HYPERTENSION, AGING, AND ATHEROSCLEROSIS

ESSENTIAL HYPERTENSION, PART I

0025-7125/97 $0.00

+ .20

HYPERTENSION, AGING, AND ATHEROSCLEROSIS The Endothelial Interface Dinko Susic, MD, PhD

Hypertension, atherosclerosis, and aging are considered major independent risk factors for cardiovascular morbidity and mortality? Although independent as risk factors, the three entities are closely related and often operate simultaneously to affect the cardiovascular system adversely, resulting eventually in damage to various organs, most notably heart, kidneys, and brain. The prevalence of hypertension and atherosclerosis increases with aging5; elevated arterial pressure aggravates atherosclerotic lesions to blood vessels, accelerates age-related pathologic changes in function and structure of target organs, and significantly increases both morbidity and mortality in the elderly.46Morphologic and functional changes that occur in the cardiovascular system with aging and hypertension are similar in many respects2,28, 54; thickening of the subendothelial layer with increased protein, lipid and cell disposition, modification of viscoelastic properties of the arterial wall, increased incidence of atherosclerosis, and alterations in the response to vasoactive stimuli are seen in both conditions. In fact, hypertension-related alterations in the cardiovascular system may be described as an accelerated aging. Thus, it is tempting to speculate that common pathophysiologic mechanisms may be operative. Many reports have clearly demonstrated that endothelial dysfunction is present, or may even precede, the onset of vascular changes associated with aging, hypertension, and atherosclerosis.2,11, 12, 14, 28, 40 This article evaluates the evidence indicating that

From the Laboratory on Hypertensive Disease, Alton Ochsner Medical Foundation, New Orleans, Louisiana

MEDICAL CLINICS OF NORTH AMERICA VOLUME 81 * NUMBER 5 SEPTEMBER 1997

1231

endothelial damage may be the underlying pathophysiologic mechanism that connects the three entities. PHYSIOLOGIC ROLE OF THE ENDOTHELIUM

Endothelium was once thought to act solely as the internal lining of blood vessels that provides nonclotting surface and functions as a semipermeable membrane allowing diffusion of macromolecules. Since 1980, however, when Furchgott and Zawadzki'O demonstrated that vasorelaxant properties of acetylcholine were dependent on the presence of an intact endothelium and showed that these effects were due to the release of a humoral substance termed endothelium-derived relaxing factor (EDRF), a great deal of evidence has accumulated to indicate clearly that endothelium, in an endocrine, paracrine, and autocrine manner, regulates vascular smooth muscle tone, platelet adhesion and aggregation, local clotting, and vascular The endothelium serves a dual role in the control of vascular tone. Endothelial cells produce and release a variety of vasoactive substances; these include both vasodilators, such as EDRF, which has now been identified as nitric oxide 37 endothelium-derived hyperpolarizing factor, and prostacyclin, and vasoconstrictors, such as cyclooxygenase-derived factors (thromboxane A, and prostaglandin H2), endothelin, and angiotensin I1 (produced locally or taken up from cir~ulation).~~ The interaction between these vasodilators and vasoconstrictors provides a local control mechanism that regulates vascular tone. It is worth noting that under physiologic conditions, endothelium mostly exerts inhibitory functions. It attenuates vascular smooth muscle contraction, prevents platelet aggregation, and retards smooth muscle growth. Of all endothelium-derived vasoactive substances, NO appears to have a predominant role in regulating vascular smooth muscle tone under physiologic condition^.'^, 48 Thus, NO is continuously released under basal conditions as well as in response to mechanical stress (shear stress) and after activation of various receptors by agonists, including acetylcholine, neurotransmitters, and hormones.25,42, 48 Inhibition of NO production induces a rapid, sustained increase in total peripheral resistance and mean arterial pressure and impairs blood flow to regional cir~ulation,4~ suggesting that its vasodilatory action is necessary to maintain normal arterial pressure. A study3 also indicates that one of the major actions of NO is to inhibit the local endothelin vasoconstrictor system. Furthermore, acetylcholine-induced vasodilation is antagonized after blocking NO production but not by indomethacin, a cyclooxygenase inhibitor, or ouabain, an inhibitor of smooth muscle cell hyperpolarization, a finding indicating that NO, and not prostacyclin or endothelium-derived hyperpolarizing factor, mediates acetylcholine-induced vascular smooth r e l a x a t i ~ nAvailable .~~ evidence also suggests that endothelin, a potent endothelium-derived vasoconstrictor, does not play a major role in regulating vascular tone under physiologic condition^.^^ Its

