The acute and chronic antihypertensive effects of ketanserin cannot be explained by blockade of vascular serotonin, type 2, receptors or alpha 1-adrenergic receptors

The love of a lifetime: 5-HT in the cardiovascular system

Serotonin [5-hydroxytryptamine (5-HT)] is an amine made from the essential amino acid tryptophan. 5-HT serves numerous functions in the body, including mood, satiety, and gastrointestinal function. Less understood is the role 5-HT plays in the cardiovascular system, although 5-HT receptors have been localized to every important cardiovascular organ and 5-HT-induced changes in physiological function attributed to activation of these receptors. This manuscript relates a few scientific stories that test the idea that 5-HT is important to the control of normal vascular tone, more so in the hypertensive condition. Currently, our laboratory is faced with two different lines of experimentation from which one could draw vastly different conclusions as to the ability of 5-HT to modify endogenous vascular tone and blood pressure. Studies point to 5-HT being important in maintaining high blood pressure, but other studies solidly support the ability of 5-HT to reduce elevated blood pressure. This work underscores that our knowledge of the functions of 5-HT in the cardiovascular system is significantly incomplete. As such, this field is an exciting one in which to be, because there are superb questions to be asked.

5-HT in systemic hypertension: foe, friend or fantasy?

Since its discovery by Erspamer in the 1930s and identification by Page in the 1950s, 5-HT (5-hydroxytryptamine; serotonin) has been an elusive candidate as a substance that plays a role in the disease of high blood pressure, also known as hypertension. In both animal and human hypertension, arterial contraction to 5-HT is profoundly enhanced. Additionally, 5-HT is a vascular smooth muscle cell mitogen. Because both increased arterial contractility and smooth muscle growth contribute to the disease of hypertension, it is logical to believe that 5-HT is a potential cause of disease, and thus a foe. However, decades of research have produced conflicting results as to the potential role of 5-HT in hypertension. This review will discuss historical findings which both support and refute the involvement of 5-HT in hypertension, and pose some new questions that may reveal novel ways for 5-HT to modify vascular control of blood pressure.

Serotonin and the heart

Serotonin is a naturally occurring vasoactive substance that has diverse cardiophysiological effects. These effects can be explained by the existence of serotonin receptor subtypes which mediate different biological actions. The vasoconstrictive actions of serotonin are mediated by 5-HT2 serotonergic receptors, and serotonin also amplifies the release and activities of other vasoconstrictors, such as angiotensin and norepinephrine. Abnormalities in the serotonergic system may play an important role in the pathophysiology of multiple cardiovascular disease states such as systemic hypertension, primary pulmonary hypertension and peripheral vascular disease. Selective 5-HT2 serotonergic receptor blockers have been developed which appear to be potent vasodilators with therapeutic potential in various cardiovascular disease states. The largest clinical experience has been collected with ketanserin, and other agents in this class are being investigated. Prolongation of the ECG QT interval with 5-HT2 serotonergic receptor blockers may pose a potential risk with these treatments in some patients.

Effects of angiotensin II and losartan in the forearm of patients with essential hypertension

OBJECTIVE

Angiotensin II type 1 receptor-mediated constrictor effects may be modulated by hypertension-related vascular changes, changes in receptor function and in neurohumoral activity.

DESIGN

The forearm blood flow (FBF) effects of angiotensin II, methoxamine, and losartan were investigated in essential hypertensive patients. Minimal forearm vascular resistance was measured to determine structural vascular changes.

METHODS

Seven hypertensive patients were selected, and seven matched normotensives. Angiotensin II (0.01-10 ng/kg per min) was infused during predilatation by sodium nitroprusside (6.1 +/- 0.6 ng/kg per min) before and during losartan infusion (0.3-3 microg/kg per min). Methoxamine (0.2-2 microg/kg per min) was co-infused with the nitric oxide synthase inhibitor NG-monomethyl-L-arginine. FBF, measured by venous occlusion plethysmography, was expressed as the change in FBF ratio (FBFinfused arm/FBFnon-infused arm).

RESULTS

Baseline FBF (infused arm) was increased by sodium nitroprusside from 2.56 +/- 0.80 to 5.46 +/- 0.92 (P<0.05) and from 2.66 +/- 0.25 to 5.42 +/- 0.40 ml/100 ml per min (P<0.05) in the hypertensive and normotensive group, respectively. Baseline forearm vascular resistance (FVR) was higher in the hypertensive than in the normotensive group [51 +/- 8 versus 33 +/- 3 mmHg/ (ml/100 ml per min); P<0.05]. Angiotensin II caused a maximal change in FBF ratio (Emax) by -70 +/- 3 and -72 +/- 6% in the hypertensive and normotensive group, respectively (NS). Tachyphylaxis did not occur. Infusions of losartan at 0.3, 1 and 3 microg/kg per min reduced the Emax values from -70 +/- 3 to -50 +/- 5, -45 +/- 5 and -15 +/- 2%, respectively, in the hypertensive group, and from -72 +/- 6 to -62 +/- 4, -45 +/- 2 and -32 +/- 2%, respectively, in the normotensive group (NS). Infusion of methoxamine significantly reduced the FBF ratio by -58 +/- 6 and -69 +/- 5% in the hypertensive and normotensive groups, respectively (NS). Minimal FVR, after forearm ischemia, was the same in hypertensives and normotensives, namely 3.2 +/- 0.7 and 3.2 +/- 0.4 mmHg/(ml per 100 ml per min), respectively (NS).

