Comparison of the vasoconstrictor effects of rizatriptan and sumatriptan in human isolated cranial arteries: immunohistological demonstration of the involvement of 5-HT1B-receptors

Abstract

Aims: We compared the vasoconstrictor effects of 5-HT with those of the selective 5-HT1B/1D-receptor agonists sumatriptan and rizatriptan in human isolated cranial (middle meningeal) arteries. In addition selective 5-HT1B- or 5-HT1D-receptor antibodies were used in combination with semiquantitative immunohistochemical techniques to compare the levels of expression of these receptors in human middle meningeal and coronary arteries.

Methods: Middle meningeal and coronary arteries were obtained (with consent) from either neurosurgical patients or donor hearts, respectively. Segments of middle meningeal artery were mounted in organ baths for isometric recording and cumulative concentration-effect curves to 5-HT, rizatriptan and sumatriptan were obtained. Frozen fresh sections of middle meningeal and coronary arteries were subjected to standard immunohistochemical techniques using specific 5-HT1B- or 5-HT1D-receptor primary antibodies and a radiolabelled secondary antibody. Data were subjected to analysis of variance (ANOVA) and nonlinear regression analysis.

Results: 5-HT, rizatriptan and sumatriptan were potent vasoconstrictors in human isolated middle meningeal artery (EC50 values=32, 90 and 71 nM, respectively). A significantly higher level of 5-HT1B-receptor immunoreactivity was detected in middle meningeal artery compared with coronary artery (ANOVA, F=7.95, DF=1,4, P<0.05).

Conclusions: Rizatriptan and sumatriptan act selectively to cause vasoconstriction in human isolated middle meningeal artery and are 10-fold more potent than in human coronary artery. The higher level of expression of 5-HT1B-receptors in middle meningeal compared with coronary artery provides a pharmacological basis for the craniovascular selectively of both rizatriptan and sumatriptan.

Figure 1

Concentration-effect curves to 5-HT (open squares), sumatriptan (closed circles) and rizatriptan (closed squares) obtained in isolated segments of middle meningeal artery. Points represent mean data expressed relative to the contraction evoked by 45 mm KCl. Vertical bars signify±s.e. mean. Curves were fitted to the data using nonlinear regression analysis.

Figure 2

Representative autoradiographs obtained in sections of human middle meningeal artery (upper panel) and human coronary artery (lower panel). Sections were labelled with either 5-HT_1B-receptor (Panel A) or 5-HT_1D-receptor (Panel B) subtype specific primary antibodies and a radiolabelled secondary antibody. Levels of immunoreactivity were quantified using densitometery and an image analysis system. 5-HT_1B-immunoreactivity, but not no. 5-HT_1D-immunoreactivity, was detected in both middle meningeal and coronary arteries.

Figure 3

Graphical summary of a three-way anova (artery–agonist–concentration) to compare overall vasoconstrictor effects of 5-HT (circles), rizatriptan (triangles) and sumatriptan (squares) in middle meningeal artery and coronary artery. All three agonists were signifciantly more effective in middle meningeal compared to coronary artery (artery–agonist interaction F = 19.4, d.f. = 2 P < 0.00009). Post-analysis of variance multiple comparisons using Newman-Keuls showed significant differences between the effects of rizatriptan and sumatriptan in the two arteries (P < 0.05).