Santos W C, Hernández-Guijo J M, Ruiz-Nuño A, Olivares R, Jurkiewicz A, Gandía L, García A G
Departamento de Farmacología, Escola Paulista de Medicina, UNIFESP, 04034-970, SP, Sao Paulo, Brazil.
Eur J Pharmacol. 2001 Apr 6;417(1-2):99-109. doi: 10.1016/s0014-2999(01)00897-4.
The blockade of exocytosis induced by the putative endogenous ligand for imidazoline receptors, agmatine, was studied by using on-line measurement of catecholamine release in bovine adrenal medullary chromaffin cells. Agmatine inhibited the acetylcholine-evoked release of catecholamines in a concentration-dependent manner (IC(50)=366 microM); the K(+)-evoked release of catecholamines was unaffected. Clonidine (100 microM) and moxonidine (100 microM) also inhibited by 75% and 50%, respectively, the acetylcholine-evoked response. In cells voltage-clamped at -80 mV, the intermittent application of acetylcholine pulses elicited whole-cell inward currents (I(ACh)) that were blocked 63% by 1 mM agmatine. The onset of blockade was very fast (tau(on) = 31 ms); the recovery of the current after washout of agmatine also occurred very rapidly (tau(off = 39 ms). Efaroxan (10 microM) did not affect the inhibition of I(ACh) elicited by 1 mM agmatine. I(ACh) was blocked 90% by 100 microM clonidine and 50% by 100 microM moxonidine. The concentration-response curve for acetylcholine to elicit inward currents was shifted to the right in a non-parallel manner by 300 microM agmatine. The blockade of I(ACh) caused by agmatine (100 microM) was similar at various holding potentials, around 50%. When intracellularly applied, agmatine did not block I(ACh). At 1 mM, agmatine blocked I(Na) by 23%, I(Ba) by 14%, I(K(Ca)) by 16%, and I(K(VD)) by 18%. In conclusion, agmatine blocks exocytosis in chromaffin cells by blocking nicotinic acetylcholine receptor currents. In contrast to previous views, these effects seem to be unrelated to imidazoline receptors.
利用在线测量牛肾上腺髓质嗜铬细胞中儿茶酚胺释放的方法，研究了咪唑啉受体的假定内源性配体胍丁胺对胞吐作用的阻断。胍丁胺以浓度依赖性方式抑制乙酰胆碱诱发的儿茶酚胺释放（IC(50)=366微摩尔）；钾离子诱发的儿茶酚胺释放不受影响。可乐定（100微摩尔）和莫索尼定（100微摩尔）也分别抑制乙酰胆碱诱发反应的75%和50%。在钳制电压为-80毫伏的细胞中，间歇性施加乙酰胆碱脉冲会引发全细胞内向电流（I(ACh)），1毫摩尔胍丁胺可阻断该电流的63%。阻断起效非常快（tau(on)=31毫秒）；洗脱胍丁胺后电流的恢复也非常迅速（tau(off)=39毫秒）。依酚氯铵（10微摩尔）不影响1毫摩尔胍丁胺对I(ACh)的抑制作用。100微摩尔可乐定可阻断I(ACh)的90%，100微摩尔莫索尼定可阻断50%。乙酰胆碱诱发内向电流的浓度-反应曲线在300微摩尔胍丁胺作用下以非平行方式右移。100微摩尔胍丁胺对I(ACh)的阻断在不同钳制电位下相似，约为50%。当细胞内施加胍丁胺时，它不阻断I(ACh)。在1毫摩尔时，胍丁胺阻断I(Na)的23%、I(Ba)的14%、I(K(Ca))的16%和I(K(VD))的18%。总之，胍丁胺通过阻断烟碱型乙酰胆碱受体电流来阻断嗜铬细胞中的胞吐作用。与先前观点相反，这些作用似乎与咪唑啉受体无关。