CONCLUSIONS

Angiotensin II type 1- and alpha1-mediated vascular effects were unchanged by essential hypertension. Baseline FVR was greater in the hypertensives than in the normotensives, while minimal FVR was the same. These results indicate that the forearm vascular bed of the patient group studied does not show important structural and renin-angiotensin system-related functional changes as a result of hypertension.

Serotonin and serotonin antagonism in cardiovascular and non-cardiovascular disease

Serotonin, or 5-hydroxytryptamine, is a naturally-occurring vasoactive substance found primarily in the brain, enterochromaffin tissue, and blood platelets. It has diffuse cardiophysiologic effects. The multiple effects of serotonin on blood vessels can be explained by the existence of 2 serotonergic receptor subtypes (the S1 receptor mediates vasodilation, and the S2 receptor vasoconstriction). Serotonin via the S2 receptor also augments the actions of several other vasoconstricting substances. Serotonin may be responsible for causing, or at least perpetuating, some forms of systemic hypertension through peripheral and central nervous system (CNS) actions. Ketanserin is a highly selective S2-serotonergic antagonist with additional alpha-adrenergic blocking activity, which has been proposed as a therapy for various cardiovascular diseases including hypertension. It has been shown to be more effective than placebo in treating hypertension and comparable in effectiveness to other antihypertensive drugs. Its major side effects relate to the CNS, and prolongation of the electrocardiogram QT interval has been described. Caution must be used when using ketanserin in patients receiving potassium- and magnesium-losing agents, because of the risk of torsades de pointes. Ketanserin has potential utility in the treatment of eclampsia, peripheral vascular disease, carcinoid syndrome, and "shock lung." The drug is not yet approved for clinical use in the United States.

Different types of centrally acting antihypertensives and their targets in the central nervous system

The central regulation of blood pressure and other cardiovascular parameters may involve the baroreceptor reflex are, including both adrenergic and serotonergic pathways, as well as amino acids, as neurotransmitters. Both adrenergic and serotonergic pathways have been recognized as targets for clinically relevant, centrally acting antihypertensives, such as clonidine, guanfacine, and alpha-methyl-DOPA. The central components of the hybrid drugs urapidil and ketanserin also involve serotonergic pathways and receptors. For urapidil the stimulation of 5-HT1A-receptors is assumed to induce peripheral sympathoinhibition, whereas for ketanserin the central mechanism is unknown in detail. More recently central imidazoline (I1) receptors have been proposed as the major target for the newer antihypertensives rilmenidine and moxonidine. Clonidine, however, is assumed to be mixed I1- and alpha2-receptor agonist. The distinction between central I1- and alpha2-receptors may potentially offer the design of new antihypertensives, acting like clonidine but with fewer side effects. Finally, the amino acid pathways should be considered as potential targets for centrally acting antihypertensives. Experimental compounds on this basis are available but clinical implications appear to be very remote. In the present survey an outline is given of the various pathways, neurotransmitters, and receptors involved in the central regulation of blood pressure. The different types of centrally acting antihypertensives are subsequently discussed on this basis.

Interactions between serotonin and endogenous and exogenous noradrenaline in the human forearm

We investigated the interactions between serotonin (5-hydroxytryptamine, 5-HT) and exogenous or endogenous noradrenaline (NA) in the forearm of normotensive volunteers (n = 24), using venous occlusion plethysmography. Endogenous release of NA was stimulated by lower body negative pressure (LBNP; -10, -20 and -40 mmHg) and by intra-arterial (i.a.) infusions of tyramine (0.1 to 1 microgram/kg/min). Exogenous NA was infused in cumulative doses (0.3 to 10 ng/kg/min). All experiments were performed in the presence of either vehicle (0.4 mL/min), sodium nitroprusside (SNP; 3 or 5 ng/kg/min), or 5-HT (0.1 or 1 ng/kg/min). NA or 5-HT (1 ng/kg/min) were also infused with the selective 5-HT2 receptor antagonist ritanserin (500 ng/kg/min). Angiotensin II (Ang II; 0.03 to 0.3 ng/kg/min) was given as a vasoconstrictor control (n = 6). Vasoconstriction to exogenous NA was significantly enhanced by 5-HT (1 ng/kg/min; p < 0.05) in one group of subjects, but this could not be reproduced in a second group. In contrast, the vasoconstrictor responses to endogenous NA remained unaffected by 5-HT. SNP caused the same degree of vasodilation as 5-HT, and was followed by a similar response to NA. Ritanserin did not alter the vasoconstriction to infused NA, nor vasoconstriction to infused NA when combined with 5-HT. The vasoconstrictor response to Ang II was not influenced by 5-HT (0.1 ng/kg/min). Apparently, a synergistic effect between 5-HT and NA in the periphery involves a nonspecific mechanism. The latter seems to be confined to infused NA. The precise role of the 5-HT2 receptor in this interaction could not be elucidated.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological treatment of portal hypertension: hemodynamic effects and prevention of bleeding

