Electrophysiological effects of various ceveratrum alkaloids on single nerve axons.

1. The effects of the ceveratrum alkaloids protoverine, germine, germine-3-acetate (GMA), germine-3,16-diacetate, germine-3,7,15,16-tetraacetate, veracevine, cevacine, cevadine and veratridine on the resting and action potential of isolated single nerve axons of the lobster, Homarus americanus, were investigated using a double sucrose-oil-gap method. 2. Cevadine had the highest potency, causing a threshold effect (i.e. a small negative after-potential) at 3×10−7 M. Protoverine had no detectable effect on resting or action potential at 10−2 M. 3. The resting potential showed slow oscillations in the presence of GMA, cevacine and cevadine. Veratridine and higher concentrations of cevacine and cevadine caused a sustained depolarization. 4. All alkaloids studied here (except protoverine) prolonged the repolarization of the action potential. With increasing concentrations, this effect showed the following modifications: a negative after-potential, repetitive after-discharge, a plateau-shaped after-depolarization. Veratridine caused negative after-potentials of the greatest duration (half-time of decline 2450–3100 msec), germine- and veracevine-induced negative after-potentials had the shortest duration (half-time of decline 6–8 msec). 5. In the veracevine-series the nature of the 3β-substituent modified 1) the duration of the negative after-potential (the order of potency in slowing the rate of decline of the induced negative after-potential was: veratroyl > angeloyl >acetyl > hydroxyl) and 2) the ratio of depolarizing concentration to threshold concentration for inducing a negative after-potential (the order of potency in increasing this ratio was: hydroxyl > acetyl > angeloyl > veratroyl). 6α-hydroxylation of germine and acetylation of the 16-hydroxyl of GMA lowered the potency for inducing a negative after-potential to less than 1/100 and to 1/300, respectively. 6. GMA was selected for additional experiments using the voltage-clamped giant axon of the squid, Loligo pealii, in order to elucidate the mechanism of action (in terms of ionic conductance changes) of a ceveratrum alkaloid characterized by a negative after-potential of relatively short duration and lack of permanently depolarizing effect (at concentrations up to 10−3 M). 7. In squid axons, GMA (10−3 M, applied to the internal perfusate) was found to induce a transitory late inward current during long-lasting depolarizations and a slowly declining inward tail current occurring upon repolarization, both currents being eliminated by additional application of tetrodotoxin (3×10−7 M) and thus probably being carried predominantly by sodium ions. After sudden repolarization to holding potentials close to the normal resting potential the time constant of the (exponentially decaying) GMA-induced tail current was 8.20±0.52 msec (mean ±S.E.; 9 axons; 23–27°C), reflecting the rate at which the GMA-induced sodium conductance is turned off. It is tentatively suggested that the comparatively low value of this time constant may be the cause for GMA-induced negative after-potentials to decay at a faster rate than those induced by veratridine.