Activation of endothelial L-arginine pathway in resistance arteries. Effect of age and hypertension

Beta-adrenoceptor agonist mediated relaxation of rat isolated resistance arteries: a role for the endothelium and nitric oxide

1. Isoprenaline (10(-9)-10(-5) M) relaxed rat isolated mesenteric resistance arteries pre-contracted with K+ (30-60 mM) (p EC50 (M) 8.03 +/- 0.40; maximum relaxation 66.79 +/- 2.43%, n = 7). This relaxation was partially attenuated by the nitric oxide (NO) synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 10(-4) M). 2. The beta 2-adrenoceptor agonist, salbutamol (10(-9)-10(-5) M), produced a modest maximum relaxation (35.93 +/- 2.93%), which was not sensitive to L-NAME. 3. The beta 1-adrenoceptor agonist, dobutamine (10(-9)-10(-5) M), relaxed arteries precontracted with K+. This relaxation was abolished by L-NAME (10(-4) M) and also by propranolol (10(-6) M), but not affected by D-NAME (10(-4) M). The inhibition by L-NAME was partially reversed by L-arginine (10(-3) M). Removal of the endothelium severely attenuated relaxation to dobutamine. 4. Contractile responses to depolarizing K+ solutions were enhanced by the addition of L-NAME, and also by removal of the endothelium. 5. The above findings demonstrate that beta 1-adrenoceptor causes relaxation via NO release from the endothelium of rat mesenteric resistance arteries. In addition, contraction to K+ is modified by release of NO from the endothelium, possibly in response to tension development.

Hypertension and left ventricular hypertrophy are associated with impaired endothelium-mediated relaxation in human coronary resistance vessels

BACKGROUND

Patients with hypertension and myocardial hypertrophy may have signs and symptoms of myocardial ischemia in the absence of obstructive coronary disease. Prior investigations have demonstrated impaired coronary flow reserve and have led to speculation that microvascular dysfunction might contribute to ischemia in these patients. Experimental studies have shown that the endothelium, an important regulator of microvascular tone, can be damaged by hypertension and is dysfunctional in cardiomyopathy. We hypothesized that endothelium-dependent vasodilation is impaired in the coronary microvasculature of patients with hypertension and ventricular hypertrophy.

METHODS AND RESULTS

We studied coronary microvascular responses in 10 patients with left ventricular hypertrophy secondary to essential hypertension (HTN) (mean arterial pressure at catheterization, 151/94 mm Hg; mean posterior wall thickness, 1.4 +/- 0.1 cm) and nine normal control subjects with no history of hypertension (mean arterial pressure at catheterization, 128/75 mm Hg; mean posterior wall thickness, 1.0 +/- 0.02 cm) using the intracoronary Doppler catheter and quantitative angiography to assess changes in coronary blood flow (CBF). All patients had normal left ventricular systolic function. To assess microvascular endothelial function, we infused the endothelium-dependent vasodilator acetylcholine (10(-8)-10(-6) M) and the endothelium-independent vasodilator adenosine (10(-6)-10(-4) M) into the left anterior descending coronary artery. In response to acetylcholine, CBF increased only 32 +/- 25% in HTN patients, whereas CBF increased 192 +/- 39% in normal control subjects (p = 0.003). CBF increased 465 +/- 93% in HTN patients and 439 +/- 41% in normal control subjects in response to adenosine (p = NS). The proportion of coronary flow reserve attributable to endothelium-dependent dilation (obtained from peak acetylcholine/peak adenosine flow response) was 48 +/- 9% in normal control subjects but only 7 +/- 8% in HTN patients (p = 0.008).

CONCLUSIONS

Endothelium-dependent vasodilation is markedly impaired in the coronary microvessels of patients with hypertension and ventricular hypertrophy. Loss of this vasodilator mechanism may contribute to disordered coronary flow regulation and the ischemic manifestations of hypertensive heart disease.