In the past 10 years, it has been clearly shown that vasoactive substances reduce portal pressure in patients or animals with portal hypertension. Some of these substances act by inducing splanchnic vasoconstriction, while others reduce hepatic and porto-systemic collateral vascular resistance and, thus, induce a portal hypotensive effect. Still others induce arterial hypotension, which causes a vasoconstrictive effect in the splanchnic territory. Since these drugs act on different vascular receptors, their combination should have a more marked effect on portal hypertension. Up to now, only nonselective beta-blockers have been used in the prevention of first gastrointestinal bleeding in patients with portal hypertension and esophageal varices and in the prevention of recurrent gastrointestinal bleeding. These trials have shown that propranolol or nadolol significantly reduce either a first episode of bleeding or recurrent bleeding. This pharmacological treatment also improves the survival rate in these patients. All of these studies have helped us to understand, in part, why gastrointestinal hemorrhage occurs in certain patients. Additional studies of beta-blockers or other substances are, nevertheless, necessary to select patients who will respond to this type of treatment. Finally, it is possible that the pharmacological treatment of portal hypertension may also be used before esophageal varices occur.

Effects of serotonin and noradrenaline on superficial hand veins in patients with primary hypertension and in healthy volunteers

In vitro and animal experiments suggest that a constitutional increase in vascular responsiveness to serotonin (5-HT) may play a role in the development of essential hypertension. We have studied the potential alterations in vascular responsiveness to the direct and catecholamine-potentiating effects of exogenous 5-HT in patients with essential hypertension, by comparing in vivo responsiveness of superficial hand veins to local infusions of 5-HT or to coinfusions of 5-HT and noradrenaline in unmedicated hypertensive patients and in healthy control subjects. The dorsal hand vein compliance technique was employed. There was no significant difference between patients and control subjects in the maximal 5-HT-induced venoconstriction or in the doses required for half-maximal venoconstriction (ED50) for 5-HT. Coinfusions of a constant dose of 5-HT caused a significant leftward shift in the dose response curve for noradrenaline as compared with noradrenaline alone. This was indicated by a 9.7 +/- 13.7 fold and a 10.4 +/- 13.8 fold increase in the ED50 for noradrenaline in the patient and control groups, respectively (p = 0.89 between study groups). Our results argue against a generalized increase in responsiveness to the direct or catecholamine-potentiating effect of 5-HT in vivo in vascular smooth muscle, associated with hypertension.

Serotonergic receptors and drugs in hypertension

The possible role of 5-hydroxytryptamine (5HT) and 5HT-receptors in hypertension, already suggested by Page in 1954, has been subject to a renaissance of interest owing to the development of antihypertensive drugs which interact with 5HT-receptors. These drugs, like ketanserin, urapidil and flesinoxan are used as tools to study the role of 5HT and its receptors in hypertension. Some arguments would plead in favour of a certain role of 5HT and 5HT-receptors in the pathogenesis and maintenance of hypertension: hyperresponsiveness of blood vessels from hypertensive patients and animals to 5HT-induced constriction; the antihypertensive/vasodilator activity of the 5HT2-receptor antagonist ketanserin; enhanced sensitivity of platelets from hypertensives to 5HT. However, there are also several arguments which do not support a causal role of 5HT in hypertensive disease: 5HT is not a generally accepted pressor agent, whereas its concentration in the circulating blood is subthreshold; the 5HT2-receptor antagonist ketanserin is the only agent of this type which lowers blood pressure, other 5HT2-receptor blockers (ritanserin; LY 53587) being inactive. The various data and arguments available do not unequivocally support a relevant role of peripheral 5HT and its receptors in hypertensive disease. 5HT2-receptor blockade may, however, have a favourable effect on the microcirculation under pathological conditions. The stimulation of central 5HT1A-receptors by drugs like urapidil, 8-OH-DPAT or flesinoxan, has been demonstrated to induce peripheral sympathoinhibition and a fall in blood pressure. This mechanism appears to be a novel target for centrally acting antihypertensives, clearly different from that of clonidine and related drugs, which are centrally acting alpha 2-adrenoceptor agonists.