IDENTIFICATION OF APAMIN BINDING-SITES IN RAT INTESTINAL-MUCOSA

Apamin, a specific blocker of one class of Ca2+-activated K+ channels, was used to detect the apamin receptors associated with K+ channels in the mucosa of the rat jejunum and colon. Two receptor sites for I-125-apamin have been identified. These sites differed in their affinity for apamin (jejunum: K(D1) = 1.1 nM and K(D2) = 170 nm; colon: K(D1) = 0.5 nm and K(D2) = 140 nM) and the maximum number of sites (jejunum: B(max1) = 111 and B(max2) = 4030; colon: B(max1) = 187 and B(max2) = 7550 fmol/mg of protein). I-125-apamin binding was stimulated by K+ ions with K0.5 = 1.0 mM and inhibited by the neuromuscular blocker tubocurarine (K(I) = 50-mu-M). We interpret these data to demonstrate that the high-affinity, low-capacity binding sites reflect the existence of apamin-sensitive K+ channels in the intestinal mucosa.