Intestinal secretagogues increase cytosolic free Ca2+ concentration and K+ conductance in a human intestinal epithelial cell line

A human intestinal epithelial cell line (Intestine 407) is known to retain receptors for intestinal secretagogues such as acetylcholine (ACh), histamine, serotonin (5-HT) and vasoactive intestinal peptide (VIP). The cells were also found to possess separate receptors for secretin and ATP, the stimulation of which elicited transient hyperpolarizations coupled to decreased membrane resistances. These responses were reversed in polarity at the K+ equilibrium potential. The hyperpolarizing responses to six agonists were reversibly inhibited by quinine or quinidine. By means of Ca2(+)-selective microelectrodes, increases in the cytosolic free Ca2+ concentration were observed in response to individual secretagogues. The time course of Ca2+ responses coincided with that of hyperpolarizing responses. The responses to ACh and 5-HT were abolished by a reduction in the extracellular Ca2+ concentration down to pCa 7 or by application of Co2+. Thus, in Intestine 407 cells, not only the intestinal secretagogues, which are believed to act via increased cytosolic Ca2+ (ACh, 5-HT and histamine), but also those which elevate cyclic AMP (VIP, secretin and ATP) induce increases in cytosolic Ca2+, thereby activating the K+ conductance. It is likely that the origin of increased cytosolic Ca2+ is mainly extracellular for ACh- and 5-HT-induced responses, whereas histamine, VIP, secretin and ATP mobilize Ca2+ from the internal compartment.