HYPERTENSION, AGING, AND ATHEROSCLEROSIS

1233

circulating level is low, possibly because of the presence of inhibitory mechanisms or the absence of adequate stimuli.47It should be noted, however, that endothelin receptor antagonists lower blood pressure in a finding that suggests a possible role of endogenetic thelin in blood pressure regulation, at least under certain conditions. Finally, although all components of the renin-angiotensin system are present in vascular tissue,u the exact physiologic role of locally formed angiotensin I1 is difficult to assess because circulating level of angiotensin I1 is relatively high. Thus, NO emerges as a dominant local regulator of vascular tone. NO is derived from L-arginine, an essential amino acid, by the action of NO synthase, an Ca' + /calmodulin-dependent enzyme, leaving citrulline as a It is a diffusible substance with a half-life of a few seconds. Synthesis of NO and endothelium-dependent vasodilation are effectively blocked by L-arginine analogues, such as L-NG-monomethyl arginine (L-NAME). The mechanism of vasodilatory action of NO involves stimulation of soluble guanylate cyclase and an increase in cyclic guanosine monophosphate (cGMP) in vascular smooth muscle cells, which, in turn, induces re1axati0n.l~In addition to its vasodilatory properties, NO is also involved in the retardation of vascular smooth muscle cell growth. ENDOTHELIAL DYSFUNCTION IN HYPERTENSION

Hypertension is associated with morphologic and functional alterations of the endothelium. Morphologic changes include subendothelial accumulation of fibrin and cellular infiltration together with swelling of endothelial whereas functional alterations involve endotheliumdependent regulation of vascular tone, including changes in processes mediated by NO, endothelin, and cyclooxygenase products. The basal formation of NO appears to be reduced in various experimental models of hypertension and in patients with essential hypertension? 18, 41, 51 as indicated by the lesser degree of vasoconstriction in response to inhibitors of NO synthesis in hypertensive than in normotensive subjects. Moreover, endothelium-dependent vasodilation to acetylcholine is reduced in peripheral vasculature of hypertensive rats? 14, 2o and the vasodilator effect of acetylcholine in the forearm of hypertensive 39, * It is patients is blunted as compared to normotensive also worth noting that reduced endothelium-dependent vasodilation in hypertensive subjects has been demonstrated with different agonists, such as acetylcholine and bradykinin, substances that act through different receptors and intracellular signaling pathways.38This finding indicates that endothelial dysfunction in hypertension cannot be attributed to a defect in muscarinic receptors. Finally, it has been shown that the effectiveness of different NO donors in reducing forearm resistance is impaired in patients with arterial hypertension.4O Thus, the mechanism of impaired endothelium-dependent vasodilatory response in hyperten-

