Pumiliotoxin-C and synthetic analogues. A new class of nicotinic antagonists

Pumiliotoxin-C (PTX-CI), a cis-decahydroquinoline alkaloid, and synthetic analogues inhibited directly and indirectly elicited twitches in rat phrenic nerve diaphragm preparations. At 40 microM the most potent analogue, 2,5-di-n-propyl-cis-decahydroquinoline (PTX-CII), reversibly blocked the indirectly elicited twitch and markedly depressed the amplitude of end-plate potentials and miniature end-plate potentials without affecting muscle membrane potential. PTX-CII did not depress the directly elicited twitch in diaphragm muscle treated with alpha-bungarotoxin. At 40 microM, the compound blocked extrajunctional acetylcholine (ACh) sensitivity in chronically denervated soleus muscle of the rat. The peak amplitude of the end-plate current in frog sartorius muscle was depressed by PTX-CII at both positive and negative membrane potentials. Nonlinearity in the current-voltage relationship appeared at membrane potentials greater than -100 mV with concentrations of PTX-CII from 2.5 to 7.5 microM. The decay phase of the end-plate current was shifted to lower values such that it decayed faster at all membrane potentials but retained voltage sensitivity. PTX-CII, the most potent of the analogues, inhibited binding of [3H]perhydrohistrionicotoxin to Torpedo electroplax membranes with an IC50 value of 1.5 microM in the absence of carbamylcholine (Carb) and 0.1 microM in the presence of Carb. The other analogues were less potent, but all showed higher affinities in the presence of Carb. PTX-CI, PTX-CII, and PTX-CIII had no effect on the binding of [125I]alpha-bungarotoxin to ACh receptors in Torpedo membranes while slightly enhancing the binding of [3H]ACh, whereas PTX-CIV gave slight inhibition of both receptor ligands at concentrations greater than 10 microM. The pumiliotoxin-C class of alkaloids therefore appears to block neuromuscular transmission primarily via interactions with sites associated with the ion channel controlled by the ACh receptor.