5-HT1B-receptors and vascular reactivity in human isolated blood vessels: assessment of the potential craniovascular selectivity of sumatriptan

Abstract

Aims: 5-HT1B-receptor mediated vasoconstriction of cranial arteries is a potential mechanism by which 5-HT1B/1D-receptor agonists such as sumatriptan produce their antimigraine effects. 5-HT1B-receptors exist in other blood vessels which may give rise to unwanted vascular effects. Therefore we examined the distribution of 5-HT1B-receptor immunoreactivity (i.r.) in human blood vessels (including target and nontarget vessels) and confirmed the functionality of this receptor protein, by comparing the vasoconstrictor effects of sumatriptan and 5-HT (the endogenous ligand) in isolated vessels.

Methods: Blood vessels (middle meningeal, pial, temporal and uterine arteries and saphenous veins) were obtained from surgical patients (with consent). Sections of the vessels were prepared for routine immunohistochemical studies using specific 5-HT1B- and 5-HT1D-receptor antibodies. For functional studies, ring segments of the vessels were mounted in organ baths for isometric tension recording.

Results: 5-HT1B-receptor i.r. was detected on the smooth muscle layer in middle meningeal, pial and uterine arteries and in saphenous vein and sumatriptan produced contractions in these vessels with potency values (mean pEC50) of 7.00, 7.08, 6.44 and 6.61, respectively, the magnitude of contraction was greatest in the cranial arteries with Emax values of 100.7, 60.3, 23.0 and 35.9%, respectively (expressed as a percentage of the reference agonist 45 mm KCl). 5-HT1B-receptor i.r. was not detected in temporal artery and sumatriptan had no effect in this artery. 5-HT1D-receptor i.r. was not detected in any of the vessels studied.

Conclusions: Sumatriptan can evoke vasoconstriction in antimigraine target vessels and also in nontarget vessels through an action at 5-HT1B-rcceptors. Sumatriptan acts preferentially to cause contraction in human cranial arteries compared with the other blood vessels we examined and this effect is likely to be shared by other drugs of this class.

Figure 1

Representative immunohistochemical findings in: (I) middle meningeal artery; (II) pial artery; (III) temporal artery; (IV) uterine artery and (V) saphenous vein. Immunoreactivity (ir) was detected using di-amine benzidine as the chromagen which gives orange/brown staining and haematoxylin was used as a counterstain to detect cell nuclei (blue/purple stain). Thin (7 µm) sections of the artery were obtained and high power micrographs of parts of these sections are shown (for middle meningeal, pial and temporals arteries magnification is × 400 and for uterine artery and saphenous vein magnification is × 200). Panels A and B show immunostaining to α-actin (smooth muscle cells) and Ulex europeaus (endothelial cells) and Panels C and D show 5-HT_1B-receptor and 5-HT_1D-receptor immunoreactivity, respectively. Dense 5-HT_1B-immunoreactivity was detected on the smooth muscle layer in middle meningeal, pial and uterine arteries and also in saphenous vein but not in temporal artery. Dense 5-HT_1B-immunoreactivity was also detected on endothelial cells in middle meningeal and pial arteries and weak 5-HT_1B-immunoreactivity was also detected on endothelial cells in saphenous vein. 5-HT_1D-immunoreactivity was not detected in any vessel.

Figure 2

Concentration-effect curves to 5-HT (▪) and sumatriptan (▴) obtained in endothelium-denuded segments of human isolated middle meningeal artery (a), pial artery (b), temporal artery (c), uterine artery (d) and saphenous vein (e). Points represent mean data expressed relative to the contraction evoked by 45 mm KCl. Vertical bars signify ±s.e. mean. Several segments obtained from the same donor were used, the details regarding the total number of segments (s) which were exposed to either 5-HT and/or to sumatriptan (and the number of individual donors from which these were obtained, n) are listed in Table 1. Curves were fitted to the data using nonlinear regression analysis.