Inhibition and stimulation of nitric oxide synthesis in the human forearm arterial bed of patients with insulin-dependent diabetes

Patients with insulin-dependent diabetes mellitus have an increased mortality and morbidity due to vascular complications. Nitric oxide from the vascular endothelium contributes to the control of normal vascular tone, and endothelial dysfunction has been implicated in the pathogenesis of diabetic vascular disease. In this study we have examined basal and stimulated nitric oxide-mediated vasodilatation in insulin-dependent diabetics and age- and sex-matched healthy controls. Drugs were infused locally into the brachial artery and forearm blood flow measured using venous occlusion plethysmography. Noradrenaline and NG-monomethyl-L-arginine produced similar reductions in resting forearm blood flow in healthy controls. However, in the diabetics, NG-monomethyl-L-arginine was significantly less effective than noradrenaline. Comparing between groups, the response to NG-monomethyl-L-arginine was also significantly less in the diabetics compared with the healthy controls. The response to sodium nitroprusside was significantly less in the diabetics compared with the healthy controls, whereas the responses to both acetylcholine and verapamil were the same in the two groups. The results provide evidence for an abnormality of basal nitric oxide-mediated dilatation in the forearm arterial bed of patients with insulin-dependent diabetes mellitus, and suggest that the vascular smooth muscle is less sensitive to nitric oxide.

Method for study of endothelium-dependent vasodilatation in rabbit intrarenal arterial network

The objectives of this study were twofold: 1) to determine whether a microdissected rabbit intrarenal arterial network (IAN), consisting mainly of interlobar, arcuate, and interlobular arteries exhibits endothelium-dependent vasodilatation, and 2) to establish a means of selectively abolishing this response. The IAN was perfused at a constant flow with heated-oxygenated Krebs-bicarbonate buffer through the main renal artery, and evoked responses were limited to vessels distal to the renal artery. A decrease in perfusion pressure reflected a vasodilator response, after vascular tone had been induced by intraarterial infusion of phenylephrine. Bolus injections of acetylcholine produced graded endothelium-dependent vasodilator responses, whereas glyceryl trinitrate caused endothelium-independent responses. Manual de-endothelialization was accomplished by gently stroking the IAN and at the same time removing any remaining glomeruli. This procedure blocked the response to acetylcholine, but not to glyceryl trinitrate (n = 6). A 10-min infusion of NG nitro-L-arginine (NArg) (4 x 10(-5)-3 x 10(-4) M) into the IAN also selectively attenuated the response to acetylcholine (n = 7). The third procedure, consisting of a 10-min infusion of 22-44 mM hydrogen peroxide into the IAN also attenuated the response to acetylcholine, but not to glyceryl trinitrate in nine of 12 experiments. This investigation demonstrates that intrarenal arteries are capable of undergoing endothelium-dependent vasodilatation, and the potential use of the IAN for further study of renal endothelium-derived vasoactive factors.

Endothelium-dependent relaxation and noradrenaline sensitivity in mesenteric resistance arteries of streptozotocin-induced diabetic rats

