Luminal ATP induces K⁺ secretion via a P2Y₂ receptor in rat distal colonic mucosa

Abstract

 We have previously investigated, in studies of rat distal colonic mucosa, the effect of ATP added to the basolateral side on ion transport and [Ca²⁺]_i. It was demonstrated that ATP acts via a P2Y₁ receptor to increase [Ca²⁺]_i and NaCl secretion. In the present study we investigated the effect of luminally added nucleotides (ATP, UTP) on transepithelial voltage (V _te) and resistance (R _te) in Ussing chamber experiments on rat distal colonic mucosa. Both nucleotides induced a rapid and transient (within 30 s) change of V _te to lumen-positive values (resting V _te: –2±1 mV; peak V _te after 100 µmol/l ATP: +2.4±1.1 mV) and a decrease of R _te from 89.9±10.3 to 83.8±9.1 Ωcm² (n=10). Similar values were obtained with luminal UTP (n=15). The estimated EC₅₀ values for both nucleotides were approximately 6 µmol/l. The ATP-induced V _te effect was nearly completely sensitive to Ba²⁺. Addition of the K⁺ channel blocker Ba²⁺ (1 mmol/l) to the luminal solution reversibly inhibited 77±4% (n=5) of the ATP-induced V _te effect. Experiments to identify the respective P2 receptor subtype revealed the following rank order of potency at 500 µmol/l agonist: UTP≥ATP>>2-methylthio-ATP=ADP>>adenosine> AMP>β,γ-methylene-ATP (n=5). This closely resembles the published rank order for the P2Y₂ receptor. Using the reverse-transcriptase polymerase chain reaction (RT-PCR) technique P2Y₂ receptor-specific mRNA was detected in total RNA extracted from isolated crypts. In summary these data indicate that luminal ATP and UTP act via a P2Y₂ receptor in the luminal membrane of colonic mucosa to elicit a transient K⁺ secretion.