Zolmitriptan stimulates a Ca2+-dependent K+ current in C6 glioma cells stably expressing recombinant human 5-HT1B receptors

Stimulation of a Ca2+-dependent K+ current by zolmitriptan. a 5-HT1B/1D rec eptor partial agonist, was investigated in C6 glioma cells stably expressin g recombinant human 5-HT1B receptors. Outward K+ currents (I-K) were examin ed in non-transfected C6 glioma cells and in cells expressing cloned human 5-HT1B receptors using the patch-clamp technique in the whole-cell configur ation. In C6 glioma cells expressing recombinant human 5-HT1B receptor, zol mitriptan increased I-K in a concentration-dependent manner (maximum increa se 16.3 +/- 7.8%, n = 5, p < 0.001) with a pD(2) value (geometric mean with 95% confidence intervals) of 7.03 (7.90-6.10). Zolmitriptan failed to elic it increases in I-K in non-transfected C6 cells. In the presence of the mix ed 5-HT1B/(1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2(-methyl4(5-methyl-1,2,4)oxadiazol-3-yl)[1,1-biphenyl]-4-carboxami de 2HCl (GR 127935, 0.1 mu M), zolmitriptan (1 mu M) failed to significantl y increase I-K in C6 cells expressing human 5-HT1B receptors confirming tha t zolmitriptan-evoked responses were indeed mediated by human 5-HT1B recept ors. In C6 cells expressing cloned human 5-HT1B receptors, zolmitriptan-ind uced increases in I-K were prevented by the calcium chelator, EGTA (5 mM) w hen included in the patch pipette (maximum increase -3.3 +/- 4.2%, n = 4, P = NS). The Ca2+-dependent K+ channel blockers, iberiotoxin (0.1 mu M) and tetraethylammonium (TEA, 1 mM), abolished zoimitriptan-induced increases in I-K (4.5 +/- 7.3%, n = 4 and -0.8 +/- 1.7%, n = 4, respectively, P = NS in each case) in C6 cells expressing human 5-HT1B receptors, confirming the i nvolvement of Ca2+-dependent K+ channels. In conclusion, the 5-HT1B/1D rece ptor partial agonist, zolmitriptan, stimulates I-K/Ca in C6 glioma cells st ably transfected with human 5-HT1B receptors suggesting an increase of hype rpolarizing current. (C) 2000 Elsevier Science B.V. All rights reserved.