1. Noradrenaline sensitivity and acetylcholine-induced relaxation were investigated in mesenteric resistance arteries of control and streptozotocin-induced diabetic rats. 2. The diabetic rats demonstrated enhanced vascular sensitivity to noradrenaline compared with age-matched controls (pEC50 5.99 +/- 0.06 for diabetic rats, n = 25, versus 5.82 +/- 0.03 for controls, n = 45, P < 0.05). 3. Significant impairment of acetylcholine-induced relaxation was observed in arteries from the diabetic animals compared with controls (pEC50 6.81 +/- 0.17 for diabetic rats, n = 21, versus 7.54 +/- 0.17 for controls, n = 45, P < 0.001). 4. The difference between acetylcholine-induced relaxation in diabetic and control arteries remained in the presence of 10 microM indomethacin (pEC50 6.41 +/- 0.11 for diabetic rats, n = 16, versus 7.59 +/- 0.08 for controls, n = 20, P < 0.001). 5. The nitric oxide synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA, 1 mM) produced profound inhibition of acetylcholine-induced relaxation in diabetic arteries but partial inhibition in controls. The incomplete inhibition of acetylcholine-induced relaxation by L-NMMA in the control arteries was the result of ineffective inhibition of nitric oxide synthase since an alternative inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 0.1 mM), led to similar inhibition to that seen in the diabetic arteries with L-NMMA. The endothelium-derived relaxing factor (EDRF)-mediated component of acetylcholine-induced relaxation determined by use of the nitric oxide synthase inhibitors was, therefore, apparently reduced in diabetic rats compared with control animals.6. In further experiments L-NAME was found to enhance the response to noradrenaline in control rats but not in diabetic animals, suggesting that the abnormal response to noradrenaline in the diabetic animals was also due to reduced EDRF release.7. Nitroprusside-induced relaxation (endothelium-independent) was similar in arteries from control anddiabetic rats (pEC5o 7.61 +/- 0.13 for diabetic arteries, n = 18, versus 7.68 +/- 0.15 in the controls, n = 20, P not significant).8. These results suggest that endothelial function is abnormal in mesenteric resistance arteries of streptozotocin-induced diabetic rats and that this is predominantly due to reduced EDRF release.

Prostaglandin H2 and thromboxane A2 are contractile factors in intrarenal arteries of spontaneously hypertensive rats

Vascular resistance is increased in the kidneys of spontaneously hypertensive rats (SHR). Previous studies have demonstrated impaired vascular relaxations of mesenteric resistance arteries of SHR because of increased production of a cyclooxygenase-dependent endothelium-derived contracting factor. To test the hypothesis that altered endothelial function contributes to the enhanced constriction in kidneys of SHR, endothelium-mediated relaxations of renal resistance arteries from 5-6-week-old prehypertensive SHR and Wistar-Kyoto rats were compared in arteriographs. Acetylcholine induced endothelium-dependent contractions in SHR arteries, while potent endothelium-dependent relaxations were noted in renal arteries from Wistar-Kyoto rats. Inhibition of cyclooxygenase (indomethacin) or blockade of prostaglandin H2-thromboxane A2 receptors (SQ 29,548) blocked acetylcholine-induced contractions in SHR renal arteries; relaxations in SHR renal arteries after either treatment were similar to those observed in renal arteries from Wistar-Kyoto rats. NG-Nitro-L-arginine inhibited acetylcholine-mediated relaxations in both SHR and Wistar-Kyoto arteries. Endothelium-independent relaxations induced by verapamil were comparable in SHR and Wistar-Kyoto arteries. Thus, the impaired response to acetylcholine in SHR renal resistance arteries may result from the release of endothelium-derived cyclooxygenase products (prostaglandin H2 or thromboxane A2), which oppose endothelium-derived nitric oxide-mediated relaxation.

Age, hypertension and hypercholesterolaemia alter endothelium-dependent vascular regulation

As a source of several vasoactive factors, the endothelium takes part in the regulation of vascular tone. The most important endothelium-derived vasoactive substances are nitric oxide, prostacyclin, endothelin-1 and contracting factors requiring the activity of cyclooxygenase. The endothelium is an obvious target organ of cardiovascular risk factors. Accordingly, functional alterations do occur with aging, hypertension and hypercholesterolaemia. All three conditions are associated with a decreased basal and simulated release of endothelium-derived nitric oxide. On the other hand, the release of endothelin-1 appears to increase with age, while the sensitivity to the peptide markedly decreases under the same conditions. In the spontaneously hypertensive rat, acetylcholine and stretch evoke the release of a cyclooxygenase-dependent endothelium-derived contracting factor, most likely prostaglandin H2. The circulating levels of endothelin-1 on the other hand are not increased in experimental and human hypertension. In the porcine coronary circulation, oxidized low-density lipoproteins selectively reduced endothelium-dependent relaxations to aggregating platelets, serotonin and thrombin which are mediated by nitric oxide. The alterations of endothelial function occurring with aging, hypertension and hypercholesterolaemia may have important clinical implications for the pathogenesis of cardiovascular disease.

