Studies on the stable inhibition of Na⁺ + K⁺-ATPase by cassaine

Exposure of rat brain Na⁺ + K⁺-ATPase (ATP phosphohydrolase E.C. 3.6.1.3) to concentrations of cassaine greater than 1 × 10^-4 M resulted in a poorly reversible inhibition of this enzyme. Inhibition did not require the presence of ATP and developed rapidly, but the final amount of inhibition observed was independent of time. The amount of inhibition observed at a given concentration of cassaine was reduced by increasing the concentration of membranes in the system. The inhibition of Na⁺ + K⁺-ATPase activity was associated with equivalent inhibition of the phosphorylation and (³H)-ouabain binding reactions of this enzyme, while the uninhibited enzyme was apparently kinetically normal. Concentrations of cassaine which produced this stable inhibition of Na⁺ + K⁺-ATPase had no effect on the Mg²⁺-activated ATPase or the NADH cytochrome-c-reductase activities of crude rat brain microsomal preparations. Cassaine inhibited the cholinesterase activity of rat brain microsomes with a Ki of about 5 × 10^-5 M, but this inhibition was fully reversible. The poorly reversible inhibitory actions of cassaine, thus, appeared specific for Na⁺ + K⁺-ATPase. Because this stable pattern of inhibition of the Na⁺ + K⁺-ATPase by cassaine required drug concentrations at least one hundred-fold greater than those which produce positive inotropic effects, it appears unlikely that this pattern of Na⁺ + K⁺-ATPase inhibition is involved in the cardiotonic actions of this drug.