Role of the 5-HT1B receptor in rat basilar and mesenteric arteries.

A Alharbi, A.M. Shaw. Glasgow Caledonian University, Glasgow, UK

The 5-HT1B receptor in bovine pulmonary arteries (BPA) contributes to the 5-HT-induced vasocontraction and also limits vasodilation to a range of vasodilators including the ββ-adrenoceptor agonist isoprenaline (McKenzie et al., 2010). The present study investigated the contribution of the 5-HT1B receptor to the 5-HT-induced contraction in systemic arteries (rat basilar and mesenteric arteries) and its influence on isoprenaline-induced relaxation in these vessels.

Wistar rats, age 12-15 weeks, were killed by CO2 gas and ring segments of basilar (1-1.5 mm in length) or mesenteric (2 mm in length) arteries were dissected and mounted in a small vessel wire myograph (DMT) for isometric recording in Krebs physiological saline. Tissues were left un-stretched for 30 minutes at 37oC before applying a tension of 1.5 mN (basilar) and 2.5 mN (mesenteric). Changes in isometric tension were recorded using Powerlab data collection and Chart 5 software. Tissues were allowed to equilibrate for 30 minutes before the addition of drugs. All tissues were first contracted with KCl (60mM, mesenteric; 80mM, basilar). The study used the following receptor antagonists: GR127935 (100nM, 5-HT1B/1D), SB224289 (1µM, 5-HT1B) and BRL15572 (100nM, 5-HT1D) on concentration response curves (CRC) to 5-HT. The effect of GR127935 was also examined on the contraction induced by a single concentration of 5-HT (1-2 μ M) soprenaline (1µM) induced relaxation was examined in vessels constricted with U46619 (100nM, basilar), phenylephrine (2µM, mesenteric), 5-HT (100-300 nM, both) or 5-HT (1-2 µM) in the presence of GR127935 (both). Results are expressed as % of the contraction to KCl or % of maximum relaxation and are presented as means ± s.e.m. Statistical analysis was carried out using Student's t-test with p < 0.05 considered significant.

5-HT induced a concentration-dependent contraction of basilar (1nM-300µM) and mesenteric arteries (1nM-300µM) that was unaffected by BRL15572. GR127935 (100nM) produced a rightward shift of the CRCs giving estimated pKB values of 8.6 (basilar) and 7.5 (mesenteric). SB224289 (1µM) caused a rightward shift of the CRC (mesenteric) giving an estimated pKB of 6.5 whereas in basilar it produced only a small rightward shift but substantially reduced the maximum contraction (pEC50 and RMAX Values: 5-HT control 6.9 ± 0.06, Rmax 134.8 ± 2.6 %; SB224289, 6.3 ± 0.06, 71.2 ± 1.6%; RMAX p<0.05, n= 6). The tone produced by a single concentration of 5-HT (approx. EC70, 300nM, mesenteric; 150-300nM, basilar; 1-2μM, basilar + GR129935) was poorly sustained in mesenteric falling to 16.9 ± 4.9 (n=4) by 50 min. In basilar arteries the presence of GR127935 significantly increased the loss of tone (50 min, control, 60 ± 3.1; GR127935 34.7±7.8 n=4, p< 0.05). Isoprenaline induced almost full relaxation of mesenteric arteries constricted with 5-HT or phenylephrine and basilar arteries constricted with U46619 or 5-HT in the presence of GR127935 but only partial relaxation (34.6 ± 3.1%) in basilar arteries constricted with 5-HT alone.

This study suggests that the 5-HT1B receptor is more prominent in rat basilar arteries than mesenteric arteries and may be particularly important in mediating the sustained tone induced by 5-HT. The study also demonstrates that the presence of 5-HT1B receptor in basilar arteries limits isoprenaline induced relaxation similar to that described in BPA.

McKenzie et al., (2010) Br. J. Pharmacol. 159, 188-200.