Mechanism by which the 5-HT1B receptor inhibits vasodilation in bovine pulmonary arteries: role of the Na+:Ca2+ exchanger (NCX) and Na+:H+ exchanger (NHX)

Activation of the 5-HT1B receptor in bovine pulmonary arteries (BPA) inhibits vasodilation mediated by various vasodilators including the -adrenoceptor agonist isoprenaline (McKenzie et al., 2010). This study investigated the relationship between the 5-HT1B receptor, the Na:Ca exchanger (NCX) and the Na:H exchanger (NHX) in isoprenaline-induced relaxation of BPA.

Bovine lungs from animals under 20 months were dissected within 50 minutes of slaughter. Ring segments 0.1-0.3cm in diameter from 4th arterial generations were dissected out and mounted in 5ml Linton organ baths under a resting tension of 2g in Krebs physiological saline solution gassed with 95/5% O2/CO2 at 37°C. Tissues were allowed to equilibrate for 1 hour before contraction with 5-HT (1  M) or U46619 (100nM). The study investigated the effect of the 5-HT1B receptor antagonist SB216641 (1µM, Price et al., 1997), the selective reverse mode NCX inhibitor KB-R7943 (10µM, Watanabe et al., 2006), the forward and reverse mode NCX inhibitor benzamil (10  M, Yamanaka et al., 2003) and the NHX inhibitor 5-(N-Ethyl-N-isopropyl) amiloride (EIPA, 100nM, Horinouchi et al., 2008) on isoprenaline- (1  M) induced relaxation. Inhibitors were pre-incubated for 40mins. Results are expressed as % of the 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.

Isoprenaline induced a partial relaxation (49.98% ± 0.82, n=22) of 5-HT-constricted BPA. The isoprenaline-induced partial relaxation was converted to almost full relaxation in the presence of the 5-HT1B receptor antagonist SB216641 (85.3% ± 0.58, n=22; p< 0.0001), the NCX reverse mode inhibitor KB-R7943 (93.8 ± 2.6, n=9, p < 0.0001) and the NHX inhibitor EIPA (84.7 ± 4.2, n = 9, p< 0.0001) but not the forward and reverse mode NCX inhibitor benzamil. The full relaxation in the presence of SB216641 (84.4 ± 3.9 , n=9) reverted to partial relaxation in the presence of benzamil (55.2 ± 3.5, n=9, p< 0.0001) but not KB-R7943. In U46619-constricted arteries isoprenaline induced almost full relaxation (89.9 ± 4.3, n = 4) but partial relaxation (48.3 ± 4.1, n=4, p<0.0001) in the presence of the 5-HT1B-receptor-selective agonist CP94253 (1  M, Koe et al., 1992) and benzamil (45.7 ± 4.3, n = 4, p<0.0001) but not KB-R7943 however the partial relaxation in the presence of CP94253 was converted to almost full relaxation by KB-R7943 (94.75 ± 4.1, n=5, p< 0.0001).

This study confirms the report by McKenzie et al. (2010) that the 5-HT1B receptor reduces isoprenaline-induced relaxation of BPA by approximately 50%. From the present data we hypothesise that around 50% of the isoprenaline-induced relaxation is mediated through stimulation of the forward mode of NCX promoting Ca2+ extrusion. Activation of the 5HT1B receptor stimulates the reverse mode of NCX opposing the isoprenaline-mediated stimulation of the forward mode of NCX and the relaxation mediated through this mechanism. Activation of NHX, presumably by providing a local increase in [Na+]i, is also required to drive the reverse mode of NCX.

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

Price et al., (1997), NaunynSchmiedebergs Arch Pharmacol 356: 312–320.

Watanabe et al., (2006) J. Pharmacol. Sci. 102, 7-16.

Yamanak et al., (2003) J. Physiol. 549, 553-562.

Horinouchi et al., (2008) J. Pharmacol. Sci. 107, 456-459.

Koe et al., 1992, Drug Development Research. 1992; 26(3): 241-250.