Intestinal epithelial crypts are the sites of ion secretion. Colonic ion secretion contributes to body K⁺ homeostasis and is under the control of numerous systemic and local signaling molecules. Also local purinergic signals control colonic ion transport. Extracellular ATP has been found to activate ion secretion. Most studies have addressed the effect of luminal ATP, which leads to activation of K⁺ secretion. The physiological source of luminal ATP in the intestine remains obscure. Epithelial cells are able to release ATP into the extracellular space by an undefined mechanism. Here we investigate, if ATP can be measured in the luminal fluid and, if activation of ion secretion coincides with the appearance of luminal ATP.

Methods: 

An inverted mouse distal colonic sac preparation was established with a small extracellular volume. A luciferase-luciferin based assay was used to quantify ATP concentrations with pharmacological inhibition of ecto-ATPases. ATP concentrations were measured every 3 min over 45 min.

Results: 

Baseline ATP concentrations in the supernatant were low (~ 0.2 nM) and increased steadily to approximately 0.8 nM during the experiments. Interestingly, the application of IBMX/forskolin to the bath resulted in an immediate and sustained (15 min) increase of luminal ATP concentration to ~ 1.7 nM.

Conclusion: 

These data indicate that activation of colonic ion secretion is paralleled by the secretion of ATP into the luminal space. We speculated that secreted ATP will serve as an activator of apical P2Y₂ and P2Y₄ receptors to trigger further activation of ion secretion.