1234

SUSIC

sion may, in addition to diminished endogenous production and release of NO, also involve a decreased responsiveness of the vasculature to NO or an exaggerated breakdown of NO. There is also evidence that impaired endothelium-dependent vasodilation, at least in some forms of hypertension, is not solely due to alterations in NO-mediated mechanisms and that increased production 30, 49 of endothelium-derived contracting factors may also parti~ipate.'~, Thus, in spontaneously hypertensive rats, the reduced vasodilatory response to various agonists (serotonin, acetylcholine, calcium ionophore) is completely or partially restored by inhibition of prostaglandin synthesis or by blockade of prostaglandin H, receptors.14,30 These results suggest that the metabolism of arachidonic acid is altered, resulting in an increased production of vasoconstrictory prostaglandins. Similarly, in patients with essential hypertension, the impaired vasodilation to acetylcholine in forearm circulation is partially restored by pretreatment with indomethacin, a prostaglandin synthesis inhibit0r.4~As already discussed, alterations in endothelin metabolism may also be a part of endothelial dysfunction in hypertension. Although the dysfunction of endothelium in hypertension is well documented, the question is whether it is the primary phenomenon or it is secondary to increased arterial pressure. Because it is present in established but not early hypertension and because it can be reversed 48 it appears that in most animal models by antihypertensive of hypertension endothelial dysfunction is the consequence rather than the cause. The only exception may be Sabra hypertension-prone rats, in which both basal and acetylcholine-stimulated production were found to be diminished, whereas hypertension-resistant rats of the same strain have exaggerated basal and stimulated NO production when compared to Wistar rats.41In patients with essential hypertension, the decreased basal NO production also appears to be a secondary phenomenon because antihypertensive therapy restores forearm arterial responsiveness to L-NAME.~~ In normotensive offspring of patients with essential hypertension, however, forearm vasodilation to acetylcholine was reported to be reduced,"*suggesting that impaired endothelium-dependent vasodilation may precede the onset of hypertension. Regardless of whether it is a primary or secondary defect, endothelial dysfunction in hypertension may participate in maintenance and progression of hypertensive disease. ENDOTHELIAL DYSFUNCTION WITH AGING

A number of studies in experimental models in animals suggest that increasing age is associated with progressive endothelial dysfunction.2,22, 31 Similarly, in otherwise healthy humans, age is associated with an impaired endothelium-dependent vasodilation to acetylcholine in forearm circulation" and in epicardial coronary arteries with varying degrees of atheroscler~sis.~~ The mechanism of this age-related endothelial dysfunction is not known. It is estimated that normal life span of

HYPERTENSION, AGING, AND ATHEROSCLEROSIS

1235

human endothelial cells is about 30 years, and it has been that newly regenerated cells lose in part their ability to release EDRF in response to various stimuli. This idea is based on the findings of in vivo animal studies showing that after removal of coronary artery endothelium, the regenerated one had a reduced endothelium-dependent responsiveness." A more recent demonstrating impaired endothelium-dependent vasodilation to acetylcholine of angiographically normal coronary arteries in subjects aged over 30 years inferentially supports the same notion. There is some evidence that in older patients an increased production of cyclooxygenase-derived vasoconstrictor factors may contribute to endothelium dysfunction because indomethacin may partially restore vasodilator response to acetylcholine." It has also been suggested" that accumulation of superoxide radicals in the vasculature of older individuals may decrease the half-life of released NO and in this way may induce endothelial dysfunction. It has been well established that superoxide anions are involved in the breakdown of EDRF.I3 At present, there are no data to indicate that endothelial dysfunction contributes to the aging process or, if it does, to what extent. In this respect, interesting is a reporP4 showing that in young (20-week-old) spontaneously hypertensive rats (SHR), a short, 3-week treatment with the NO synthase inhibitor L-NAME induced severe nephrosclerosis that in terms of morphologic and functional characteristics was identical to nephrosclerosis that by natural progression occurs in aged (73-week-old) SHR.I9 Furthermore, short-term treatment (3 weeks) with angiotensin converting enzyme inhibitors reversed nephrosclerotic changes in aged SHRI9 and prevented or reversed L-NAME-induced nephrosclerosis in young SHR.35 The author has also examined the possible influence of defective NO production on age-related changes in morphology and function of the cardiovascular system (D Susic, ED Frohlich, unpublished observations). To this end, the cardiovascular effects of long-term (6 months) blockade of NO production, using L-NAME, a competitive inhibitor of NO synthesis, were examined. Because L-NAME induces hypertension, which by itself may affect the cardiovascular system, a low dose (10 mg/L of drinking fluid) of L-NAME was employed so that animals remained normotensive throughout the 6-month course of the experiment. Three groups of male, 8 weeks old at the beginning of the experiment, Wistar rats were studied: The control group was given tap water and standard rat chow, a group was given L-NAME in drinking water and standard food, and a group was given L-NAME in drinking water and rat chow with the addition of 2% cholesterol. After 6 months of respective treatments, a study of systemic hemodynamics and histologic examination of the heart, aorta, and kidneys were performed. Histologic examination revealed no difference between the groups; hernodynamic study demonstrated that, when compared to control animals, L-NAMEtreated rats had significantly higher total peripheral resistance and somewhat lower cardiac index (Fig. l), suggesting that endothelium-derived NO may have a role in long-term regulation of vascular tone.