Endothelium-dependent relaxation and L-arginine metabolism in genetic hypertension

This study characterizes the effects of L-arginine and NG-monomethyl L-arginine on dilator responsiveness of vascular tissue from Wistar-Kyoto rats and stroke-prone spontaneously hypertensive rats. Rings of abdominal aorta were suspended in tissue baths for measurement of isometric force. After contraction induced by phenylephrine, cumulative addition of acetylcholine, L-arginine, or A23187 to the muscle bath caused a similar relaxation of aortic rings in both animal groups. To test the hypothesis that arginine metabolism is altered in hypertension, aortic rings were incubated with NG-monomethyl L-arginine. NG-monomethyl L-arginine (10-300 microM) did not affect contractile responses to phenylephrine (10(-10) to 10(-4) M) in either animal group (EC50, 10(-7) M). Exposure of aortic rings to NG-monomethyl L-arginine resulted in a greater inhibition of relaxation response to acetylcholine (10(-10) to 10(-6) M) in hypertensive animals. NG-monomethyl L-arginine (300 microM) caused complete inhibition of relaxation to acetylcholine in the hypertension group. Incubation with L-arginine (10-100 microM) overcame the inhibition of acetylcholine-induced relaxation produced by NG-monomethyl L-arginine in both groups. Exposure of aortic ring segments to NG-monomethyl L-arginine attenuated relaxation responses to A23187 (10(-10) to 3 x 10(-6) M) in both groups. L-Arginine-induced reversal of the inhibitory effect of NG-monomethyl L-arginine on the relaxation responses to A23187 was similar between groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased endothelium-dependent hyperpolarization to acetylcholine in smooth muscle of the mesenteric artery of spontaneously hypertensive rats

The endothelium-dependent vascular relaxation to acetylcholine (ACh) in spontaneously hypertensive rats (SHR) may be impaired because of an imbalance of endothelium-derived relaxing factor and contracting factor. However, the role of the endothelium-dependent hyperpolarization remains undetermined. We examined the ACh-induced hyperpolarization and its contribution to relaxation in arteries of SHR. Membrane potentials were recorded from the mesenteric artery trunk of 6-8-month-old male SHR and also Wistar-Kyoto (WKY) rats. Endothelium-dependent hyperpolarization to ACh was unaffected by NG-nitro-L-arginine, indomethacin, or glibenclamide; was reduced by tetraethylammonium or high K+ solution; and was enhanced by low K+ solution or methylene blue, thereby indicating that hyperpolarization is not mediated by nitric oxide (endothelium-derived relaxing factor) but is presumably mediated by a hyperpolarizing factor and is due to an opening of K+ channels that probably differ from the ATP-sensitive ones. Hyperpolarizations to ACh were markedly reduced in SHR compared with findings in WKY rats (maximum, 8 +/- 1 versus 17 +/- 1 mV). In addition, under conditions of depolarization with norepinephrine (10(-5) M), the ACh-induced hyperpolarization was even less and transient in SHR, while it was large and sustained in WKY rats (6 +/- 1 versus 29 +/- 2 mV). Endothelium-dependent relaxations to ACh in arterial rings precontracted with 10(-5) M norepinephrine were far less in SHR than in WKY rats, even in the presence of indomethacin. Furthermore, high K+ solution showed smaller inhibitory effects on the relaxations in SHR than in WKY rats. Endothelium-independent hyperpolarizations and relaxations to cromakalim, a K+ channel opener, were similar between SHR and WKY rats. It would thus appear that the endothelium-dependent hyperpolarization to ACh is reduced in SHR and this would, in part, account for the impaired relaxation to ACh in SHR mesenteric arteries.

