Inhibition of thromboxane A(2)-induced Cl(-) secretion by antidiarrhea drug loperamide in isolated rat colon.

The antitumor drug irinotecan clinically causes severe diarrhea as a side effect. Thromboxane A(2) (TXA(2)), released by irinotecan, has been shown to be a novel physiological stimulant of Cl(-) secretion in the rat colon. Herein, we examined the effect of loperamide, an antidiarrhea drug, on Cl(-) secretion induced by irinotecan; 9, 11-epithio-11,12-methano-thromboxane A(2) (STA(2)), a stable TXA(2) analog; and prostaglandin E(2) (PGE(2)) by using isolated mucosae of the rat colon. In the presence of atropine, loperamide in a concentration-dependent manner inhibited the Cl(-) secretion induced by irinotecan, STA(2), and PGE(2). However, the drug inhibited more effectively the irinotecan- and STA(2)-induced secretion (IC(50) = 0. 7 and 1.2 microM, respectively) than the PGE(2)-induced secretion (IC(50) = 23 microM). Naloxone, an opiate antagonist, did not affect the antisecretory action of loperamide. Similar to the case for loperamide, W-7, a specific calmodulin antagonist, inhibited more effectively the STA(2)-induced Cl(-) secretion (IC(50) = 5 microM) than the PGE(2)-induced secretion (IC(50) = 36 microM). W-5, a low-affinity calmodulin antagonist (a dechlorinated control analog of W-7), also inhibited the STA(2)-induced secretion, but this effect was much less than that of W-7. STA(2)-induced increase in the intracellular free Ca(2+) concentration of single colonic crypt cells was not affected by loperamide. We suggest that loperamide efficiently inhibits the TXA(2)-induced secretion by blocking the calmodulin system in the colonic epithelium. The present results may explain why coadministration of loperamide with irinotecan is clinically efficient for avoiding the irinotecan-induced side effect of diarrhea.