1236

SUSIC

118

24

m

22

20

110

8 c c

108

E 3

16 14

114

m

;

.-

E

18

12 102

10

Control

L-NAME

L-NAME+ Chol

Control

L-NAME

L-NAME+ Chol

Control

L-NAME

L-NAME+ Chol

0 6.5

0

385 6.0

.

:

.-C

9 E

-8

.-C

m

4.5

E

375

E Z? 365

5.5 E 5.0

4.0

355 345

Control

L-NAME

L-NAME+ Chol

Figure 1. Mean arterial pressure (MAP), cardiac index, total peripheral resistance, and heart rate in control rats, rats given low-dose L-NAME (L-NAME) for 6 months, and rats given low-dose L-NAME and food containing 2% cholesterol (L-NAME + Chol). *P<0.05 when compared to controls.

ENDOTHELIUM DYSFUNCTION IN ATHEROSCLEROSIS

In established atherosclerosis, there is a characteristic defect in vessel response to vasoactive agents; endothelium-dependent vasodilation to various agonists is reduced,& and even paradoxic vasoconstriction to acetylcholine may be elicited.24Furthermore, dyslipidemia, with an increase in low-density lipoproteins and a decrease in high-density lipoproteins, is associated with a marked endothelial dysfunction, even when atherosclerosis is not evident.12,25 Apparently the oxidized lowdensity lipoproteins play a major role in inducing endothelium dysfunction because native low-density lipoproteins do not influence endothelium-mediated vasodilation.21 Reduction in low-density lipoproteins with therapy improves abnormal endothelium-dependent vasodilation in atheroscler~sis~~ and reduces the number of adverse coronary events before any significant regression of atherosclerosis occurs.12Thus, the available evidence clearly indicates that endothelial dysfunction associated with atherosclerosis significantly contributes to its clinical manifestations. It is also of interest to note that some major risk factors for the development of atherosclerosis, such as cigarette smoking or diabetes mellitus, are independently associated with endothelial dysfunction.12 The mechanism of impaired endothelium-dependent vascular responses in atherosclerosis is not clear. It is possible that the production or release

HYPERTENSION, AGING, AND ATHEROSCLEROSIS

1237

of NO is diminished, as suggested by findings that release of EDRF (determined in bioassay) by atherosclerotic porcine coronary arteries is reduced.44An increased NO breakdown, possibly owing to an increased production of superoxide radicals in atherosclerotic vascular wall, has also been i m p l i ~ a t e dFurthermore, .~~ an increased release of endothelin may contribute to endothelium dysfunction in atheroscler~sis.~~ ENDOTHELIAL DYSFUNCTION: A LINK BETWEEN HYPERTENSION, AGING, AND ATHEROSCLEROSIS A great deal of evidence, obtained in experiments using animals as well as in studies involving human subjects, clearly demonstrates that endothelial dysfunction, particularly in the L-arginine-NO pathway, invariably develops in hypertension, in atherosclerotic disease, and during the aging process. It is also evident that altered function of endothelium significantly contributes to clinical manifestations of the three entities. There is little, if any, clear evidence, however, that altered function of endothelium is a primary factor in the pathogenesis of hypertension, the atherosclerotic process, or age-related disorders. Thus, it seems that endothelial dysfunction is only a consequence of various disorders, although it may contribute to the maintenance and progression of primary diseases. Consequently, it would appear that hypertension, atherosclerosis, and aging are connected only through sharing clinical manifestations of endothelial dysfunction. Although, at present, there are no clear data to indicate that endothelium dysfunction is a common causative factor in the development of hypertension, atherosclerosis, and age-related disorders, there is mounting evidence that alterations in turnover of endothelium-derived NO may link the three entities in terms of basic pathophysiologic disturbances and even therapeutic approaches. Increased intravascular generation and accumulation of superoxide radicals are common features of hypertension, atherosclerosis, and aging.', ", 32 Accumulation of superoxide radicals, in turn, accelerates NO breakdown and contributes to endothelial dysfunction.13 NO acts as a superoxide radical scavenger, thus providing protection against oxidative stress and its adverse eff e c t ~ Consequently .~~ the stimulation of endothelial NO production or release may have dual beneficial effects. First, it improves or restores endothelial function, and second, it may provide protection against tissue damage induced by superoxide radicals. Because bradykinin stimulates endothelial NO production and release, this, in fact, may explain the beneficial effects of angiotensin converting enzyme inhibition that cannot be solely attributed to decreased angiotensin I1 generation and diminished bradykinin degradation by itself. Thus, the ability of angiotensin converting enzyme inhibitors to restore endothelium-dependent vasodilation in hypertension and atherosclerosis,5zto prevent endothelial dysfunction in heart failure? 33 to reduce neointimal thickening and maintain endothelial function in an experimental model of atherosclero-

