Influence of sodium substitutes on 5-HT-mediated effects at mouse 5-HT3 receptors.
1 The influence of sodium ion substitutes on the 5-hydroxytryptamine (5-HT)-induced flux of the organic cation [14C]guanidinium through the ion channel of the mouse 5-HT3 receptor and on the competition of 5-HT with the selective 5-HT3 receptor antagonist [3H]GR 65630 was studied, unless stated otherwise, in mouse neuroblastoma N1E-115 cells. 2 Under physiological conditions (135 mm sodium), 5-HT induced a concentration-dependent [14C]guanidinium influx with an EC50 (1.3 microm) similar to that in electrophysiological studies. 3 The stepwise replacement of sodium by increasing concentrations of the organic cation hydroxyethyl trimethylammonium (choline) concentration dependently caused both a rightward shift of the 5-HT concentration-response curve and an increase in the maximum effect of 5-HT. Complete replacement of sodium resulted in a 34-fold lower potency of 5-HT and an almost two times higher maximal response. A low potency of 5-HT in choline buffer was also observed in other 5-HT3 receptor-expressing rodent cell lines (NG 108-15 or NCB 20). 4 Replacement of Na+ by Li+ left the potency and maximal effects of 5-HT almost unchanged. Replacement by tris (hydroxymethyl) methylamine (Tris), tetramethylammonium (TMA) or N-methyl-d-glucamine (NMDG) caused an increase in maximal response to 5-HT similar to that caused by choline. The potency of 5-HT was only slightly reduced by Tris, to a high degree decreased by TMA (comparable to the decrease by choline), but not influenced by NMDG. 5 The potency of 5-HT in inhibiting [3H]GR65630 binding to intact cells was 35-fold lower when sodium was completely replaced by choline, but remained unchanged after replacement by NMDG. 6 The results are compatible with the suggestion that choline competes with 5-HT for the 5-HT3 receptor; the increase in maximal response may be partly due to a choline-mediated delay of the 5-HT-induced desensitization. For studies of 5-HT-evoked [14C]guanidinium flux through 5-HT3 receptor channels, NMDG appears to be an 'ideal' sodium substituent since it increases the signal-to-noise ratio without interfering with 5-HT binding.
Barann
M
M
Institut für Pharmakologie und Toxikologie, Universität Bonn, Reuterstrasse 2b, Bonn D-53113, Germany. barann@mailer.meb.uni-bonn.de
Schmidt
K
K
Göthert
M
M
Urban
B W
BW
Bönisch
H
H
D000818:Animals; D001667:Binding, Competitive; D002250:Carbon Radioisotopes; D045744:Cell Line, Tumor; D019791:Guanidine / Q000378:metabolism*; D051379:Mice; D011869:Radioligand Assay; D044406:Receptors, Serotonin, 5-HT3 / Q000378:metabolism*; D012701:Serotonin / Q000494:pharmacology*; D058827:Serotonin 5-HT3 Receptor Agonists; D058831:Serotonin 5-HT3 Receptor Antagonists; D012702:Serotonin Antagonists / Q000494:pharmacology; D017366:Serotonin Receptor Agonists / Q000494:pharmacology; D015222:Sodium Channels / Q000378:metabolism*
D002250:Carbon Radioisotopes; D044406:Receptors, Serotonin, 5-HT3; D058827:Serotonin 5-HT3 Receptor Agonists; D058831:Serotonin 5-HT3 Receptor Antagonists; D012702:Serotonin Antagonists; D017366:Serotonin Receptor Agonists; D015222:Sodium Channels; D012701:Serotonin; D019791:Guanidine