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 Ca-45(2+) uptake into chromaffin cells stimulated with high K+ (70 mM, 1 min) was blocked by otilonium with an IC50 Of 7.6 mu M. The drug inhibited the Ca-45(2+) uptake stimulated by the nicotinic AChR agonist, dimethylphenylpiperazinium (DMPP) with a 79 fold higher potency (IC50 = 0.096 mu M). 3 Whole-cell Ba2+ currents (I-Ba) through Ca2+ channels of voltage-clamped chromaffin cells were blocked by otilonium with an IC50 of 6.4 mu M, very close to that of K+-evoked Ca-45(2+) 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 mu M DMPP) applied at 30 s intervals were blocked by otilonium in a concentration-dependent manner, showing an IC50 of 0.36 mu M. 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 I-DMPP, Na+ currents (I-Na) and Ca2+ currents (I-Ca), 1 mu M otilonium blocked 87% I-DMPP, 7% I-Na and 13% I-Ca. 6 Otilonium inhibited the K+-evoked catecholamine secretory response of superfused bovine chromaffin cells with an IC50 of 10 mu M, very close to the IC50 for blockade of K+-induced Ca-45(2+) uptake and I-Ba. 7 Otilonium inhibited the secretory responses induced by 10 s pulses of 50 mu M DMPP with an IC50 Of 7.4 nM. Hexamethonium blocked the DMPP-evoked responses with an IC50 of 29.8 mu M, 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.