sis7 and to postpone age-related endothelial dysfunction? may be related to increased production or release of NO induced by elevated tissue levels of bradykinin. Furthermore, it has been demonstrated that long-term angiotensin converting enzyme inhibition diminishes agerelated alterations in the cardiovascular system and improves survival in aging mice, and these effects have been related to diminished oxidative damage.4,l6 CONCLUSIONS It has been well established that endothelial cells produce and release a number of active substances that in an endocrine, paracrine, and autocrine manner regulate vascular smooth muscle tone, platelet adhesion and aggregation, local clotting, and vascular growth. Under physiologic conditions, the major active substance released by endothelial cells appears to be NO, which regulates vascular smooth muscle tone and growth. Alterations in endothelial function, involving mostly NO pathways, become apparent under a number of conditions, including hypertension, atherosclerosis, and aging. It appears that under these conditions endothelial dysfunction is a secondary event, although it contributes significantly to the maintenance and progression of the primary disorder. Therapeutic interventions that augment production and release of endothelium-derived NO may to a great extent ameliorate adverse cardiovascular changes associated with hypertension, atherosclerosis, or aging.

References 1. Alexander R W Hypertension and the pathogenesis of atherosclerosis: Oxidative stress and the mediation of arterial inflammatory response: A new perspective. Hypertension 25:155, 1995 2. Atkinson J: Effect of aging and chronic angiotensin I converting enzyme inhibition on the endothelial function of the mesenteric arterial bed of the rat. Am J Cardiol 76:19E, 1995 3. Banting JD, Friberg P, Adams MA: Acute hypertension after nitric oxide synthase inhibition is mediated primarily by increased endothelin vasoconstriction. J Hypertens 14:975, 1996 4. DeCavanagh EM, Inserra F, Ferder L, et al: Superoxide dismutase and glutathione peroxidase activities are increased by enalapril and captopril in mouse liver. FEBS Lett 13:361, 1995 5. Docherty JR: Cardiovascular responses in ageing: A review. Pharmacol Rev 42:103, 1990 6. Dohy Y, Thiel M, Biihler FR, et al: Activation of the endothelial L-arginine pathway in pressurized mesenteric resistance arteries: Effect of age and hypertension. Hypertension 15:170, 1990 7. Drexler H, Kurz S, Jeserich M, et al: Effect of chronic angiotensin-converting enzyme inhibition on endothelial function in patients with chronic heart failure. Am J Cardiol 7613E, 1995 8. Dusting GJ, Hyland R, Hickey H, et al: Angiotensin-converting enzyme inhibitors reduce neointimal thickening and maintain endothelial nitric oxide function in rabbit carotid arteries. Am J Cardiol 76:24E, 1995

