Levine S A, Donowitz M, Watson A J, Sharp G W, Crane J K, Weikel C S
Department of Physiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Am J Physiol. 1991 Oct;261(4 Pt 1):G592-601. doi: 10.1152/ajpgi.1991.261.4.G592.
STa, the heat-stable enterotoxin of Escherichia coli, is a specific activator of membrane-bound guanylyl cyclase and stimulates secretion of Cl- in a human colonic carcinoma cell line (T84). We investigated the effect of the cholinergic agent carbachol on the secretory response to STa. T84 cell monolayers were studied under voltage-clamped conditions in modified Ussing chambers. Simultaneous addition of STa and carbachol resulted in a biphasic synergistic response characterized by a brief peak in short-circuit current (Isc) followed by a prolonged plateau phase lasting up to 90 min. A synergistic response was also seen with sequential addition of the agonists, and was altered by the order and timing of agonist addition. Pretreatment with STa enhanced the synergistic response to carbachol, while the reverse order of additions produced synergy only when STa was added during or immediately after the Isc response to carbachol. Synergy occurred only with a concentration of STa sufficient to produce an Isc response alone. However, a concentration of carbachol that caused neither an increase in Isc nor intracellular Ca2+ mobilization was sufficient to evoke a synergistic response. Addition of 8-bromoguanosine 3',5'-cyclic monophosphate also produced a synergistic Isc response with carbachol, although maximal synergism was seen with simultaneous addition. Augmentation of the intracellular Ca2+ response to carbachol by STa is not the mechanism of synergy. Although the mechanism of synergy is not understood, these studies suggest that STa-induced cGMP interacts with other second messengers to produce the synergistic response, and that multiple intracellular mediators may influence the ability of STa to cause disease.
大肠杆菌热稳定肠毒素（STa）是膜结合鸟苷酸环化酶的特异性激活剂，可刺激人结肠癌细胞系（T84）分泌氯离子。我们研究了胆碱能药物卡巴胆碱对STa分泌反应的影响。在改良的尤斯灌流小室中，在电压钳制条件下研究了T84细胞单层。同时添加STa和卡巴胆碱会产生双相协同反应，其特征是短路电流（Isc）出现短暂峰值，随后是长达90分钟的延长平台期。依次添加激动剂也观察到协同反应，并且该反应会因激动剂添加的顺序和时间而改变。用STa预处理可增强对卡巴胆碱的协同反应，而相反的添加顺序仅在对卡巴胆碱的Isc反应期间或之后立即添加STa时才产生协同作用。协同作用仅在足以单独产生Isc反应的STa浓度下发生。然而，既不引起Isc增加也不引起细胞内Ca2+动员的卡巴胆碱浓度足以引发协同反应。添加8-溴鸟苷3'，5'-环一磷酸也与卡巴胆碱产生协同的Isc反应，尽管同时添加时观察到最大协同作用。STa增强对卡巴胆碱的细胞内Ca2+反应不是协同作用的机制。尽管协同作用的机制尚不清楚，但这些研究表明，STa诱导的cGMP与其他第二信使相互作用以产生协同反应，并且多种细胞内介质可能影响STa致病的能力。
