Escherichia coli heat stable enterotoxin (ST.) caused Cl-secretion across T84 cell monolayers in a dose-dependent manner only when applied to the apical membrane surface and not when applied to the basolateral surface. Measurement of cAMP, cGMP, and free cytosolic Ca2l in response to STa suggested that cGMP alone mediated the Cl-secretory response. Studies utilizing blockers of the Na+,K+-ATPase pump, a Na+,K+,ClI cotransport system, a K+ channel, and a Cl-channel suggest that all of them participate in the Clsecretory process induced by ST.. The results suggest that the Cl-secretory response induced by STa is mediated by cGMP after the enterotoxin binds to its receptor on the apical membrane. The enterotoxin, by increasing cGMP, opens a K+ channel on the basolateral membrane as well as a Cl-channel on the apical membrane. The activation of these ion exit mechanisms, together with activations of the Na+,K+,Cl-cotransporter and the Na+,K+-ATPase pump drives Cl-exit through the Cl-channel on the apical membrane.

Confluent T8 monolayers grown on permeable supports and mounted in a modified Ussing chamber secrete chloride (Cl-) in response to prostaglandin El. The threshold stimulation was observed at 10-' M and a maximal effect at 10' M. Unidirectional flux studies showed an increase in both serosal to mucosal and mucosal to serosal CI-fluxes with 10' M prostaglandin El; the increase in serosal to mucosal Cl-flux exceeded the increase in mucosal to serosal flux, resulting in net Cl-secretion.Na' transport was not affected in either direction and the changes in net Cla flux correlated well with the changes in short circuit current. To identify the electrolyte transport pathways involved in the Cla secretory process, the effect of prostaglandin El on ion fluxes was tested in the presence of putative inhibitors. Bumetanide was used as an inhibitor for the basolaterally localized Na',K+,Ca-cotransport system whose existence and bumetanide sensitivity have been verified in earlier studies (Dharmsathaphorn et al. 1984. J. Clin. Invest. 75:462-471). Barium was used as an inhibitor for the K+ efflux pathway on the basolateral membrane whose existence and barium sensitivity were demonstrated in this study by preloading the monolayers with "Rb+ (as a tracer for K+) and simultaneously measuring MRb+ efflux into both serosal and mucosal reservoirs. Both bumetanide and barium inhibited the net chloride secretion induced by prostaglandin El suggesting the involvement of the Na+,K+,Ca-cotransport and a K+ efflux pathways on the basolateral membrane in the Clsecretory process. The activation of another Cl-transport pathway on the apical membrane by prostaglandin El was suggested by Cl-uptake studies. Our findings indicate that the prostaglandin El-stimulated Cl-secretion, which is associated with an increase in cyclic AMP level, intimately involves (a) a bumetanide-sensitive Na+,K+,Cl-cotransport pathway that serves as a C1-uptake step across the basolateral membrane, (b) the stimulation of a barium-sensitive K+ efflux mechanism on the basolateral membrane that most likely acts to recycle K+, and (c) the activation of a Cl-transport pathway on the apical membrane that serves as a Cl-exit pathway.