An attempt to distinguish between the actions of neuromuscular blocking drugs on the acetylcholine receptor and on its associated ionic channel

Abstract

The effects of lobeline and tubocurarine on the voltage-clamped endplates of frog sartorius and cutaneous pectoris muscles were examined at room temperature (20-230C The ability of lobeline to shorten ti/2 and to remove the voltage dependence of ti/2 was partially antagonized by Mg2+ (13 mM) As expected, when lobeline or tubocurarine was removed from the bath or when acetylcholine release from the motor nerve terminals was increased by 4-aminopyridine (20 AM) and Ca2+ (10 mM) (in the presence of lobeline or tubocurarine), the amplitude of e.p.c.s increased as a function of time. However, the ti/2 of the decay phase of the e.p.c.s remained shortened (i.e., unaltered from the earlier treatment). These results suggest that both tubocurarine and lobeline have at least two distinct postjunctional actions including: (j) a block of the acetylcholine receptor and (ii) a block of the ionic channel associated with the acetylcholine receptor. Tubocurarine acts both on the acetylcholine (AcCho) receptor and on the AcCho receptor-activated ionic channel at the neuromuscular junction (1-3) to block transmission. The neuromuscular blocking properties of lobeline (4,5) suggest that lobeline, like tubocurarine, may have blocking actions both at the receptor and at the receptor-activated ionic channels. We compared the actions on endplate currents (e.p.c.s) of lobeline with those of tubocurarine and found that both lobeline and tubocurarine act on the ionic channel.In addition, we were able to discriminate between the actions of the drugs on the AcCho receptor and on the ionic channels. By increasing AcCho release, we could selectively antagonize the blocking action of lobeline and tubocurarine on the receptor without affecting the action on the ionic channel. Conversely, in the case of lobeline, we found that Mg2+ selectively antagonizes the blockade' of the ionic channel. Furthermore, by studying the washout of lobeline and tubocurarine from the The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 5003endplates, we demonstrated that the reversal of blockade of the receptor occurs long before the reversal of the blockade of the channel. Some results have been reported (6, 7). METHODS Drugs used were lobeline-HCI (Sigma), tubocurarine (Sigma), and 4-aminopyridine-(4-AP) (J. T. Baker).All the experiments reported here were done on voltageclamped endplates of frog (Rana pipiens) sartorius or cutaneous pectoris muscles at 20-230C in Ringer s solution of the following composition (mM): NaCl, 98.5; KCI, 2.5; CaCl2, 2; NaH2PO4, 1.9; Na2HPO4, 4.8. Ringer's solution containing 10 mM Ca2+ had the following composition (mM): NaCl, 98.5; KCI, 2.5; CaC12, 10; Tris, 2.0.The two-microelectrode voltage clamp described by Deguchi and Narahashi (8) Where mean values of several experiments were presented, the standard error of the mean is included. RESULTSLobeline (50 AM) and tu...

Blockade by 2,2′,2″‐tripyridine of the nicotinic acetylcholine receptor channels in embryonic Xenopus muscle cells

“…The peak amplitude of ACh-induced currents in the whole cell recording was reduced by 2,2',2"-tripyridine in a concentration-dependent manner which corresponded to the depression of both types of low-and high-conductance ACh channels in the single channel recordings. Similar effects have been reported for (+)-tubocurarine (Katz & Miledi, 1978;Lambert et al, 1980;Gibb & Marshall, 1984) and other neuromusclar blocking agents (Colquhoun & Sheridan,198 1). 2,2',2"-Tripyridine increased the run-down of ACh-induced currents at higher frequency stimulations of 7 Hz and 30 Hz, which could be accounted for either by enhancement of the rate of agonist-induced desensitization or by a direct action on the receptor-activated ionic channel.…”

“…The action of the nondepolarizing neuromuscular blocking agents, (+ )-tubocurarine and lobeline on both the ACh receptor and ACh receptor-activated ionic channel at the neuromuscular junction have been studied in some detail (Katz & Miledi, 1978;Lambert et al, 1980;Gibb & Marshall, 1984). Initial work indicated that (+)-tubocurarine competitively blocked ACh receptors with no effect on the channel lifetime (Katz & Miledi, 1978), despite some earlier evidence that the drug shortened endplate current (e.p.c) decay (Beranek & Vyskocil, 1968) and blocked AChactivated channels in Aplysia neurone (Marty et al, 1976).…”

Pre‐and post‐junctional effects of tubocurarine and other nicotinic antagonists during repetitive stimulation in the rat.

“…Blockade of the ACh-activated ion channel is one such mechanism and has been demonstrated for a wide range of compounds exemplified by the barbiturates (Adams, 1976(Adams, , 1981 and local anaesthetics (Beam, 1976a, b;Ruff, 1976Ruff, , 1977Ruff, , 1982. Tubocurarine possesses such an action in the frog (Manalis, 1977;Katz & Miledi, 1978;Colquhoun et al 1979;Lambert, Volle & Henderson, 1980;Shaker, Eldefrawi, Aguayo, Warnick, Albuquerque & Eldefrawi, 1982) and this action has been postulated as the mechanism by which tubocurarine produces e.p.c. train run-down at the neuromuscular junction (Dreyer, 1982).…”

Regulation of Brain Nicotinic Receptors by Chronic Agonist Infusion

“…Lobeline also has been classified as a nicotinelike agonist (Barlow and Johnson, 1989); its cataleptic actions can be reversed by the nicotinic antagonist mecamylamine (Zetler, 1968, 197 1). Lobeline is not, however, a pure nicotinic agonist, because some of its actions can be blocked by the muscarinic antagonist scopolamine (Zetler,197 l), and it has neuromuscular and sodium channel blocking effects (Lambert et al, 1980).…”