HYPERTENSION, AGING, AND ATHEROSCLEROSIS

1239

9. Frohlich ED, Apstein C, Chobanian AV, et al: The heart in hypertension. N Engl J Med 327998, 1992 10. Furchgott RF, Zawadzki JV:The obligatory role of endothelial cells in the relaxation of arterial smooth muscle by acetylcholine. Nature 288:373, 1980 11. Gerhard M, Roddy MA, Craeger SJ, et al: Aging progressively impairs endothelium dependent vasodilatation in forearm resistance vessels of humans. Hypertension 27849, 1996 12. Glasser SP, Selwyn AP, Ganz P: Atherosclerosis: Risk factors and the vascular endothelium. Am Heart J 131:379, 1996 13. Gryglewski RJ, Palmer RMJ, Moncada S: Superoxide anion is involved in the breakdown of endothelium-derived vascular relaxing factor. Nature 320454, 1986 14. Huang A, Koller A: Both nitric oxide and prostaglandin-mediated responses are impaired in skeletal muscle arterioles of hypertensive rats. J Hypertens 149387, 1996 15. Ignarro LJ, Byrns RE, Buga GM, et al: Pharmacological evidence that endotheliumderived relaxing factor is nitric oxide: Use of pyrogallol and superoxide dysmutase to study endothelium-dependent and nitric oxyde-elicited vascular smooth muscle relaxation. J Pharmacol Exp Ther 244:181, 1988 16. Inserra F, Romano L, Ercole L, et al: Cardiovascular changes by long-term inhibition of the renin-angiotensin system in aging. Hypertension 25:437, 1995 17. Johnson RA, Freeman RH: Sustained hypertension in the rat induced by chronic blockade of nitric oxide production. Am J Hypertens 5:919, 1992 18. Kitazono T, Faraci FM, Heistad D D L-Arginine restores dilator responses of the basilar artery to acetylcholine during chronic hypertension. Hypertension 27893, 1996 19. Komatsu K, Frohlich ED, On0 H, et al: Glomerular dynamics and morphology of aged SHR Effects of angiotensin-converting enzyme inhibitor. Hypertension 25:207, 1995 20. Konishi M, Su C: Role of endothelium in dilator responses of spontaneously hypertensive rat arteries. Hypertension 5:881, 1983 21. Kugiyama K, Kerns SA, Morrisett JD, et a1 Impairment of endothelium-dependent relaxation by lysolecithin in modified low-density lipoproteins. Nature 334:160, 1990 22. Kiing CF, Liischer TF: Different mechanism of endothelial dysfunction with aging and hypertension in rat aorta. Hypertension 25:194, 1995 23. Lilly LS, Pratt RE, Alexander RW, et al: Renin expression by vascular endothelial cells in culture. Circ Res 57312, 1985 24. Ludmer PL, Selwyn AP, Shook TL, et al: Paradoxical vasoconstriction induced by acetylcholine in atherosclerotic coronary arteries. N Engl J Med 315:1046, 1986 25. Liischer TF, &bey RK: Endothelium and platelet-derived vasoactive substances: Role in the regulation of vascular tone and growth. In Laragh JH, Brenner BM (eds): Hypertension: Pathophysiology, Diagnosis and Management, ed 2. New York, Raven Press, 1995, p 609 26. Liischer TF, Seo BG, Biihler FR Potential role of endothelin in hypertension: Controversy on endothelin in hypertension. Hypertension 21:752, 1993 27. Lyons D, Webster J, Benjamin N The effect of antihypertensive therapy on responsiveness to local intra-arterial NG-monomethyl-L-arginine in patients with essential hypertension. J Hypertens 12:1047, 1994 28. Marin J: Age related changes in vascular responses: A review. Mech Ageing Dev 79:71, 1995 29. Massoudy P, Becker BF, Gerlach E: Nitric oxide accounts for postischemic cardioprotection resulting from angiotensin-converting enzyme inhibition: Indirect evidence for a radical scavenger effect in isolated guinea pig heart. J Cardiovasc Pharmacol 25:440, 1955 30. Mayhan WG, Faraci FM, Heistad DD: Impairment of endothelium dependent responses of cerebral arterioles in chronic hypertension. Am J Physiol 253:H1435, 1987 31. Mayhan WG, Faraci FM, Baumbach GL, et al: Effects of aging on responses of cerebral arterioles. Am J Physiol 258:H1138, 1990 32. Minor RL, Myers RR, Guerra R, et al: Diet-induced atherosclerosis increases the release of nitrogen oxides from rabbit aorta. J Clin Invest 86:2109, 1990 33. Mulder P, Devaux B, Fertak L, et al: Vascular and myocardial protective effects of converting enzyme inhibition in experimental heart failure. Am J Cardiol 76:28E, 1995

