Otilonium: a potent blocker of neuronal nicotinic ACh receptors in bovine chromaffin cells

1. Otilonium, a clinically useful spasmolytic, behaves as a potent blocker of neuronal nicotinic acetylcholine receptors (AChR) as well as a mild wide-spectrum Ca2+ channel blocker in bovine adrenal chromaffin cells. 2. 45Ca2+ uptake into chromaffin cells stimulated with high K+ (70 mM, 1 min) was blocked by otilonium with an IC50 of 7.6 microM. The drug inhibited the 45Ca2+ uptake stimulated by the nicotinic AChR agonist, dimethylphenylpiperazinium (DMPP) with a 79 fold higher potency (IC50 = 0.096 microM). 3. Whole-cell Ba2+ currents (IBa) through Ca2+ channels of voltage-clamped chromaffin cells were blocked by otilonium with an IC50 of 6.4 microM, very close to that of K(+)-evoked 45Ca2+ uptake. Blockade developed in 10-20 s, almost as a single step and was rapidly and almost fully reversible. 4. Whole-cell nicotinic AChR-mediated currents (250 ms pulses of 100 microM DMPP) applied at 30 s intervals were blocked by otilonium in a concentration-dependent manner, showing an IC50 of 0.36 microM. Blockade was induced in a step-wise manner. Wash out of otilonium allowed a slow recovery of the current, also in discrete steps. 5. In experiments with recordings in the same cells of whole-cell IDMPP, Na+ currents (INa) and Ca2+ currents (ICa), 1 microM otilonium blocked 87% IDMPP, 7% INa and 13% ICa. 6. Otilonium inhibited the K(+)-evoked Catecholamine secretory response of superfused bovine chromaffin cells with an IC50 of 10 microM, very close to the IC50 for blockade of K(+)-induced 45Ca2+ uptake and IBa. 7. Otilonium inhibited the secretory responses induced by 10 s pulses of 50 microM DMPP with an IC50 of 7.4 nM. Hexamethonium blocked the DMPP-evoked responses with an IC50 of 29.8 microM, 4,000 fold higher than that of otilonium. 8. In conclusion, otilonium is a potent blocker of nicotinic AChR-mediated responses. The drugs also blocked various subtypes of neuronal voltage-dependent Ca2+ channels at a considerably lower potency. Na+ channels were unaffected by otilonium. This extraordinary potency of otilonium in blocking nicotinic AChR, unrecognised until now, might account in part for its well known spasmolytic effects.