MODULATION OF K-ACID IN T84 CELLS - I - INHIBITION OF THE CA2+-DEPENDENT K+ CHANNEL( CHANNELS BY ARACHIDONIC)

The Cl- secretory response of colonic cells to Ca2+-mediated agonists is transient despite a sustained elevation of intracellular Ca2+. We e valuated the effects of second messengers proposed to limit Ca2+-media ted Cl- secretion on the basolateral membrane, Ca2+-dependent K+ chann el (K-Ca) in colonic secretory cells, T84. Neither protein kinase C (P KC) nor inositol tetrakisphosphate (1,3,4,5 or 3,4,5,6 form) affected K-Ca in excised inside-out patches. In contrast, arachidonic acid (AA; 3 mu M) potently inhibited K-Ca, reducing NPo, the product of number of channels and channel open probability, by 95%. The apparent inhibit ion constant for this AA effect was 425 nM. AA inhibited K-Ca in the p resence of both indomethacin and nordihydroguaiaretic acid, blockers o f the cyclooxygenase and lipoxygenase pathways. In the presence of alb umin, the effect of AA on K-Ca was reversed. A similar effect of AA wa s observed on K-Ca during outside-out recording. We determined also th e effect of the cis-unsaturated fatty acid linoleate, the trans-unsatu rated fatty acid elaidate, and the saturated fatty acid myristate. At 3 mu M, all of these fatty acids inhibited K-Ca, reducing NPo by 72-86 %. Finally, the effect of the cytosolic phospholipase A(2) inhibitor a rachidonyltrifluoromethyl ketone (AACOCF(3)) on the carbachol-induced short-circuit current (I-sc) response was determined. In the presence of AACOCF(3), the peak carbachol-induced I-sc response was increased s imilar to 2.5-fold. Our results suggest that AA generation induced by Ca2+-mediated agonists may contribute to the dissociation observed bet ween the rise in intracellular Ca2+ evoked by these agonists and the a ssociated Cl- secretory response.