1240

SUSIC

34. Ono H, Ono Y, Frohlich E D Nitric oxyde synthase inhibition in spontaneously hypertensive rats: Systemic, renal, and glomerular hemodynamics. Hypertension 26249,1995 35. 0x10 H, On0 Y, Frohlich E D ACE inhibition prevents and reverses L-NAME-exacerbated nephrosclerosis in spontaneously hypertensive rats. Hypertension 27176, 1996 36. Palmer RMJ, Ashton DS, Moncada S Vascular endothelial cells synthesize nitric oxyde from L-arginine. Nature 333:664, 1988 37. Palmer RMJ, Ferrige AG, Moncada S Nitric oxyde release accounts for the biological activity of endothelium-derived relaxing factor. Nature 327524, 1987 38. Panza JA, Garcia CE, Kilcoyne CM, et a1 Impaired endothelium-dependent vasodilation in patients with essential hypertension: Evidence that nitric oxide abnormality is not localized to a single signal transduction pathway. Circulation 91:1732, 1995 39. Panza JA, Quyyumi AA, Brush JE, et a1 Abnormal endothelium-dependent vascular relaxation in patients with essential hypertension. N Engl J Med 323:22, 1990 40. Preik M, Kelm M, Feelisch M, et al: Impaired effectiveness of nitric oxide-donors in resistance arteries of patients with arterial hypertension. J Hypertens 14:903, 1996 41. Rees D, Ben-Ishay D, Moncada S: Nitric oxide and the regulation of blood pressure in the hypertension-prone and hypertension-resistant Sabra rat. Hypertension 28:367, 1966 42. Rubanyi GM, Romero JC, Vanhoutte PM: Flow-induced release of endothelium-derived relaxing factor. Am J Physiol 250:H1145, 1986 43. Shimokawa H, Aarhus LL, Vanhoutte PM. Porcine coronary arteries with regenerated endothelium have a reduced endothelium-responsiveness to aggregation platelets and serotonin. Circ Res 61:256, 1987 44. Shimokawa H, Vanhoutte PM: Impaired endothelium-dependent relaxation to aggregating platelets and related vasoactive substances in porcine coronary arteries in hypercholesterolemia and in atherosclerosis. Circ Res 64900, 1989 45. Sigmon DH, Beierwaltes W Nitric oxyde influences blood flow distribution in renovascular hypertension. Hypertension 24(suppl I):I34, 1994 46. Smith W M Epidemiology of hypertension in older patients. Am J Med 38(suppl 3B):2, 1988 47. Stewart DJ, Langleben D, Cernacek P: Endothelin release is inhibited by coculture of endothelial cells with cells of vascular media. Am J Physiol 259:H2928, 1993 48. Taddei S, Salvetti A: Pathogenetic factors in hypertension: Endothelial factors. Clin Exp Hypertens 18:323, 1996 49. Taddei S, Virdis A, Matei P, et al: Vasodilatation to acetylcholine in primary and secondary forms of human hypertension. Hypertension 21:929, 1993 50. Treasere CB, Klein L, Weitraub WS, et al: Beneficial effects of cholesterol-lowering therapy on the coronary endothelium in patients with coronary artery disease. N Engl J Med 332:481, 1995 51. Vallance P, Collier J, Moncada S Effects of endothelium-derived nitric oxyde on peripheral arteriolar tone in man. Lancet 2997, 1989 52. Vanhoutte PM, Boulanger CM, Mombouli JV: Endothelium-derived relaxing factors and converting enzyme inhibition. Am J Cardiol 763E, 1995 53. Vita JA, Treasure CB, Nabel EG, et al: Coronary vasomotor response to acetylcholine relates to risk factors for coronary artery disease. Circulation 81:491, 1990 54. Wadsworth RM: Calcium and vascular reactivity in ageing and hypertension. J Hypertens 8:975, 1990 55. Yasue H, Matsuyama K, Okumura K, et al: Responses of angiographically normal human coronary arteries to intracoronary injection of acetylcholine by age and segment. Circulation 81:482, 1990

Address reprint requests to Dinko Susic, MD, PhD Laboratory on Hypertensive Disease Richard Freeman Research Institute Alton Ochsner Medical Foundation 1520 Jefferson Highway New Orleans, LA 70121