Modulation of K+ channels by arachidonic acid in T84 cells. I. Inhibition of the Ca2+-dependent K+ channel

Abstract

The Cl− secretory response of colonic cells to Ca2+-mediated agonists is transient despite a sustained elevation of intracellular Ca2+. We evaluated the effects of second messengers proposed to limit Ca2+-mediated Cl− secretion on the basolateral membrane, Ca2+-dependent K+ channel (KCa) in colonic secretory cells, T84. Neither protein kinase C (PKC) nor inositol tetrakisphosphate (1,3,4,5 or 3,4,5,6 form) affected KCa in excised inside-out patches. In contrast, arachidonic acid (AA; 3 μM) potently inhibited KCa, reducing NP o, the product of number of channels and channel open probability, by 95%. The apparent inhibition constant for this AA effect was 425 nM. AA inhibited KCa in the presence of both indomethacin and nordihydroguaiaretic acid, blockers of the cyclooxygenase and lipoxygenase pathways. In the presence of albumin, the effect of AA on KCa was reversed. A similar effect of AA was observed on KCa during outside-out recording. We determined also the effect of the cis-unsaturated fatty acid linoleate, the trans-unsaturated fatty acid elaidate, and the saturated fatty acid myristate. At 3 μM, all of these fatty acids inhibited KCa, reducing NP o by 72–86%. Finally, the effect of the cytosolic phospholipase A2 inhibitor arachidonyltrifluoromethyl ketone (AACOCF3) on the carbachol-induced short-circuit current ( I sc) response was determined. In the presence of AACOCF3, the peak carbachol-induced I sc response was increased ∼2.5-fold. Our results suggest that AA generation induced by Ca2+-mediated agonists may contribute to the dissociation observed between the rise in intracellular Ca2+ evoked by these agonists and the associated Cl− secretory response.