Agonist-induced inhibition of inositol-trisphosphate-activated IK(Ca) in NG108-15 neuroblastoma hybrid cells.

IK(Ca) activated by intracellular ionophoresis of inositol trisphosphate (IP3) or pressure-applied acetylcholine was inhibited by bradykinin and acetylcholine in NG108-15 cells transfected with m1 receptors. The inhibition of the IP3-evoked current was complete at 10 microM acetylcholine. This inhibition was not seen if the current was evoked by intracellular ionophoresis of calcium ions. Only receptors the activate the phosphoinositide system in these cells produced this inhibition, i.e. transfected muscarinic m1 and m3 and bradykinin receptors, but not muscarinic m2, m4 or adrenergic alpha 2 receptors. This inhibition was not sensitive to pertussis toxin or staurosporine. The concentrations of acetylcholine needed to inhibit the evoked current were identical to those needed to raise intracellular calcium but tenfold less than those needed for the agonist to activate IK(Ca). In a normal calcium-containing superfusate, recovery from inhibition required around 8 min (half-time 4 min) after removal of acetylcholine. When the experiment was performed in calcium-free medium no recovery was seen after 8 min washing in drug-free solution, but complete recovery was seen within 3 min (half-time 1.5 min) after adding calcium. Responses to repeated pressure applications of acetylcholine could be reversibly inhibited by acetylcholine and bradykinin. It seems, then, that there is no direct action of acetylcholine or bradykinin on the IK(Ca) channels themselves but that concentrations below those needed to activate IK(Ca) can empty and inhibit the IP3-sensitive calcium store. This may provide a mechanism for heterologous desensitization for phospholipase-C-linked receptor-mediated responses.