Endothelin stimulated by angiotensin II augments contractility of spontaneously hypertensive rat resistance arteries

In cultured endothelial cells, endothelin is produced after stimulation with angiotensin II. The effects of angiotensin II and endothelin-1 on vascular sensitivity to norepinephrine were studied in perfused rat mesenteric resistance arteries. Expression of endothelin messenger RNA (mRNA) was determined in endothelial cells obtained from the mesenteric circulation. Perfusion (5 hours) of the arteries with angiotensin II (10(-7) M) potentiated contractions in arteries with endothelium induced by norepinephrine in spontaneously hypertensive rats but not Wistar-Kyoto rats. The potentiation was inhibited by phosphoramidon and an endothelin antibody. Short-term stimulation (1 hour) with angiotensin II did not cause the potentiation. Stimulation with angiotensin I (10(-7) M; 5 hours) caused a potentiation prevented by captopril. In endothelial cells collected from the mesenteric arterial bed of spontaneously hypertensive rats, endothelin-specific mRNA was constitutively expressed, and the level of endothelin transcripts was increased by angiotensin II (10(-7) M). Threshold concentrations of exogenous endothelin-1 potentiated contractions induced by norepinephrine in arteries with and without endothelium of spontaneously hypertensive rats but not Wistar-Kyoto rats. Thus, angiotensin II stimulates the endothelial production of endothelin in situ and therapy potentiates contractions to norepinephrine in mesenteric resistance arteries of spontaneously hypertensive rats. This suggests that vascular endothelin production acts as an amplifier of the pressor effects of the renin-angiotensin system that may play an important role in hypertension.

L-arginine abrogates salt-sensitive hypertension in Dahl/Rapp rats

This study examined the contribution of nitric oxide (NO) to the susceptibility or resistance to the hypertensive effects of high sodium chloride (8.0% NaCl) intake in young Dahl/Rapp salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats. Using NG-monomethyl-L-arginine (L-NMMA) as a probe for NO production in vivo, we found that increasing dietary sodium chloride increased NO activity in salt-resistant rats, but not in salt-sensitive rats. Exogenous L-arginine, the substrate for NO synthesis, decreased blood pressure to normotensive levels in salt-sensitive rats made hypertensive for 2 wk from 8.0% NaCl chow. D-arginine had no effect on blood pressure of these rats and L-arginine did not change blood pressure of salt-resistant rats. Intraperitoneal injections of L-arginine and its precursor, L-citrulline, and oral L-arginine, but not D-arginine, prevented the increase in blood pressure in salt-sensitive rats on the high salt chow over 2 wk of observation. In contrast, L-arginine did not alter the development of hypertension in spontaneously hypertensive rats. Mean urinary cGMP levels were higher in salt-sensitive rats on oral L-arginine than salt-sensitive rats on D-arginine. Infusion of L-NMMA acutely decreased, whereas intravenous L-arginine rapidly increased, urinary cGMP in both groups. L-arginine and L-citrulline increased production of NO and prevented salt-sensitive hypertension in Dahl/Rapp rats.

Release of nitric oxide from human vascular smooth muscle cells

It has recently been shown that nitric oxide (NO) or a labile NO-releasing compound is produced in endothelial cells. In the present study we measured the NO-release from human vascular smooth muscle cells in culture. The cells released an average 2.2 x 10(-9) moles nitric oxide per 10(8) cells in ten minutes with a large variation between different cell lines and passages without stimulators. The NO-release was markedly reduced by the inhibitor of NO-formation NG-momomethyl-L-arginine (L-NMMA, 10(-5) M) to 3% of the control levels (p less than 0.02, n = 4), but unaffected by acetylcholine, bradykinin or endothelin -1, -2 or -3. In a microperfusion system the compound released from human vascular smooth muscle cells caused profound relaxation of isolated rat mesenteric resistance arteries. Thus, human vascular smooth muscle cells in culture produce and release biologically active NO from L-arginine.

