Abstract
The putative coupling between stably expressed recombinant h 5-HT1B or h 5-HT1D receptors and K+ channels which regulate excitability was investigated in C6 glioma cells. Outward K+ currents (I K) were examined in non-transfected C6 glioma cells and in cells expressing cloned h 5-HT1B or h 5-HT1D receptors using the patch-clamp technique in the whole-cell configuration. I K was elicited by a depolarizing step from a holding potential of –60 mV. In C6 glioma cells expressing either recombinant h 5-HT1B or h 5-HT1D receptors, sumatriptan similarly increased I K in a concentration-dependent manner (maximum increase 19.4±7.2%, n=8, P<0.05 and 25.1±3.9%, n=6, P<0.001, respectively) with EC50 values (geometric mean with 95% confidence intervals in parentheses) of 56.3 nM (7.9–140 nM) and 68.7 nM (16–120 nM), respectively. Sumatriptan failed to elicit increases in I K in non-transfected cells, confirming a specific involvement of the respective membrane h 5-HT1B and h 5-HT1D receptors in transfected C6 cells. In the presence of the mixed 5-HT1B/D receptor antagonist GR 127935 (0.1 µM), sumatriptan (1 µM) failed to significantly increase I K in C6 cells expressing h 5-HT1B receptors (–7.5±3.5%, P=NS, n=6), although a higher concentration of GR 127935 (1 µM) was required to significantly inhibit sumatriptan-evoked increases in I K in C6 cells expressing h 5-HT1D receptors (–1.8±3.5%, P=NS, n=6), confirming that sumatriptan-evoked responses were indeed mediated by h 5-HT1B and h 5-HT1D receptors, respectively. In C6 cells expressing either cloned h 5-HT1B or h 5-HT1D receptors, sumatriptan-induced increases in I K were prevented by the calcium chelator EGTA (5 mM) when included in the patch pipette (maximum increase 0.57±0.6%, n=3, P=NS and –2.8±1.6%, n=5, P=NS, respectively). In C6 cells expressing cloned h 5-HT1B receptors, sumatriptan (1 µM) similarly failed to significantly increase I K in the presence of dibutyryl cAMP (10 µM) or when a nominally Ca2+-free medium was included in the patch pipette (–19.4±5.1%, n=5 and –5.2±4.3%, n=5, respectively, P=NS in each case). In addition, the Ca2+-dependent K+ channel blockers iberiotoxin (0.1 µM) and tetraethylammonium (TEA, 1 mM) abolished sumatriptan-induced increases in I K (–0.5±1.0%, n=4 and –3.9±3.1%, n=4, respectively, P=NS in each case) in C6 cells expressing h 5-HT1B receptors, confirming the involvement of Ca2+-dependent K+ channels. In C6 cells expressing cloned h 5-HT1B receptors, sumatriptan (1 µM) similarly failed to significantly increase I k after 30-min incubation with thapsigargin (1 µM) or when heparin (2 mg/ml) was included in the patch pipette (1.1±0.4%, n=5 and 1.2±2.4%, n=5, respectively, P=NS). In conclusion, evidence is provided that both recombinant h 5-HT1B and h 5-HT1D receptors stably transfected in C6 glioma cells are positively coupled to Ca2+-dependent K+ channels, and the outward hyperpolarizing current mediated by these channels is dependent upon IP3 receptor-mediated intracellular Ca2+ release.
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Received: 15 April 1998 / Accepted: 9 September 1998
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Grand, B., Panissié, A., Pauwels, P. et al. Activation of recombinant h 5-HT1B and h 5-HT1D receptors stably expressed in C6 glioma cells produces increases in Ca2+-dependent K+ current. Naunyn-Schmiedeberg's Arch Pharmacol 358, 608–615 (1998). https://doi.org/10.1007/PL00005301
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DOI: https://doi.org/10.1007/PL00005301