A novel type of ATP block on a Ca2+-activated K+ channel from bullfrog erythrocytes

Using the patch-clamp technique, we have identified an intermediate conduct ance Ca2+-activated K+ channel from bullfrog (Rana catesbeiana) erythrocyte s and have investigated the regulation of channel activity by cytosolic ATP , The channel was highly selective for K+ over Na+, gave a linear I-V relat ionship with symmetrical 117.5 mM K+ solutions and had a single-channel con ductance of 60 pS, Channel activity was dependent on Ca2+ concentration (K- 1/2 = 600 nM) but voltage-independent, These basic characteristics are simi lar to those of human and frog erythrocyte Ca2+-activated K+ (Gardos) chann els previously reported. However, cytoplasmic application of ATP reduced ch annel activity with block exhibiting a novel bell-shaped concentration depe ndence. The channel was inhibited most by similar to 10 mu M ATP (P-O reduc ed to 5% of control) but less blocked by lower and higher concentrations of ATP, Moreover, the novel type of ATP block did not require Mg2+, was indep endent of PKA or PKC, and was mimicked by a nonhydrolyzable ATP analog, AMP -PNP. This suggests that ATP exerts its effect by direct binding to sites o n the channel or associated regulatory proteins, but not by phosphorylation of either of these components.