Endothelin in hypertensive resistance arteries. Intraluminal and extraluminal dysfunction

The effects of intraluminal and extraluminal endothelin-1 and its interactions with endothelium-derived relaxing factor were studied in perfused mesenteric resistance arteries of Wistar-Kyoto rats and spontaneously hypertensive rats. Changes in intraluminal diameter were recorded. In adult Wistar-Kyoto rats, but not spontaneously hypertensive rats, low concentrations of intraluminal endothelin-1 (10(-10) to 10(-9) M) caused relaxations of quiescent arteries blocked by indomethacin. After endothelial removal, intraluminal endothelin-1 evoked concentration-dependent contractions in both strains. Extraluminal endothelin-1 caused greater contractions of arteries with endothelium than intraluminal endothelin-1, and the sensitivity was lower in adult hypertensive rats; endothelial removal enhanced the contractions to extraluminal endothelin-1 to a greater extent in hypertensive than in normotensive rats. In arteries without endothelium, intraluminal and extraluminal endothelin-1 caused comparable contractions, but the sensitivity was reduced in adult but not young hypertensive as compared with normotensive rats. Both young spontaneously hypertensive and normotensive rats exhibited a high sensitivity to the peptide. In arteries precontracted with endothelin-1, endothelium-dependent relaxation to intraluminal acetylcholine was reduced in hypertensive as compared with normotensive rats, whereas relaxations to extraluminal acetylcholine were increased in hypertensive rats. Thus, endothelin-1 interacts with both vascular smooth muscle and the endothelium. The sensitivity of vascular smooth muscle to endothelin-1 is reduced in adult hypertensive rats. Intraluminal activation of the endothelium by endothelin-1 or acetylcholine is reduced in spontaneously hypertensive rats, whereas extraluminal activation causes more pronounced responses in hypertensive than in normotensive rats, suggesting a prominent dysfunction of the intraluminal surface of the endothelium in hypertension.

Renovascular hypertension impairs formation of endothelium-derived relaxing factors and sensitivity to endothelin-1 in resistance arteries

1. Endothelium-dependent vascular regulation was investigated in mesenteric resistance arteries of Goldblatt two kidney-one clip (2K1C) renovascular hypertensive rats. 2. Third order branches of mesenteric arteries were dissected free and mounted on glass cannulae in an organ chamber. Changes in vascular diameter were measured in pressurized and perfused arteries with a video dimension analyzer. 3. Acetylcholine evoked endothelium-dependent relaxations that were much more pronounced with intraluminal than with extraluminal application. In 2K1C rats the relaxation induced by intraluminal, but not extraluminal acetylcholine was decreased compared to normotensive Wistar Kyoto rats (WKY). Increased duration of hypertension further decreased the response to intraluminal but not extraluminal acetylcholine. 4. Endothelin-1 and noradrenaline caused contractions which were augmented by removal of the endothelium. This augmentation was reduced in 2K1C rats compared to WKY; the difference was small with noradrenaline but more pronounced with endothelin-1. 5. In arteries without endothelium the sensitivity, but not the maximal contraction to endothelin-1 was lower in 2K1C rats, while the response to noradrenaline was not different in 2K1C rats and WKY. The sensitivity to the peptide was not further affected by increasing the duration of hypertension. 6. Thus, renovascular hypertension leads to an impaired intraluminal, but not extraluminal activation of the release of endothelium-derived relaxing factor and a decreased inhibitory effect of the endothelium against endothelin-1- and noradrenaline-induced contractions in mesenteric resistance arteries. Furthermore, the sensitivity, but not the maximal response of vascular smooth muscle to endothelin-1 was reduced.

