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

Modulation of vasoactivity and platelet aggregation by selective 5-HT receptor antagonism in humans

BACKGROUND

Distinct serotonin [5-hydroxytryptamine (5-HT)] receptors are involved in platelet aggregation and vasoconstriction. Compounds that simultaneously and selectively inhibit the pertaining 5-HT receptors may therefore represent a therapeutic strategy for arterial thrombosis, observed frequently after atherosclerotic plaque rupture. The vasoactive and antiplatelet effects of the combined 5-HT1B and 5-HT2A receptor blocker SL65.0472-00 were investigated in humans to elucidate the functional involvement of these receptors.

METHODS

Twenty-four volunteers, divided into 2 groups of 12, received an oral dose of 20 mg of SL65.0472-00 or placebo in a randomized, double-blind, crossover study. Pre dose and at 2, 4, and 6 hours after dosing, intra-arterial infusions of 5-HT1B agonist sumatriptan (n = 12) or 5-HT (n = 12) were administered. Forearm blood flow (FBF) was measured using plethysmography, and platelet aggregation was measured using whole blood aggregometry induced by a combination of a threshold collagen concentration and an excess of 5-HT. Treatments were compared using analysis of variance.

RESULTS

After placebo treatment, infusion of 1 ng·kg·min 5-HT induced vasodilatation (FBF +80% change from baseline), whereas infusion of 30 and 80 ng·kg·min 5-HT resulted in vasoconstriction (FBF -25% and -50%). After SL65.0472-00 treatment, all 5-HT doses induced vasodilatation (FBF +25%-60%). Sumatriptan dose dependently decreased FBF (maximally -35%), but this effect was not altered by SL65.0472-00 treatment. 5-HT-induced platelet aggregation was effectively inhibited by 90% after SL65.0472-00 treatment for at least 6 hours.

CONCLUSIONS

SL65.0472-00 has potent antagonistic effect on 5-HT-induced vasoconstriction and platelet aggregation but not on sumatriptan-induced vasoconstriction. This suggests that in humans, SL65.0472-00 is a 5-HT2A blocker without clear 5-HT1B antagonistic activity.

Stimulated nitric oxide release and nitric oxide sensitivity in forearm arterial vasculature during normotensive and preeclamptic pregnancy

OBJECTIVE

We sought to determine whether the enhanced forearm vascular activity of nitric oxide during pregnancy and preeclampsia is associated with altered smooth muscle sensitivity to nitric oxide or with stimulated nitric oxide release.

STUDY DESIGN

Forearm blood flow responses to brachial artery infusion of glyceryl trinitrate (a nitric oxide donor), serotonin (an endothelium-dependent nitric oxide-mediated agonist), and ritodrine (a beta-adrenergic receptor agonist) were studied in 10 nonpregnant women, 12 pregnant women, and 7 women with preeclampsia by means of strain-gauge plethysmography. Responses to each drug (maximum dilator response and the sum of the percentage of dilator responses to each drug) were compared by analysis of variance.

RESULTS

Compared with nonpregnant women, pregnant subjects showed reduced responses to serotonin (summary response, 117 +/- 19 vs 221 +/- 30; P <.05). Responses to serotonin were reduced in the group with preeclampsia compared with those in the nonpregnant group (summary response, 71 +/- 28; P <.05) but did not differ from the responses in pregnant women. There were no differences between responses to glyceryl trinitrate and responses to ritodrine in any of the groups.

CONCLUSION

Vascular smooth muscle sensitivity to nitric oxide is not altered in normal pregnancy or preeclampsia, but dilator responses to serotonin appear blunted. Alterations in serotonin receptor coupling to nitric oxide synthase, or a limitation of availability of the substrate for nitric oxide synthase (L-arginine) during pregnancy, could account for the reduction in stimulated nitric oxide release.