References ADAMS. P.R. (1977). Voltagejump analysis of procaine action at frog end-plate. J. Physiol. Lond. 268, 291-318. COLQUHOUN, D., DREYER, F. & SHERIDAN, R.E. (1978). The action of tubocurarine at the neuromuscular junc- tion. J. PhYsiol. Lond. 284, 171-172P. KATZ, B., & MILEDI, R. (1978). A re-examination of curare action at the motor end-plate. Proc. Roy. Soc. B. in press. RANG, H.P. & RITTER, J.M. (1969). A new kind of drug antagonism: evidence that agonists cause a molecular change in acetylcholine receptors. Mol. Pharmacol. 5, 394-411. Two modes of action of ganglionic blocking drugs P. ASCHER, W.A. LARGE & H.P. RANG Department of Pharmacology, St. George's Hospital Medical School, London, S Wl 7 ORE Non-depolarizing ganglion blocking drugs are usually thought to block acetylcholine receptors competitive- ly, though Blackman (1959) suggested that hex- amethonium acts by blocking ionic channels rather than by competition. We have used a two microelectrode voltage clamp technique on rat submandibular ganglion cells (Ascher, Large & Rang, 1978) to investigate the actions of blocking drugs in more detail. With the membrane potential held constant applica- tion of carbachol (by perfusion or micro- iontophoresis) causes an inward current whose magnitude increases markedly with hyper- polarization. Some antagonists (trimetaphan, 2.5.M; surugatoxin, 0.2 gM) reduce the carbachol-induced current equally at all membrane potentials (i.e. the block is not voltage-dependent). Others (tubocurarine, 5 gM; hexamethonium 2 gM; decamethonium, 50 gM) reduce the current proportionately more as the membrane is hyperpolarized (voltage-dependent block). Defining A as the ratio of current in the absence and in the presence of antagonist, voltage dependence can be expressed as (A_80-1)/(A50_-l) where the subscript represents membrane potential. Values obtained were: trimetaphan 1.18, surugatoxin 0.86, tubocurarine 3.13, hexamethonium 3.17, decamethonium 2.87. Marked voltage dependence suggests, but does not prove, a mechanism other than competitive block. With competitive block A-1 should decrease as the agonist concentration is increased. We tested this by measuring (Aiarge-1)/(AsmaII-) with alternate large and small responses to iontophoretically applied car- bachol, differing 3-4 fold in amplitude. With trimetaphan this ratio is slightly less than one; with voltage-dependent antagonists this ratio ranges from 1.3 to 1.7, which is not consistent with competitive block. This pattern of block increasing with agonist con- centration and with membrane potential favours a channel block mechanism such as that described for procaine and other agents at the neuromuscular junc- tion (Adams, 1977; Feltz, Large & Trautmann, 1977, Neher & Steinbach, 1978) and for various acetylcholine antagonists on Aplysia neurones (Ascher, Marty & Neild, 1978). Kinetic studies were carried out with voltage jumps, in which the change of current with time was followed after a step in membrane potential from -5OmV to -8OmV. With carbachol alone the current increases to a new level within 10 ms (the limit of resolution of our voltage clamp) and no relaxation is detectable. With hexamethonium or tubocurarine present, the current increases rapidly and then decreases exponentially with a time constant of 1-2 s. This relaxation becomes faster if the antagonist concentration is in- creased, as expected for a slowly-dissociating channel blocker. With decamethonium the slow relaxation is in the opposite direction and becomes slower if the decamethonium concentration is increased, as predicted for a rapidly dissociating channel blocker (Adams, 1977, Ascher, Large & Rang, 1978). It is concluded that tubocurarine, hexamethonium and decamethonium act mainly or entirely by blocking ionic channels, whereas trimetaphan and surugatoxin probably block receptors. Surugatoxin was provided by Professor D.A. Brown. The work was supported by the Medical Research Council. References ADAMS, P.R. (1977). Voltage-jump analysis of procaine action at frog end-plate. J. Physiol. Lond., 268, 291-318. ASCHER, P., LARGE, W.A. & RANG, H.P. (1978). The action of ganglion-blocking drugs studied by voltage clamp. J. Physiol. Lond., 280, 17P. ASCHER, P., MARTY, A. & NEILD, T.O. (1978). The mode of action of antagonists of the excitatory response to acetylcholine in Aphysia neurones. J. Physiol. Lond., 278, 207-235. BLACKMAN, J.G. (1959). Thp pharmacology of depressor bases. PhD thesis, University of New Zealand. FELTZ, A., LARGE, W.A. & TRAUTMANN, A. (1977). Analysis of atropine action at the frog neuromuscular junction. J. Physiol. Lond., 269, 109-130. NEHER, E. & STEINBACH, J.H. (1978). Local anaesthetics transiently block currents through single acetylcholine- receptor channels. J. Physiol. Lond., 277, 153-176. 79P View publication stats View publication stats