The endothelial L-arginine/nitric oxide pathway and the renal circulation

Endothelial cells contain an enzyme(s) which produces nitric oxide from L-arginine in response to a variety of mechanical stimuli as well as to autacoids and local and circulating hormones. Nitric oxide is a potent vasodilator and inhibitor of platelet function; it exerts its effects via activation of soluble guanylate cyclase and subsequent formation of cyclic 3'-5'-guanosine monophosphate. In the kidney, activation of the endothelial L-arginine pathway is associated with increases in renal blood flow, diuresis and natriuresis, while the glomerular filtration rate remains constant. The activity of the endothelial L-arginine pathway is impaired in hypertension and during chronic therapy with cyclosporine A. In addition, diabetes and atherosclerosis impair this pathway. Thus, the endothelial L-arginine pathway plays an important role in the local regulation of blood flow. Alterations in the activity of this pathway may play an important role in the pathophysiology of hypertension and renal disease.

Regional coronary haemodynamic effects of two inhibitors of nitric oxide synthesis in anaesthetized, open-chest dogs

1. The role of endothelial nitric oxide synthesis from L-arginine in the regulation of coronary vascular tone and myocardial tissue perfusion was evaluated in anaesthetized, open-chest dogs. Coronary blood flow was measured with an electromagnetic flow probe placed around the left circumflex coronary artery. Coronary vascular resistance was calculated from mean arterial blood pressure and mean coronary blood flow, whereas regional myocardial tissue flow was determined by use of the radioactive microspheres technique. 2. NG-monomethyl L-arginine (L-NMMA) and NG-nitro-L-arginine methyl ester (L-NAME), administered directly into the left circumflex artery, induced a small increase in arterial blood pressure and an increase in coronary vascular resistance. However, myocardial tissue perfusion, assessed by the microspheres technique (whether subendocardial, subepicardial, or transmural), was unaffected by L-NMMA or L-NAME. 3. Acetylcholine, administered intracoronarily, induced an increase in left circumflex coronary blood flow and a decrease in coronary vascular resistance, without affecting systemic haemodynamics. This coronary vasodilator effect of acetylcholine was markedly inhibited by L-NMMA and L-NAME, the latter being a more potent antagonist than the former. 4. These results indicate that the endothelial L-arginine pathway is largely responsible for the coronary vasodilator effect of acetylcholine. However, although basal release of nitric oxide from L-arginine apparently contributes to the regulation of resting coronary vascular tone, blockade of this pathway does not affect myocardial tissue perfusion, possibly because of compensatory mechanisms occurring at the level of small arterioles and/or capillaries.

Hormonal and local factors in hypertension

The cause of primary (essential) hypertension remains unknown, but a number of circulating hormones and endothelium-derived factors are probably involved. This review summarizes recent evidence on the roles of hyperinsulinemia, the renin-angiotensin system, atrial natriuretic factor, and three endothelium-derived factors--prostacyclin, endothelium-derived relaxing factor, and endothelin.

Progression of coronary endothelial dysfunction in man and its potential clinical significance

