Role of luminal ATP in regulating electrogenic Na+ absorption in guinea pig distal colon

Extracellular ATP regulates a variety of functions in epithelial tissues by activating the membrane P2-receptor. The purpose of this study was to investigate the autocrine/paracrine regulation by luminal ATP of electrogenic amiloride-sensitive Na+ absorption in the distal colon from guinea pigs treated with aldosterone by measuring the amiloride-sensitive short-circuit current (I-sc) and Na-22(+) flux in vitro with the Ussing chamber technique. ATP added to the luminal side inhibited the amiloride-sensitive I-sc and Na-22(+) absorption to a similar degree. The concentration dependence of the inhibitory effect of ATP on amiloride-sensitive I-sc had an IC50 value of 20-30 muM, with the maximum inhibition being similar to50%. The effects of different nucleotides and of a nucleoside were also studied, the order of potency being ATP = UTP > ADP > adenosine. The effects of ATP were slightly, but significantly, reduced in the presence of suramin in the luminal solution. The inhibitory effect of luminal ATP was more potent in the absence of both Mg2+ and Ca2+ from the luminal solution. Pretreatment of the tissue with ionomycin or thapsigargin in the absence of serosal Ca2+ did not affect the percent inhibition of amiloride-sensitive I-sc induced by ATP. Mechanical perturbation with a hypotonic luminal solution caused a reduction in amiloride-sensitive I-sc, this effect being prevented by the presence of hexokinase, an ATP-scavenging enzyme. These results suggest that ATP released into the luminal side by hypotonic stimulation could exert an inhibitory effect on the electrogenic Na+ absorption. This effect was probably mediated by a P2Y(2) receptor on the apical membrane of colonic epithelial cells, and a change in the intracellular Ca2+ concentration may not be necessary for this process.