGMP, and transmembrane potential were measured in isolated canine coronary arteries as indications of the respective contribution of nitric oxide and endothelium-derived hyperpolarizing factor. In rings of coronary artery with endothelium, relaxations to bradykinin were potentiated by the ACE inhibitors cilazaprilat and perindoprilat. NG-Nitro-L-arginine (NLA), a nitric oxide synthase inhibitor, impaired relaxations to bradykinin. But the presence of ACE inhibitors partially restored this activity. Bradykinin stimulated the production of cGMP, and this was enhanced significantly by ACE inhibitors, indicating an augmented release of nitric oxide. NLA abolished the increase induced by bradykinin irrespective of the presence of ACE inhibitors. Electrophysiological studies revealed that bradykinin elicited an endothelium-dependent hyperpolarization of vascular smooth muscle that was insensitive to NLA and potentiated by ACE inhibitors. The bradykinin-induced hyperpolarization and NLA-resistant relaxations were transient and impaired by potassium depolarization. Thus, production of endothelium-derived hyperpolarizing factor may account for the NLA-resistant relaxations of canine coronary arteries. The relaxations induced by bradykinin were unaffected by the B1 kinin receptor antagonist des-Arg9,[Leu8]-bradykinin either in the absence or in the presence of NLA but were antagonized by the B2 kinin receptor antagonist D-Arg[Hyp3,D-Phe7]-bradykinin. Molecular exclusion chromatography of 125I-labeled [Tyr8]-bradykinin and its degradation products demonstrated that the breakdown of the kinin by isolated coronary arteries was prevented in the presence of perindoprilat.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-dependent effects of converting-enzyme inhibitors

Angiotensin-converting enzyme (ACE) inhibitors were designed to prevent the vasoconstrictor influence of the activated renin-angiotensin system. However, it has long been suspected that the vasodilator actions of these compounds are not entirely related to inhibition of the generation of angiotensin II. Bradykinin, which is rapidly degraded by ACE, stimulates the release of endothelium-derived vasodilator mediators, including nitric oxide, endothelium-derived hyperpolarizing factor, and prostacyclin. These mediators do not contribute to the vasodilator effect of bradykinin in every arterial bed. However, the prevention by ACE inhibitors of the degradation of bradykinin-induces an augmentation of the production of these substances and thus potentiates the dilatation evoked by the peptide. The existence of a local kallikrein-kinin system in the vascular wall has been demonstrated, and locally generated kinins contribute to the acute vasodilator actions of ACE inhibitors. ACE inhibitors can potentiate endothelium-dependent dilatations evoked by neurohumoral mediators that are not substrates for ACE. Thus, the vasodilator properties of ACE inhibitors not only reflect inhibition of the renin-angiotensin system but also depend on the enhanced production of endothelium-derived mediators.

Characterization of endothelium-dependent relaxations resistant to nitro-L-arginine in the porcine coronary artery

1 Previous studies, demonstrated that endothelium‐dependent relaxations which are resistant to nitro‐l‐arginine (an inhibitor of nitric oxide synthase) are accompanied by membrane hyperpolarization in the porcine coronary artery. The present experiments were designed to characterize further this type of endothelium‐dependent relaxation in response to bradykinin by measuring isometric force in isolated rings of that artery. The experiments were performed in the presence of indomethacin to rule out vasoactive prostanoids. 2 Bradykinin induced comparable endothelium‐dependent relaxations of proximal and distal rings of porcine coronary arteries contracted with prostaglandin F2α in the presence of nitro‐l‐arginine. 3 Bradykinin and SIN 1 (a donor of nitric oxide) reduced contractions induced by prostaglandin F2α in an additive fashion in the presence of nitro‐l‐arginine. 4 Bradykinin (in the presence of nitro‐l‐arginine) relaxed the tissues contracted with tetraethylammonium, prostaglandin F2α, phorbol 12, 13‐diacetate or endothelin, with similar pD2 values. 5 The time course of the relaxations induced by bradykinin (in the presence of nitro‐l‐arginine) and UK 14304 (an α2‐adrenoceptor agonist, in the absence of the inhibitor of nitric oxide synthase) were comparable. 6 These results suggest that, in the porcine coronary artery, nitro‐l‐arginine‐resistant relaxations (a) are distributed similarly in the proximal and distal parts of the artery, (b) contribute to inhibition of vascular smooth muscle with nitric oxide in an additive fashion, (c) occur during contractions induced by various contractile agents and (d) do not precede those mediated by nitric oxide.

Calmodulin antagonists inhibit endothelium-dependent hyperpolarization in the canine coronary artery

1 The effects of the calmodulin antagonists, calmidazolium and fendiline were investigated on endothelium‐dependent hyperpolarization in the canine coronary artery. The membrane potential of vascular smooth muscle cells was measured with the microelectrode technique. 2 Smooth muscle cells of the canine coronary artery had a resting membrane potential of −50 mV. Bradykinin and the Ca2+‐ionophore, A23187, induced concentration‐ and endothelium‐dependent hyperpolarization. The hyperpolarization induced by a supramaximal concentration of bradykinin (10−6 m) reached approximately 20 mV. 3 Calmidazolium (10−5 m) and fendiline (10−4 m) inhibited hyperpolarization induced by bradykinin and A23187. By contrast, calmidazolium did not affect the hyperpolarization induced by lemakalim, an opener of ATP‐sensitive K+‐channels. 4 These observations suggest that calmodulin is involved in the generation of endothelium‐dependent membrane hyperpolarization of vascular smooth muscle.