Endothelial injury represents an important factor in the initiation of atherosclerosis and is associated with abnormal vasomotor responses (= dysfunctional endothelium) to a variety of stimuli. Therefore, the evaluation of endothelial function may provide a means to detect early vascular alteration preceding overt atherosclerotic lesions. To test this clinically important hypothesis, we studied the coronary vasomotor responses to three different endothelium-dependent stimuli in patients with different early stages of coronary artery disease: first, increasing doses of intracoronary infusion of acetylcholine (ACH) (10(-8), 10(-7), 10(-6) M); second, transient increases in coronary flow causing flow-dependent dilation by injection of papaverine into the midportion of the left descending artery (exposing the proximal segment of this vessel to increased flow, but not to papaverine); and third, sympathetic stimulation by cold pressor test. Coronary diameters were assessed by repeated coronary angiography and quantitative angiography, blood flow velocity (and subsequent calculation of blood flow) was obtained by intracoronary Doppler. In normal individuals, all three stimuli elicited epicardial artery dilation. In patients with smooth coronary arteries, but hypercholesterolemia, a substantial vasoconstriction was observed in response to ACH, whereas the response to cold pressor test and increases in flow was normal, that is, vasodilation occurred. In patients with angiographically visible atherosclerosis, acetylcholine and cold pressor test exerted coronary vasoconstriction. Moreover, flow-dependent, endothelium-mediated dilation was attenuated in those patients demonstrating visible luminal irregularities in the vessel under study. In patients with hypercholesterolemia, substantial endothelial dysfunction (as assessed by attenuated blood-flow increase in response to acetylcholine) was demonstrated in the coronary microcirculation. Thus, progression of endothelial dysfunction occurs during the early course of development of coronary atherosclerosis. Utilizing quantitative coronary angiography and the intracoronary Doppler technique, these early functional alterations can be identified safely at a stage when atherosclerotic lesions are not detectable by angiography. This may be useful in designing early effective interventions which restore endothelial function and prevent the occurrence of overt atherosclerosis.

Endothelium-dependent control of vascular tone: effects of age, hypertension and lipids

As a source of several vasoactive factors, the endothelium takes part in the regulation of vascular tone. The most important endothelium-derived vasoactive substances are nitric oxide, prostacyclin, endothelin-1 and contracting factors requiring the activity of cyclooxygenase. The endothelium is an obvious target organ of cardiovascular risk factors. Accordingly, functional alterations do occur with aging, hypertension, and lipids. All three conditions are associated with a decreased basal and stimulated release of endothelium-derived nitric oxide. On the other hand, the release of endothelin-1 appears to increase with age, while the sensitivity to the peptide markedly decreases under the same conditions. In the spontaneously hypertensive rat, acetylcholine and stretch evoke the release of cyclooxygenase-dependent endothelium-derived contracting factor, most likely prostaglandin H2. The sensitivity and circulating levels of endothelin-1, on the other hand, are reduced in this experimental model of hypertension. In the porcine coronary circulation, oxidized low-density lipoproteins selectively reduce endothelium-dependent relaxations to aggregating platelets, serotonin, and thrombin which are mediated by nitric oxide. The alterations of endothelial function occurring with aging, hypertension, and hyperlipidemia may have important clinical implications for the pathogenesis of cardiovascular disease.

Aging differentially affects direct and indirect actions of endothelin-1 in perfused mesenteric arteries of the rat

1. The effects of age on the vascular action of endothelin-1 were studied in mesenteric resistance arteries of 4, 9 and 27 month old Fischer 344 rats. 2. Third order branches (about 200 microns in diameter) of mesenteric resistance arteries were dissected free and mounted on glass cannulae in organ chambers. Changes in intraluminal diameter of the perfused and pressurized vessels were continuously measured with a video dimension analyzer. 3. Endothelin-1 (10(-14) - 3 x 10(-8) M) caused contractions that were augmented after removal of the endothelium. The inhibitory effects of the endothelium were greater in young than in old rats. 4. The sensitivity of vascular smooth muscle to endothelin-1 decreased with age, while the maximal response was maintained. In contrast, the contractions to noradrenaline were unaffected by aging. 5. Threshold concentrations of endothelin-1 potentiated the contractions evoked by low and moderate concentrations of noradrenaline (10(-7) - 10(-6) M) in old, but not in young, rats. 6. Endothelium-dependent relaxations to acetylcholine inhibited maximal contractions to endothelin-1 and this effect decreased with age. In contrast, the relaxations to the nitric oxide donor, 3-morpholinosydnonimine (SIN-1; the active metabolite of molsidomine), did not differ in the three age groups. 7. Aging specifically decreases the direct contractile effects of endothelin-1 and the inhibitory effects of the endothelium against these contractions, while the indirect (potentiating) effects of the peptide become more pronounced.