Inhibition of Na+,K+-ATPase by Ouabain Opens Calcium Channels Coupled to Acetylcholine Release in Guinea Pig Myenteric Plexus

Abstract: Ouabain, an Na⁺,K⁺-ATPase inhibitor, increases the release of acetylcholine (ACh) from various preparations in a Ca²⁺-independent way. However, in other preparations the release of ACh evoked by ouabain is dependent on the presence of extracellular calcium. In the present study, we have labeled the ACh of myenteric plexus longitudinal muscles of guinea pig ileum and compared the effect of calcium channel blockers on ouabain-evoked release of [³H]ACh. Release of [³H]ACh evoked by ouabain is dose dependent and decreased markedly in the absence of calcium or in the presence of cadmium, a nonspecific calcium channel blocker. N-type calcium channel blockage by the ω-conotoxins GVIA (selective N-type calcium channel blocker) and MVIIC (a nonselective calcium channel blocker) inhibited by 45 and 55%, respectively, the release of [³H]ACh. L-type calcium channel suppression by low concentrations of verapamil, nifedipine, and diltiazem had no effect on the release of [³H]ACh. The release of transmitter was also not affected significantly by nickel, a T-type calcium channel blocker. In addition, ω-agatoxin-IVA, at concentrations that block P- and Q-type calcium channels, did not affect significantly the release of [³H]ACh. Thus, extracellular Ca²⁺ is essential for the release of ACh induced by ouabain from guinea pig ileum myenteric plexus. In this preparation, the N-type calcium channel plays a dominant role in transmitter release evoked by inhibition of Na⁺,K⁺-ATPase, but other routes of calcium entry in addition to these channels can also support the release of neurotransmitter induced by ouabain.