Potentiation by trandolaprilat of the endothelium-dependent hyperpolarization induced by bradykinin

Summary In canine coronary arteries. bradykinin evokes endothelium-dependent relaxations that are mediated by nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). The converting-enzyme inhibitor trandolaprilat potentiates the endothelium-dependent relaxations evoked by bradykinin in this tissue. The present experiments were designed to determine whether or not facilitated release of EDHF contributes to the augmented response to bradykinin in the presence of trandolaprilat. Organ-chamber studies were performed to measure changes in isometric tension in rings of canine coronary arteries. In the presence of nitro-Larginine, an inhibitor of NO synthase, trandolaprilat augmented the endothelium-dependent relaxations evoked by bradykinin. These relaxations were not inhibited by the K+-channel inhibitors tetraethylammonium. 4-aminopyridine, or glibenclamide, but were abolished in high-potassium solution. The membrane potential in individual smooth-muscle cells of coronary artery was measured by means of glass microelectrodes. Trandolaprilat potentiated the endothelium-dependent hyperpolarizations evoked by a subthreshold concentration of bradykinin, and these endothelium-dependent hyperpolarizations were inhibited by high-potassium solution. These experiments demonstrate that EDHF contributes to the relaxation evoked by bradykinin in the canine coronary artery and that trandolaprilat potentiates the release of this factor. This effect of trandolaprilat may contribute to its vasodilator properties.

Potentiation of endothelium-dependent hyperpolarization to serotonin by dietary intake of NC 020, a defined fish oil, in the porcine coronary artery

The membrane potential of vascular smooth muscle cells of the porcine coronary artery was measured to examine whether serotonin evokes endothelium-dependent hyperpolarization, and if it does, whether the electrical responses are modulated by the chronic dietary intake of NC 020, a defined fish oil. Serotonin induced transient, concentration-dependent hyperpolarizations of coronary arterial smooth muscle cells. The hyperpolarization was observed in tissues with, but not in those without endothelium. In coronary arteries obtained from pigs fed chronically with NC 020, serotonin induced significantly larger hyperpolarizations than those observed in control arteries. These results suggest that endothelium-dependent hyperpolarization may contribute to the endothelium-dependent relaxation evoked by serotonin and to its potentiation by the dietary intake of fish oil (NC 020).

Electrical and mechanical changes during anoxic contractions of the isolated canine basilar artery.

Isometric tension and membrane potential were measured to determine the electrophysiological events occurring during anoxia in the isolated canine basilar artery. Anoxia induced transient contractions which were inhibited by the Ca2+-channel inhibitor, diltiazem, and were abolished in Ca2+-free solution. Anoxic contractions were accompanied by membrane depolarizations, which were resistant to diltiazem. When matched contractions were obtained with anoxia and high K+, the level of membrane depolarization was smaller during anoxic contractions. These results support the importance of voltage-dependent Ca2 + influx in the generation of anoxic contractions in the canine basilar artery. However, membrane depolarization does not fully account for these anoxic contractions.

The sydnonimine C87-3754 evokes endothelium-independent relaxations and prevents endothelium-dependent contractions in blood vessels of the dog.

Experiments were designed to compare the relaxing activities of the new sydnonimine C87–3754 with SIN-1 in arteries and veins of the dog, and to determine whether C87–3754 can prevent endothelium-dependent contractions. Rings of coronary and femoral arteries, and saphenous veins were suspended in organ chambers for the measurement of changes in isometric tension. SIN-1 and C87–3754 evoked concentration-dependent relaxations in all rings of blood vessels contracted with a submaximal concentration of either prostaglandin F2α, endothelin-1, phenylephrine, or norepinephrine. In both arteries and veins, the concentration- relaxation curves to C87–3754 were shifted significantly to the right (by two to three logarithmic units) of that to SIN-1. The presence of endothelium significantly inhibited the relaxations to SIN-1 but did not affect those to C87–3754. The treatment of coronary arteries with methylene blue or oxy-hemoglobin significantly impaired the relaxations to SIN-1 and C87–3754. Neither C87–3754 nor its prodrug pirsidomine (CAS 936) affected the membrane potential in coronary arteries. The endothelium-dependent contractions evoked by nitro L-arginine, arachidonic acid, and the calcium ionophore A23187 in basilar arteries of the dog were inhibited by C87– 3754. These results indicate that the sydnonimine C87–3754 is a dilator of both arterial and venous smooth muscle, and can prevent endothelium-mediated contractions in cerebral arteries of the dog. The inhibition of vascular tone is likely to involve the activation of soluble guanylate cyclase, causing enhanced production of cyclic guanosine monophosphate in the smooth muscle without a change in membrane potential.

Calcium and the Production of Endothelium-Derived Vasoactive Factors

Endothelial cells play a major role in the local control of the vascular system. They form a semipermeable barrier which regulates the exchange of nutrients and waste products between the blood and the underlying smooth muscle. These cells also influence the tone of the blood vessel wall, in part, by releasing potent and short-lasting relaxing factors such as prostanoids, nitric oxide and endothelium-derived hyperpolarizing factor, as well as contractile factors such as thromboxane / endoperoxides, superoxide anions, and endothelin(s). The present review will discuss the biological properties of the vasoactive factors and emphasize the importance of calcium ions for both the production of the factors by the endothelial cells and the modulation of the vascular tone that they achieve.